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22 Dec 1997

Volume 107, Issue 24, pp. 10365-10830

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Laser velocity modulation spectroscopy of TiCl+: Observation of the A 3Δ(3d2) state and deperturbation of the X 3Φ−A 3Δ complex

C. Focsa, B. Pinchemel, J.-L. Féménias, and T. R. Huet

J. Chem. Phys. 107, 10365 (1997); http://dx.doi.org/10.1063/1.475313 (8 pages) | Cited 3 times

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A single mode cw dye laser along with velocity modulation was used to record weak bands of TiCl+ in the spectral region 17 100–18 600 cm−1. The ions were produced in the positive column of an ac glow discharge with a gas mixture of He/TiCl4. These new bands have been identified as linking the upper already known [17.8]3Δ(3d4s) state to respectively the X 3Φ(v = 1) state and to a newly observed A 3Δ(v = 0) state located 350 cm−1 above the X 3Φ(v = 0) state. As suggested by Focsa et al. (J. Chem. Phys. 106, 9044 (1997)) the A 3Δ(3d2) state was found to be responsible for the perturbations observed in the X 3Φ(3d2) state. The spin-orbit components of the A 3Δ(v = 0) and the X 3Φ(v = 1) states revealed interaction leading to homogeneous and heterogeneous perturbations. A set of deperturbed parameters has been determined with the help of a full matrix-based handling of the A 3Δ(v = 0) and the X 3Φ(v = 0,1) states. Study of the contributions of the basis functions to the eigenvectors of the diagonalized matrix displayed evidence of a reversal of the leading character of the Ω = 3 spin-orbit components of the A 3Δ(v = 0) and the X 3Φ(v = 1) states. A remarkable example of avoided crossing has been observed between the A 3Δ1(v=0) and the X 3Φ2(v = 1) spin-orbit components. © 1997 American Institute of Physics.
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33.20.Kf Visible spectra
33.80.Be Level crossing and optical pumping
52.80.Hc Glow; corona
02.10.Ud Linear algebra
02.10.Xm Multilinear algebra

Ab initio quartic force fields for anions: A benchmark study on 16OH−, 18OH−, and 16OD−

Timothy J. Lee and Christopher E. Dateo

J. Chem. Phys. 107, 10373 (1997); http://dx.doi.org/10.1063/1.474200 (8 pages) | Cited 3 times

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The singles and doubles coupled-cluster method that includes a perturbational correction for connected triple excitations, denoted CCSD(T), is used in conjunction with several one-particle basis sets to determine quartic force fields for OH−. Basis set convergence properties of the various rovibrational spectroscopic constants is studied. To assure numerical stability of the quartic force fields and to investigate variational calculations of vibrational energy levels, sextic force fields have also been evaluated. The largest one-particle basis set employed contains up through h functions on oxygen and g functions on hydrogen. Agreement between experiment and theory is excellent and it is shown that the CCSD(T)/spdfg approach performs as well for OH− as it previously did for H2O. Due to the inherent difficulty in high resolution studies on molecular anions, it is concluded that currently the CCSD(T)/spdfg approach may be the most reliable technique for the determination of accurate rovibrational spectroscopic properties of small- to medium-sized anions. © 1997 American Institute of Physics.
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31.15.A- Ab initio calculations
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
31.15.bw Coupled-cluster theory
33.20.Vq Vibration-rotation analysis

Dynamic solvent effects on the vibrational overtone dephasing in molecular liquids: Subquadratic quantum number dependence

N. Gayathri, S. Bhattacharyya, and B. Bagchi

J. Chem. Phys. 107, 10381 (1997); http://dx.doi.org/10.1063/1.474201 (10 pages) | Cited 14 times

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A theoretical study of vibrational dephasing of molecular vibrations in liquids is presented with an aim to understand the experimentally observed sub-quadratic quantum number (n) dependence of the vibrational dephasing rate, in systems like CH3I and CHCl3 and their deuterated analogues. The analysis is based on Oxtoby’s theory of vibrational dephasing but with a detailed microscopic description of the frequency dependent frictional forces on the vibrational mode. The friction on the normal coordinate in liquids is found to have a pronounced biphasic behavior with a dominant Gaussian initial component followed by a slow exponential-like relaxation. While the exponential relaxation usually assumed in Kubo’s stochastic theory leads to a quadratic n dependence of the dephasing rate, the biphasic friction is shown to give rise to the sub-quadratic n dependence. As the biphasic frictional response is expected to be a generic feature of the friction on any vibrational coordinate in dense liquids, the sub-quadratic quantum number dependence is predicted to be common to most ultrafast overtone dephasing. In addition, the calculated rates (without any adjustable parameter), are found to be in good agreement with the experimental results for the C-I stretching mode in liquid CH3I and for the C-H stretching in liquid CHCl3. © 1997 American Institute of Physics.
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63.50.-x Vibrational states in disordered systems
78.30.C- Liquids

