A. P. Blokhin

Semiclassical description of purely rotational recurrences for collisionless asymmetric top molecules: new results

Abstract A semiclassical approach, based on the discrete WKB approximation, is developed for the description of purely rotational recurrences in thermal ensembles of collisionless asymmetric top molecules. Exact non-perturbative expressions are derived for the periods of the J-, H-, and K-type transients. These formulas automatically account for the phenomenon of increasing the period of the J-type transients for oblate asymmetric tops and decreasing that for prolate asymmetric tops in parallel with the enhancement of molecular asymmetry. The asymmetry corrections are demonstrated to magnify the period of the K- and H-type transients, irrespective of the form of the molecular top. The polarity of the A- and C-type transients is established. New kinds of transients are predicted and characterized by their periods and polarity. The most important of these is demonstrated to be the P-type transients arising for planar molecules.

POLARIZED FLUORESCENCE OF POLYATOMIC FRAGMENTS PRODUCED THROUGH PHOTODISSOCIATION OF POLYATOMIC MOLECULES IN THE GAS PHASE

A combined theoretical and experimental study is carried of the polarized emission of polyatomic products produced through photodissociation of polyatomic molecules. A general approach, based on the formalism of dissociation kernels and orientational correlation functions, is developed to predict anisotropy of the fluorescence of photoproducts. We consider the most general case of asymmetric top parent and product molecules. The rotational predissociation effect is taken into account. Various kinds of photoreactions are studied: those when fragments after dissociation are in the electronically excited states and those when fragments are in the ground electronic states so that additional laser pulse is necessary to excite their fluorescence. Particular attention is concentrated on those practically important extreme cases, when predissociation times and lifetimes of the electronically excited states of photoproducts are short or long as compared to the averaged period of free rotation. The steady state pol...

Effect of the intensity of collisions on the orientational relaxation of spherical top molecules

The influence is investigated of the correlation of angular momenta before and after instantaneous impulsive collision on the course of the orientational relaxation of spherical top molecules. To do this, a Markovian master equation with a collision kernel by Keilson and Storer is solved semianalytically for orientational correlation functions of the first and second rank through the recursive relations. A particular case of the equation contains the J diffusion model and the rotational Fokker-Planck equation. The results of these calculations are compared with those of molecular dynamics simulations of the rough sphere fluid and good agreement is found.

Dynamics of optically induced anisotropy in an ensemble of asymmetric top molecules in the gas phase

A complex theoretical and experimental investigation of the evolution of optically induced anisotropy in an ensemble of free asymmetric tops under collisionless conditions is performed. The dynamics of the orientational correlation functions is studied and the results of femtosecond measurements of the relaxation of the anisotropy of perylene fluorescence in the gas phase are reported. The results obtained are interpreted in terms of the theory developed. It is shown that, in contrast to steady-state anisotropy, the time kinetics of optically induced anisotropy yields more complete and typical data on the dynamics of vector correlations in an ensemble of molecules.

Femtosecond Dynamics of Optically Induced Anisotropy of Complex Molecules in the Gas Phase

The evolution of optically induced anisotropy in an equilibrium ensemble of free asymmetric tops under collisionless conditions has been investigated theoretically and experimentally. The dynamics of the orientation correlation functions has been studied, the results of femtosecond experiments on measurement of relaxation of the optically induced anisotropy for perylene in the gas phase are presented, and their interpretation within the framework of the theory developed is given. It is shown that in contrast to stationary anisotropy, its time kinetics has more complete characteristic information about the dynamics of vector correlations in a molecular ensemble.

Rotational Brownian motion of spherical molecules the Fokker-Planck equation with memory

The rotational Fokker-Planck equation with memory is considered for a spherical top molecule. Angular momenta and rotational energy correlation functions and also orientational correlation functions in the high and low friction limit are calculated analytically. A convenient semianalytical method of solution of the equation through recursive relations is proposed. The method is applicable for an arbitrary memory kernel of the equation and representative calculations for the exponential memory kernel are presented to simultaneously keep track of the behavior of the angular momenta and orientational correlation functions vs. intensity and correlation time of the Brownian torques.

Depolarization of fluorescence of polyatomic molecules in noble gas solvents

Abstract The collisional depolarization of fluorescence is studied for p -quarterphenyl (PQP) in He, Ar, Xe solvents, under pressures ranging from zero to nearly atmospheric. The results are interpreted within the Keilson–Storer model of the orientational relaxation and smooth rigid body collision dynamics. This allows us to estimate the rate of the angular momentum scrambling due to encounters of PQP with its partners. The collisions are shown to be neither strong nor weak, so that the averaged number of encounters giving rise to the PQP angular momentum randomization equals to 33 (PQP–He), 4.5 (PQP–Ar), and 2.1 (PQP–Xe).

Influence of the finite duration and efficiency of collisions on the orientational relaxation times of non-spherical molecules in the hindered rotation limit

The present work deals with the influence of the efficiency and finite duration of intermolecular interactions on the orientational relaxation times (ORTs) in the hindered rotation limit. To this end, a single-particle non-markovian Zwanzig-type master equation is used. The most general case of asymmetric top molecules is considered and a number of analytical results is derived. A possibility is demonstrated to extract non-trivial information about the peculiarities of molecular rotation and interaction by examining variation of ORTs and ratios of ORTs of different ranks vs. the angular momentum relaxation time. The collision efficiency is proved to influence these dependencies markedly. This effect should be necessarily taken into account in describing the orientational relaxation.

Mechanism of photodissociation of polyatomic molecules through the polarized fluorescence of fragments

Abstract To describe the polarized fluorescence of polyatomic photoproducts, a new model, based on the assumption of free rotation of products after photofragmentation, is proposed and checked against preliminary experimental data.

Polarized luminescence of photoproducts as a probe of the photofragmentation dynamics

Abstract To describe the contribution of the parent molecule rotation and the interfragment repulsive forces into the rotational excitation of molecular fragments produced through the photodisociation process, a simple model is proposed. The model allows one to estimate the magnitude of the additional angular momentum, that originates from the interfragment repulsive forces, by observation of the polarized luminescence of photoproducts.