Index of content:
Volume 92, Issue 6, 15 March 1990

A study of the structure and dynamics of the hydronium ion by high resolution infrared laser spectroscopy. III. The ν_{3} band of D_{3}O^{+}
View Description Hide Description294 transitions were measured by velocitymodulation spectroscopy using a difference frequency laser system with a D_{2}/O_{2} discharge and assigned to the ν_{3} (antisymmetric stretch) band of D_{3}O^{+}. A simultaneous analysis of this data with the ν_{2} band spectra reported by Sears, e t a l. [J. Chem. Phys. 8 3, 2676 (1985)] yielded a refined set of parameters for both the ground and the excited states of both bands. The ν_{3} bandcenters are 2629.6512(39) and 2624.2376(47) cm^{−} ^{1} for the s–s and a–a inversion components, respectively.

Infrared spectra of hydrogen dimers
View Description Hide DescriptionWeakly bound complexes of molecular hydrogen have been studied using an infrared Fourier transformspectrometer and a long absorption path (112 m) through equilibrium gas mixtures at temperatures ∼20 K. Spectra of the dimers (H_{2})_{2}, (HD)_{2}, and (D_{2})_{2} were recorded, along with those of the mixed species H_{2}–D_{2} and HD–D_{2}. They lie in the 2900–8700 cm^{−} ^{1} (1.15–3.35 μm) spectral region corresponding to the monomer fundamental and first overtone bands. Since the rotation and vibration of the monomers remain largely unperturbed in a dimer, each dimer band is centered around a monomer rotation–vibration transition. The structure within these bands is associated with motions of the dimer as a whole, and particularly its end‐over‐end rotation, denoted by l. The spectra are quite detailed and well resolved, especially for the heavier isotopes HD–D_{2}, and (D_{2})_{2}, and in many cases, their rotational structure can be fully assigned. Although a detailed analysis is beyond the scope of the present paper, it should ultimately be possible to obtain precise multidimensional pair potential and induced dipole moment surfaces for molecular hydrogen directly from this experiment.

Fermi resonances in overtone spectra of bromoform, CHBr_{3}
View Description Hide DescriptionA curvilinear internal coordinate Hamiltonian is used to analyze Fermi resonances between CH stretching and bending vibrations in bromoform. Besides the usual Fermi resonance terms, cubic and quartic kinetic and potential energy terms in bending variables are included in the Hamiltonian. The eigenvalues are obtained variationally with a Morse oscillator basis set for the stretch and a two‐dimensional harmonic oscillator basis set for the bend. The potential energy parameters are optimized with the nonlinear least squares method. A bond dipole model is used to calculate infrared absorption intensities. The model reproduces well the observed vibrational term values and infrared absorption intensities.

Molecular relaxation properties studied by Rayleigh–Brillouin scattering from aqueous solutions of ammonium nitrate salts
View Description Hide DescriptionRayleigh–Brillouin spectra of aqueous solutions of hydroxyl ammonium nitrate (HAN) and triethanol ammonium nitrate (TEAN) were studied at atmospheric pressure over a temperature range from 205 to 298 K. The hypersonic velocity and absorption coefficient of the phonons were calculated from the spectra as a function of temperature. The absorption due to viscosity and internal energy exchange through the relaxation mechanisms were considered. We find that both mechanisms are responsible for the absorption, but at the lower temperature, the one related to internal energy dominates. The free volume model was used to estimate the activation energy. This value is quite consistent with values calculated from the relaxation time. The effects of relaxation and the water concentration on mixtures of aqueous solutions of HAN and TEAN are also discussed.

A molecular beam‐optical Stark study of the A ^{2}Π–X ^{2}Σ band system of YO
View Description Hide DescriptionThe permanent electric dipole moments of yttrium monoxide YO in its X ^{2}Σ^{+}, A ^{2}Π_{3} _{/} _{2}, and A ^{2}Π_{1} _{/} _{2} states have been experimentally determined from an analysis of the Stark shifts in the optical spectrum of a molecular beam sample. The determined values are 4.45(7), 3.68(2), and 3.22(8) D, respectively. The results are discussed in terms of bonding models for YO and are compared to recent theoretical predictions.