Vacuum ultraviolet photoionization and dissociative photoionization of W(CO)6

Fei Qi, Shihe Yang, Liusi Sheng, Hui Gao, Yunwu Zhang, and Shuqin Yu

J. Chem. Phys. 107, 10391 (1997); http://dx.doi.org/10.1063/1.474202 (8 pages) | Cited 4 times

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Photoionization of W(CO)6 in the photon energy range of 8–40 eV produced a variety of intermediate ions of the form (WCm(CO)n)x+. Photoionization efficiency (PIE) curves of these ions have been measured for the first time by using a time-of-flight mass spectrometer (TOFMS) with vaccum ultraviolet (VUV) photons from a synchrotron radiation source. Appearance potentials (AP) of all the observed ions have been determined from their PIE curves. Based on these AP values, we obtained a series of bond dissociation energy (BDE) data for the intermediate ions produced by the dissociative photoionization. © 1997 American Institute of Physics.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Ta Mass spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation

High resolution optothermal spectroscopy of pyridine in the S1 state

M. Becucci, N. M. Lakin, G. Pietraperzia, P. R. Salvi, E. Castellucci, and E. R. Th. Kerstel

J. Chem. Phys. 107, 10399 (1997); http://dx.doi.org/10.1063/1.474203 (7 pages) | Cited 6 times

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The optothermal technique has been utilized to obtain the first high resolution spectrum of pyridine in the region of the S1←S0 electronic transition. Rotational profiles for several vibronic bands (000,6a01,16b026a01,6a02,1201) were measured and found to be severely homogeneously broadened with linewidths of the order of 3–5 GHz, in agreement with previous lifetime measurements. Rotational constants of pyridine in the excited S1 vibronic levels were extracted by a band contour analysis. The values obtained are in good agreement with results from ab initio calculations, also presented here. © 1997 American Institute of Physics.
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33.80.-b Photon interactions with molecules
31.15.A- Ab initio calculations
33.50.Dq Fluorescence and phosphorescence spectra
33.70.Jg Line and band widths, shapes, and shifts

Dipole moments and hyperfine interactions in scandium monosulfide, ScS

T. C. Steimle, A. J. Marr, and D. M. Goodridge

J. Chem. Phys. 107, 10406 (1997); http://dx.doi.org/10.1063/1.474204 (9 pages) | Cited 9 times

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A supersonic molecular beam of scandium monosulfide (ScS), produced using a laser ablation/reaction source, was interrogated by laser induced fluorescence. The fine and magnetic hyperfine interactions in the (0,0) B 2Σ+–X 2Σ+ band system (origin=12 940.402 cm−1) were analyzed. The magnitude of the Stark effect was measured for the (RR24+RQ24)(0) and (PP13+PQ23)(1) branch features, giving dipole moments of μ(B 2Σ+) = 5.60±0.04 D and μ(X 2Σ+) = 5.64±0.04 D. The spectroscopic parameters and dipole moments are used to develop a molecular orbital bonding model and are compared with the isovalent molecules ScO and ScNH. © 1997 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.57.+c Magneto-optical and electro-optical spectra and effects
37.20.+j Atomic and molecular beam sources and techniques

Isotropic and anisotropic interaction induced scattering in liquid argon

Victor Teboul and Yves Le Duff

J. Chem. Phys. 107, 10415 (1997); http://dx.doi.org/10.1063/1.474205 (5 pages) | Cited 8 times

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The collision induced scattering (CIS) spectra have been studied for liquid argon at 130 K and 615 amagat. For the first time, isotropic CIS intensities are measured and the depolarized CIS spectrum is obtained up to 370 cm−1. Molecular dynamics simulations are performed for several models of polarizabilities and intermolecular potentials. They show that theoretical polarizabilities deduced from self consistent field calculations are in agreement with both depolarized and isotropic CIS experimental spectral shape. © 1997 American Institute of Physics.
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61.25.Bi Liquid noble gases
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
32.70.Fw Absolute and relative intensities
61.20.Ja Computer simulation of liquid structure

