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Physical Review A

Stimulated photon echo for angular analysis of elastic and depolarizing Yb/emph>– noble-gas collisions

J. Keller, J.-L. Le Gouët

Published Sep 1, 1985• 22 citations
Publisher

Abstract

Elastic and depolarizing collisions between excited ytterbium atoms (6s6p ${}^{3}$${P}_{1}$ level) and noble gases (He, Ne, and Xe) are investigated using a stimulated-photon-echo technique. The method is sensitive to collisional velocity changes concerning both the total population ${\ensuremath{\rho}}^{0}$ and the alignment ${\ensuremath{\rho}}^{2}$ of the level. More specifically, it enables us to compare precisely the velocity changes associated with elastic scattering and with alignment-preserving scattering. This represents an improvement with regard to previous works. A detailed theoretical discussion of depolarizing collisions in laser spectroscopy is presented. A simplified density-matrix equation is derived under appropriate approximations. Stimulated photon-echo data for elastic scattering are interpreted within the limits of small-angle scattering in a long-range van der Waals potential; total elastic scattering cross sections are derived from comparison between experiment and calculation: ${\mathit{\ensuremath{\sigma}}}_{\mathrm{He}}^{\mathrm{el}}$=220(20) A${\r{}}^{2}$, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Ne}}^{\mathrm{el}}$=320(20) A${\r{}}^{2}$, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Xe}}^{\mathrm{el}}$=1240(60) A${\r{}}^{2}$. Total rates for destruction of alignment are measured in an auxiliary standard photon-echo experiment; the corresponding cross sections are ${\mathit{\ensuremath{\sigma}}}_{\mathrm{He}}^{(2)}$=94(3) A${\r{}}^{2}$, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Ne}}^{(2)}$=123(7) A${\r{}}^{2}$, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Xe}}^{(2)}$=300(20) A${\r{}}^{2}$. Stimulated photon-echo data for depolarizing collisions are successfully compared with a calculation that we develop starting from a long-range anisotropic model potential.

DOI
10.1103/physreva.32.1624

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Study of collisional decay rates of population, orientation, and alignment using stimulated photon echo in SF 6 molecular gas

