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15 Dec 1993

Volume 99, Issue 12, pp. 9337-10089

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The Ar–HF intermolecular potential: Overtone spectroscopy and ab initio calculations

Huan‐C. Chang, Fu‐Ming Tao, William Klemperer, Catherine Healey, and Jeremy M. Hutson

J. Chem. Phys. 99, 9337 (1993); http://dx.doi.org/10.1063/1.465518 (13 pages) | Cited 53 times

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The vibrational dependence of the intermolecular potential of Ar–HF is investigated through the spectra of levels correlating with HF(v=3). We have previously reported measurements of the (vbKn)=(3000), (3100), and (3110) levels of Ar–HF using intracavity laser‐induced fluorescence in a slit supersonic jet [J. Chem. Phys. 98, 2497 (1993)]. These levels are found to be well reproduced (within 0.1 cm−1) by the Ar–HF H6(4,3,2) potential [J. Chem. Phys. 96, 6752 (1992)]. The second overtone experiments are extended to include the (3002) state which is coupled to (3110) through Coriolis interaction, and the (3210) state which is more sensitive to higher‐order anisotropic terms in the potential. The observations establish that the level (3002) lies 0.229 cm−1 below (3110), with upper state rotational constant B=0.085 89 cm−1. This is in good accord with the predictions of the H6(4,3,2) potential. The (3210) state lies at 11 484.745 cm−1 with B=0.099 79 cm−1. The band origin is 1.7 cm−1 higher than predicted, and thus contains important new information on the vibrational dependence of the potential. Several detailed features of the spectra can be explained using the H6(4,3,2) potential. The Q‐branch lines of the (3210)←(0000) band show evidence of a weak perturbation, which can be explained in terms of mixing with the (3112) state. The (3210) spectrum exhibits parity‐dependent rotational predissociation and the widths of the P‐ and R‐branch lines and the magnitude of the l‐type doubling can be explained in terms of coupling to the (3200) state, which is estimated to lie 4 cm−1 below the (3210) state.
The Q‐branch lines show a predissociation cutoff above Q(16); this is in reasonable agreement with the predictions of the H6(4,3,2) potential, but suggests that the binding energy calculated for the potential may be about 1 cm−1 too large. To examine the potential further, high‐level ab initio calculations are performed, with an efficient basis set incorporating bond functions. The calculations give a well depth of 92%–95% of that of the H6(4,3,2) potential at θ=0° for v=0 and v=3, respectively; this is in line with earlier results on rare gas pairs. The calculations also reproduce the anisotropy of the H6(4,3,2) potential and its vibrational dependence. The dependence of the intermolecular potential on HF bond length is found explicitly.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.A- Ab initio calculations

Rotational analysis of n=4–7 Rydberg states of CO observed by ion‐dip spectroscopy

Masaaki Komatsu, Takayuki Ebata, and Naohiko Mikami

J. Chem. Phys. 99, 9350 (1993); http://dx.doi.org/10.1063/1.465519 (16 pages) | Cited 15 times

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Rotationally resolved spectra of the 5s–7s, 5p–7p, 5d, 6d, and 4f–6f Rydberg states (v=1) of jet cooled CO have been measured by ion‐dip spectroscopy with triple resonance excitation via the 3sσ B 1Σ+(v=1) state. The dip spectra due to the high Rydberg (v=1)←B 1Σ+(v=1) transition revealed the states most of which have not been observed by other spectroscopy. By the rotational analysis of the dip spectra, electronic term values, quantum defects, and rotational constants were obtained. For the np Rydberg states, the change of the angular momentum coupling between the p Rydberg electron and the CO+ core was clearly observed as a function of the principal quantum number, n, indicating a transition from Hund’s case (b) to Hund’s case (d). It was found that the rotational constant of the nsσ state increases with n, while that of the ndσ state decreases. The changes in the rotational constants were interpreted by the mixing between the nsσ state and the (n−1)dσ state and the mixing coefficients for those states were determined. For the 4f, 5f, and 6f Rydberg states, only the rotational levels belonging to e‐symmetry were observed in the dip spectrum. It indicates the selective predissociation through the low lying D′ 1Σ+ state.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Sn Rotational analysis

Spectroscopic probing of aerosol particle interfaces

Jian‐Xiang Zhang and Pamela M. Aker

J. Chem. Phys. 99, 9366 (1993); http://dx.doi.org/10.1063/1.465520 (10 pages) | Cited 10 times

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A novel nonlinear Raman spectroscopic technique, which can be used to monitor the chemical composition of an aerosol particle interface, is described. The technique is called morphology‐dependent stimulated Raman scattering (MDSRS). Experimental results show that there is a quantifiable relation between MDSRS signal size and the concentration of species being probed. We outline the analytic form of the nonlinear gain equation and prove its reliability by modeling MDSRS signal strengths.
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82.70.Rr Aerosols and foams
42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering

