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7 Oct 2006

Volume 125, Issue 13, Articles (13xxxx)

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back to top Gas Phase Collision Processes

Reaction dynamics of OH+(math)+C2H2 studied with crossed beams and density functional theory calculations

Li Liu, Courtney Martin, and James M. Farrar

J. Chem. Phys. 125, 133117 (2006); http://dx.doi.org/10.1063/1.2212417 (7 pages) | Cited 1 time

Online Publication Date: 4 October 2006

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The reactions between OH+(math) and C2H2 have been studied using crossed ion and molecular beams and density functional theory calculations. Both charge transfer and proton transfer channels are observed. Products formed by carbon-carbon bond cleavage analogous to those formed in the isoelectronic O(math)+C2H2 reaction, e.g., mathH2+HCO+, are not observed. The center of mass flux distributions of both product ions at three different energies are highly asymmetric, with maxima close to the velocity and direction of the precursor acetylene beam, characteristic of direct reactions. The internal energy distributions of the charge transfer products are independent of collision energy and are peaked at the reaction exothermicity, inconsistent with either the existence of favorable Franck-Condon factors or energy resonance. In proton transfer, almost the entire reaction exothermicity is transformed into product internal excitation, consistent with mixed energy release in which the proton is transferred with both the breaking and forming bonds extended. Most of the incremental translational energy in the two higher-energy experiments appears in product translational energy, providing an example of induced repulsive energy release.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Hf Product distribution
82.20.Db Transition state theory and statistical theories of rate constants

Dynamics of ionization of H2 by Ne*(math) investigated by electron spectroscopy

Joseph H. Noroski and P. E. Siska

J. Chem. Phys. 125, 133118 (2006); http://dx.doi.org/10.1063/1.2206781 (9 pages) | Cited 1 time

Online Publication Date: 5 October 2006

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The Penning ionization reaction Ne*(2p53s math)+H2→[NeH2]++e− has been studied in crossed supersonic molecular beams with electron-energy analysis at four collision energies E = 1.83, 2.50, 3.16, and 3.89 kcal/mol. The electron kinetic-energy spectra, which directly reflect the ionizing transition region, show resolved peaks assignable to v′ = 0–4 of H2+. The vibrational populations deviate systematically from Franck-Condon behavior, suggesting that the discrete-continuum coupling increases with H2 bond stretching. Each peak displays both increasing breadth and increasing blueshift with increasing E, and the blueshift also increases with increasing v′. The first two properties are consistent with a predominantly repulsive excited-state potential-energy surface, while the last is speculated to be a reflection of the rHH dependence of the ionic surface. Quantum scattering calculations based on ab initio potential surfaces for the excited and ionic states in spherical and infinite-order-sudden rigid rotor approximations are in semiquantitative agreement with the measurements. Discrepancies suggest changes in the imaginary, absorptive part of the excited surface, which probably can be best effected by multiproperty fitting calculations.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Ej Quantum theory of reaction cross section

Direct ab initio molecular dynamics study on a microsolvated SN2 reaction of OH−(H2O) with CH3Cl

Hiroto Tachikawa

J. Chem. Phys. 125, 133119 (2006); http://dx.doi.org/10.1063/1.2229208 (10 pages) | Cited 9 times

Online Publication Date: 5 October 2006

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Reaction dynamics for a microsolvated SN2 reaction OH−(H2O)+CH3Cl have been investigated by means of the direct ab initio molecular dynamics method. The relative center-of-mass collision energies were chosen as 10, 15, and 25 kcal/mol. Three reaction channels were found as products. These are (1) a channel leading to complete dissociation (the products are CH3OH+Cl−+H2O: denoted by channel I), (2) a solvation channel (the products are Cl−(H2O)+CH3OH: channel II), and (3) a complex formation channel (the products are CH3OH⋯H2O+Cl−: channel III). The branching ratios for the three channels were drastically changed as a function of center-of-mass collision energy. The ratio of complete dissociation channel (channel I) increased with increasing collision energy, whereas that of channel III decreased. The solvation channel (channel II) was minor at all collision energies. The selectivity of the reaction channels and the mechanism are discussed on the basis of the theoretical results.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Bc State selected dynamics and product distribution
82.20.Wt Computational modeling; simulation
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

Quasiclassical trajectory study of the reaction H+CH4(ν3 = 0,1)→CH3+H2 using a new ab initio potential energy surface