Shaping molecular beams with intense light

Tamar Seideman

J. Chem. Phys. 107, 10420 (1997); http://dx.doi.org/10.1063/1.474206 (10 pages) | Cited 32 times

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We discuss the possibility of manipulating the center-of-mass motion of molecules using the nonlinear interaction of a moderately intense, long-pulse laser field with the molecular polarizability tensor. Recent theoretical work demonstrating the possibility of focusing and trapping molecules is extended to consider the effects of circularly and elliptically polarized light and the effect of nonspherical laser optics. The aberrations affecting the quality of the “molecular lens” are analyzed and the means by which they can be minimized are discussed. Molecular focusing is extended to a general field of molecular optics; the possibilities of steering, reflecting, and collimating molecular beams are illustrated. Application of the mechanical force of light to disperse and separate species according to their mass, velocity, or quantum state is proposed. © 1997 American Institute of Physics.
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37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
37.20.+j Atomic and molecular beam sources and techniques
07.77.-n Atomic, molecular, and charged-particle sources and detectors
41.85.Ct Particle beam shaping, beam splitting

Dressed states of molecules and microwave–infrared double-resonance spectroscopic techniques employing an electric quadrupole focusing field

Chung Yi Lee and Brooks H. Pate

J. Chem. Phys. 107, 10430 (1997); http://dx.doi.org/10.1063/1.474207 (10 pages) | Cited 10 times

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High-sensitivity, microwave–infrared double-resonance measurements can be made in molecular-beam spectrometers employing a single state-focusing device. The key feature of the double-resonance technique is the achievement of large signal modulations of infrared signals using microwave transitions, even in cases where the infrared transition cannot be saturated. A series of measurements is presented that shows that the technique is based on the transition moment and state-focusing properties of dressed molecular states in the presence of a strong microwave field. Using a state-focusing device, the spectroscopic measurements are doubly sensitive to the composition of the dressed states. The technique can be extended to other types of spectroscopy, such as electronic spectroscopy and the spectroscopy of weakly bound complexes. © 1997 American Institute of Physics.
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33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
07.60.Rd Visible and ultraviolet spectrometers

Photon emission from gas phase fullerenes excited by 193 nm laser radiation

P. Heszler, J. O. Carlsson, and P. Demirev

J. Chem. Phys. 107, 10440 (1997); http://dx.doi.org/10.1063/1.474208 (6 pages) | Cited 9 times

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Continuous, black-body-type light emission is observed upon irradiation of gas phase C60 and C70 by 193 nm ArF excimer laser at fluences from 3 to 80 mJ/cm2 in Ar and He ambient. Cluster temperatures are estimated by calibrating the detection system against a tungsten filament and applying Wien’s displacement law. Time-resolved spectroscopic measurements show that the initial internal temperature of the irradiated fullerenes (around 2800 K) decreases linearly, while the emitted light intensity decreases exponentially with time, respectively. Excited C60 and C70 molecules are predominantly cooled via inelastic collisions with noble gas atoms above ∼ 0.5 mbar ambient pressure and below ∼ 2800 K temperature. The quenching rates are ∼ 7.1 bar−1 s−1 for C60, and ∼ 6.4 bar−1 s−1 for C70 in Ar, and 81 bar−1 s−1 for C60 in He ambient, respectively, determined from Stern–Vollmer type relations. The inelastic quenching cross section for He gas ( ∼ 4.4×10−23 cm2) is ∼ 3.7 times higher than for Ar. This observation may provide further insight on the mechanisms of fullerene synthesis by coalescence of hot carbon vapor in a noble gas atmosphere. At laser fluence above 30–40 mJ/cm2 the fullerene temperature saturates at ∼ 2800 K, indicating a kind of phase-transition—“boiling” of the excited fullerenes. The cluster temperature stabilizes by other cooling mechanisms like electron and/or C2 “evaporation” (i.e., ionization and/or fragmentation). The unperturbed (i.e., extrapolated to zero ambient pressure) lifetime of the temperature-stabilized species is 100±25 μs for C60 and 44±4 μs for C70, respectively, at high (80 mJ/cm2) laser fluences. The measured two- and three-photon multiplicities of the excitation at low laser fluences (<15 mJ/cm2) are in good agreement with the observed cluster temperatures. © 1997 American Institute of Physics.
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36.40.-c Atomic and molecular clusters
81.05.ub Fullerenes and related materials
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment
33.50.Hv Radiationless transitions, quenching
33.50.Dq Fluorescence and phosphorescence spectra