High resolution electronic spectroscopy of ZnCH3 and CdCH3

Timothy M. Cerny, Xue Qing Tan, James M. Williamson, Eric S. J. Robles, Andrew M. Ellis, and Terry A. Miller

J. Chem. Phys. 99, 9376 (1993); http://dx.doi.org/10.1063/1.465521 (13 pages) | Cited 29 times

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ZnCH3 and CdCH3 radicals have been prepared in a cold supersonic free jet expansion and their laser‐induced‐fluorescence spectrum recorded for the math 2E←math 2A1 electronic transition. These spectra show well resolved rotational and spin structure, which has been completely analyzed. This analysis yields the rotational constants and the components of the spin–rotation tensors in the math and math states of both radicals. The observed constants are discussed in terms of the electronic structure of the radicals. It is demonstrated that the upper F2 spin–orbit component of the math 2E state of CdCH3 is strongly perturbed by another, dissociative electronic state. This leads to some predissociation of the math 2E3/2 component and a broadening of its lines. The rotational and fine structure in this state is also quite perturbed leading to an unusual, but still interpretable spectrum.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Vibrational band shape analysis of the C–H vibration of CH2I2 molecules in liquid CCl4

G. Moser, A. Asenbaum, and G. Döge

J. Chem. Phys. 99, 9389 (1993); http://dx.doi.org/10.1063/1.465522 (5 pages) | Cited 7 times

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The vibrational band shapes and their parameters of the C–H vibration of CH2I2 diluted with liquid CCl4 were measured as a function of composition. The linewidth increases from 8.7 cm−1 with decreasing mole fraction X of CH2I2 reaches a maximum value of 15.92 cm−1 near X=0.4 and decreases again in the limit of the diluted case. The frequency of the band center increases with decreasing X from 2965.37 to 2984.75 cm−1. These results are compared with the model of Knapp and Fischer for the concentration dependence of the vibrational linewidths and shifts. For the line shift, good agreement is found between theory and experiment by introducing a microscopic mole fraction. The model can be used to get an estimation on the difference between macroscopic and microscopic concentration. The predictions for the band shape are rather satisfactory in the whole concentration range. Deviations are due to the assumption that the band shapes in the model are Lorentzian for both neat liquids, whereas the experimental line shape in neat CH2I2 is not.
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33.70.Jg Line and band widths, shapes, and shifts
78.30.C- Liquids

Pure rotational spectrum and structure of the benzene–CO van der Waals complex

Th. Brupbacher and A. Bauder

J. Chem. Phys. 99, 9394 (1993); http://dx.doi.org/10.1063/1.465523 (6 pages) | Cited 18 times

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Pure rotational transitions of four isotopic species of the benzene–CO van der Waals complex have been observed in the microwave region between 8–18 GHz. Double resonance experiments on the parent species have been used to confirm the assignment of the spectrum resembling that of a symmetric top. The structure of the complex, with CO above the plane of benzene, has been determined from the measured moments of inertia for the isotopic species. Only an upper limit for the barrier to internal rotation of CO may be derived in the absence of transitions in excited internal rotation states.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
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)
33.20.Bx Radio-frequency and microwave spectra

Competing mechanisms for intramolecular vibrational redistribution in the ν14 asymmetric methyl stretch band of trans‐ethanol

G. A. Bethardy and David S. Perry

J. Chem. Phys. 99, 9400 (1993); http://dx.doi.org/10.1063/1.465524 (12 pages) | Cited 13 times

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The extensively perturbed spectrum of the asymmetric methyl stretching vibration of trans‐ethanol near 2990 cm−1 has been reinvestigated via direct absorption infrared spectroscopy at a resolution of 30 MHz. A ground state combination difference analysis of the vibrational state mixing is presented for the upper state levels Ka′ = 0–2 and J′=0–4. The analysis indicates that the rotationless 000 level is anharmonically coupled to the dark bath states. The effective number of perturbing states in each rovibrational transition increases with both J and Ka providing evidence for rotational involvement in intramolecular vibrational redistribution (IVR). The decrease of the average dilution factor from ϕd=0.41 at Ka′ = 0 to ϕd=0.09 at Ka′ = 2 and the increase of the average interaction width from Δϵ=0.04 cm−1 at Ka′ = 0 to Δϵ=0.19 cm−1 at Ka′ = 2 indicate an a‐type Coriolis component to the bright‐bath coupling. In the Ka′ = 0 series the dilution factor decreases rapidly from ϕd=0.92 at J′=0 to ϕd=0.14 at J′=3 indicating that b,c‐type Coriolis coupling also plays a significant role in the IVR process. The effective level density ρeffc for all of the observed transitions lie above the total vibrational state density ρvib=9 levels per cm−1 and most are closer to the total rovibrational state density ρrovib=(2J+1)ρvib. This suggests that following a coherent preparation of the asymmetric methyl stretching vibration, the ensuing dynamics explores all of the energetically accessible vibrational phase space of both the gauche and trans forms and much of the accessible rovibrational phase space, i.e., that the Ka quantum number is at least partially destroyed. The C–H stretch is deduced to decay with a 59 ps IVR lifetime to the asymptotic probability of 0.24.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.70.Fd Absolute and relative line and band intensities
33.20.Ea Infrared spectra

