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Physical Review A

Polarization spectra of excited-state-Mg(3p)–rare-gas-atom optical collisions

Rosemary A. Lasell, D. A. Olsgaard, Mark D. Havey, Dmitriy V. Kuprianov

Published Jul 1, 1994• 9 citations
Publisher

Abstract

Experimental, polarization-dependent excitation spectra for excited-state-Mg--rare-gas-atom optical collisions are reported. In these first studies of the process, polarized Mg atoms in the 3p $^{1}$${\mathit{P}}_{1}$ level are produced by absorption of linearly polarized light tuned to the 3s $^{1}$${\mathit{S}}_{0}$\ensuremath{\rightarrow}3p $^{1}$${\mathit{P}}_{1}$ resonance transition at 285.2 nm. Detuning-dependent, collision-induced polarization spectra are measured in a \ifmmode\pm\else\textpm\fi{}200-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range around the Mg 3p $^{1}$${\mathit{P}}_{1}$\ensuremath{\rightarrow}5s $^{1}$${\mathit{S}}_{0}$ transition at 571.2 nm. The spectra correspond to probing transient Mg-Ne and Mg-Ar molecules on 3p $^{1}\mathrm{\ensuremath{\Pi}}_{1}$\ensuremath{\rightarrow}5s $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$ and 3p $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$\ensuremath{\rightarrow}5s $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$ electronic transitions. Measurements of these excited-state polarization spectra for Mg-Ne optical collisions reveal that for detunings to the red of the atomic Mg 3p $^{1}$${\mathit{P}}_{1}$\ensuremath{\rightarrow}5s $^{1}$${\mathit{S}}_{0}$ transition, electronic linear polarization greater than 50% survives far into the molecular regime. This represents a direct measure of the polarization important to alignment-dependent inelastic processes in alkaline-earth-metal--rare-gas-atom collisions. The polarization spectra are discussed in terms of existing information on the interatomic potentials and through an axial recoil limit for the polarization degree for parallel and perpendicular molecular transitions. Rate coefficients k for disalignment of Mg 3p $^{1}$${\mathit{P}}_{1}$ atoms by collisions with Ar [k=9.4(5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s] and with Ne [k=6.5(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s] are also extracted from the data.

