Ionic State Relaxation Processes in VUV-Excited Polyatomic Molecules

Sydney Leach,Gérald Dujardin

doi:10.1155/lc.2.285

Abstract

Advantages of molecular ions for the study of electronic radiationless transitions are discussed. Several coincidence techniques for measuring key relaxation parameters are briefly described. The different cases of radiationless transition studies of molecular ions are outlined and examples mentioned. Coincidence measurements between threshold photoelectrons and ion fluorescence photons emitted by VUV-excited hexafluorobenzene are presented. The radiative, kr, and nonradiative, knr, relaxation rates for specific vibrational levels in the B∼2A2u and C∼2B2u states of C6F6+ are derived from the measured lifetimes and quantum yields of ion fluorescence. The kr rate for excitation of the C∼, 0° level is found to be about 25% of that for the B∼2A2u state where kr = (18±3) × 106 s−1. The knr rates for the 1n and 1n21 progressions in the B∼ state increase with n but exhibit a mode selective behavior. Model calculations of knr confirm the mode selectivity. These calculations were carried out on a noncommunicating, harmonic oscillator basis. Progressive deviations between experimental and calculated results are interpreted as indicating the gradual onset of vibrational redistribution with increasing vibrational energy. The results illustrate a new method for demonstrating the growing in of intramolecular vibrational relaxation.

Highlights

Excited electronic states of molecular ions can relax by three different processes: (1) radiative transitions to lower electronic states; (2) isoenergetic radiationless transitions to bound states; (3) isoenergetic radiationless transitions to dissociation continua
Our discussion will be limited to the collisionless cases of electronic nonradiative transitions (ENRT) and vibrational nonradiative transitions (VNRT)
In the quasi-equilibrium theory of mass spectra[2] it is assumed that molecular ions in electronic excited states undergo ENRT and rapidly convert their electronic energy into vibrational energy of the ground state. This can subsequently lead to vibrational predissociation if sufficient vibrational energy is available, and the intramolecular dynamics (VNRT) allows the ion to attain a dissociative configuration faster than any competitive process that tends to deactivate the high vibrational levels of the ground state

Summary

INTRODUCTION

Excited electronic states of molecular ions can relax by three different processes: (1) radiative transitions to lower electronic states; (2) isoenergetic radiationless transitions to bound states; (3) isoenergetic radiationless transitions to dissociation continua. In the quasi-equilibrium theory of mass spectra[2] it is assumed that molecular ions in electronic excited states undergo ENRT and rapidly convert their electronic energy into vibrational energy of the ground state. This can subsequently lead to vibrational predissociation if sufficient vibrational energy is available, and the intramolecular dynamics (VNRT) allows the ion to attain a dissociative configuration faster than any competitive process that tends to deactivate the high vibrational levels of the ground state.

ADVANTAGES OF MOLECULAR IONS FOR THE STUDY OF RADIATIONLESS TRANSITIONS

Structural advantages

Instrumental advantages

Findings

RADIATIONLESS TRANSITION STUDIES ON MOLECULAR IONS’ GENERAL REMARKS