Zhen Xie, Joel M. Bowman, and Xiubin Zhang

J. Chem. Phys. 125, 133120 (2006); http://dx.doi.org/10.1063/1.2238871 (8 pages) | Cited 25 times

Online Publication Date: 5 October 2006

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Detailed quasiclassical trajectory calculations of the reaction H+CH4(ν3 = 0,1)→CH3+H2 using a slightly updated version of a recent ab initio-based CH5 potential energy surface [ X. Zhang et al., J. Chem. Phys. 124, 021104 (2006) ] are reported. The reaction cross sections are calculated at initial relative translational energies of 1.52, 1.85, and 2.20 eV in order to make direct comparison with experiment. The relative reaction cross section enhancement ratio due to the excitation of the C–H antisymmetric stretch varies from 2.2 to 3.0 over this energy range, in good agreement with the experimental result of 3.0±1.5 [ J. P. Camden et al., J. Chem. Phys. 123, 134301 (2005) ]. The laboratory-frame speed and center-of-mass angular distributions of CH3 are calculated as are the vibrational and rotational distributions of H2 and CH3. We confirm that this reaction occurs with a combination of stripping and rebound mechanisms by presenting the impact parameter dependence of these distributions and also by direct examination of trajectories.
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82.20.Kh Potential energy surfaces for chemical reactions
82.20.Fd Collision theories; trajectory models
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Crossed molecular beam studies on the reaction dynamics of O(math)+N2O

Yu-Ju Lu, Chi-Wei Liang, and Jim J. Lin

J. Chem. Phys. 125, 133121 (2006); http://dx.doi.org/10.1063/1.2202828 (8 pages) | Cited 1 time

Online Publication Date: 5 October 2006

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The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N2+O2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(math) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N2+O2 channel (fT = 0.57) is much higher than that of the NO+NO channel (fT = 0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The mathO product of the isotope exchange channel, math+mathO→math+mathO, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
82.20.Tr Kinetic isotope effects including muonium
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
back to top Spectroscopy

The bending vibrational levels of the acetylene cation: A case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Sheunn-Jiun Tang, Yung-Ching Chou, Jim Jr-Min Lin, and Yen-Chu Hsu

J. Chem. Phys. 125, 133201 (2006); http://dx.doi.org/10.1063/1.2199827 (15 pages) | Cited 10 times

Online Publication Date: 2 October 2006

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Forty three vibronic levels of C2H2+, math math, with υ4 = 0–6, υ5 = 0–3, and K = 0–4, lying at energies of 0–3520 cm−1 above the zero-point level, have been recorded at rotational resolution. These levels were observed by double resonance, using 1+1′ two-color pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. The intermediate states were single rovibrational levels chosen from the math math, 4ν3 (K = 1–2), 5ν3 (K = 1), ν2+4ν3 (K = 0), and 47 206 cm−1 (K = 1) levels of C2H2. Seven of the trans-bending levels of C2H2+ (υ4 = 0–3, K = 0–2) had been reported previously by Pratt et al. [ J. Chem. Phys. 99, 6233 (1993) ]; our results for these levels agree well with theirs. A full analysis has been carried out, including the Renner-Teller effect and the vibrational anharmonicity for both the trans- and cis-bending vibrations. The rotational structure of the lowest 16 vibronic levels (consisting of the complete set of levels with υ4+υ5 ⩽ 2, except for the unobserved upper math component of the 2ν4 overtone) could be fitted by least squares using 16 parameters to give an rms deviation of 0.21 cm−1. The vibronic coupling parameter ε5 (about whose magnitude there has been controversy) was determined to be −0.02737. For the higher vibronic levels, an additional parameter, r45, was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the υ4+υ5 = 3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with υ4+υ5 ⩽ 5, an rms deviation of 0.34 cm−1 was obtained using 20 parameters. An interesting finding is that Hund’s spin-coupling cases (a) and (b) both occur in the Σu components of the ν4+2ν5 combination level. The ionization potential of C2H2 (from the lowest rotational level of the ground state to the lowest rotational level of the cation) is found to be 91 953.77±0.09 cm−1 (3σ).
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33.20.Tp Vibrational analysis
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Probing the electronic structure of UO+ with high-resolution photoelectron spectroscopy

Vasiliy Goncharov, Leonid A. Kaledin, and Michael C. Heaven

J. Chem. Phys. 125, 133202 (2006); http://dx.doi.org/10.1063/1.2213262 (8 pages) | Cited 14 times

Online Publication Date: 2 October 2006

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The pulsed field ionization–zero kinetic energy photoelectron technique has been used to observe the low-lying energy levels of UO+. Rotationally resolved spectra were recorded for the ground state and the first nine electronically excited states. Extensive vibrational progressions were characterized. Ω+ assignments were unambiguously determined from the first rotational lines identified in each vibronic band. Term energies, vibrational frequencies, and anharmonicity constants for low-lying energy levels of UO+ are reported. In addition, accurate values for the ionization energies for UO [48643.8(2) cm−1] and U [49957.6(2) cm−1] were determined. The pattern of low-lying electronic states for UO+ indicates that they originate from the U3+(5f3)O2− configuration, where the uranium ion-centered interactions between the 5f electrons are significantly stronger than interactions with the intramolecular electric field. The latter lifts the degeneracy of U3+ ion-core states, but the atomic angular momentum quantum numbers remain reasonably well defined.
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33.60.+q Photoelectron spectra
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra

Hui Wu, Wen-kai Zhang, Wei Gan, Zhi-feng Cui, and Hong-fei Wang

J. Chem. Phys. 125, 133203 (2006); http://dx.doi.org/10.1063/1.2352746 (12 pages) | Cited 9 times

Online Publication Date: 3 October 2006

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Knowledge of the ratios between different polarizability βi′j′k′ tensor elements of a chemical group in a molecule is crucial for quantitative interpretation and polarization analysis of its sum frequency generation vibrational spectroscopy (SFG-VS) spectrum at interface. The bond additivity model (BAM) or the hyperpolarizability derivative model along with experimentally obtained Raman depolarization ratios has been widely used to obtain such tensor ratios for the CH3, CH2, and CH groups. Successfully, such treatment can quantitatively reproduce the intensity polarization dependence in SFG-VS spectra for the symmetric (SS) and asymmetric (AS) stretching modes of CH3 and CH2 groups, respectively. However, the relative intensities between the SS and AS modes usually do not agree with each other within this model even for some of the simplest molecular systems, such as the air/methanol interface. This fact certainly has cast uncertainties on the effectiveness and conclusions based on the BAM. One of such examples is that the AS mode of CH3 group has never been observed in SFG-VS spectra from the air/methanol interface, while this AS mode is usually very strong for SFG-VS spectra from the air/ethanol interface, other short chain alcohol, as well as long chain surfactants. In order to answer these questions, an empirical approach from known Raman and IR spectra is used to make corrections to the BAM. With the corrected ratios between the βi′j′k′ tensor elements of the SS and AS modes, all features in the SFG-VS spectra of the air/methanol and air/ethanol interfaces can be quantitatively interpreted. This empirical approach not only provides new understandings of the effectiveness and limitations of the bond additivity model but also provides a practical way for its application in SFG-VS studies of molecular interfaces.
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68.03.Kn Dynamics (capillary waves)
68.03.Hj Liquid surface structure: measurements and simulations
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
63.50.-x Vibrational states in disordered systems
78.30.-j Infrared and Raman spectra

Gold as hydrogen: Structural and electronic properties and chemical bonding in Si3Au3+/0/− and comparisons to Si3H3+/0/−

Boggavarapu Kiran, Xi Li, Hua-Jin Zhai, and Lai-Sheng Wang

J. Chem. Phys. 125, 133204 (2006); http://dx.doi.org/10.1063/1.2216707 (7 pages) | Cited 19 times

Online Publication Date: 4 October 2006

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A single Au atom has been shown to behave like H in its bonding to Si in several mono- and disilicon gold clusters. In the current work, we investigate the Au/H analogy in trisilicon gold clusters, Si3Au3+/0/−. Photoelectron spectroscopy and density functional calculations are combined to examine the geometric and electronic structure of Si3Au3−. We find that there are three isomers competing for the ground state of Si3Au3− as is the case for Si3H3−. Extensive structural searches show that the potential energy surfaces of the trisilicon gold clusters (Si3Au3−, Si3Au3, and Si3Au3+) are similar to those of the corresponding silicon hydrides. The lowest energy isomers for Si3Au3− and Si3Au3 are structurally similar to a Si3Au four-membered ring serving as a common structural motif. For Si3Au3+, the 2π aromatic cyclotrisilenylium auride ion, analogous to the aromatic cyclotrisilenylium ion (Si3H3+), is the most stable species. Comparison of the structures and chemical bonding between Si3Au3+/0/− and the corresponding silicon hydrides further extends the isolobal analogy between Au and H.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Cg Electronic and magnetic properties of clusters
33.60.+q Photoelectron spectra
31.15.E- Density-functional theory
31.50.Bc Potential energy surfaces for ground electronic states
33.15.Fm Bond strengths, dissociation energies

Decay dynamics of the long-range math math state of D2 and H2: Experiment and theory