The interpretation of vibrational spectra of ionic melts

Evangelia A. Pavlatou, Paul A. Madden, and Mark Wilson

J. Chem. Phys. 107, 10446 (1997); http://dx.doi.org/10.1063/1.474209 (12 pages) | Cited 32 times

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Computer simulations of the short-time, vibrational dynamics of the network-forming ionic melts, LaCl3 and ZnCl2, and of their mixtures with network-breaking alkali halides are described. In the mixtures, high frequency peaks in the vibrational density of states are shown to be describable in terms of the normal coordinates of vibration of transient molecular ion species, like LaCl63− and ZnCl42−. Novel simulation methods are presented which allow this association to be established. In the pure melts, the vibrational motions retain a strong aspect of this local polyhedral unit vibrational character, but the effects of network-induced coupling between the vibrations of different units become pronounced, particularly in ZnCl2. The calculated vibrational spectra are compared with extensive Raman data on these systems, and with infrared and neutron spectra in pure ZnCl2. For the mixtures, remarkably good agreement with experiment is found, confirming the high quality of the representation of the interionic interactions obtained with the polarizable ion model potentials used. For the melts, there are discrepancies between the peak frequencies observed in the vibrational DOS and the Raman spectra. These discrepancies are likely to be due to the network-induced couplings, whose effect on the Raman (and infrared) spectra is not fully included in the calculated DOS. © 1997 American Institute of Physics.
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78.30.C- Liquids
61.25.-f Studies of specific liquid structures

Vibrational self-consistent field method for many-mode systems: A new approach and application to the vibrations of CO adsorbed on Cu(100)

Stuart Carter, Susan J. Culik, and Joel M. Bowman

J. Chem. Phys. 107, 10458 (1997); http://dx.doi.org/10.1063/1.474210 (12 pages) | Cited 155 times

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We report calculations of the vibrational energies of CO–Cu(100) using a new code to perform vibrational self-consistent field (VSCF) and state-mixing calculations for many-mode systems. The major new feature of the code is the representation of the potential. Unlike recent implementations of the VSCF method, the potential is not expanded in terms of normal coordinates as a multinomial series about a minimum. The full potential, in normal coordinates, is used in the Watson Hamiltonian. This approach, while rigorous, can lead to prohibitively large numerical quadratures, and so we suggest a novel representation of the potential as an expansion in all two-mode, or all three-mode, or all four-mode coupling terms. The new code is tested against previous exact calculations of vibrational states of HCO, and also against previous VSCF calculations that used a fourth-order, normal coordinate force field representation of the global HCO potential. The new code is applied to calculations of the vibrations of CO adsorbed to Cu(100). We explicitly treat nine modes corresponding to the motion of the C and O atoms and the Cu atom that is bonded to C. The potential used is a semi-empirical one developed by Tully and co-workers [J. C. Tully, M. Gomez, and M. Head-Gordon, J. Vac. Sci. Technol. A 11, 1914 (1993)], and is used fully, i.e., without recourse to multinomial expansion in displacement coordinates. We test the convergence of the results with respect to the number of modes coupled and find that the errors in the two-mode coupling representation vary from 0.6 to 6 cm−1 for the fundamentals but grow to 30 cm−1 for overtone and combination states. The errors in the three-mode representation of the potential are less than 0.2 cm−1 for the fundamentals and no larger than 2.5 cm−1 for high overtone/combination states with as much as 9 quanta of excitation. We calculate the thermally broadened spectra of the CO-stretch fundamental, the CO–Cu frustrated rotation and the CO–Cu frustrated translation over the temperature range 50–350 K. We compare the temperature dependence of the average frequency and standard deviation of these modes with experiment, and find semiquantitative agreement. © 1997 American Institute of Physics.  
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68.08.-p Liquid-solid interfaces
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
33.15.Mt Rotation, vibration, and vibration-rotation constants

Computing vibrational energy relaxation for high-frequency modes in condensed environments