Inert gas clusters of C_{6}F^{+} _{6}: The evolution from isolated ion to solid matrix
View Description Hide DescriptionSimultaneous laser‐induced fluorescence(LIF)spectra and time‐of‐flight mass spectra have been recorded for ionic clusters, C_{6}F^{+} _{6}⋅R_{ n } where R=He, Ne, and Ar. These spectra span the regime of clusters extending from the isolated ion to the ion located in the corresponding inert‐gas matrix (except He). The conclusions of these studies include the following. Abundant clusters with n=1 and 2 exist in symmetrical forms with one atom above and below the benzene plane. Such configurations appear, however, to be evolutionary dead ends with respect to the ultimate matrix structure. Rather, the latter likely corresponds to several inert‐gas atoms sharing more or less equally the cationic charge on each side of the ring. From our results, it may be speculated that most of the essential features of the matrix LIFspectra are obtained with the completion of what is roughly the first solvent shell in the cluster, 6–10 atoms, depending upon the inert gas.

Resonance Raman quantum yields for CS_{2} in solution: Dynamics of solvent‐induced spectral broadening
View Description Hide DescriptionElectronic absorption spectra, resonance Raman spectra and quantum yields, and total emission yields have been measured for the S _{3}←S _{0} transition of CS_{2} in cyclohexane, pentane, acetonitrile, hexadecane, and perfluorohexane solvents. The solution‐phase absorption spectra are significantly broadened and redshifted relative to the vapor. The solution‐phase S _{3} state lifetimes inferred from the total emission yields are 0.6–1.0 ps, close to the vapor‐phase lifetime, while the resonance Raman quantum yields imply electronic dephasing times of 25–50 fs in solution. This rapid dephasing due to intermolecular (solvent–solute) interactions is sufficient to account for almost all of the increased electronic spectral breadth in solution. The data are analyzed quantitatively with the aid of a stochastic theory of line broadening that accounts for solvent memory effects, and evidence is found for non‐Markovian (nonexponential in time) electronic dephasing. Possible physical origins for the rapid electronic dephasing in solution are suggested, and the relevance of these results to other dynamical processes in liquids such as activated barrier crossing and electron transfer reactions is discussed.

Pseudolocal modes of guest molecules in mixed molecular crystals: Photon echo experiments and computer simulations
View Description Hide DescriptionThe temperature‐dependent optical dephasing of anthracene, 9‐methylanthracene, and 2‐methylanthracene monomers in phenanthrene host crystals has been measured using photon echo experiments. Despite large linear electron–acoustic phonon coupling, all three systems dephase because of coupling to pseudolocal modes (local motions of the guest molecule). Computer simulations of the three systems calculate the pseudolocal mode eigenvalues and eigenvectors. In contrast to previous discussions in the literature which describe pseudolocal modes as librations, the predicted eigenvalues are in reasonable agreement with the measured pseudolocal mode energies. The predicted eigenvectors are combinations of translational motion along the long molecular axis and rotational motion about the out‐of‐plane axis of the guest. Differences in site energies for various locations and orientations of the methyl group are calculated.

Femtosecond transition‐state absorption spectroscopy of Bi atoms produced by photodissociation of gaseous Bi_{2} molecules
View Description Hide DescriptionFemtosecond transition‐state absorption spectroscopy has been performed on Bi atoms produced by the 308 nm photodissociation of Bi_{2} molecules contained in bismuth vapor. The transient spectra obtained are all clearly identifiable as a t o m i c, yet they display striking asymmetries in line shapes and enhancements in intensity that clearly demonstrate that they are spectral signatures of atoms still in the force fields of their receding partners.