Vibrational ab initio calculations and spectra of C–H bonds of trimethylboron

Carlos Manzanares, Victor M. Blunt, and Jingping Peng

J. Chem. Phys. 99, 9412 (1993); http://dx.doi.org/10.1063/1.465475 (8 pages) | Cited 5 times

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Visible laser photoacoustic and near‐IR spectra of the overtones of the C–H stretches of (CH3)3B in the gas phase are reported. Two bands are assigned to nonequivalent methyl C–H bonds. The interaction of an empty 2p orbital of the boron atom with the C–H bonds of a methyl group changes the strength of the C–H bonds during the internal rotation. The most intense, higher energy absorption band in each overtone region is assigned to the CH bonds in the molecular plane (C–H∥) and the least intense, lower energy absorption band to the CH bonds out of the molecular plane (C–H⊥). To interpret the experimental results, overtone transitions are described in terms of the local mode model. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to particular transitions. Ab initio molecular orbital calculations were also performed. Equilibrium geometries, vibrational frequencies, and infrared intensities were calculated at the Hartree–Fock level using the 3–21G and 6–31G∗ split valence basis set. Several geometries were calculated. The minimum energy corresponds to a geometry in which the CH3 groups are aligned with a single CH bond in the molecular plane (C3h symmetry). Another geometry in which two CH3 groups are aligned with a C–H bond in the plane of the molecule and one CH3 group is aligned with a C–H bond perpendicular to the molecular plane is found to be the saddle point. A third geometry in which each CH3 group has a single CH bond aligned perpendicular to the molecular plane (C3v symmetry) is higher in energy than the first two and does not correspond to a minimum. Calculations were performed on the deuterated molecule (CHD2)B(CD3)2 using the most stable C3h conformation and the conformation of the saddle point. In this way, the isolated C–H∥ and C–H⊥ bond lengths, the corresponding C–H stretching force constants, and the vibrational frequencies were obtained.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.20.Ea Infrared spectra
07.77.-n Atomic, molecular, and charged-particle sources and detectors
37.20.+j Atomic and molecular beam sources and techniques
31.15.V- Electron correlation calculations for atoms, ions and molecules

Stark effect measurement in samarium monoxide: Dipole moments of the [16.6]1 and X0− states

C. Linton, A. M. James, and B. Simard

J. Chem. Phys. 99, 9420 (1993); http://dx.doi.org/10.1063/1.465476 (8 pages) | Cited 4 times

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The permanent electric dipole moments of the [16.6]1 (the Ω=1 state lying near 16 600 cm−1) and X0− (ground) electronic states of 152SmO and 154SmO have been determined in a pulsed molecular beam by measuring the Stark shifts of the R(0) and R(1) lines of the (0,0) band in the [16.6]1–X0− transition. Electric fields up to 7.9 kV/cm were used. The Stark measurements also gave a precise determination of the Ω doubling of the J=1 level of the [16.6]1 state. The magnitudes of the dipole moments for the X0− and [16.6]1 states were determined to be 3.517(20) and 4.022(24) D for 152SmO, and 3.451(28) and 3.967(40) D for 154SmO (2σ error bounds). The splitting due to the Ω doubling of the J=1 level of the [16.6]1 state was determined to be 0.0433(24) cm−1 for 152SmO and 0.0380(50) cm−1 for 154SmO. A field dependent perturbation affecting the J=1, MJ=0 level of the [16.6]1 state of the 154SmO isotopomer was observed and analyzed.
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33.57.+c Magneto-optical and electro-optical spectra and effects

Structural transitions and thermally averaged infrared spectra of small methanol clusters