Dorita Rostkier-Edelstein, Peter Graf, and Abraham Nitzan

J. Chem. Phys. 107, 10470 (1997); http://dx.doi.org/10.1063/1.475323 (10 pages) | Cited 28 times

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In this paper we consider vibrational relaxation of high-frequency impurity modes in condensed environments as a computational problem. Linear response theory provides convenient routes for this computation: The vibrational relaxation rate is obtained as a Fourier transform of a force–force time correlation function. However, numerical difficulties arise for processes characterized by a direct relaxation of high-frequency modes into an environment characterized by a relatively low cutoff frequency. It is shown that modern signal processing procedures can significantly enhance the efficiency and accuracy of the needed computation. Since the relevant “signal” can be very small, the computation can be very sensitive to boundary conditions, and care must be taken to avoid artifacts. The computation may be facilitated by using the expected functional form, exponential dependence on the impurity frequency for high frequency, and fitting the parameters of this form from the simulation. It is emphasized that this exponential dependence seems to be the correct functional form, in spite of theoretical arguments in favor of a Gaussian dependence. The main difficulty in the numerical evaluation of the relaxation rate of high-frequency modes results from the fact that at low temperature the dynamical behavior of such modes is essentially quantum mechanical. We demonstrate this issue by considering vibrational relaxation of an impurity CO molecule in a low-temperature Ar matrix. The results obtained for this system by estimating the quantum correction to the classical force–force correlation function are consistent with experimental results, which indicate that under these conditions the relaxation of the vibrationally excited CO is dominated by radiative decay. © 1997 American Institute of Physics.
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34.50.Ez Rotational and vibrational energy transfer
02.30.Nw Fourier analysis

Stochastic theory of combined radiative and nonradiative transport

Mário N. Berberan-Santos, Eduardo J. Nunes Pereira, and José M. G. Martinho

J. Chem. Phys. 107, 10480 (1997); http://dx.doi.org/10.1063/1.474211 (5 pages) | Cited 6 times

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A stochastic theory of combined radiative and nonradiative transport is presented. The stochastic approach is physically clear and versatile, allowing the consideration of the combined effect of radiative and nonradiative transport, carried out here for the first time. The stochastic approach is formulated for delta-pulse excitation and for the photostationary state. General equations for the intensity, polarization, and anisotropy decays are derived. © 1997 American Institute of Physics.
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05.60.-k Transport processes
44.40.+a Thermal radiation
02.50.Ey Stochastic processes

Absorption line shapes and solvation dynamics of CH3I in supercritical Ar

S. A. Egorov, M. D. Stephens, and J. L. Skinner

J. Chem. Phys. 107, 10485 (1997); http://dx.doi.org/10.1063/1.474212 (7 pages) | Cited 21 times

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We consider the electronic spectroscopy of dilute CH3I in supercritical Ar fluid. Absorption line shapes for the B←X transition of CH3I have been measured previously in low-density argon, which yielded results for the CH3I/Ar pair potentials. Using these potentials, Kalbfleisch et al. [J. Chem. Phys. 105, 7034 (1996)] have performed molecular dynamics simulations to calculate the absorption line shapes at higher densities, and also the solvation correlation function. We compare the results of several analytic theories to the simulated line shapes and solvation correlation functions. © 1997 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation
33.20.Ea Infrared spectra
31.70.Dk Environmental and solvent effects
33.70.Jg Line and band widths, shapes, and shifts

Spectroscopic characterization of the metastable 3pπ 3Π0+,0− valence states and the 4s3Σ+ Rydberg states of the MgKr and MgXe van der Waals molecules

John G. Kaup, Allen W. K. Leung, and W. H. Breckenridge

J. Chem. Phys. 107, 10492 (1997); http://dx.doi.org/10.1063/1.474213 (14 pages) | Cited 8 times

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The first metastable valence excited states and the first Rydberg states of the MgKr and MgXe molecules have been characterized by resonance two-photon photoionization (R2PI) spectroscopy. The Mg(3s3p 3PJ)⋅RG(3Π0+,0−) metastable states, produced by expanding the products of a laser-ablated magnesium rod in Kr/Ar or Xe/Ar gas mixtures into a supersonic expansion, were excited by a dye laser pulse to several vibrational levels of the Mg(3s4s 3S1)⋅RG(3Σ+) Rydberg states, with detection by ionization with a second dye laser pulse. Spectroscopic constants, bond energies, and bond lengths are reported for both states of MgKr and MgXe. The 3Σ+ Rydberg states are much more strongly bound than the lower 3Π0− valence states, and in fact are essentially as strongly bound as the ground states of the analogous MgRG+ ions, characterized previously in the same apparatus. This clearly indicates that the RG atoms can readily penetrate the diffuse Mg(4s) Rydberg electron cloud. The interesting and unusual spin–orbit and “spin–spin” effects observed are attributed to mixing of some RG character into wave functions of predominantly Mg∗ excited state character. Bonding and spin–orbit interactions in the MgAr, MgKr, and MgXe first triplet metastable and Rydberg states are discussed. © 1997 American Institute of Physics.
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31.50.Df Potential energy surfaces for excited electronic states
33.80.-b Photon interactions with molecules
33.80.Eh Autoionization, photoionization, and photodetachment