On the forced catalytic oscillator
View Description Hide DescriptionCatalyticoscillators are usually characterized by dynamic variables that cannot be perturbed directly, by wide separation of time scales and by either soft or hard bifurcation to periodicity. Analysis of a simple relaxation oscillator subject to a square‐wave variation in a parameter reveals a structure similar to that known for the circle map. Qualitative analysis of periodic forcing around a hard‐bifurcation boundary is also presented. These results are compared with motions obtained by a periodic change in the composition of the environment surrounding a Pt wire catalyzing NH_{3}oxidation. The unperturbed system exhibits the three features described above. Harmonic quasiperiods and narrow subharmonic bands are mapped in the forced system.

Photofragmentation of the Ne⋅⋅⋅ICl complex: A three‐dimensional quantum mechanical study
View Description Hide DescriptionConverged three‐dimensional quantum mechanical calculations for photofragmentation of the Ne⋅⋅ICl van der Waals molecule in the energy region of the electronically excited B(^{3}∏_{0+ }) state of ICl are presented and compared with experiments. Lifetimes and final state distributions for the ICl fragments were determined for vibrational predissociation from the lowest van der Waals level in the B(v’=2) channel. Good agreement between theory and experiment was achieved using a sum of atom–atom pairwise potentials. This potential energy surface predicts the equilibrium geometry of the complex to be bent at 140° with the Ne atom towards the Cl end of ICl. The diabatic vibrational golden rule (DVGR) approximation, as well as the rotational infinite order sudden approximation (RIOSA), have been tested again the full 3D calculations. Analysis of the quasibound wave function reveals that the highly inverted rotational distribution of the ICl fragments observed in the experiment, is not due to zero‐point bending motion. It is more likely to be due to a rotational rainbow effect enhanced by the favorable initial geometry of the complex. The effect of the excitation of the bending van der Waals mode in the complex has also been studied. As compared with the lowest level, a longer lifetime and a different rotational distribution of the fragments is predicted. The results presented in this work not only elucidates many dynamical aspects of vibrational predissociation for the Ne⋅⋅⋅ICl complex, but also provide benchmark data for the study of other theoretical methods and approximations.

Picosecond real‐time studies of mode‐specific vibrational predissociation
View Description Hide DescriptionThe vibrational predissociation of several van der Waals complexes of t‐stilbene has been studied by directly measuring, in real time, the fluorescence intensity from the initial reactant state and from the individual product states formed in the dissociation process after exciting single vibrational levels of the complex. With the aid of a kinetic model involving sequential processes, the individual rates for intramolecular vibrational redistribution and vibrational predissociation in the overall dissociation process are resolved and distinguished in several cases. In the stilbene–He complex, the dissociation is significantly faster from low energy out‐of‐plane modes than it is from a higher energy in‐plane mode.

Spectroscopy and dynamics of the highly excited nonrotating three‐dimensional H^{+} _{3} molecular ion
View Description Hide DescriptionA study of the bound states of the H^{+} _{3} molecular ion at zero total angular momentum is presented. Wave functions are shown for the accurate a b i n i t i o Meyer–Botschwina–Burton potential energy surface and the more approximate diatomics in molecules (DIM) surface. The qualitative behavior is similar for the two potentials. The analytic form of the DIM surface enables a study that reaches energies as high as the dissociation threshold. Quantum states are found to localize regularly around the horseshoe periodic orbits found in previous classical studies. There is good agreement between a semiclassical periodic orbit quantization formula and the exact quantum energies. The antisymmetric stretch frequency with respect to the orbit is estimated classically and quantum mechanically and found to be in agreement with a previous estimate. A three‐dimensional stability analysis of the horseshoe orbit is presented and used as a basis for the semiclassical theory. The implications on the assignment of the coarse grained photodissociation spectrum measured by Carrington and Kennedy are discussed.

