J. A. Beswick

Vibrational predissociation lifetimes of the van der Waals molecule HeI2

In this paper we present a quantum mechanical study of the rates of vibrational predissociation of the T‐shaped HeI2(B) van der Waals molecule. The van der Waals bond is characterized in terms of Morse type atom–atom interaction and by a Buckingham type atom–atom potential. The dynamics of vibrational predissociation is shown to be insensitive to the long‐range part of the van der Waals potential and the Morse form is adequate for the description of this process. The close‐coupling equations for nuclear motion are solved by standard numerical methods and the vibrational predissociation rates are related to the widths of the resulting resonances. The superlinear theoretical dependence of the vibrational predissociation rates on the excess vibrational energy of the molecular I2 bond is in good agreement with the experimental data. The relative contribution of intramolecular and of intermolecular terms to this superlinear dependence are elucidated, demonstrating the effects of the anharmonicity of the molecu...

Photofragmentation of the Ne⋯ICl complex: A three-dimensional quantum mechanical study

Converged 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....

Theory of mode specific vibrational predissociation: The HF dimer

Quantum mechanical close coupling calculations on (HF)2 vibrational predissociation are presented. The model considers vibrational excitation of the proton donor monomer. The other momomer is frozen at its equilibrium position. The linewidth (lifetime) and final state rotational distribution of the initially vibrationally excited HF were calculated using the artificial channel method. The potential surface was taken from Cournoyer and Jorgensen. The calculated linewidth is in good agreement with recent experimental measurements. This result is drastically different from what is obtained in the collinear model, merely reflecting the important influence of the rotational–vibrational coupling in the highly anisotropic systems. The final rotational distribution of the initially excited HF monomer is highly inverted. The possibility of a rotational in conjunction with a conventional vibrational HF laser is raised.

A wave packet Golden Rule treatment of vibrational predissociation

The time‐dependent wave packet technique is applied to the Golden Rule treatment of vibrational predissociation. The wave packet at time zero is taken as the product of the quasibound wave function and the coupling inducing predissociation. The rate for vibrational predissociation can then be obtained by Fourier transform into the energy domain of the time‐dependent wave packet autocorrelation function. The method has been applied to a model triatomic van der Waals molecule. It is shown that when the bound‐state components of the wave packet are projected out, the time‐dependent version of the Golden Rule approximation provides an alternative efficient technique to treat intramolecular decay.

Angular distributions and separation energies of predissociation photofragments of O2+ (b 4Σg−, v′=4, N′, F′)

Kinetic energy distributions have been measured for O+ photofragments resulting from transitions O2+ (a 4Πu, v″, J″, Ω″) →O2+ (b 3Σg−,v′=4,N′,F′) →O+(4S0) +O(3P), where for the first time in the study of photodissociation all relevant quantum numbers involved are specified. Both 16,16O2+ and 16,18O2+ were studied. The measurements were made using a laser‐ion coaxial‐beams photofragment spectrometer, a single‐mode laser, and velocity tuning of the absorption wavelength. Experimental values of the anisotropy parameter β, which describes the angular distributions of the photofragments, were obtained by fitting the experimental energy distributions with calculated distributions, obtained from a Monte Carlo computer simulation of ion trajectories in the apparatus. Values of β were thus determined with an uncertainty of ±0.05 or less. They are compared with theoretical values of β obtained by a generalization to Hund’s case (b) of the theory previously developed by R. N. Zare. In a similar manner, values of the...

Intermolecular and intramolecular V→V transfer in the vibrational predissociation of some polyatomic van der Waals molecules

In this paper we present a theoretical study of vibrational predissociation (VP) on the ground state potential surface of van der Waals complexes consisting of polyatomic molecules. The intermolecular interaction was represented by separate contributions of atom–atom interactions, which were expressed in terms of Morse potentials. The VP process was handled utilizing the distorted wave approximation to describe a zero‐order Hamiltonian, which is separable in the intramolecular and in the intermolecular motion, while the residual interaction, which induces the reactive process, corresponds to the deviations of the intermolecular interaction from its value at the frozen intramolecular equilibrium configurations of the two constituents. Model calculations of the VP dynamics were performed for the He⋅⋅⋅N2O complex and for the (N2O)2 dimer, where the N2O unit is initially excited to the (001) vibrational state. These calculations were performed for the linear configuration as well as for the T‐shaped configura...

On the orientation of molecular photofragments produced in highly excited rotational states

The degree of orientation of highly excited rotational states of molecular fragments produced by photodissociation with circularly polarized light is studied quantum mechanically. It is shown that a significant orientation of the fragments’ angular momentum j can be obtained when two or more dissociative continua correlated to the same final state of the products are excited simultaneously. In addition, the coherently excited continua should correspond to different helicity states, that is, to different projections of j on the reaction coordinate R (the vector joining the centers of mass of the fragments). The particular cases of an initial total angular momentum equal to zero as well as the axial recoil limit are discussed. The theory is applied to a simplified model of the photodissociation of ICN in the A continuum. The calculations have been performed by integration of the time independent quantum close‐coupling equations for the coupling between the rotation of CN and the reaction coordinate R, using...

Electronic predissociation in N2O+ model calculations for the Ã2Σ+ state

Abstract Presented are quantum mechanical model calculations for the electronic predissociation of N 2 O + (A 2 Σ + ). The distorted wave half-collision model in the collinear configuration is used to calculate predissociation linewidths and final vibrational distributions. An aprroximate potential surface ion dissociation is obtained by fitting some or the available experimental results.

Vibrational predissociation of highly vibrationally excited van der Waals molecules. A quantum-mechanical distorted-wave treatment for vibration and sudden approximation for rotation

Abstract In this paper a three-dimensional quantum model to study vibrational predissociation of van der Waals molecules is applied to the I 2 He complex for very high vibrational (50 ≤ υ 1 ≤ 64) of the I 2 subunit. The vibrations are described in the distorted-wave diabatic approximation while rotations are treated in the sudden approximation. The rates for vibrational predissociation as a function of the initial diatomic (I 2 ) vibrational excitation show a maximum at ν 1 ≈ 57, which is found to be in good agreement with recent experimental data.

A distorted wave calculation for electronic energy transfer in molecular collisions. Application to the N2(A 3Σu+)+CO(X 1Σ+) →N2(X 1Σg+) +CO(a 3Π) system

The electronic energy transfer in collisions between two diatomic molecules at thermal energy is treated by a distorted‐wave type calculation. The predominating role of short‐range interactions is assumed. Potential energy curves and coupling terms have been estimated by configuration interaction between locally excited states. The method is applied to the system: N2(A 3Σu+)+CO(X 1Σ+) →N2(X 1Σg+) +CO(a 3Π). The results are compared with the available experimental data. A general study of the dependence of the rate constants and cross sections on the energy gap and temperature is given. Finally a comparison with the golden‐rule model and other quantum mechanical approximate calculations is presented.