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Very-high-resolution photon-echo measurements of depolarizing collisions

Arjun G. Yodh, THOMAS W. MOSSBERG, John E. Thomas

Published Jun 9, 1986• 0 citations
OpenAlex

Abstract

The collisional scattering of magnetically polarized atoms is of fundamental interest since the scattering process couples translational and internal degrees of freedom.1 Photon-echo techniques have recently been used to study this coupling by preparing active atoms in a superposition of magnetic sublevels and then quantitatively correlating collisional velocity changes with collisional depolarization.2–5 These experiments provide previously unmeasured information about the collision process. For example, they elucidate the differential relaxation experienced by different types of magnetic order (k-multipoles) and clarify the structure of polarization preserving collision kernels.

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The collisional scattering of magnetically polarized atoms is of fundamental interest since the scattering process couples translational and internal degrees of freedom.1 Photon-echo techniques have recently been used to study this coupling by preparing active atoms in a superposition of magnetic sublevels and then quantitatively correlating collisional velocity changes with collisional depolarization.2–5 These experiments provide previously unmeasured information about the collision process. For example, they elucidate the differential relaxation experienced by different types of magnetic order (k-multipoles) and clarify the structure of polarization preserving collision kernels.
Among various photon echo phenomena which have been predicted and observed in vapours [1], stimulated photon echo (three pulse echo) is a very attractive tool to investigate angular dependence of collisional processes. Already applied to the study of small angle elastic scattering [2] this method may also prove adequate for studying quasi-resonant inelastic processes involving excited species such as depolarizing collisions, collisional transitions between hyperfine, fine or rotational sublevels, energy pooling collisions. For all these processes total cross sections have already been obtained in the past by different experimental ways. However they deserve to be reexamined in the perspective of their angular dependence.
Isolated-multipole-moment photon-echo techniques for atomic collision studies are described and analyzed. The techniques employ three-pulse photon echoes observed in a polarization rotation configuration such that the echo intensity depends on the square of an isolated J=1 multipole moment for which the velocity-changing kernel is diagonal. This scheme is applied to resolve clearly classical and wave-mechanical features of 3P1^{3}\mathrm{P}_{1}3P1​ orientation kernels for 174gas^{174}\mathrm{gas}174gas scattering. The diagonality (in the sense to be described) of the one-dimensional kernels chosen for study is discussed physically and derived in detail.
Measurements have been made of the polarization of characteristic radiation of excited states, formed in charge exchange collisions, in delayed time coincidence with the neutral particles formed and scattered during the collision. Such information directly provides ratios of populations of orbital magnetic sublevels and phase differences between scattering amplitudes. This relative phase data, in conjunction with an appropriate model of the collision process, has provided insights into the evolution of the electronic charge distribution during the collision process. These techniques are also being applied to ion-molecule charge transfer collisions.
A stimulated photon echo technique with specially selected linear polarizations of the coherent resonant driver pulses is used to study depolarizing collisions in the molecular gas SF 6 and in mixtures of it with buffer He and Xe. The collisional decay rates of the population, orientation, and alignment in an ensemble of gas particles are determined for the first time in a single experiment. These relaxation rates are measured as a function of the longitudinal translational velocities of the resonant particles. To within the experimental accuracy, no significant dependence of the collisional decay rates on the translational velocities of the particles was observed. This result confirms the conventional theoretical approach to depolarizing collisions. In pure SF 6 the decay rates for the orientation and alignment were lower than the relaxation constant for collisions involving a change in the longitudinal velocity (elastic collisions) that is known from experimental observations of the ordinary photon echo. This means that only some of the elastic collisions participate in destroying the multipole moments of the levels. Evidence is found that the relaxation of the multipole moments created by polarized radiation in a resonant medium of molecular SF 6 gas depends on j , the total angular momentum of the level.
An experimental investigation of the Cs $6s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}\ensuremath{\rightarrow}6p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}\ensuremath{\rightarrow}10s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}$ double-resonance two-photon polarization spectrum has been made. In the investigation, a linear polarization degree was measured to extract the alignment depolarization cross section in the cesium 6p2P3∕26p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}6p2P3∕2​ level due to collisions between ground-level argon and excited 6p2P3∕26p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}6p2P3∕2​ cesium atoms over the Zeeman coherences. The spectra show strong depolarization of the Cs 6p2P3∕26p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}6p2P3∕2​ level. The measured alignment depolarization cross section value of 186 (58)A˚2(58)\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{2}(58)A˚2 is in good agreement with the experiment and theoretical predictions.