Stephen C. Ross, Toshio Yoshinari, Yoshihiro Ogi, and Koichi Tsukiyama

J. Chem. Phys. 125, 133205 (2006); http://dx.doi.org/10.1063/1.2264331 (24 pages) | Cited 7 times

Online Publication Date: 5 October 2006

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We present accurate experimental measurements of the lifetimes of rovibrational levels of the long-range math math state for both D2 and H2, obtained directly from the observation of the time-dependent decay of the fluorescence from these excited levels. These results improve upon and extend those of Reinhold et al. [J. Chem. Phys. 112, 10754 (2000) ]. Several decay pathways are open to these levels including fluorescence, predissociation, and autoionization. We present theoretical results for each of these processes, each calculated using the simplest but still appropriate level of theory. In particular, the theoretical calculations provide a quantitative explanation of the dramatic vibrational dependence of the observed lifetimes, the isotope dependence of the lifetimes for levels well localized within the math potential well and therefore not subject to significant tunneling, and an insight into the role of enhanced tunneling in autoionization. In these calculations each of the rovibrational levels of the math state is treated individually, without having to engage in a global coupled-state calculation.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.20.Vq Vibration-rotation analysis
33.50.Dq Fluorescence and phosphorescence spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Eh Autoionization, photoionization, and photodetachment

The calculated infrared spectrum of Cl−H2O using a new full dimensional ab initio potential surface and dipole moment surface

Jaime Rheinecker and Joel M. Bowman

J. Chem. Phys. 125, 133206 (2006); http://dx.doi.org/10.1063/1.2209675 (10 pages) | Cited 8 times

Online Publication Date: 5 October 2006

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We report a full dimensional, ab initio-based global potential energy surface (PES) and dipole moment surface for Cl−H2O. Both surfaces are symmetric with respect to interchange of the H atoms. The PES is a fit to thousands of electronic energies calculated using the coupled-cluster method [CCSD(T)] with a moderately large basis (aug-cc-pVTZ). Vibrational energies and wave functions are accurately obtained using MULTIMODE. The wave function and dipole moment surface are used to calculate and analyze the pure infrared spectrum at 0 K which is compared with experiment. Vibrational energies and the infrared spectra for DOD and HOD/DOH are also presented.
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33.20.Ea Infrared spectra
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
31.50.-x Potential energy surfaces
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
36.40.Mr Spectroscopy and geometrical structure of clusters
back to top Photodissociation and Photoionization

Vibrationally mediated photodissociation of ethene isotopic variants preexcited to the fourth C–H stretch overtone

Evgeny Bespechansky, Alexander Portnov, Amir Zwielly, Salman Rosenwaks, and Ilana Bar

J. Chem. Phys. 125, 133301 (2006); http://dx.doi.org/10.1063/1.2217743 (8 pages) | Cited 7 times

Online Publication Date: 2 October 2006

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H and D photofragments produced via vibrationally mediated photodissociation of jet-cooled normal ethene (C2H4), 1,2-trans-d2-ethene (HDCCDH), and 1,1-d2-ethene (CH2CD2), initially excited to the fourth C–H stretch overtone region, were studied for the first time. H and D vibrational action spectra and Doppler profiles were measured. The action spectra include partially resolved features due to rotational cooling, while the monitored room temperature photoacoustic spectra exhibit only a very broad feature in each species. Simulation of the spectral contours allowed determination of the band types and origins, limited precision rotational constants, and linewidths, providing time scales for energy redistribution. The H and D Doppler profiles correspond to low average translational energies and show slight preferential C–H over C–D bond cleavage in the deuterated variants. The propensities toward H photofragments emerge even though the energy flow out of the initially prepared C–H stretch is on a picosecond time scale and the photodissociation occurs following internal conversion, indicating a more effective release of the light H atoms.
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33.20.Tp Vibrational analysis
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)
33.70.Jg Line and band widths, shapes, and shifts
33.80.Gj Diffuse spectra; predissociation, photodissociation

Collision-free photochemistry of methylazide: Observation of unimolecular decomposition of singlet methylnitrene

Christopher Larson, Yuanyuan Ji, Petros Samartzis, Alec M. Wodtke, Shih-Huang Lee, Jim Jr-Min Lin, Chanchal Chaudhuri, and Tao-Tsung Ching

J. Chem. Phys. 125, 133302 (2006); http://dx.doi.org/10.1063/1.2215598 (7 pages) | Cited 6 times

Online Publication Date: 2 October 2006

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Methylazide photolysis at 248 nm has been investigated by ionizing photofragments with synchrotron radiation in a photofragmentation translational spectroscopy study. CH3N and N2 were the only observed primary products. The translational energy release suggests a simple bond rupture mechanism forming singlet methylnitrene, mathH3N, and N2. Thus, these experiments reveal the unimolecular decomposition of this highly unstable species. We explain our observations through a mechanism which is initiated by the isomerization of mathH3N to a highly internally excited methanimine H2CNH isomer, which decomposes by 1,1-H2 elimination forming HNC+H2 as well as sequential H-atom loss (N–H followed by C–H bond cleavage), to form HCN. No evidence for dynamics on the triplet manifold of surfaces is found.
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82.50.-m Photochemistry
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Qt Isomerization and rearrangement