U. Buck, B. Schmidt, and J. G. Siebers

J. Chem. Phys. 99, 9428 (1993); http://dx.doi.org/10.1063/1.465477 (10 pages) | Cited 14 times

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Classical Monte Carlo and molecular dynamics (MD) simulations were carried out to investigate the structures, the infrared spectra, and the rigid–nonrigid transitions of small methanol clusters (CH3OH)n for n=3–6. The study was motivated by experimental results for these clusters from size specific infrared (IR) dissociation spectroscopy. The MD simulations revealed the following transitions: The trimer passes from a rigid ring configuration into a series of nonrigid open chain structures starting at 197 K. For n=4 and 5 such transitions occur between rings and rapidly fluctuating ring structures at T=357 and 243 K, respectively. For n=6 first a pure isomeric transition between the two energetically lowest isomers of S6 and C2 symmetry is found at 35 K, and then a similar transition to a nonrigid behavior as is observed for n=4 and 5 is seen at 197 K. The measured spectra display in all cases the rigid lowest energy configurations.
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36.40.-c Atomic and molecular clusters
33.20.Ea Infrared spectra

Intensities of CH‐ and CD‐stretching overtones in 1,3‐butadiene and 1,3‐butadiene‐d6

Henrik G. Kjaergaard, David M. Turnbull, and Bryan R. Henry

J. Chem. Phys. 99, 9438 (1993); http://dx.doi.org/10.1063/1.465478 (15 pages) | Cited 20 times

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Gas phase vibrational overtone spectra of 1,3‐butadiene are recorded in the ΔvCH=2–6 regions by conventional near infrared–visible spectroscopy, and in the ΔvCH=4–7 regions by intracavity dye/titanium:sapphire, laser photoacoustic spectroscopy (ICL‐PAS). Gas phase vibrational overtone spectra of 1,3‐butadiene‐d6 are recorded in the ΔvCD=2–5 regions with conventional spectroscopy and in the ΔvCD=5–8 regions by ICL‐PAS. Oscillator strengths are calculated from wave functions that are obtained from a harmonically coupled anharmonic oscillator (HCAO) local mode model and from a dipole moment function that is obtained from ab initio calculations. The experimental oscillator strengths are compared to the values that are calculated for both the CH‐ and CD‐stretching components of the spectrum. Our simple calculations, which contain no adjustable parameters, are in very good agreement with the relative intensities of the peaks corresponding to the three different CH oscillators in 1,3‐butadiene. As expected, the local mode description is not as good for the CD oscillators in 1,3‐butadiene‐d6. Nonetheless, the calculations can provide a reasonable explanation of the CD‐stretching intensity distribution in the higher overtone spectra of 1,3‐butadiene‐d6. Small hydrogen impurities in the fully deuterated sample give rise to isolated CH‐stretching overtones. The relative intensities of the CD peaks and the CH impurity peaks in the 1,3‐butadiene‐d6 sample spectra are predicted by the calculations. A comparison of the 1,3‐butadiene‐d6 sample spectra in the CH‐stretching region with the CH‐stretching overtone spectra in 1,3‐butadiene dramatically illustrates the effects of vibrational coupling between CH oscillators.
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33.20.Ea Infrared spectra
33.20.Kf Visible spectra
07.77.-n Atomic, molecular, and charged-particle sources and detectors
37.20.+j Atomic and molecular beam sources and techniques
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

The effect of internal rotation on the methyl CH‐stretching overtone spectra of toluene and the xylenes

Louis Anastasakos and Timothy A. Wildman

J. Chem. Phys. 99, 9453 (1993); http://dx.doi.org/10.1063/1.465479 (7 pages) | Cited 11 times

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The structure of methyl CH‐stretching overtone bands in the vibrational spectra of methylbenzenes was investigated theoretically. The anharmonic CH‐stretching vibration, described by a Morse potential, was represented in terms of a harmonic basis while hindered internal rotation of the methyl group was represented by a rigid rotor attached to an infinitely massive frame. Relatively weak coupling between the anharmonic CH vibration and the hindered internal rotation is sufficient to shift the positions of rovibrational lines from a PQR‐like rotational contour to patterns similar to those observed experimentally. For high rotational barriers, as in o‐xylene, the rovibrational transitions form two bands associated with conformationally nonequivalent CH‐bonds, consistent with the conformational preference established by microwave spectroscopy and molecular orbital calculations. For nearly free internal rotation, as in toluene, m‐xylene and p‐xylene, a prominent middle band is also present. This ‘‘free rotor’’ band corresponds to rotational transitions between states high above the barrier and disappears as the barrier height increases. The outer bands correspond to transitions for which either the initial or the final state is below or near the barrier height in energy. Contrary to earlier suggestions, the band structure is not indicative of the conformational preference of the methyl group in toluene. In fact, the calculated spectra of nearly free internal rotors are insensitive to this preference.
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33.20.Vq Vibration-rotation analysis
33.20.Bx Radio-frequency and microwave spectra

Dynamics of CS2 in the large spectral bandwidth stimulated Rayleigh‐wing scattering

