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Volume 105, Issue 19, 15 November 1996

Saturation behavior in degenerate four‐wave mixing with nonmonochromatic, non‐Lorentzian fields
View Description Hide DescriptionWe report experimental studies of the degenerate four‐wave mixing (DFWM) response in weak field (no saturation) and strong field (saturation) regimes for the case of nonmonochromatic, non‐Lorentzian fields. The saturation behavior of line‐center intensities and linewidths of rovibrational transitions in the ν_{3}/ν_{2}+ν_{4}+ν_{5} Fermi dyad region of jet‐cooled C_{2}H_{2} are in good agreement with nondegenerate two‐level saturation models, suggesting that the on‐resonance DFWM response is weakly dependent on laser line shape. We find that DFWMline shapes in the weak field regime are determined by the infrared power spectrum and cubic intensity dependence, as illustrated using broadband (0.35 cm^{−1}) and narrowband (0.04 cm^{−1}) infrared radiation. The saturation intensity (I ^{0} _{sat}) is observed to decrease with decreasing bandwidth, in qualitative agreement with theoretical predictions.

Field and temperature dependencies of free carrier photogeneration efficiencies of molecular glasses
View Description Hide DescriptionThe field and temperature dependencies of free carrier photogeneration efficiencies of vapor deposited molecular glasses have been studied by time‐of‐flight photocurrent techniques. The measured photogeneration efficiencies were analyzed by a theory of geminate recombination due to Onsager. In spite of the wide distribution of the charge mobilities and molecular dipole moments of the materials studied, thermalization distances and primary quantum yields were 27–36 Å and 10^{−3} to 10^{−2}, respectively. The results suggest that the thermalization process in molecular glasses is not driven by processes that determine bulk transport properties.

Rotational spectra, structures, and dynamics of small Ar_{ m }–(H_{2}O)_{ n } clusters: The Ar_{2}–H_{2}O trimer
View Description Hide DescriptionRotational spectra have been observed for the Ar_{2}–H_{2}O trimer and several of its isotopomers with the Balle/Flygare pulsed nozzle, Fourier transformmicrowave spectrometer. Analysis gives a planar T‐shaped structure with C _{2v } symmetry and the bidentate protons pointed at the argons. Two sets of asymmetric top transitions were found for the species with H_{2}O, D_{2}O, or H_{2} ^{18}O. Several lines of evidence support assigning them to internal rotor states of the water, the upper set to Σ(0_{00}) and the lower to Σ(1_{01}) as in the Ar–H_{2}O dimer. Support includes: Hyperfine interaction constants which differ for the two states of water; Systematic aspects of the rotational constants such as B’s that are little affected by isotopic substitution; and MMC calculations which indicate the importance of rovibrational coupling. The Ar–Ar distance for the Σ(0_{00}) state of the trimer is estimated to be 3.822 Å and the center of mass (Ar_{2}) to center of mass (H_{2}O) distance to be 3.173 Å. An intriguing result is finding the hyperfine interaction constants of the water in the trimer to be very nearly the same as those reported earlier for the dimer. This shows the two dynamic states of the water are the same in the dimer and trimer, as is the average projection of the water C _{2} axis onto the inertial frame.

