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The Journal of Chemical Physics

Coherence effects in half collisions: Collision induced fluorescence in Hg(2537 Å)–Ar

D. Segal, I. D. Harris

Published Feb 15, 1991• 10 citations
Publisher

Abstract

We have measured the polarization of collisionally redistributed light as a function of detuning from line center for the Hg3P1–1S0 transition in the presence of Ar. We present a polarization profile for the red wing between 5 and 18 cm−1 and polarizations for two detunings in the blue wing. We compare this type of experiment to the complementary method of photodissociation and discuss the issue of coherence between excitation path-ways. The data are analyzed using a simple molecular reorientation model suggested by others which includes coherent excitation. The model is shown to be unable to completely account for the high polarizations measured in this experiment. We suggest reasons for the failure of this model.

DOI
10.1063/1.459846

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We have measured the polarization of collisionally redistributed light as a function of detuning from line center for the Hg3P1–1S0 transition in the presence of Ar. We present a polarization profile for the red wing between 5 and 18 cm−1 and polarizations for two detunings in the blue wing. We compare this type of experiment to the complementary method of photodissociation and discuss the issue of coherence between excitation path-ways. The data are analyzed using a simple molecular reorientation model suggested by others which includes coherent excitation. The model is shown to be unable to completely account for the high polarizations measured in this experiment. We suggest reasons for the failure of this model.
The authors have measured the polarisation of collisionally redistributed light as a function of detuning from line centre for the Hg 3 P 1 - 1 S 0 transition in the presence of Kr. These are the first such results for a J=0 to 1 system for which the interatomic potentials are well documented. They present a polarisation profile for the red wing between 5 and 40 cm -1 and polarisation measurements for two individual detunings in the blue wing. The data have been corrected for various mechanisms which affect the polarisation but which do not depend on the original scattering event. The application of some of these corrections has been discussed elsewhere for the CW case. Since they report the first pulsed collisional redistribution experiments involving depolarisation measurements they discuss the differences between the pulsed and CW cases in detail.
We have measured the alignment of Hg(6 3^{3}3P1{\mathit{P}}_{1}P1​) atoms following off-resonant excitation of the Hg 6 1S0^{1}\mathrm{S}_{0}1S0​--6 3P1^{3}\mathrm{P}_{1}3P1​ transition in the presence of Kr. We present a polarization spectrum for both red and blue wings out to detunings beyond 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This represents a considerable extension to the existing results, which has been made possible by the use of a pump-and-probe excitation scheme. In the experiment a pulsed laser, detuned from the resonance, creates an alignment in the upper state. This is probed by tuning a delayed second laser to a higher transition and monitoring laser-induced fluorescence from the two-step process. The alignment of the asymptotic Hg(6 3^{3}3P1{\mathit{P}}_{1}P1​) atoms is determined by comparison of the signals obtained for different relative polarizations of the lasers. We discuss the advantages of this method and the relation of the raw data to the polarizations presented. The results are discussed within the context of a simple quasiclassical model of the process.
When detuned laser light is absorbed during a collision, the subsequent fluorescence will not be 100% because of rotation of the excited molecular orbitals as the collision proceeds. A semiclassical locking/decoupling model of this successfully predicted the polarization curve in the red-wing of for Sr+Ar collisions, but was much less successful for Sr+He. The averaged over all phases. Inspired by a quantal treatment effects in photofragmentation, we introduce simple phase terms for evolutionary interference between parallel and components of the molecular orbitals, from the Condon to their decoupling as atomic orbitals. The resulting interference terms have a major effect on Sr-He depolarization, bringing it into good agreement with brute-force quantal computations.
The depolarisation of the redistributed Na D1and D2line fluorescence caused by Na-Ar collisions after far-wing excitation was investigated. In the limit of zero Ar density the J=1/2orientation and J=3/2alignment reflect the distribution among Zeeman sublevels of Na 32PJproduced after absorption of a photon into one or several excited Na-Ar molecular states at small internuclear separation, followed by a mixing between these states during dissociation. Measurements of these quantities as functions of the detuning up to Delta omegaJ=50 cm-1in the red wings are compared with quantum-mechanical coupled-channel (CC) calculations using realistic molecular potentials and thermal averaging, and with predictions from the factorisation approximation (FA) of the theory of redistribution. Fair agreement with CC calculations and FA predictions was obtained within the impact region, proving the validity of the factorisation assumption there. Outside this region the overall depolarisation is primarily due to reorientation of the Na 3P electronic angular momentum and is reproduced qualitatively by the CC calculations but not by the FA theory. Additional depolarisation connected with the fine-structure coupling was observed.
We have measured the far-wing collisional redistribution line shape, the redistributed fluorescence polarization, and the collisional alignment decay rates for barium perturbed by argon and xenon. The experiment was performed with a heated gas cell ($T\ensuremath{\sim}900$ K) with perturber-gas densities in the 1---30-Torr range and laser light detuned \ifmmode\pm\else\textpm\fi{} 3--- 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$ from the 5535-\AA{} Ba I $6^{1}S\ensuremath{-}6^{1}P$ resonance line. Metal-vapor densities were determined by a Rayleigh scattering technique, which is outlined in detail. By correlating structure in the line shape with that of the polarization, we are able to make definitive statements about the interatomic potentials. The Ba data show strong similarities with previous experimental results for similar two-electron atoms, namely Ca, Sr, and Hg, and so our conclusions are of relevance to these systems as well.
The polarisation of atomic line fluorescence resulting from the excitation of a collisional complex is calculated on the basis of a simple molecular picture for the pair of colliding atoms. The red-wing region is treated in detail as arising from excitation of both Pi and Sigma molecular states at different internuclear distances, and the blue wing is identified as predominantly due to the Sigma state. A calculation for the strontium 4607 AA line gives good qualitative agreement with the experiment.