Ultraviolet photodissociation of the van der Waals dimer (CH3I)2 revisited. II. Pathways giving rise to neutral molecular iodine

Konstantin V. Vidma, Alexey V. Baklanov, Yongwei Zhang, and David H. Parker

J. Chem. Phys. 125, 133303 (2006); http://dx.doi.org/10.1063/1.2345365 (8 pages) | Cited 6 times

Online Publication Date: 2 October 2006

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The formation of neutral I2 by the photodissociation of the methyl iodide dimer, (CH3I)2, excited within the A band at 249.5 nm is evaluated using velocity map imaging. In previous work [ J. Chem. Phys. 122, 204301 (2005) ], we showed that the formation of I2+ from photodissociation of the methyl iodide dimer takes place via ionic channels (through the formation of (CH3I)2+). It is thus not possible to detect neutral I2 by monitoring I2+. Neutral I2 is detected in this study by monitoring I atoms arising from the photodissociation of I2. Iodine atoms from I2 photodissociation have a characteristic kinetic energy and angular anisotropy, which is registered using velocity map imaging. We use a two-color probe scheme involving the photodissociation of nascent I2 at 499 nm, which gives rise to I atoms that are ionized by (2+1) resonance enhanced multiphoton ionization at 304.67 nm. Our estimate of the yield of nascent I2 is based on the comparison with the signal from I2 at a known concentration. Using molecular beams with a small fraction of CH3I (1% in the expanded mixture) where smaller clusters should prevail, the production of I2 was found to be negligible. An upper estimate for the quantum yield of I2 from (CH3I)2 dimers was found to be less than 0.4%. Experiments with a higher fraction of CH3I (4% in the expanded mixture), which favor the formation of larger clusters, revealed an observable formation of I2, with an estimated translational temperature of about 820 K. We suggest that this observed I2 signal arises from the photodissociation of several CH3I molecules in the larger cluster by the same UV pulse, followed by recombination of two nascent iodine atoms is responsible for neutral I2 production.
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82.50.Hp Processes caused by visible and UV light
82.50.Pt Multiphoton processes
82.20.Hf Product distribution

Rovibrationally selected and resolved state-to-state photoionization of ethylene using the infrared-vacuum ultraviolet pulsed field ionization-photoelectron method

Xi Xing, Mi-Kyung Bahng, Peng Wang, Kai-Chung Lau, Sun Jong Baek, and C. Y. Ng

J. Chem. Phys. 125, 133304 (2006); http://dx.doi.org/10.1063/1.2213261 (14 pages) | Cited 11 times

Online Publication Date: 2 October 2006

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By preparing ethylene [C2H4(math math)] in selected rotational levels of the ν11(b1u), ν2+ν12(b1u), or ν9(b2u) vibrational state with infrared (IR) laser photoexcitation prior to vacuum ultraviolet (VUV) laser photoionization, we have recorded rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra for C2H4+(math math) in the energy region of 0–3000 cm−1 above the ionization energy (IE) of C2H4(math math). Here, ν2(ag), ν9(b2u), ν11(b1u), and ν12(b1u) represent the C–C stretching, CH2 stretching, CH2 stretching, and CH2 bending modes of C2H4(math math), respectively. The fully rovibrationally resolved spectra have allowed unambiguous symmetry assignments of the observed vibrational bands, which in turn have provided valuable information on the photoionization dynamics of C2H4. The IR-VUV photoionization of C2H4(math math) via the ν11(b1u) or ν2+ν12(b1u) vibrational states is found to predominantly produce vibrational states of C2H4+(math math) with b1u symmetry, which cannot be observed in single-photon VUV-PFI-PE measurements of C2H4(math math). The analysis of the observed IR-VUV-PFI-PE bands has provided the IE(C2H4) = 84 790.2(2) cm−1 and accurate vibrational frequencies for the ν4+(au)[84.1(2) cm−1], ν12+(b1u)[1411.7(2) cm−1], ν4++ν12+(b1g)[1482.5(2) cm−1], ν2+(ag)[1488.3(2) cm−1], ν2++ν4+(au)[1559.2(2) cm−1], 2ν4++ν12+(b1u)[1848.5(2) cm−1], 4ν4++ν12+(b1u)[2558.8(2) cm−1], ν2++ν12+(b1u)[2872.7(2) cm−1], and ν11+(b1u)[2978.7(2) cm−1] vibrational states of C2H4+(math math), where ν4+ is the ion torsional state. The IE(C2H4) and the ν4+(au), ν2+(ag), and ν2++ν4+(au) frequencies are in excellent accord with those obtained in previous single-photon VUV-PFI-PE measurements. The other ion vibrational frequencies represent new experimental determinations. We have also performed high-level ab initio anharmonic vibrational frequency calculations for C2H4(math math) and C2H4+(math math) at the CCSD(T)/aug-cc-pVQZ level for guidance in the assignment of the IR-VUV-PFI-PE spectra. All theoretical vibrational frequencies for the neutral and ion, except the ion torsional frequency, are found to agree with experimental vibrational frequencies to better than 1%.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.60.+q Photoelectron spectra
33.20.Vq Vibration-rotation analysis
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
33.20.Tp Vibrational analysis