An accurate modified Kramers–Kronig transformation from reflectance to phase shift on attenuated total reflection
View Description Hide DescriptionA new and simple procedure is presented for the calculation of the infrared real, n, and imaginary, k, refractive indexspectra from s‐polarized attenuated total reflection(ATR)spectra by a modified Kramers–Kronig transform of the reflectance to the phase shift on reflection. The procedure consists of two parts, first a new modified Kramers–Kronig (KK) transform, and second a new, wave number‐dependent, correction to the phase shift. The procedure was tested with ATRspectra which were calculated from refractive indexspectra that were synthesized under the classical damped harmonic oscillator model. The procedure is far more accurate than previous procedures for the real case of a wave number‐dependent refractive index of the incident medium, and yields n and k values that are accurate to ≤0.1% provided that no errors are introduced by the omission of significant reflection bands. This new procedure can be used to obtain optical constants from any ATR experiment that yields the spectrum of R _{ s }, the reflectance polarized perpendicular to the plane of incidence. In this laboratory R _{ s }spectra are obtained from samples held in the Spectra‐Tech CIRCLE cell in a Bruker IFS 113 V spectrometer. Accordingly the ATRspectra used to test the new procedure were calculated for the optical configuration of this system, which is m reflections at 45° incidence with equal intensities of s‐ and p‐polarized light and retention of polarization between reflections. For the previously studied [J. S. Plaskett and P. N. Schatz, J. Chem. Phys. 38, 612 (1963); J. A. Bardwell and M. J. Dignan, ibid. 83, 5468 (1985)], but unreal, case of constant refractive index of the incident medium, n _{0}, the new transform gave better results than either of two previously studied procedures. In this case the phase shift at each wave number was corrected by a constant which ensured that the correct phase shift was obtained at the highest wave number in the transform, 7800 or 8000 cm^{−1}. In contrast to a previous study [J. Chem. Phys. 83, 5468 (1985)] it was found that the normal KK transform is inferior for this case to a previous modified KK transform [J. Chem. Phys. 38, 612 (1963)], and it is also inferior to the new modified KK transform. Further, the new transform has only the usual singularity of a KK transform, and this makes it numerically superior to the previous modified KK transform which has an additional singularity at 0 cm^{−1}. For the real case, in which the refractive index of the incident medium changes with wave number, the new transform was used with a new simple wave number‐dependent additive correction to the phase shift. This new correction is calculated with the actual value of n _{0} at each wave number. For molecular liquids such as methanol and benzene the new transform is markedly superior to the previous two transforms. It yields real and imaginary refractive index values that are accurate to better than 0.1% provided the reflection spectrum is known down to 2 cm^{−1}. The latter condition is rarely fulfilled, and the effect of the omission of low wave number bands is illustrated. A method to reduce the impact of missing low‐wave number parts of the reflectance spectrum is described, and its effectiveness is illustrated.

The infrared spectroscopy and dynamics of OCO–HCl and SCO–HCl: An example of mode specific intermolecular energy transfer
View Description Hide DescriptionOptothermal near infrared laser spectroscopy has been used to study the OCO–HCl and SCO–HCl complexes by exciting the H–Cl stretch using an F‐center laser. In both cases, the two isotopic forms associated with H^{35}Cl and H^{37}Cl have been observed. All of the observed spectra are consistent with and analyzed in terms of a linear structure. Vibrational predissociation is observed to be abnormally fast in the OCO–HCl complex, considering the rather weak coupling between the intermolecular degrees of freedom and the H–Cl stretch suggested by the small vibrational frequency shift associated with complex formation. Comparisons are made between the two systems studied here that indicate that this anomalous rate is due to a near resonance between the energy available to the fragments after the photodissociation of the complex and the asymmetric stretch of the CO_{2} fragment.

On the interrelation between nuclear dynamics and spectral line shapes in clusters
View Description Hide DescriptionWe analyzespectral absorption line shapes simulated using the molecular dynamicsspectral density method. We explore three classes of line shapes: (1) the region of the 0–0 S _{0}→S _{1}(ππ*) transition of perylene⋅Ar_{ N } clusters, (2) the Xe^{1} S _{0}→^{3} P _{1} transition of XeAr_{ N } clusters, and (3) the photoelectron spectrum of the Li_{4}F_{4} cluster in the valence region. These spectra represent examples for weak, unresolved, and extensive vibrational progressions, which have been analyzed and assigned. Employing a simplified model for the energy gap autocorrelation function allows for an understanding of the different behaviors and for a classification of the interrelation between nuclear dynamics and spectralline shapes. With decreasing the characteristic decay time of the transition dipole autocorrelation function, the line shape passes the limiting cases of the model in the order fast modulation limit→vibrational progression limit→slow modulation limit, with the vibrational progression limit extending the limiting cases of the Kubo stochastic model of line shapes. Some simple qualitative rules have been extracted to predict the overall character of a line shape.

The torsional spectrum of CH_{3}CD_{3}
View Description Hide DescriptionThe torsional spectrum of gaseous CH_{3}CD_{3} has been measured between 230 and 280 cm^{−1} under relatively large pressure‐path length conditions. The observations were made using a modified Bomem spectrometer at a resolution of 0.016 cm^{−1} and an absorption path of 20 m. The gas temperature and pressure were 190 K and 82 Torr, respectively. The prominent features of the spectrum are the P, Q, and R branches of the torsional fundamental (v _{6}=1←0). The P and R branches consist of broad peaks from 0.3 to 0.5 cm^{−1} wide showing partially resolved rotational structure. A frequency analysis combining the present far‐infrared frequencies with molecular beam,microwave, and mm‐wave measurements from previously reported experiments yielded an improved set of torsion–rotation parameters. An intensity analysis was carried out to obtain the torsional dipole moments. The torsional dipole components μ^{ T } _{⊥} and (μ^{ T } _{∥}−μ^{ T } _{⊥}) were determined to be 14.0(1.5) and −39.6(4.0) μD, respectively. The conditions under which there is agreement between the experimental values and those calculated from theoretical expressions are discussed.

