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Journal of Physics B Atomic and Molecular Physics

Collision-induced two-photon forbidden 3s21S-3snp1P transitions and collisional enhancement of 3s21S-3sns1S transitions in Mg I

J -Y Zhang, K. T. Lu

Published Oct 14, 1987• 14 citations
Publisher

Abstract

The collision-induced two-photon forbidden 3s2 1S-3snp 1P transitions (n>11) and collisional enhancement of 3s2 1S-3sns 1S transitions (n>8) of Mg I have been observed and studied as functions of Mg vapour pressure and perturbing gas (Ar) pressure by using two-photon resonant ionisation and thermionic diode techniques. The occurrence of forbidden transitions and collisional enhancement of 3s2 1S-3sns 1S transitions is qualitatively attributed to the formation of a Rydberg Mg2 excimer and a MgxAry Fermi complex. Quantitatively, the above phenomena are interpreted as collisional mixing of high Rydberg states of Mg. The mixing coefficients are estimated by using perturbation theory and the semiclassical (JWKB) approximation. The theoretical estimation is in good agreement with the experimental result.

DOI
10.1088/0022-3700/20/19/017

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•Oct 1, 1984•0 citations
The collision-induced two-photon forbidden 3s2 1S-3snp 1P transitions (n>11) and collisional enhancement of 3s2 1S-3sns 1S transitions (n>8) of Mg I have been observed and studied as functions of Mg vapour pressure and perturbing gas (Ar) pressure by using two-photon resonant ionisation and thermionic diode techniques. The occurrence of forbidden transitions and collisional enhancement of 3s2 1S-3sns 1S transitions is qualitatively attributed to the formation of a Rydberg Mg2 excimer and a MgxAry Fermi complex. Quantitatively, the above phenomena are interpreted as collisional mixing of high Rydberg states of Mg. The mixing coefficients are estimated by using perturbation theory and the semiclassical (JWKB) approximation. The theoretical estimation is in good agreement with the experimental result.
An anomalous resonant dip of two-photon ionisation with 3s3p 1 P as an intermediate state and a stimulated emission by the 3s4s 3 S-3s3p 3 P transition, at 518.37 nm following the pulsed laser excitation near the 3s 2 1 S-3s3p 1 P resonance at 285.21 nm in magnesium vapour are observed and studied. The mechanism of the ionisation dip is attributed to formation and predissociation of the A 1 Sigma + u Mg 2 dimer, which transfer the population from the laser-pumped 3s3p 1 P level to 3s3p 3 P levels. Re-excitation of the produced 3s3p 3 P atoms and the subsequent radiative processes are presumed to explain the generation of stimulated emission. A theoretical model is employed to interpret the experimental phenomena.
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.
The authors present experimental results on the ionisation probabilities of highly excited Rydberg S and D states of caesium in collisions with ground-state Cs atoms. The measurements were done in a thermionic diode using two-photon excitation of Rydberg states with principal quantum numbers in the range 18 to 60. Over the investigated range (0.17 to 12 mTorr) a simple exponential dependence of the ionisation probabilities on binding energy and pressure was found. The authors also measured the intensity ratios of the fine-structure doublets in 6S 1/2 to nD two-photon transitions and find good agreement with theoretical calculations. From the results they conclude that the collisional ionisation probabilities are independent of orbital and total angular momentum.