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7 Oct 2006

Volume 125, Issue 13, Articles (13xxxx)

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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

Two-color visible/vacuum ultraviolet photoelectron imaging dynamics of Br2

Jürgen Plenge, Christophe Nicolas, Allison G. Caster, Musahid Ahmed, and Stephen R. Leone

J. Chem. Phys. 125, 133315 (2006); http://dx.doi.org/10.1063/1.2217375 (6 pages) | Cited 4 times

Online Publication Date: 4 October 2006

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An experimental two-color photoionization dynamics study of laser-excited Br2 molecules is presented, combining pulsed visible laser excitation and tunable vacuum ultraviolet (VUV) synchrotron radiation with photoelectron imaging. The X math–B math transition in Br2 is excited at 527 nm corresponding predominantly to excitation of the v′ = 28 vibrational level in the B math state. Tunable VUV undulator radiation in the energy range of 8.40–10.15 eV is subsequently used to ionize the excited molecules to the X math state of the ion, and the ionic ground state is probed by photoelectron imaging. Similar experiments are performed using single-photon synchrotron ionization in the photon energy range of 10.75–12.50 eV without any laser excitation. Photoelectron kinetic energy distributions are extracted from the photoelectron images. In the case of two-color photoionization using resonant excitation of the intermediate B math state, a broad distribution of photoelectron kinetic energies is observed, and in some cases even a bimodal distribution, which depends on the VUV photon energy. In contrast, for single-photon ionization, a single nearly Gaussian-shaped distribution is observed, which shifts to higher energy with photon energy. Simulated spectra based on Franck-Condon factors for the transitions Br2(X math,v″ = 0)–Br2+(X math,v+) and Br2(B math,v′ = 28)–Br2+(X math,v+) are generated. Comparison of these calculated spectra with the measured images suggests that the differences in the kinetic energy distributions for the two ionization processes reflect the different extensions of the vibrational wave functions in the v″ = 0 electronic ground state (X math) versus the electronically and vibrationally excited state (B math,v′ = 28).
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33.60.+q Photoelectron spectra
33.80.Eh Autoionization, photoionization, and photodetachment
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Anisotropy of photofragment recoil as a function of dissociation lifetime, excitation frequency, rotational level, and rotational constant

Hahkjoon Kim, Kristin S. Dooley, Simon W. North, Gregory E. Hall, and P. L. Houston

J. Chem. Phys. 125, 133316 (2006); http://dx.doi.org/10.1063/1.2216708 (10 pages) | Cited 10 times

Online Publication Date: 4 October 2006

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Quantum mechanical calculations of photofragment angular distributions have been performed as a function of the frequency of excitation, the lifetime of the dissociative state, the rotational level, and the rotational constant. In the limit of high J values and white, incoherent excitation, the general results are found to agree exactly with both those of Mukamel and Jortner [J. Chem. Phys. 61, 5348 (1974)] and those of Jonah [J. Chem. Phys. 55, 1915 (1971)] . Example calculations describe how the anisotropy is dependent on the degree of broadening, the rotational constant, the initial rotational level, and the frequency of excitation. Applications are also made to interpret experimental results on the photodissociation of ClO via the 11-0, 10-0, and 6-0 bands of the A math–X math transition and on the photodissociation of O2 via the 0-0 band of the E math–X math transition.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Jg Line and band widths, shapes, and shifts
34.10.+x General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.)

Dynamics of the 193 nm photodissociation of dichlorocarbene

Seung Keun Shin and Paul J. Dagdigian

J. Chem. Phys. 125, 133317 (2006); http://dx.doi.org/10.1063/1.2212416 (9 pages) | Cited 9 times

Online Publication Date: 4 October 2006

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The dynamics of the 193 nm photodissociation of the CCl2 molecule have been investigated in a molecular beam experiment. The CCl2 parent molecule was generated in a molecular beam by pyrolysis of CHCl3, and both CCl2 and the CCl photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section was estimated from the reduction of the CCl2 signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A math-X math band system. Most of the energy available to the CCl(X math)+Cl fragments appears as translational energy. The CCl fragment rotational energy is much less than predicted in an impulsive model. The excited electronic state appears to dissociate indirectly, through coupling with a repulsive state arising from the ground-state CCl(X math)+Cl asymptote. The identity of the initially excited electronic state is discussed on the basis of what is known about the CCl2 electronic states.
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82.50.Hp Processes caused by visible and UV light
82.80.Dx Analytical methods involving electronic spectroscopy
34.50.Lf Chemical reactions

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of phenol

Michael G. D. Nix, Adam L. Devine, Bríd Cronin, Richard N. Dixon, and Michael N. R. Ashfold