Interplay of multiple vibrational spectral densities in femtosecond nonlinear spectroscopy of liquids
View Description Hide DescriptionThe multimode Brownian oscillator model of nonlinear response functions is generalized to include a multielectronic level system interacting with several spectral densities representing solvent and vibrational coupling to electronic energies, transition dipoles, and permanent dipoles. Applications to resonant and off‐resonant transient grating as well as to infrared and fifth‐order Raman photon echoes illustrate how the various spectral densities may be probed separately.

A study on the structure and vibrations of diphenylamine by resonance‐enhanced multiphoton ionization spectroscopy and ab initio calculations
View Description Hide DescriptionLaser‐desorption jet‐cooling has been applied in combination with mass‐selective gas‐phase spectroscopic techniques to study the structure and low‐frequency vibrations of diphenylamine (DPA). Two‐color (1+1′) resonance‐enhancedmultiphoton ionization has been used to measure the vibrationally resolved excitation spectrum of the S _{1}←S _{0} transition in the 305–309 nm region. Ion‐dip measurements have been performed to determine the vibrational structure in the electronic ground state. The electronic spectra of DPA are dominated by long progressions in low‐frequency vibrations involving the motion of the phenyl rings as a whole. For the interpretation of the experimental data ab initio calculations have been performed at the Hartree–Fock level for the S _{0}‐state and using single‐excitation configuration interaction for the S _{1}‐state. The DPA molecule is found to change from a pyramidal geometry around the N‐atom with unequal torsional angles of the phenyl groups in the S _{0}‐state to a planar geometry with equal torsional angles in the S _{1}‐state. The two most prominent vibrational motions are the in‐phase wagging and the in‐phase torsion of the phenyl rings. In addition, the resonance‐enhancedmultiphoton ionization spectra of the S _{1}←S _{0} transition in the DPA‐Ar, DPA‐Kr, and DPA‐Xe van der Waals complexes have been measured. From these spectra it is inferred that there is a coupling between the van der Waals modes and the low‐frequency intra‐molecular modes of DPA.

On a theoretical model for the Renner–Teller effect in tetra‐atomic molecules
View Description Hide DescriptionA model for the ab initio treatment of the Renner–Teller effect in tetra‐atomic molecules is elaborated. It is based on the approach developed by Petelin and Kiselev [Int. J. Quantum Chem. 6, 701 (1972)]. Particular attention is paid to Π electronic states. Perturbative formulas are derived for several coupling cases. The model is checked by means of ab initio calculations at various levels of sophistication. Results of computations of various quantities related to the model are presented for the X ^{2}Π_{ u } states of B_{2}H^{+} _{2} and C_{2}H^{+} _{2}. The reliability of the basis assumptions is demonstrated by comparing the results obtained in the framework of the model considered with those of independent ab initio calculations.

Two‐exciton spectroscopy of photosynthetic antenna complexes: Collective oscillator analysis
View Description Hide DescriptionThe linear and third‐order polarizabilities of the light‐harvesting antennae of photosynthetic bacteria and green plants are calculated using an equation of motion approach which maps the system onto a coupled set of anharmonic excitonic oscillators. The oscillator representation is shown to have several advantages over the traditional picture based on properties of individual global eigenstates of the aggregate; besides a considerable reduction of computational effort, the dynamics of excitations in the two‐exciton band is conveniently analyzed in terms of single‐exciton Green’s functions and the two‐exciton scattering matrix.