Vibrational relaxation of OH(X ^{2}Π_{ i }, v=1–3) by O_{2}
View Description Hide DescriptionRate constants for OH(X ^{2}Π_{ i }, v=1–3) vibrational relaxation induced by nonreactive collision with O_{2} have been measured. OH(v) is created by the H+O_{3} →OH(v≤9)+O_{2}reaction in an electron‐irradiated O_{3}, H_{2}, Ar mixture. OH(v) fundamental and first overtone IR emission is observed using time‐resolved Fourier spectroscopy.Spectral fitting followed by kinetic fitting of the resultant populations using a single‐quantum relaxation model yields rate constants of k _{ v=1} =(1.3±0.4)×10^{−} ^{1} ^{3}, k _{ v=2}=(2.1±0.3)×10^{−13}, k _{ v=3}=(2.9±0.8) ×10^{−13} (all units are in cm^{3} /s). Our measurements are consistent with and extend published results on the same system, as well as predictions made by Schwartz–Slawsky–Herzfeld theory.

Rate constants for the reaction, H+D_{2}→HD+D, over the temperature range, 724–2061 K, by the flash photolysis‐shock tube technique
View Description Hide DescriptionThermal rate constants measured by the flash photolysis‐shock tube (FP‐ST) technique are reported for the reaction, H+D_{2}→HD+D, over the temperature range, 724–2061 K. H‐atom concentration has been monitored by atomic resonance absorption spectroscopy (aras). The results can be represented by the Arrhenius expression: k _{1}=(3.95±0.35)×10^{−} ^{1} ^{0} exp(−5919±95 K/T) cm^{3} molecule^{−} ^{1} s^{−} ^{1}, to within ±25% over the temperature range. These results are then combined with lower temperature direct determinations, and a three parameter expression is derived which expresses the rate behavior between 256–2061 K: k _{1}=1.69×10^{−} ^{1} ^{7} T ^{1.10} exp(−3527 K/T) cm^{3} molecule^{−} ^{1} s^{−} ^{1}. The experimental results are then compared to theoretical calculations that utilize a b i n i t i o potential energy surfaces that are presumably the most exact that have ever been determined. Thus, the theoretical to experimental comparison constitutes a stringent test of the a b i n i t i o surfaces and the dynamical calculations in which they are used. The conclusion from this comparison is that transition state theory supplies a high quality prediction for the rate behavior, being within ±30% of the experimental data over the entire temperature range.

Time dependent quantum mechanics using picosecond time steps: Application to predissociation of HeI_{2}
View Description Hide DescriptionTime‐dependent quantum mechanics has proved to be a useful conceptual and calculational tool for a wide range of femtosecond time scale processes. In this paper we develop a method of extending time‐dependent methodology to processes taking many picoseconds, through use of the interaction representation of quantum mechanics. Two implementations are presented, one in the energy frame and one in the coordinate frame; both of these implementations scale as N ^{2} with the number of basis functions, as opposed to N ^{3} for a matrix diagonalization. The interaction representation approach allows the use of time steps much greater than those that can be used in the Schrödinger representation, and is expected to be useful for the broad class of problems where the Hamiltonian can be partitioned into H _{0}+V, where V≪H _{0}. Converged quantum mechanical results are presented for the predissociation of T‐shaped HeI_{2} from highly excited vibrational levels, for times up to 200 ps.

A study of the neon–methane interaction potential as determined from low energy molecular beam experiments
View Description Hide DescriptionAn exhaustive search of the parameter space for the Ne–CH_{4} interaction potential is made in an attempt to determine a potential which will reproduce the results of molecular beamscattering experiments. The experiments were performed at low energy, well below the threshold for vibrational excitation. It is shown that the effects of the long‐range part of the potential were not seen in the experiment. It is further shown that the anisotropy is well represented as far as the experiments are concerned by a fourth rank expansion. In the light of this and earlier studies, it is concluded that a rigid top model for methane is not capable of describing the experimental results even though the collision energy is well below the threshold for vibrational excitation. This is in marked contrast to atom–diatomic molecule scattering.