A least-action variational method for determining tunneling paths in multidimensional system

Tetsuya Taketsugu and Kimihiko Hirao

J. Chem. Phys. 107, 10506 (1997); http://dx.doi.org/10.1063/1.474214 (9 pages) | Cited 8 times

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We propose a least-action variational method to determine the optimal tunneling paths in multidimensional polyatomic reactions on the basis of the minimum energy path (MEP) and the least motion path (LMP). Taking into account geometrical features of the MEP (i.e., variations of path tangent and path curvature or variations of reaction plane along the MEP), the tunneling path is determined to minimize the amount of exponential damping of the nuclear wave function in the passage through the classically forbidden region. As a demonstration, the method is applied to a polyatomic reaction, NH3+OH→NH2+H2O, in which there are highly curved regions on the MEP before and after a transition state. It is shown that the imaginary action integral calculated along the variationally determined tunneling path decreases extensively in comparison with that calculated along the MEP. © 1997 American Institute of Physics.
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82.30.-b Specific chemical reactions; reaction mechanisms
82.20.Kh Potential energy surfaces for chemical reactions
02.30.Xx Calculus of variations
02.30.Yy Control theory

Nonradiative lifetimes for LiH in the A state using adiabatic and diabatic schemes

F. X. Gadéa, H. Berriche, O. Roncero, P. Villarreal, and G. Delgado Barrio

J. Chem. Phys. 107, 10515 (1997); http://dx.doi.org/10.1063/1.474215 (8 pages) | Cited 32 times

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Accurate positions and nonradiative lifetimes of states belonging to the adiabatic A state of LiH are estimated. The results coming from a Golden Rule treatment in the adiabatic scheme present excellent agreement with those obtained through a diabatic close coupling calculation. That confirms the accuracy reached in both approaches and also in the treatment of the diabatic–adiabatic transformation. It involves, in particular, an effective phase control that is needed to properly estimate nonadiabatic couplings. Also, a powerful numerical procedure to obtain energy profiles in the diabatic close coupling frame is described and applied in this work. © 1997 American Institute of Physics.
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33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Vibrational energy relaxation of the cyanide ion in water

Peter Hamm, Manho Lim, and Robin M. Hochstrasser

J. Chem. Phys. 107, 10523 (1997); http://dx.doi.org/10.1063/1.474216 (9 pages) | Cited 76 times

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The vibrational relaxation time of the cyanide ion in H2O and in D2O was measured by IR-pump–IR-probe experiments. The isotopic composition of the ion was varied in order vary the oscillation frequency of the CN− vibrational mode. In D2O, the vibrational relaxation rate is accelerated from 120 to 71 ps when increasing the vibrational frequency from 2004 cm−1 (13C15N) to 2079 cm−1 (12C14N−). In H2O, time constants between 31 and 28 ps were observed. The systematic dependence of the relaxation rates on the vibrational frequency provides a small portion of the friction spectrum. A significant correlation between vibrational relaxation time of the solute and the IR absorption cross section of the solvent was found, providing experimental evidences for a dominating contribution to vibrational relaxation of Coulomb interactions and the importance of coupling to internal solvent modes. In addition, the infrared bandwidths and the orientational diffusion times are reported. All experimental observables T1, T2, and τ2R related to the vibrational transitions of CN− are now experimentally available and can be compared with model theoretical calculations. © 1997 American Institute of Physics.
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34.50.Ez Rotational and vibrational energy transfer
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.30.Gs Hyperfine interactions and isotope effects
33.20.Tp Vibrational analysis
33.20.Ea Infrared spectra

Isomerization of stilbene in the gas phase: Theoretical study of isotopic and clustering effects