Microwave spectroscopy and interaction potential of the long‐range He⋯Kr^{+} ion: An example of Hund’s case (e)
View Description Hide DescriptionWe have observed a microwave spectrum of the HeKr^{+} ion in which all of the observed levels lie within a few cm^{−1} of either the first or second dissociation limit. We use an ion beam technique in which HeKr^{+} ions, formed by electron impact, are mass analyzed. Passage of the ion beam through an electric field lens results in selective fragmentation of energy levels lying close to dissociation. Kr^{+} ions formed in the lens are separated from all other ions by means of an electrostatic analyzer, and are detected with an electron multiplier. Microwaveradiation induces transitions which result in population transfer and produce detected changes in the electric field‐induced Kr^{+} fragment ion current. Additional transitions have been detected by a microwave–microwave double resonance method, and we have also made extensive use of the Zeeman effects produced by small applied coaxial magnetic fields to identify the J quantum numbers of the levels involved. Coupled channel calculations of the bound states of the He⋯Kr^{+} ion are carried out, fully including all the couplings between different electronic states correlating with He+Kr^{+} (^{2} P _{3/2} and ^{2} P _{1/2}). The calculations allow the spectra to be assigned to pure rotational transitions involving levels in the X, A _{1}, and A _{2} states that lie within 2.5 cm^{−1} of the dissociation limits. Because of a systematic near degeneracy between vibrational levels in the X and A _{1} states, the long‐range He⋯Kr^{+} ion provides a very good example of Hund’s case (e) in the form introduced by Mulliken, in which there are no projection quantum numbers onto the interatomic axis. Mulliken’s case (e) is rather different from the Rydberg case (e) described by Lefebvre–Brion, and this is the first time that Mulliken’s case (e) has been observed. The spectra allow the interaction potential for He⋯Kr^{+} to be determined accurately, for the first time, by least‐squares fitting of potential parameters to the experimental line frequencies and g factors. The resulting interaction potential (designated MAL1) is compared with that previously determined for He⋯Ar^{+}: the He⋯Kr^{+} potential is significantly shallower, because the long‐range ion‐induced dipole C _{4} coefficient is the same for the two systems but the larger Kr^{+} ion prevents the He atom approaching as close.

Resonance Raman spectrum of HOCl—A time‐dependent quantum dynamical treatment
View Description Hide DescriptionThe emission spectrum of the dissociating HOCl molecule, following the 2 ^{1} A ^{′}←X̃ ^{1} A ^{′} excitation, is computed using a rigorous time‐dependent quantum mechanical method. The theory needed to include all the polarization and angular momentum effects within the time‐dependent framework is developed and applied for the first time. Detailed predictions are made of the emission or resonanceRaman spectrum of the dissociating molecule both ‘‘on resonance,’’ near the peak of the first absorption band and ‘‘off resonance,’’ in the tails of the band. If the exciting radiation is linearly polarized, and the direction of polarization of the emitted radiation is also measured, then four different ‘‘cases’’ or geometric arrangements of the two polarization directions relative to each other can be identified. The different signals which result are computed and discussed. Expressions are given for the Raman amplitudes as a product of a dynamic and a geometric factor.

Quantum mechanical tunneling through a time‐dependent barrier
View Description Hide DescriptionWe present a numerical investigation of quantum mechanical tunneling process in a time‐dependent (fluctuating) barrier using a one dimensional model of Eckart barrier. The tunneling probability is calculated for two cases in which (1) the height of the barrier is undergoing harmonic oscillation with frequency ω and (2) the location of the barrier is undergoing harmonic oscillation with frequency ω. It is observed in both cases that the quantum mechanical tunneling probability exhibits a maximum as a function of the oscillating frequency ω between the low and high frequency limits. The physical origin and process underlying this resonantlike phenomenon are proposed in this paper based on the current model study.

The influence of multiple scattering processes on the electron mobility in low density methanol gas
View Description Hide DescriptionThe electron drift mobility μ has been measured in CH_{3}OH gas in the temperature range 303≤T≤363 K and in the number density range 2.4×10^{17}≤n≤2.1×10^{19} cm^{−3}. It is usually assumed—although there is no foundation for it—that at such low gas densities the ‘‘zero‐field’’ density‐normalized mobility (μn) does not depend on n, i.e. it can be described by the so‐called Lorentz single collision approximation [see, e.g., N. Gee and G. R. Freeman, Can. J. Chem. 61, 1664 (1983)]. We observed, however, a density dependence of (μn) which can be explained approximately in terms of coherent and incoherent multiple scattering corrections where the coherent contribution due to correlations between the scatterers predominates at lower temperatures.

Diabatic approach to the close‐coupling wave packet method in reactive scattering
View Description Hide DescriptionThis article explores a diabatic formulation of electronically adiabatic quantum reactive scattering problems. In particular, a diabatic approach to the close‐coupling wave packet method is developed which allows us to perform, efficiently, accurate calculations using natural coordinate systems for both the reactant and product configurations. The efficiency and accuracy of the method are investigated in terms of the size of the basis sets, as well as in terms of the symmetry of the diabatic coupling and demonstrated for a model H+H_{2} collinear reaction. Extension of the method to three‐dimensional problems is discussed.

