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Journal of Physics B Atomic Molecular and Optical Physics

Collision-induced two-photon forbidden 6s21S-6snp1P transitions in Ba I

Wu Dong-hong, Yufen Yang, K. T. Lu

Published May 14, 1990• 9 citations
Publisher

Abstract

By using two-photon resonant ionisation and a thermionic diode detector, the collision-induced two-photon forbidden 6s 2 1 S-6snp 1 P transitions (n>29) have been observed at P(Ba) approximately 0.13 Torr and P(Ar) approximately 15 Torr. The forbidden transitions are attributed to the dimer excimer Ba 2 processes. The minimum quantum number, n approximately 28 is calculated by using the l-mixing model and perturbation theory, which is in good agreement with the authors' observation.

DOI
10.1088/0953-4075/23/9/006

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By using two-photon resonant ionisation and a thermionic diode detector, the collision-induced two-photon forbidden 6s 2 1 S-6snp 1 P transitions (n>29) have been observed at P(Ba) approximately 0.13 Torr and P(Ar) approximately 15 Torr. The forbidden transitions are attributed to the dimer excimer Ba 2 processes. The minimum quantum number, n approximately 28 is calculated by using the l-mixing model and perturbation theory, which is in good agreement with the authors' observation.
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.
The authors have investigated both theoretically and experimentally the intensities of the forbidden two-photon lines 6S 1/2 to 41P 3/2 ,61P 3/2 in an atomic beam of caesium exposed to an electric field. At low field strengths the line intensities depend quadratically on the electric field and are quantitatively well described by Stark mixing parameters obtained from perturbation theory. At higher field strengths the line intensities exhibit an unexpected linear field dependence which is in good agreement with the results from a more general calculation. The authors applied the results obtained to measure the components of a small spurious electric field in a thermionic ionisation detector.
Investigates theoretically a new aspect of radiative collisions on two-photon ionisation of colliding atoms of different species A and B. The authors derive the main characteristics of the resonance appearing in the two-photon ionisation cross section of atom B when the laser frequency is tuned on a resonance transition of atom A which cannot itself be ionised by two-photon absorption. Under some realistic conditions and for reasonable values of the light energy density flux F ( approximately 1 MW cm -2 ), the cross section of the process considered varies as F 2/5 while its width varies as F 1/2 . These predictions are in reasonable agreement with a recent observation on the mixture Sr(A)-Cs(B) by Brechignac et al. (1980).
The authors have observed the spectra for the production of Ba + and Ba 2+ between 550 and 670 nm. They observe sharp resonances for the production of Ba 2+ which generally do not coincide with peaks in the Ba + production. While they can easily assign most of the resonances in the Ba + spectra to two-photon resonances with bound states it is not possible to assign most of the Ba 2+ peaks to two- or three-photon Ba + transitions. This suggests that the production of Ba 2+ proceeds both via Ba autoionising states and low-lying Ba + states.