Gidon Gershinsky and Eli Pollak

J. Chem. Phys. 107, 10532 (1997); http://dx.doi.org/10.1063/1.474217 (7 pages) | Cited 2 times

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This paper presents a continuation of our previous theoretical studies on the rate of isomerization of trans-stilbene from the first excited electronic state based on the potential energy surface of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. Harmonic RRKM computations and molecular dynamics and Monte Carlo based classical rates are presented for deuterated isotopes of stilbene as well as hexane clusters of stilbene of varying size. Good agreement with experiment is found for energy dependent rates of d12 vs h12 stilbene. However, we find that the rate for d2 stilbene is greater than for d10 stilbene in contradiction to the experimental observations. For the hexane clusters we find that addition of hexane molecules causes a systematic decrease in the rate, in agreement with experiment. © 1997 American Institute of Physics.
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36.40.-c Atomic and molecular clusters
82.30.Qt Isomerization and rearrangement
31.30.Gs Hyperfine interactions and isotope effects
82.20.Tr Kinetic isotope effects including muonium
31.50.Df Potential energy surfaces for excited electronic states

Kramers theory of chemical reactions in a slowly adjusting environment

A. M. Berezhkovskii, V. Yu. Zitserman, S.-Y. Sheu, D.-Y. Yang, J. Kuo, and S. H. Lin

J. Chem. Phys. 107, 10539 (1997); http://dx.doi.org/10.1063/1.474218 (16 pages) | Cited 6 times

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When describing the reaction dynamics in a slowly relaxing environment, one has to include slow nonreactive modes of the environment in an explicit consideration along with the “chemical” mode intrinsically responsible for the chemical transformation. This is done within the framework of the Kramers approach to condensed phase chemical reaction dynamics. The problem is studied under the condition of high friction of the nonreactive mode (slow adjustment) while friction of the chemical mode covers the whole range from weak to high friction. It is found that the reaction dynamics and, hence, the kinetics depend strongly on the strength of the coupling of the reactive and the nonreactive modes. For strong mode coupling the rate constant monotonically decreases with the increase of the friction of the chemical mode. Such behavior is quite distinctive from one for fast adjustment of the environment when the rate constant demonstrates a turnover behavior. Turnover behavior takes place for moderate strength mode coupling. This case has its own interesting specific features: (1) When friction of the chemical mode tends towards zero, the reaction rate remains finite due to the energy diffusion of the chemical mode induced by the motion of the nonreactive mode. (2) For a certain range of the friction coefficients particles escape the reactant domain on a path that avoids the saddle on the potential surface. This saddle-point avoidance is accompanied by violation of the Arrhenius law in the sense that the activation energy becomes a function of the friction intensity. (3) There is a range of the friction coefficients, where the kinetics is multiexponential. Analytical expressions for the rate constant are obtained for those conditions when the kinetics is single exponential. They show how the rate constant depends on the friction coefficients as well as on the parameters of the potential surface. © 1997 American Institute of Physics.
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82.30.-b Specific chemical reactions; reaction mechanisms
82.20.Kh Potential energy surfaces for chemical reactions

The reactions of silver clusters with ethylene and ethylene oxide: Infrared and photoionization studies of Agn(C2H4)m, Agn(C2H4O)m and their deuterated analogs

Geoffrey M. Koretsky and Mark B. Knickelbein

J. Chem. Phys. 107, 10555 (1997); http://dx.doi.org/10.1063/1.474219 (12 pages) | Cited 27 times

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Ethylene and ethylene oxide react readily with silver clusters at 70 K to form the addition complexes, Agn(C2H4)m and Agn(C2H4O)m, respectively. The infrared photodissociation spectra of Agn(C2H4)m and Agn(C2D4)m (n = 3–7) recorded in the 9–11 μm region show several characteristic vibrational bands of ethylene lying near their gas phase frequencies. Photoionization spectroscopy studies reveal that the ionization potentials (IPs) of the complexes decrease monotonically with adsorption of additional ethylene molecules. Together, these results imply that as on macroscopic silver surfaces, ethylene adsorbs molecularly to small silver clusters, with a net donation of electron density into the underlying cluster. Similarly, silver cluster–ethylene oxide complexes display IPs that decrease with increasing adsorbate coverage. The infrared depletion spectra of Agn(C2D4O)m complexes reveal a single feature at 949 cm−1, assigned to the ν4(a1) fundamental of C2D4O. These results verify that ethylene oxide adsorbs molecularly to silver clusters, with the oxygen atom oriented toward the silver cluster. © 1997 American Institute of Physics.
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36.40.-c Atomic and molecular clusters
82.30.Nr Association, addition, insertion, cluster formation
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Ea Infrared spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Determination of reaction geometries