Geneviève Rivoire and Dadi Wang

J. Chem. Phys. 99, 9460 (1993); http://dx.doi.org/10.1063/1.465480 (5 pages) | Cited 12 times

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A theoretical model for the large bandwidth stimulated Rayleigh‐wing scattering [D. Wang and G. Rivoire, J. Chem. Phys. 98, 9279 (1993)] has been built, based on the existence of a translational coherent collective movement driven by an intense optical field. Its rise and decay times deduced from the comparison with the experimental spectral observation are as short as 115 fs.
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33.70.Jg Line and band widths, shapes, and shifts
42.65.Es Stimulated Brillouin and Rayleigh scattering

Vibronic analysis of the math→math laser‐induced fluorescence of jet‐cooled methoxy (CH3O) radical

Yin‐Yu Lee, Gwo‐Huei Wann, and Yuan‐Pern Lee

J. Chem. Phys. 99, 9465 (1993); http://dx.doi.org/10.1063/1.465481 (7 pages) | Cited 26 times

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The dispersed fluorescence of the math 2A1–math 2E system of CH3O and 13CH3O in a supersonic jet was recorded after excitation of various vibrational levels of the math state. Analysis of the spectra yielded ν2″ = 1412, ν3″ = 1047, ν5″ = 1494, and ν6″ = 653 cm−1 for the math state of 12CH3O, and ν2″ = 1408, ν3″ = 1027, ν5″ = 1488, and ν6″ = 649 cm−1 for 13CH3O. The least‐squares fitting of the observed 300–306 lines (with spin–orbit splitting removed) yielded ωe=1057±3 cm−1 and ωexe=7.0±0.7 cm−1 for ν3″ of 12CH3O. Several tentative assignments were also made on the basis of the improved fluorescence spectra. The Fermi‐resonant levels 1289 and 1319 cm−1 above v=0 of the math state were also reassigned; the former is better represented as v2′ = 1, whereas the latter is represented as v3′ = 2.
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33.50.Dq Fluorescence and phosphorescence spectra

Characterization of the first excited 1Π1 and the ground X 1Σ+ states of MgXe. I. Analysis of the 1Π1←X 1Σ+ bound–bound transitions

John G. McCaffrey, David J. Funk, and W. H. Breckenridge

J. Chem. Phys. 99, 9472 (1993); http://dx.doi.org/10.1063/1.465482 (10 pages) | Cited 10 times

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Laser induced fluorescence (LIF) excitation spectra recorded for the vibrational bands in the Mg(3s3p 1P1)⋅Xe(1Π1)←Mg(3s3s 1S0)⋅Xe (X 1Σ+) system have been analyzed, yielding absolute vibrational assignments and values of ωexe=1.585±0.02 and ωe=97.5±1.0 cm−1 for the 1Π1 state of 24Mg132Xe. From a Birge–Sponer extrapolation, the well depth of this state is estimated to be 1500 cm−1. Simulations of rotationally structured spectra of three of the most intense vibrational bands are consistent with R″e=4.56±0.12 Å for the X 1Σ+ state. From Morse function extrapolation of the excited state rotational constants from the simulations, and Franck–Condon intensity simulations of the 1Π1←X 1Σ+ vibrational progressions, R′e for the 1Π1 state is estimated to be 3.07±0.10 Å. The 1Π1 state of MgXe fluoresces strongly. The corresponding 1Π1 states of ZnXe and CdXe do not fluoresce, but ‘‘action’’ spectra from the production (via predissociation) of metal atom 3PJ states are observed. Possible reasons for these differences are discussed in terms of spin–orbit induced predissociation. It is concluded that predissociation of the MgXe(1Π1) state is not observed because the crossing between the repulsive 3∑+1 and the attractive 1Π1 potential curves does not occur until energies higher than those accessible experimentally. Possible reasons for the behavior of the diatomic MgXe(1Π1) state vs that of Mg(3s3p 1P1) isolated in solid Xe, where production of Mg(3s3p 3PJ) states competes with Mg(3s3p 1P1) fluorescence, are also discussed. Finally, the attractive ‘‘bonding’’ interactions in the MgXe(1Π1) state are analyzed in terms of electrostatic interactions and compared with those for other Π‐type states of metal/rare‐gas van der Waals diatomic molecules.
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33.50.Dq Fluorescence and phosphorescence spectra
33.20.Tp Vibrational analysis
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Rovibronic interactions in NO2 around 17 700 cm−1 observed by Zeeman effect and anticrossing experiments