Photodissociation and photoisomerization of α-fluorotoluene and 4-fluorotoluene in a molecular beam

Cheng-Liang Huang, Jyh-Chiang Jiang, Yuri A. Dyakov, Ming-Fu Lin, Chien-Ming Tseng, S. H. Lin, Yuan T. Lee, and Chi-Kung Ni

J. Chem. Phys. 125, 133305 (2006); http://dx.doi.org/10.1063/1.2219445 (8 pages)

Online Publication Date: 2 October 2006

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The photodissociation of jet-cooled α-fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for α-fluorotoluene at both 193 and 248 nm, including two major channels C6H5CH2F→C6H5CH2 (or C7H7)+F and C6H5CH2F→C6H5CH (or C7H6)+HF and two minor channels C6H5CH2F→C6H5CHF+H and C6H5CH2F→C6H5+CH2F. The vuv wavelength dependence of the C7H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of α-fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6H4FCH3→C6H4FCH2+H and C6H4FCH3→C6H4F+CH3 and two minor channels C6H4FCH3→C6H5CH2 (or C7H7)+F and C6H4FCH3→C6H5CH (or C7H6)+HF. The dissociation rates for α-fluorotoluene at 193 and 248 nm are 3.3×107 and 5.6×105 s−1, respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0×106 s−1. An ab initio calculation demonstrates that the barrier height for isomerization from α-fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between α-fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between α-fluorotoluene and 4-fluorotoluene.
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82.50.Hp Processes caused by visible and UV light
82.30.Qt Isomerization and rearrangement
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.-w Chemical kinetics and dynamics

Unimolecular dissociation of the propargyl radical intermediate of the CH+C2H2 and C+C2H3 reactions

Laura R. McCunn, Benjamin L. FitzPatrick, Maria J. Krisch, Laurie J. Butler, Chi-Wei Liang, and Jim J. Lin

J. Chem. Phys. 125, 133306 (2006); http://dx.doi.org/10.1063/1.2353821 (11 pages) | Cited 10 times

Online Publication Date: 2 October 2006

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This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5/−10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical’s dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C–H fission channel were also observed in the photolysis of propargyl chloride.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.60.Cx Enthalpies of combustion, reaction, and formation
82.50.-m Photochemistry

Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene

Narayanan Kuthirummal, Fedor M. Rudakov, Conor L. Evans, and Peter M. Weber

J. Chem. Phys. 125, 133307 (2006); http://dx.doi.org/10.1063/1.2345203 (8 pages) | Cited 12 times

Online Publication Date: 2 October 2006

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The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 math state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm−1, which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 math state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 math state that has been implicated as an intermediate step between the initially excited 1 math state and the ground electronic state. The rise and decay times for the 2 math state were found to be 55 and 84 fs, respectively.
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82.50.Pt Multiphoton processes
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
82.53.Kp Coherent spectroscopy of atoms and molecules
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.60.+q Photoelectron spectra

The photodissociation dynamics of ozone at 193 nm: An O(math) angular momentum polarization study

M. Brouard, R. Cireasa, A. P. Clark, G. C. Groenenboom, G. Hancock, S. J. Horrocks, F. Quadrini, G. A. D. Ritchie, and C. Vallance

J. Chem. Phys. 125, 133308 (2006); http://dx.doi.org/10.1063/1.2210009 (16 pages) | Cited 18 times

Online Publication Date: 3 October 2006

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Polarized laser photolysis, coupled with resonantly enhanced multiphoton ionization detection of O(math) and velocity-map ion imaging, has been used to investigate the photodissociation dynamics of ozone at 193 nm. The use of multiple pump and probe laser polarization geometries and probe transitions has enabled a comprehensive characterization of the angular momentum polarization of the O(math) photofragments, in addition to providing high-resolution information about their speed and angular distributions. Images obtained at the probe laser wavelength of around 205 nm indicate dissociation primarily via the Hartley band, involving absorption to, and diabatic dissociation on, the math math(3 math) potential energy surface. Rather different O(math) speed and electronic angular momentum spatial distributions are observed at 193 nm, suggesting that the dominant excitation at these photon energies is to a state of different symmetry from that giving rise to the Hartley band and also indicating the participation of at least one other state in the dissociation process. Evidence for a contribution from absorption into the tail of the Hartley band at 193 nm is also presented. A particularly surprising result is the observation of nonzero, albeit small values for all three rank K = 1 orientation moments of the angular momentum distribution. The polarization results obtained at 193 and 205 nm, together with those observed previously at longer wavelengths, are interpreted using an analysis of the long range quadrupole-quadrupole interaction between the O(math) and O2(math) species.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Pt Multiphoton processes
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Eh Autoionization, photoionization, and photodetachment
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Dissociative photodetachment dynamics of the iodide-aniline cluster

