Slow Relaxation Processes in SF6 Gas After Pumping by a CO2 Laser Pulse

Iwao Kitazima,Hiroshi Iwasawa

doi:10.1155/lc.11.39

Abstract

The vibrational energy relaxation and distribution in SF6 at the collisional condition of 2 Torr was measured with the pump-probe method by use of two tunable CO2 300 ns-pulse lasers at the delay time up to 400 μs. The 10.6 μs P(30) pump intensity was 0.4 J/cm2. The probed spectra, divided into two parts, the induced transmission at the P(10)–P(22) and the absorption at the P(24)–P(32), were red-shifted by 50 μs and then slowly back to the initial. The time variation of induced signals was divided into the four regions due to the following processes; (I) the formation of nonthermal distribution in the ν3 mode and quasi-continuum with 1 μs, (II) the broad redistribution by the rise-up of vibrational temperature in 50 μs-duration, (III) the molecular diffusion or expansion out of the pumped region locally heated up by the V–T relaxation in another 60 μs, (IV) the gas cooling corresponding to the energy relaxation or population depletion over 400 μs. So, this pump-probe method at the collisional conditions will easily give us some informations, with a short gas cell and a suitable time scale, on those slow processes.

Highlights

Today it is supposed that the vibrational distribution in the state is nonequilibrium and that the energy relaxation depends nonlinearly on the rate of excitation.5’5 The molecular collision effects on excitation and relaxation must be considered because, collisions are inevitable in most experiments such as a saturable absorber cell usually filled with more than a few Torr of SF6 gas for the laser pulse duration of a few hundred nanosecond
We have studied on the properties of nonlinear absorption in SF6 for a TEA CO2 laser and found that the absorption spectra was red-shifted with increasing the laser intensity and that the significant difference between the additive effects of He and N2 suggested the stochastization limit effectively lower down near 2000 cm-1 in the collisional condition
The probed spectra are divided into two parts, the induced transmission at the

Summary

INTRODUCTION

Q-switching in 1967.1 In addition, after the experiment of the multiphoton dissociation in 1971,2 a number of studies of nonlinear absorption or multiphoton excitation in SF6 were reported on several CO210.6 #m P-branch lines.[3,4,5] The SF6 molecule, a typical heavy polyatomic one, has 15 vibrational modes divided into 6 parts (Vl-V6), which result in formation of the quasi-continuum from a comparatively low energy level (3000 or 5000 cm-1) to the dissociation limit (40,000 cm-1) due to stochastization of vibrational energy by anharmonic shift, splitting and interaction between the modes. Here, we performed the pump-probe measurement on the vibrational energy relaxation by use of two tunable CO2 pulse lasers9’1 with good time-resolution, without saturation effect by the probe laser itself, and avoiding some coherent interactions between the two beams, instead of the more conventionally used CW probe laser.6-8’11-14 We observed the time evolution of absorption spectrum or the induced absorption and transmission by the probe pulse after pumped the SF6 molecules. One can clearly probe, with the delayed weak laser pulse, the time variation of the distribution after pumping by the intense laser pulse

EXPERIMENTAL SETUP

EXPERIMENTAL RESULTS

CONCLUSIONS