Antoine Delon, Patrick Dupre, and Rémy Jost

J. Chem. Phys. 99, 9482 (1993); http://dx.doi.org/10.1063/1.465483 (14 pages) | Cited 20 times

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We have observed the Zeeman effect on N=1, K=0 rotational levels of seven vibronic levels of NO2 located between 17 438 and 17 842 cm−1. We have used a supersonic jet, (Trot≊4 K) located inside a 5 MW Bitter coil of 100 mm bore which allows magnetic field scans up to 8 T. CW monomode ring dye laser excitation allows a resolution of about 300 MHz limited by the residual Doppler effect. We have observed the evolution of the Zeeman energy levels versus the field. The values of high field Landé factors range from 1.80 to 1.98, significantly lower than the free spin value (2.0023). The standard perturbation theory of Curl [Mol. Phys. 9, 585 (1965)], which relates Landé factor and the spin‐splitting constant ϵ, does not fit the observed results. In addition, 54 anticrossings due to rovibronic interactions have been observed. The corresponding matrix elements range from about 50 MHz (limited by field inhomogeneities) up to 15 GHz, (0.5 cm−1). The expected number of anticrossings in the magnetic field range scanned (from the known rovibronic density of state and from first‐order ‘‘spin–rotation’’ interaction selection rules), is only 27. We explain the additional anticrossings by higher order interactions. In fact, the distribution of observed matrix elements is smooth, without any gap between first order and higher order matrix elements. In this case, we have assumed that the first order matrix elements are the larger ones.
With this assumption, we have determined the average reduced matrix element of first‐order spin–rotation interaction: 0.73±0.15 cm−1. These off diagonal spin–rotation interactions are expected to be roughly independent of the N rotational quantum number. This contrasts with the diagonal electronic‐spin interactions (spin splittings) which increase linearly with N but which are significantly weaker than off‐diagonal interactions at least for the N=1, K=0 levels studied here. We show that these rovibronic interactions (by both first order and higher order) induce the numerous irregularities previously observed in the zero field jet cooled excitation spectrum of NO2. Moreover, the average reduced matrix element of first order spin–rotation interaction observed in the zero field spectrum from 16 500 to 18 500 cm−1 is about 0.76±0.25 cm−1 in agreement with the above‐mentioned high field measurement.  
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.57.+c Magneto-optical and electro-optical spectra and effects

Two‐dimensional femtosecond vibrational spectroscopy of liquids

Yoshitaka Tanimura and Shaul Mukamel

J. Chem. Phys. 99, 9496 (1993); http://dx.doi.org/10.1063/1.465484 (16 pages) | Cited 287 times

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The nonlinear optical response of liquids subjected to a series of N femtosecond laser pulses is calculated using a multimode harmonic model for nuclear motions, with nonlinear coupling to the radiation field through the coordinate dependence of the electronic polarizability. Using electronically off‐resonant optical fields, this multidimensional spectroscopy is shown to provide direct information regarding the homogeneous or the inhomogeneous nature of the spectral density obtained from optical birefringence measurements. Complementary information can be obtained using infrared pulses where the multiple time correlation functions of the nuclear dipole moment (rather than the electronic polarizability) are being probed.
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33.70.Jg Line and band widths, shapes, and shifts
78.30.C- Liquids
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
42.65.-k Nonlinear optics

Theoretical potential energy functions and rovibronic spectrum of electronically excited states of HCO+

Bernhard Weis and Koichi Yamashita

J. Chem. Phys. 99, 9512 (1993); http://dx.doi.org/10.1063/1.465485 (9 pages) | Cited 8 times

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Three‐dimensional potential energy functions (PEFs) and rovibronic spectra for several low‐lying electronically excited singlet states of HCO+ have been investigated based on the ab initio complete active space self‐consistent field (CASSCF) method. The calculated electronic transition moments show that the ultraviolet (UV) absorption spectrum of the ion should be dominated by the transition C 1Π–X 1Σ+. Analytical representations of the PEFs for the two components of the degenerated C 1Π state have been used in beyond Born–Oppenheimer calculations of the rovibronic energy levels of HCO+ and DCO+ by a Renner–Teller variational approach which accounts for anharmonicity, rotation–vibration, and electronic angular momentum coupling effects. Since the excited states of HCO+ are experimentally unknown up to now, the calculated spectroscopic constants, the vertical and adiabatic excitation energies, and the absorption spectra should provide valuable information for future experimental characterizations of the excited states.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.15.V- Electron correlation calculations for atoms, ions and molecules

Relaxation of vibrationally excited HCl molecules in the H2O–HCl collision complex