M. Shane Bowen, Maurizio Becucci, and Robert E. Continetti

J. Chem. Phys. 125, 133309 (2006); http://dx.doi.org/10.1063/1.2210010 (9 pages) | Cited 4 times

Online Publication Date: 3 October 2006

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The photodetachment dynamics of the iodide-aniline cluster, I−(C6H5NH2), were investigated using photoelectron-photofragment coincidence spectroscopy at several photon energies between 3.60 and 4.82 eV in concert with density functional theory calculations. Direct photodetachment from the solvated I− chromophore and a wavelength-independent autodetachment process were observed. Autodetachment is attributed to a charge-transfer-to-solvent reaction in which incipient continuum electrons photodetached from I− are temporarily captured by the nascent neutral iodine-aniline cluster configured in the anion geometry. Subsequent dissociation of the neutral cluster removes the stabilization, leading to autodetachment of the excess electron. The dependence of the dissociative photodetachment (DPD) and autodetachment dynamics on the final spin-orbit electronic state of the iodine fragment is characterized. The dissociation dynamics of the neutral fragments correlated with autodetached electrons were found to be identical to the DPD dynamics of the I atom product spin-orbit state closest to threshold at a given photon energy, lending support to the proposed sequential mechanism.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Sx Diffusion and dynamics of clusters
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.60.+q Photoelectron spectra
31.15.E- Density-functional theory

Dissociation of heme from gaseous myoglobin ions studied by infrared multiphoton dissociation spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry

Yi-Sheng Wang, Sahadevan Sabu, Shih-Chia Wei, C.-M. Josh Kao, Xianglei Kong, Shing-Chih Liau, Chau-Chung Han, Huan-Cheng Chang, Shih-Yu Tu, A. H. Kung, and John Z. H. Zhang

J. Chem. Phys. 125, 133310 (2006); http://dx.doi.org/10.1063/1.2221696 (7 pages) | Cited 3 times

Online Publication Date: 3 October 2006

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Detachment of heme prosthetic groups from gaseous myoglobin ions has been studied by collision-induced dissociation and infrared multiphoton dissociation in combination with Fourier-transform ion cyclotron resonance mass spectrometry. Multiply charged holomyoglobin ions (hMbn+) were generated by electrospray ionization and transferred to an ion cyclotron resonance cell, where the ions of interest were isolated and fragmented by either collision with Ar atoms or irradiation with 3 μm photons, producing apomyoglobin ions (aMbn+). Both charged heme loss (with [Fe(III)-heme]+ and aMb(n−1)+ as the products) and neutral heme loss (with [Fe(II)-heme] and aMbn+ as the products) were detected concurrently for hMbn+ produced from a myoglobin solution pretreated with reducing reagents. By reference to Ea = 0.9 eV determined by blackbody infrared radiative dissociation for charged heme loss of ferric hMbn+, an activation energy of 1.1 eV was deduced for neutral heme loss of ferrous hMbn+ with n = 9 and 10.
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87.14.E- Proteins
87.15.M- Spectra of biomolecules
87.15.N- Properties of solutions of macromolecules
36.20.Kd Electronic structure and spectra
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Gj Diffuse spectra; predissociation, photodissociation

Photodissociation dynamics of CBr4 at 267 nm by means of ion velocity imaging

Jamila R. Greene, Joseph S. Francisco, Dadong Xu, Jianhua Huang, and William M. Jackson

J. Chem. Phys. 125, 133311 (2006); http://dx.doi.org/10.1063/1.2213260 (9 pages) | Cited 2 times

Online Publication Date: 3 October 2006

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The photodissociation dynamics of CBr4 at 267 nm has been studied using time of flight (TOF) mass spectrometry and ion velocity imaging techniques. The photochemical products are detected with resonance enhanced multiphoton ionization (REMPI) as well as single-photon vacuum ultraviolet ionization at 118 nm. REMPI at 266.65 and 266.71 nm was used to detect the ground Br(math) and spin-orbit excited Br(math) atoms, respectively. The translational energy and angular distributions are consistent with direct dissociation from an excited triplet state and indirect dissociation from high vibrational levels on the singlet ground state surface. Br2+ ions are also observed in the TOF spectra with a focused 267 nm laser. The counter fragment, CBr2+, is observed when this photolysis laser is unfocused, and photons at 118 nm are used to ionize the radical products. The translational energy distributions of the CBr2+ and Br2+ products can be momentum matched, which indicates that molecular Br2 elimination is one of the primary dissociation channels.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Hp Processes caused by visible and UV light
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ta Mass spectra
33.20.Tp Vibrational analysis