Karl-Heinz Gericke, Christoph Kreher, and Ernst Albrecht Reinsch

J. Chem. Phys. 107, 10567 (1997); http://dx.doi.org/10.1063/1.474220 (6 pages) | Cited 4 times

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Using polarized light the reaction geometry of selected species can be controlled even in bulk experiments. One reactant A is generated in a photodissociation process and its spatial distribution is completely described by the anisotropy parameter β. The other molecular reactant B is excited in a specific rovibrational state. Its spatial distribution is given by the J- and branch-dependent alignment parameter A0(2). Equations have been developed that allow a relatively easy conversion of experimental results to the angle of attack, γ. The unnormalized probability of an attack of A on B under an angle γ is given by the simple expression P(γ)∝[1+⅕βA0(2)P2(cos γ)P2(cos δ)] where δ is the angle between the math vectors of the dissociating and the exciting laser beam. As an example, we have studied the reaction of A+HCN→HA+CN with A=H,Cl. The experimental results prove a preferred linear reaction geometry, i.e, an end-on attack of atom A on the terminating hydrogen atom of the HCN reactant. However, the cone of acceptance is higher for the Cl+HCN reaction than for the H+HCN one. © 1997 American Institute of Physics.
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82.30.-b Specific chemical reactions; reaction mechanisms
33.15.Bh General molecular conformation and symmetry; stereochemistry

Infrared-depletion spectroscopy study on hydrogen-bonded fluorobenzene–methanol clusters

S. Djafari, H.-D. Barth, K. Buchhold, and B. Brutschy

J. Chem. Phys. 107, 10573 (1997); http://dx.doi.org/10.1063/1.474221 (9 pages) | Cited 28 times

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Infrared-depletion spectroscopy, a double resonance method combining infrared- with resonant two-photon ionization (R2PI)-spectroscopy has been applied to mixed molecular aggregates of fluorobenzene⋅(methanol)n (Fb⋅MeOH) with n ⩽ 4. From the IR spectra in the region of the OH stretching vibration of methanol it can be shown that the solvent moiety forms subclusters on one side of the aromatic ring. For Fb⋅(MeOH)3 the methanol trimer exhibits a linear as well as a cyclic structure. The different shifts of the UV bands of these two isomeric clusters reveal the diverse character of the hydrogen bonds involved in their formation. Furthermore it can be shown that for both isomeric forms the product anisole+ is formed in equal quantities by an intracluster SN2 reaction following the photoionization of the chromophore. Whereas for Fb⋅(MeOH)2 this reaction is in competition with evaporative fragmentation it is the dominant reaction channel for the Fb⋅(MeOH)3 cluster. © 1997 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Ea Infrared spectra
33.80.Eh Autoionization, photoionization, and photodetachment
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.15.Fm Bond strengths, dissociation energies
33.15.Bh General molecular conformation and symmetry; stereochemistry

Photodissociation dynamics of OClO: O(3PJ) state and energy distributions

Melanie Roth, Christof Maul, and Karl-Heinz Gericke

J. Chem. Phys. 107, 10582 (1997); http://dx.doi.org/10.1063/1.474222 (10 pages) | Cited 15 times

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The photodissociation dynamics of OClO in the near ultraviolet (UV) was investigated in a state specific and energy selective manner. At a dissociation wavelength of 308 nm, OClO(math 2B1) was excited to the OClO( 2A2(18,0,0)) state, from which it decays into ClO(X 2Π3/2,1/2) and O(3P2,1,0). The nascent oxygen fragments were detected spin selectively by resonant enhanced multiphoton ionization and time of flight measurements (REMPI-TOF). Based on the measurements and the conservation of energy and linear momentum, the internal energy of the ClO partner fragment was obtained. On average, more than 60% of the available energy is transferred into internal excitation of the ClO radical. Nearly the whole internal energy is vibrational energy with vibrational levels populated up to the energetic limit. Besides, the internal energy depends on the oxygen spin-orbit state because the fraction of highly excited ClO fragments increases with increasing total angular momentum J. The bimodal behavior of the fragment energy distribution indicates two different dissociation pathways, in which one leads to ClO radicals excited up to v = 10 and the other one up to v = 15. Furthermore, the decay is anisotropic, which was proved by polarization experiments. This is a hint for a short decay time estimated to be in the order of a few hundred femtoseconds. © 1997 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Wz Other multiphoton processes
33.80.Eh Autoionization, photoionization, and photodetachment
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