J. Ree, Y. H. Kim, and H. K. Shin

J. Chem. Phys. 99, 9521 (1993); http://dx.doi.org/10.1063/1.465486 (11 pages) | Cited 1 time

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The temperature dependence of the relaxation of HCl(v=1) by H2O in a complex‐mode collision is studied in a semiclassical approach. The de‐excitation probability takes a maximum value near room temperature, and it decreases logarithmically with increasing temperature. The dependence is nearly linear. Below room temperature, the relaxation becomes less efficient. This unusual temperature dependence is a result of the vibrational relaxation occurring in complex‐mode collisions, which are dominated by large impact parameter interactions. The principal pathway for the removal of vibrational energy is the H–Cl oscillatory and librational motions along the O–H–Cl configuration. When these hindered motions gain the energy, they undergo transitions to free rotational states. The decreased energy transfer efficiency at low temperature is due to the slowing of rotational motions. Energy transfer to the O–Cl large‐amplitude motion is of minor importance.
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34.50.Ez Rotational and vibrational energy transfer
03.65.Sq Semiclassical theories and applications

Classical trajectory simulation of the cluster–atom association reaction I–Arn+I→I2+nAr. II. Diffusion of captured iodine and evaporative cooling of I2

Xiche Hu and Craig C. Martens

J. Chem. Phys. 99, 9532 (1993); http://dx.doi.org/10.1063/1.465487 (15 pages) | Cited 7 times

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This is Part II of a series of papers in which we address the role of microscopic solvation in the association reaction between a free iodine atom and an iodine doped van der Waals cluster: I+I(Ar)n→I2+nAr. The influence of microscopic solvation on the I+I to I2 reactivity, reaction mechanism, energetics, and product energy partitioning is the major focus of our study. The overall reaction for I+I(Ar)12→I2+12Ar can be characterized by three fundamental processes: (1) capture of the incident iodine atom by the I(Ar)12 cluster; (2) diffusive migration of the captured I atom on the surface or in the interior of the cluster, leading ultimately to an encounter with the other I atom to form a highly excited I∗2 molecule; (3) vibrational relaxation of the nascent I∗2 product, leading to evaporative cooling and decomposition of the cluster. Part I [J. Chem. Phys. 98, 8551 (1993)] dealt with the capture process. This article focuses on the chemical dynamics of the subsequent processes of diffusion, vibrational energy transfer, and evaporative cooling. The stabilization of the chemically activated I∗2 molecule through evaporative cooling eliminate the need of a third body collision as required in isolation gas phase recombination. The overall distribution of final energies is nonstatistical for the chemically activated I∗2Arn. The final vibrational energy of I2 exhibits a nonthermal structure even after all the argon atoms are evaporated. In addition to monoatomic sequential evaporation, a ‘‘fissioning’’ mechanism, leading to the formation of at least one multiatom fragment, is observed. The relationship between structure and dynamics is explored. The dynamics of vibrational relaxation, diffusion of the captured iodine, evaporation, and fragmentation pattern, final I2 energy partitioning are found to be strongly dependent upon structure and temperature of the doped cluster. A spectroscopic experimental verification of the above observations is also proposed.
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82.30.Nr Association, addition, insertion, cluster formation
36.40.-c Atomic and molecular clusters
82.20.Fd Collision theories; trajectory models
82.20.Wt Computational modeling; simulation

Flux–flux correlation function study of resonance effects in reactive collision

Victor Ryaboy and Roland Lefebvre

J. Chem. Phys. 99, 9547 (1993); http://dx.doi.org/10.1063/1.465488 (6 pages) | Cited 10 times

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Thermal rate constants for a one‐dimensional model of a reactive collision involving a transient resonance are calculated by using autocorrelation functions of the flux operator in a finite basis set representation [Miller, Schwartz, and Tromp (MST), J. Chem. Phys. 79, 4889 (1983)] and performing either integration over time (MST) or Pade extrapolation to zero of an energy parameter [Lefebvre, Ryaboy, and Moiseyev, J. Chem. Phys. 98, 8601 (1993)]. The two procedures prove to be equally successful. We observe that in the time dependent approach, the correlation function of the reactive flux operator shows, as expected, damped oscillations with a period which slightly depends on the temperature. However, these oscillations are decaying on a time scale that is significantly shorter than the resonance lifetime. This finding shows that the flux–flux correlation function approach is applicable to calculations of thermal rate constants for reactions which proceed via formation of intermediate complexes as well as to studies of short time direct reactive processes.
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82.20.Wt Computational modeling; simulation
82.40.Bj Oscillations, chaos, and bifurcations

The vibrational predissociation of cis‐methyl nitrite in the S1 state: A comparison of exact quantum mechanical wave packet calculations with classical trajectory calculations and detailed experimental results