DOI
10.1103/physreva.50.423

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Experimental, polarization-dependent excitation spectra for excited-state-Mg--rare-gas-atom optical collisions are reported. In these first studies of the process, polarized Mg atoms in the 3p ^{1}$${\mathit{P}}_{1} level are produced by absorption of linearly polarized light tuned to the 3s ^{1}$${\mathit{S}}_{0}\ensuremath{\rightarrow}3p ^{1}$${\mathit{P}}_{1} resonance transition at 285.2 nm. Detuning-dependent, collision-induced polarization spectra are measured in a \ifmmode\pm\else\textpm\fi{}200-cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\ensuremath−1 range around the Mg 3p ^{1}$${\mathit{P}}_{1}\ensuremath{\rightarrow}5s ^{1}$${\mathit{S}}_{0} transition at 571.2 nm. The spectra correspond to probing transient Mg-Ne and Mg-Ar molecules on 3p 1\ensuremathΠ1^{1}\mathrm{\ensuremath{\Pi}}_{1}1\ensuremathΠ1​\ensuremath{\rightarrow}5s 1\ensuremathΣ0+^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}1\ensuremathΣ0+​ and 3p 1\ensuremathΣ0+^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}1\ensuremathΣ0+​\ensuremath{\rightarrow}5s 1\ensuremathΣ0+^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}1\ensuremathΣ0+​ electronic transitions. Measurements of these excited-state polarization spectra for Mg-Ne optical collisions reveal that for detunings to the red of the atomic Mg 3p ^{1}$${\mathit{P}}_{1}\ensuremath{\rightarrow}5s ^{1}$${\mathit{S}}_{0} transition, electronic linear polarization greater than 50% survives far into the molecular regime. This represents a direct measure of the polarization important to alignment-dependent inelastic processes in alkaline-earth-metal--rare-gas-atom collisions. The polarization spectra are discussed in terms of existing information on the interatomic potentials and through an axial recoil limit for the polarization degree for parallel and perpendicular molecular transitions. Rate coefficients k for disalignment of Mg 3p ^{1}$${\mathit{P}}_{1} atoms by collisions with Ar [k=9.4(5)\ifmmode\times\else\texttimes\fi{}10\ensuremath−10{10}^{\mathrm{\ensuremath{-}}10}10\ensuremath−10 cm3{\mathrm{cm}}^{3}cm3/s] and with Ne [k=6.5(7)\ifmmode\times\else\texttimes\fi{}10\ensuremath−10{10}^{\mathrm{\ensuremath{-}}10}10\ensuremath−10 cm3{\mathrm{cm}}^{3}cm3/s] are also extracted from the data.
Experimental spectra for excited-state optical collisions between Mg(3p1P1)\mathrm{Mg}(3p{}^{1}{P}_{1})Mg(3p1P1​) atoms and rare-gas atoms are presented. Aligned Mg atoms are produced in the 3p1P13p{}^{1}{P}_{1}3p1P1​ level by excitation with linearly polarized light. Collisions between the excited metal atoms and Ne or Ar rare-gas atoms are probed in a $1000\ensuremath{-}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range in the vicinity of the Mg $3p{}^{1}{P}_{1}\ensuremath{\rightarrow}5s{}^{1}{S}_{0}$ and $3p{}^{1}{P}_{1}\ensuremath{\rightarrow}4d{}^{1}{D}_{2}$ transitions. The probe radiation is linearly polarized either along or perpendicular to the alignment axis, leading to a linear polarization for the optical collision process as a function of detuning of the probe from the atomic resonances. Substantial spectral variation of the linear polarization reflects the different molecular transitions responsible for the excitation process, and the evolution of the atomic alignment into the molecular regime. In the present case, the transitions correspond to excitation from the Mg--rare-gas $3p{}^{1}{\ensuremath{\Pi}}_{1}$ and $3p{}^{1}{\ensuremath{\Sigma}}_{0}^{+}$ molecular terms to the $5s{}^{1}{\ensuremath{\Sigma}}_{0}^{+},$ $4d{}^{1}{\ensuremath{\Delta}}_{2},$ $4d{}^{1}{\ensuremath{\Pi}}_{1},$ and $4d{}^{1}{\ensuremath{\Sigma}}_{0}^{+}$ terms. Experimental details of the approach, and the results obtained, will be described. Discussion of the polarization spectra is made through theoretical modeling focusing on the effects of the symmetry of the molecular states involved in the process. The modeling employs the semiclassical theory developed earlier. For a range of qualitatively correct interaction potentials and corresponding Condon points for the photo-excitation channels, good agreement between experiment and theoretical estimates is obtained.
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We have analyzed by perturbation theory technique two-photon polarization-dependent fractional optical collisions. In its general form the cross section of the collision is expressed as the overlap integral of its spectral profile with the spectrum of the second order correlation function of the electromagnetic field. Based on the semiclassical expansion of radial wave functions and of a retarded Green function we have expressed the spectral profile of the cross section in terms of well understood semiclassical characteristics of the process such as the rotational transformations of the transition dipole moments and the transition amplitudes describing the adiabatic or nonadiabatic dynamics of the electronic subsystem along classical trajectories. As a practical example, we have calculated the Mg-He ${(3s}^{2}{}^{1}{S}_{0}\ensuremath{\rightarrow}3p{}^{1}{P}_{1}\ensuremath{\rightarrow}5s{}^{1}{S}_{0},4d{}^{1}{D}_{2})$ fractional optical collision cross section. The partial wave analysis for excitation up to the 5s1S05s{}^{1}{S}_{0}5s1S0​ state has shown the nonvalidity of quasistatic approximation based on successive single-photon transitions for some frequency detunings. For example, for the positive detuning from two-photon atomic resonance we have obtained the dominant contribution coming from the direct two-photon Franck-Condon transitions. Such a contribution gives the magnitude of the polarization ratio, characterizing the dependence of the cross section on mutual laser polarizations at the first and at the second steps of the photoexcitation, different than for successive single-photon Franck-Condon transitions. In the more complicated case of excitation up to the 4d1D24d{}^{1}{D}_{2}4d1D2​ state of magnesium the role of the interference between different photoexcitation channels becomes important. The polarization ratio has a stable value close to 17\frac{1}{7}71​ in a broad spectral range in the blue wing of the second frequency detuning. There is a strong dependence of the polarization ratio in the red wing of the second detuning. Such spectral behavior can be explained by the partial selection in the red wing of the atomic resonance line, perturbed by the collisions of those Zeeman transitions in which contributions in the polarization ratio have different orders and different signs.