Born–Oppenheimer type separation in the study of the dynamics: Application to photodetachment of ClHCl^{−} and ClDCl^{−}
View Description Hide DescriptionThe Born–Oppenheimer type separation between light and heavy nuclear motions is applied to the study of the dynamics of the ClHCl and ClDCl systems. Approximate and elaborate approaches are used to calculate the photodetachmentspectra of the ClHCl^{−} and ClDCl^{−} anions. The approximate approach is based on a Born–Oppenheimer approximation where the nuclear wave function of the ClHCl or ClDCl systems is obtained as a single product of two wave functions which describe the motion of the light H(D) nucleus for clamped chlorine nuclei and the motion of the heavy chlorine nuclei, respectively. The elaborate approach is a multistate description which goes beyond the Born–Oppenheimer approximation. The relevant close coupling equations are solved numerically subject to proper photodissociation asymptotic conditions. The results obtained in this approach confirm the validity of the approximate one and provide additional information on the ClH+Cl and ClD+Cl decay channels. Resonances are obtained at energies of 0.640 eV and 0.899 eV for the ClHCl system, and at 0.538 eV and 0.715 eV for the ClDCl system. They are identified as shape or Feshbach resonances and analyzed in terms of the normal modes of a linear symmetrical triatomic molecule.

Further study on collisional quantum interference effect in energy transfer within CO singlet–triplet mixed states
View Description Hide DescriptionIn our previous study [G‐H Sha, J‐B. He, B. Jiang, and C‐H. Zhang, J. Chem. Phys. 102, 2772 (1995)], a concept of interference phase angle (θ_{ ST }) has been formally introduced to define the coherenceeffect between singlet and triplet energy transfer channels for mixed states. In this contribution, we have measured θ_{ ST } for various monoatomic (He, Ne, Ar) and diatomic (H_{2}, N_{2}) collision partners at temperatures of 77 K and 470 K. Via a new data processing approach, θ_{ ST } is fitted more accurately than earlier approximations. Our experimental results show that θ_{ ST } increases with the polarizability of monoatomic collision partners, while for diatomic collision partners θ_{ ST } is significantly higher than that for monoatomic ones. θ_{ ST } at 470 K for He and Ne is higher than that of 77 K, while an adverse temperature effect on θ_{ ST } has been found for N_{2}. The influence of intermolecular potential and possible complex formation between excited CO and N_{2} or H_{2} has been discussed.

Vibrational frequency shift of HF in helium clusters: Quantum simulation and experiment
View Description Hide DescriptionWe report accurate variational and diffusionquantum Monte Carlo calculations for the size dependence of the vibrational frequency shift of HF molecules embedded in helium clusters with up to n=198 helium atoms. The frequency shift exhibits a strong initial size dependence and saturates at a redshift of about 2.7 ± 0.1 cm^{−1} for clusters with over 100 atoms. This value is in good agreement with our experimental redshift of 2.65 ± 0.15 cm^{−1} for clusters with over 1000 atoms. The helium cluster is found to undergo significant structural changes upon embedding of HF. The density in the nearest neighbor shell exceeds the bulk helium density by a factor of two. A second nearest neighbor density maximum and a peripheral density plateau very close to the bulk helium value is found. In spite of the anisotropic interaction between HF and helium all clusters have almost perfectly spherical helium density profiles and indicate close to free rotor behavior of HF inside the cluster. The cluster size dependence of the redshift can be qualitatively described by an induced dipole model.

A density‐functional study of the intermolecular interactions of benzene
View Description Hide DescriptionWe have tested the performance of three frequently used density functionals(LDA, LDA+B, and LDA+B+LYP) in a study of the intermolecular interactions of benzene. Molecular geometries are satisfactory, with the gradient‐corrected density functionals yielding slightly better results. The quadrupole moment is significantly underestimated by all three functionals. LDA performs fortuitously comparatively well for both binding energies and geometries of the dimer and the solid, whereas in LDA+B, and LDA+B+LYP the dimer interaction is purely repulsive, leading to the complete absence of cohesion in the solid. These results are consistent with density‐functional theory calculations for noble gas dimers. However, when the dispersion energy calculated from a model potential is included, LDA fails. Binding energies are overestimated by unacceptable amounts, and intermolecular distances are too small. In contrast, dispersion corrected LDA+B and LDA+B+LYP perform reasonably well, although discrepancies are still large when measured on the thermal energy scale at room temperature.