Photodissociation of nitrous oxide starting from excited bending levels

Hiroshi Kawamata, Hiroshi Kohguchi, Tatsuhiro Nishide, and Toshinori Suzuki

J. Chem. Phys. 125, 133312 (2006); http://dx.doi.org/10.1063/1.2264362 (10 pages) | Cited 15 times

Online Publication Date: 3 October 2006

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The photodissociation dynamics of N2O in the wavelength region of 203–205 nm was studied by velocity map ion imaging. A speed resolution of 0.8% was obtained using standard projection imaging and subpixel centroiding calculations. To investigate N2O dissociation starting from the excited bending levels in the ground electronic state, a supersonic molecular beam and an effusive beam were used. The photoabsorption transition probability from the first excited bending level in the wavelength region of 203–205 nm was estimated to be seven times greater than that from the ground vibrational level.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states

Vector properties of the O(math) fragment produced from the photolysis of ozone in the wavelength range of 298 to 320 nm

S. J. Horrocks, P. J. Pearson, and G. A. D. Ritchie

J. Chem. Phys. 125, 133313 (2006); http://dx.doi.org/10.1063/1.2201746 (11 pages) | Cited 7 times

Online Publication Date: 3 October 2006

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The speed averaged translational anisotropy and electronic angular momentum polarization of the O(math) atomic fragment formed from the photodissociation of ozone in the atmospherically important long wavelength region of the Hartley band (298 to 320 nm) have been measured using resonance enhanced multiphoton ionization time of flight mass spectrometry. The translational anisotropy parameter, β, is found to decline from 1.1 for photolysis at 300 nm to a minimum value of 0 at 310 nm which is the threshold for production of O(math) in conjunction with the O2(a mathv = 0) molecular cofragment. For photolysis wavelengths greater than 310 nm, O(math) is formed from the dissociation of internally excited ozone molecules. The corresponding β parameters are markedly lower than for atomic fragments produced with the same speed from the photolysis of ground state ozone molecules. This result is consistent with two different pathways contributing to the photolysis of internally excited ozone at the longest wavelengths studied corresponding to initial internal excitation either in the symmetric or asymmetric stretching vibration. In addition, the polarization of the atomic angular momentum has been determined with the incoherent polarization parameters a02(‖) and a02(⊥) increasing from values of −0.53 and −0.62 at 300 nm to −0.37 and −0.19 at 317 nm, consistent with the increasing contribution from the photolysis of internally excited ozone as the dissociation wavelength lengthens. Evaluation of these alignment parameters allows the populations of the magnetic substrates, mj, to be determined. For example, for a photolysis wavelength of 303 nm the populations of mj = 0,±1,±2 are in the ratio of 0.36: 0.56: 0.08 and this ratio is essentially independent of the photolysis wavelength. The coherent contribution to the atomic polarization is quantified by the Re{a12(‖,⊥)} and Im{a11(‖,⊥)} parameters and these are found to vary from −0.21 and 0.21 at 300 nm to −0.04 and 0.24 at 313 nm, respectively.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Hp Processes caused by visible and UV light
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ta Mass spectra
33.20.Tp Vibrational analysis

Enhanced selectivity and yield in multichannel photodissociation reactions: Application to CH3I

Ioannis Thanopulos and Moshe Shapiro

J. Chem. Phys. 125, 133314 (2006); http://dx.doi.org/10.1063/1.2336768 (8 pages) | Cited 2 times

Online Publication Date: 4 October 2006

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We develop a method to improve the population transfer and final-channel control of multichannel photodissociation reactions. The method is applied to the photodissociation of methyl iodide, CH3(v)+I*(math)←CH3I→CH3(v)+I(math). Our method is based on simultaneously exciting many two-photon pathways that lead to the same final outcome, each proceeding via a different intermediate bound state. The selectivity of the final product state(s) is a result of coherently controlled interference between the quantum pathways. The improvement in the population transfer yield from the ground state to the selected dissociative channel(s) is made possible by executing the process in an adiabatic fashion.
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82.37.Vb Single molecule photochemistry
82.20.Hf Product distribution
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Wz Other multiphoton processes
33.80.Be Level crossing and optical pumping
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