A new accurate (time‐independent) method for treating three‐dimensional reactive collisions: The application of optical potentials and projection operators
View Description Hide DescriptionThis work describes a new (time‐independent) approach to the study of atom–diatom reactive collisions in three dimensions. The method is based on the idea of converting a reactive multiarrangement problem into an inelastic single‐arrangement problem. This conversion is done by applying optical potentials which are located at all exits of the reagents arrangement. The reactive transition probabilities are calculated applying flux formulas. The method is reminiscent of a previous time‐dependent method successfully applied for both collinear and three‐dimensional reactive collisions.

Beyond the linear approximations of the conventional approaches to the theory of chemical relaxation
View Description Hide DescriptionThe nonlinear coupling between the reacting system and its molecular bath results in a generalized Langevin equation with a memory kernel which is nonstationary as well as dependent on the reaction coordinate. In a preceding paper by Grigolini [J. Chem. Phys. 8 9, 4300 (1988)] a theory was developed to determine the reaction rate of a physical system characterized by a nonlinear interaction between system and bath. It is here shown that the local linearization adopted in that paper extends to this nonlinear condition the linear theory of Grote and Hynes, disregards also nonlinear effects, which does not conflict with the conservation of the Smoluchowski structure necessary to apply the standard first passage time approach. Here a clear distinction is made between the second‐order local linearization (SOLL) and the infinite‐order local linearization (IOLL). When deriving the Kramers equation from a microscopic description, it is possible to go beyond the SOLL approximation without contravening the basic requirement of keeping our description within a standard Fokker–Planck form.
Thus, the influence of nonstationary memory kernel as well as that of the anharmonic contribution of the reaction potential can be conveniently described. The next step, of basic importance for a simple expression of the chemical reaction rate in the space diffusion regime to be found, consists of deriving the Smoluchowski equation. This must be taken in a careful way so that in the linear case the Grote and Hynes theory is recovered. The study of the simple linear case shows indeed that the contraction over the variable velocity of a Kramers equation which is not fully renormalized does not lead to a correctly renormalized Smoluchowski equation, even if the IOLL is applied. A simple rule to take into account the effects of higher‐order terms is then found. In the linear case, this simple rule leads to a result coincident with the exactly renormalized structure. In the nonlinear case, at the second order in the interaction between system and bath, the novel expression coincides with the results provided by the current methods to take into account the anharmonic effects produced by colored noises. The final step of our approach consists of deriving the Smoluchowski equation from this fully renormalized Kramers equation by adopting the IOLL aproximation. The final result is more general than those previously derived by Grigolini, thereby also naturally including the Grote and Hynes theory.

Reactions of O^{−}+ N_{2}O at 300 K: The totally labeled experiments
View Description Hide DescriptionThe tandem flowing afterglow‐selected ion flow tube was employed to study the isotopically labeled reactions of O^{−}+N_{2}O→NO^{−}+NO at 300 K. In the reactions of ^{1} ^{6}O^{−}+^{1} ^{5}N^{1} ^{4}N^{1} ^{6}O, ^{1} ^{6}O^{−}+^{1} ^{4}N^{1} ^{5}N^{1} ^{6}O and ^{1} ^{8}O^{−}+^{1} ^{4}N^{1} ^{4}N^{1} ^{6}O, both of the possible NO^{−} products are formed with equal probability, indicating that equilibration is achieved within the reaction complex before dissociation. In the totally labeled reactions of ^{1} ^{8}O^{−}+^{1} ^{5}N^{1} ^{4}N^{1} ^{6}O and ^{1} ^{8}O^{−}+^{1} ^{4}N^{1} ^{5}N^{1} ^{6}O all possible NO^{−} products are observed which strongly supports the formation of both trigonal and linear N_{2}O^{−} _{2} intermediates along the reaction path. The reaction mechanism is discussed and these results are compared with those of other workers.