L. S. Vasilenko, Н. Н. Рубцова, Хворостов Евгений Борисович

Stimulated photon echo technique with specially chosen linear polarizations of pumping pulses of exciting radiation is applied to investigation of depolarizing collisions in molecular gas SF6 and its mixtures with buffer gases He and Xe. For the first time, the collisional decay rates of population, orientation and alignment of SF6 levels are determined simultaneously. Translational velocity dependence of orientation and alignment collisional decay rates was under investigation. This velocity dependence proved to be negligible within the frames of experimental accuracy both in pure SF6 and in mixtures, which is in agreement with standard theoretical approach accepted for depolarizing collisions. Decay rates of orientation and alignment in pure SF6 have proved lower than velocity changing (elastic) collisions relaxation rate, which allows to conclude, that only some part of elastic collisions participates in destruction of multipole polarization moments of resonant levels. The evidence is obtained for j-dependence of multipole moments relaxation rates SF6.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Oct 21, 1998•0 citations
Elastic and depolarizing collisions between excited ytterbium atoms (6s6p {}^{3}$${P}_{1} level) and noble gases (He, Ne, and Xe) are investigated using a stimulated-photon-echo technique. The method is sensitive to collisional velocity changes concerning both the total population \ensuremathρ0{\ensuremath{\rho}}^{0}\ensuremathρ0 and the alignment \ensuremathρ2{\ensuremath{\rho}}^{2}\ensuremathρ2 of the level. More specifically, it enables us to compare precisely the velocity changes associated with elastic scattering and with alignment-preserving scattering. This represents an improvement with regard to previous works. A detailed theoretical discussion of depolarizing collisions in laser spectroscopy is presented. A simplified density-matrix equation is derived under appropriate approximations. Stimulated photon-echo data for elastic scattering are interpreted within the limits of small-angle scattering in a long-range van der Waals potential; total elastic scattering cross sections are derived from comparison between experiment and calculation: \ensuremathσHeel{\mathit{\ensuremath{\sigma}}}_{\mathrm{He}}^{\mathrm{el}}\ensuremathσHeel​=220(20) A˚2{\r{}}^{2}˚2, \ensuremathσNeel{\mathit{\ensuremath{\sigma}}}_{\mathrm{Ne}}^{\mathrm{el}}\ensuremathσNeel​=320(20) A˚2{\r{}}^{2}˚2, \ensuremathσXeel{\mathit{\ensuremath{\sigma}}}_{\mathrm{Xe}}^{\mathrm{el}}\ensuremathσXeel​=1240(60) A˚2{\r{}}^{2}˚2. Total rates for destruction of alignment are measured in an auxiliary standard photon-echo experiment; the corresponding cross sections are \ensuremathσHe(2){\mathit{\ensuremath{\sigma}}}_{\mathrm{He}}^{(2)}\ensuremathσHe(2)​=94(3) A˚2{\r{}}^{2}˚2, \ensuremathσNe(2){\mathit{\ensuremath{\sigma}}}_{\mathrm{Ne}}^{(2)}\ensuremathσNe(2)​=123(7) A˚2{\r{}}^{2}˚2, \ensuremathσXe(2){\mathit{\ensuremath{\sigma}}}_{\mathrm{Xe}}^{(2)}\ensuremathσXe(2)​=300(20) A˚2{\r{}}^{2}˚2. Stimulated photon-echo data for depolarizing collisions are successfully compared with a calculation that we develop starting from a long-range anisotropic model potential.
With a three-level configuration in ytterbium vapor, we have performed the first quantitative investigation of single excited-level scattering using a stimulated photon echo (three-pulse echo). We have studied the collisional relaxation of the echo signal by rare-gas perturbers and have derived a theoretical expression for the relaxation rate. Total elastic cross sections for collisions between Yb(6s6p3P1\mathrm{Yb}(6s6p^{3}P_{1}Yb(6s6p3P1​) and Ne or Xe have been obtained.
Elastic and depolarizing collisions between excited strontium (5s 5p 3P1) and noble gases are investigated in a broadband stimulated photon echo (SPE) experiment. The stimulated photon echo signal produced under broadband (incoherent) excitation is calculated in the weak field regime both for correlated and for uncorrelated pulses using a simple statistical model for the light field. The relaxation of the echo signal due to elastic collisions is considered and the complete development of the corresponding propagator is derived. The collisional velocity changes concerning the total population and the alignment are compared precisely in the SPE experiment and the corresponding relaxation rates are measured. A simple long-range Van der Waals potential accounts for population scattering data ; total elastic scattering cross sections are deduced from the comparison of experiment and calculation (σelAr = 583(27) Å2; σ elHe = 190(6) Å2). The collisional model is extended for light perturbers to include both the small angle classical scattering region and the diffractive scattering region. Total rates for destruction of alignment are measured in an auxiliary photon echo experiment (σ (2)He = 91 (4) Å2; ; σ(2) Ne = 102 (14) A2 ; σ(2)Ar = 267 (13 ) Å2 ; σ(2)Kr = 329 (21) Å2 ; σ(2)Xc = 370 (13) Å2). SPE data for depolarization are compared with a calculation based upon a long-range anisotropic model potential.
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.
The method of a stimulated photon echo with specially selected linear polarizations of pumping pulses of a coherent resonant radiation is applied to a research of depolarizing collisions in molecular gas SF6 and its mixtures with buffer gases He and Xe. For the first time in one experiment the rates of a collisional decay of three lowest moments of polarization (population, orientation and alignment) are defined in an ensemble of gas particles. These rates of a relaxation are measured as functions on longitudinal velocities of resonant particles. In the frames of an experimental accuracy any noticeable dependence on velocities of particles was not detected both for orientation and alignment neither in pure resonant gas SF6 nor in mixtures. This result confirms the standard theoretical approach to depolarizing collisions. In pure SF6 decay rates of orientation and alignment have proved lesser than relaxation rate constant due to velocity changing (elastic) collisions. It means, that only the part of elastic collisions participates in destruction of multipole polarization moments of resonant levels. In comparison with our previous work the evidence is obtained that the relaxation of multipole moments seems to be j-dependent one in a medium of molecular gas SF6.
Isotropic depolarizing collisions are studied using a stimulated photon echo with a specific polarization of the excitation radiation pulses in a mixture of ytterbium with krypton for the J = 1 ⇄ J = 0 transition of 174Yb. The difference between the relaxation rates of orientation and alignment γ (2) − γ (1) of the 3 P 1(6s6p) 174Yb level is measured as a function of the krypton pressure. The collision photon echo at the J = 1 ⇄ J = 0 transition induced by the anisotropic relaxation is studied for the Yb + Xe mixture. The power of the collision echo increases from zero with the addition of a buffer gas to ytterbium, reaches an optimal level, and decreases with an increase in the buffer gas pressure. The polarization of this collision-induced echo differs from the polarization of the conventional echo. The experimental results are in qualitative agreement with theoretical predictions.
Stimulated photon echo technique with specially chosen linear polarizations of pumping pulses of exciting radiation is applied to investigation of depolarizing collisions in molecular gas SF6 and its mixtures with buffer gases He and Xe. For the first time, the collisional decay rates of population, orientation and alignment of SF6 levels are determined simultaneously. Translational velocity dependence of orientation and alignment collisional decay rates was under investigation. This velocity dependence proved to be negligible within the frames of experimental accuracy both in pure SF6 and in mixtures, which is in agreement with standard theoretical approach accepted for depolarizing collisions. Decay rates of orientation and alignment in pure SF6 have proved lower than velocity changing (elastic) collisions relaxation rate, which allows to conclude, that only some part of elastic collisions participates in destruction of multipole polarization moments of resonant levels. The evidence is obtained for j-dependence of multipole moments relaxation rates SF6.