J. Chem. Phys. 125, 133318 (2006); http://dx.doi.org/10.1063/1.2353818 (13 pages) | Cited 57 times

Online Publication Date: 4 October 2006

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The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145 ≥ λphot ≥ 206.00 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Many of the total kinetic energy release (TKER) spectra so derived show structure, the analysis of which confirms the importance of O–H bond fission and reveals that the resulting phenoxyl cofragments are formed in a very limited subset of their available vibrational state density. Spectra recorded at λphot ≥ 248 nm show a feature centered at TKER ∼ 6500 cm−1. These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1←S0 (π*←π) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S0) state levels carrying sufficient O–H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S0 and S2(mathσ*) potential energy surfaces (PESs) at larger RO–H, en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes ν16a and ν11 can both promote nonadiabatic coupling in the vicinity of the S0/S2 CI. Spectra recorded at λphot ⩽ 248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER ∼ 12 000 cm−1. These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1(mathπ*) and repulsive S2(mathσ*) PESs. Parent mode ν16b is identified as the dominant coupling mode at the S1/S2 CI, and the resulting phenoxyl radical cofragments display a long progression in ν18b, the C–O in-plane wagging mode. Analysis of all structured TKER spectra yields D0(H–OC6H5) = 30 015±40 cm−1. The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of mathσ* states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the mathσ* state is optically dark. (ii) The probability of observing strikingly mode-specific product formation, even in “indirect” predissociations, if the fragmentation is driven by ultrafast nonadiabatic couplings via CIs between excited (and ground) state PESs.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)
31.50.Bc Potential energy surfaces for ground electronic states
31.50.Gh Surface crossings, non-adiabatic couplings

248 nm photolysis of CH2Br2 by using cavity ring-down absorption spectroscopy: Br2 molecular elimination at room temperature

Pei-Ying Wei, Yuan-Ping Chang, Wei-Bin Lee, Zhengfa Hu, Hong-Yi Huang, King-Chuen Lin, K. T. Chen, and A. H. H. Chang

J. Chem. Phys. 125, 133319 (2006); http://dx.doi.org/10.1063/1.2218514 (8 pages) | Cited 11 times

Online Publication Date: 5 October 2006

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Following photodissociation of CH2Br2 at 248 nm, Br2 molecular elimination is detected by using a tunable laser beam, as crossed perpendicular to the photolyzing laser beam in a ring-down cell, probing the Br2 fragment in the B math-X math transition. The nascent vibrational population is obtained, yielding a population ratio of Br2(v = 1)/Br2(v = 0) to be 0.7±0.2. The quantum yield for the Br2 elimination reaction is determined to be 0.2±0.1. Nevertheless, when CH2Br2 is prepared in a supersonic molecular beam under cold temperature, photofragmentation gives no Br2 detectable in a time-of-flight mass spectrometer. With the aid of ab initio potential energy calculations, a plausible pathway is proposed. Upon excitation to the math or math state, C–Br bond elongation may change the molecular symmetry of Cs and enhance the resultant 1 math-math math (or 1 math-math math as C2v is used) coupling to facilitate the process of internal conversion, followed by asynchronous concerted photodissociation. Temperature dependence measurements lend support to the proposed pathway.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.50.-m Photochemistry
82.20.Kh Potential energy surfaces for chemical reactions

Explosive photodissociation of methane induced by ultrafast intense laser

Fanao Kong, Qi Luo, Huailiang Xu, Mehdi Sharifi, Di Song, and See Leang Chin

J. Chem. Phys. 125, 133320 (2006); http://dx.doi.org/10.1063/1.2204919 (5 pages) | Cited 14 times

Online Publication Date: 5 October 2006

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A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2×1014 W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n = 3→2), CH (A math, B math, and C math→X math), or C2 (d math→a math) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A→X) emission of methane on a log-log scale has a slope of 10±1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.-m Photochemistry
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
82.33.Vx Reactions in flames, combustion, and explosions
33.50.Dq Fluorescence and phosphorescence spectra
33.15.Fm Bond strengths, dissociation energies

The time scale for electronic reorganization upon sudden ionization of the water and water-methanol hydrogen bonded dimers and of the weakly bound NO dimer

F. Remacle and R. D. Levine

J. Chem. Phys. 125, 133321 (2006); http://dx.doi.org/10.1063/1.2227023 (7 pages) | Cited 11 times

Online Publication Date: 6 October 2006

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When the valence molecular orbital is localized sudden ionization can cause the nascent hole to move rapidly even before any relaxation of the geometry occurs. Hydrogen bonded clusters offer suitable test systems where the hole is initially localized on one moiety. Computational studies are reported for the water dimer and water-methanol bimer. The local ionization potential of water is different in the methanol-water and water-methanol conformers and this difference is very clearly reflected in the dynamics of charge migration. For the NO dimer the results are that its structure is symmetric so that the two NO molecules are equivalent and do not exhibit the required localization. The role of symmetry is also evident in the charge propagation for holes created in different orbitals. Localization of the initial hole distribution even if absent in the bare molecule can still be induced by the intense electric field of a sudden photoionization. This effect is computationally studied for the NO dimer in the presence of a static electric field.
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36.40.Qv Stability and fragmentation of clusters
36.40.Sx Diffusion and dynamics of clusters
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Fm Bond strengths, dissociation energies
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