Agathe Untch, Reinhard Schinke, René Cotting, and J. Robert Huber

J. Chem. Phys. 99, 9553 (1993); http://dx.doi.org/10.1063/1.465489 (14 pages) | Cited 44 times

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We present quantum mechanical wave packet calculations for the vibrational predissociation of cis‐CH3ONO in the S1 state including three degrees of freedom—the CH3O–NO dissociation bond, the N=O stretching coordinate, and the CH3O–N–O bending angle. We calculate the autocorrelation function, the absorption spectrum, the lifetimes of the excited complex as a function of the internal excitation, and the final vibrational‐rotational state distributions of the NO fragment. The lifetimes and the product state distributions are compared with experimental data as well as with previous results obtained from classical trajectory calculations. The calculated vibrational state distributions of the NO product satisfactorily reproduce the systematic variation with the initially prepared quasibound state of the CH3ONO(S1) complex found experimentally; however, they are considerably narrower than the experimental distributions. The theoretical rotational state distributions of NO, all being highly inverted and having the overall shape of a Gaussian, agree well with the experimental data; this is the case for several quasibound vibrational states of CH3ONO(S1) as well as several final vibrational states of the NO product. In general, the classical trajectory calculations parallel the quantum mechanical results. The existing differences have to be attributed to the inability of the purely classical treatment in reproducing subtle quantum effects if the dissociation proceeds through a relatively long‐lived complex. While the calculations yield satisfactory agreement with the experimental NO state distributions including the envelope of the absorption spectrum, they disagree with the experiment in that the resonance widths are about one order of magnitude narrower than in the measured spectrum. Additional calculations for which the torsional angle of NO with respect to the intermolecular dissociation vector R is approximately taken into account as a fourth coordinate reveals that dephasing by out‐of‐plane motion can explain most of this discrepancy.
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33.80.Gj Diffuse spectra; predissociation, photodissociation

A comparison of time‐dependent and time‐independent quantum reactive scattering—Li+HF→LiF+H model calculations

Gabriel G. Balint‐Kurti, Fahrettin Gögtas, Steven P. Mort, Alison R. Offer, Antonio Laganà, and Osvaldo Gervasi

J. Chem. Phys. 99, 9567 (1993); http://dx.doi.org/10.1063/1.465490 (18 pages) | Cited 36 times

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Reactive scattering probabilities are computed over a wide range of collision energies for a model system based on the Li+HF→LiF+H reaction using both grid based time‐dependent and time‐independent quantum mechanical methods. The computations are carried out using a fixed Li–F–H angle which is chosen to be that at which the barrier to the chemical reaction is lowest. The calculated reaction probabilities for this system display many sharp features as a function of energy which are ascribed to scattering resonances. The time‐independent calculations have been carried out on a very dense energy grid, thus permitting detailed comparison between time‐independent and time‐dependent methods (in the latter case, a single computation of the wave packet dynamics provides information on the energy dependence over a given energy range). The results show that the time‐dependent calculations are capable of reproducing even the sharpest resonance features computed using the time‐independent method. The time‐dependent techniques are conceptually very simple and therefore easily implemented. The results presented also demonstrate that the grid based time‐dependent quantum mechanical methods used here are able to describe threshold energy dependence of reaction probabilities where the exit channel kinetic energy is effectively zero. The nature of some of the resonance structures are investigated by computing the time‐independent continuum wave functions at the ‘‘resonance’’ energies thus mapping out the nodal structure of the wave functions. The good agreement between time‐independent and time‐dependent methods is shown to be maintained when a centrifugal barrier is added to the potential to simulate the effect of nonzero orbital angular momentum.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
03.65.Nk Scattering theory

Reaction path analysis of the rate of unimolecular isomerization

Soonmin Jang and Stuart A. Rice

J. Chem. Phys. 99, 9585 (1993); http://dx.doi.org/10.1063/1.466208 (6 pages) | Cited 6 times

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We show that a reaction path Hamiltonian can be used, with the basic concepts of the Davis–Gray analysis of unimolecular reaction rate, to generate an accurate description of the dynamics of a model isomerization reaction.
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82.30.Qt Isomerization and rearrangement
82.20.Rp State to state energy transfer

Diffusion‐influenced reaction kinetics on fractal structures

A. V. Barzykin and M. Tachiya

J. Chem. Phys. 99, 9591 (1993); http://dx.doi.org/10.1063/1.465491 (7 pages) | Cited 17 times

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A theory of diffusion‐influenced geminate and pseudo‐first‐order bulk reactions on fractal structures is presented within the Smoluchowski framework. The formalism is based on the solution of the backward equation for the pair survival probability, generalized to noninteger dimensionality using the effective potential approximation. Possible anomalous diffusion is taken into account by assuming a new time variable, ensuring linear asymptotics of the mean‐squared displacement. The results for both absorbing and radiation inner boundary conditions are derived for noninteracting reactants and compared with those well known for integer dimensions. The asymptotic analysis for arbitrary potential is also carried out.
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82.40.Bj Oscillations, chaos, and bifurcations
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