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IEEE Journal of Quantum Electronics

A theoretical study of the two-photon ionization of metastable He with linearly and circularly polarized laser light

R. Sharma, K. C. Mathur

Published Oct 1, 1978• 5 citations
Publisher

Abstract

Generalized cross sections\hat{\sigma}_{L}and\hat{\sigma}_{C}for the two-photon ionization of He from the metastable2^{1}Sstate are calculated for linearly and circularly polarized incident laser light, respectively, in the wavelength range 3725-5100 Å. Significant enhancement of cross sections occurs through real3^{1}Pand4^{1}Pp intermediate states. The ratio\hat{\sigma}_{C}/\hat{\sigma}_{L}shows a minima at the wavelengths close to 3743 and 4450 Å.

DOI
10.1109/jqe.1978.1069689

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Generalized cross sections\hat{\sigma}{L}and\hat{\sigma}{C}for the two-photon ionization of He from the metastable2^{1}Sstate are calculated for linearly and circularly polarized incident laser light, respectively, in the wavelength range 3725-5100 Å. Significant enhancement of cross sections occurs through real3^{1}Pand4^{1}Pp intermediate states. The ratio\hat{\sigma}{C}/\hat{\sigma}{L}shows a minima at the wavelengths close to 3743 and 4450 Å.
Multipole interference effects are predicted in the generalized cross sections, and in the ratio $\frac{{\stackrel{^}{\ensuremath{\sigma}}}_{C}}{{\stackrel{^}{\ensuremath{\sigma}}}_{L}}$ of the cross sections for circularly and linearly polarized laser light for the case of the two-photon ionization of the n=2n=2n=2 singlet metastable helium atoms in the wavelength range 5036-5050 \AA{}.
The polarization dependency of relative cross-sections of the two-photon REMPI dipole transition (2$\sp1$S-3$\sp1$P) and quadrupole transition (2$\sp1$S-3$\sp1$D) is studied experimentally. Strict attention is paid to the measurement statistics, particularly to those associated with the ionizing laser beam. The cross section profiles are fitted to Voight functions that are generated from the measurement of experimental parameters. Ratios of the circular to linear polarization ion yields are also reported. These ratios are compared to theoretical predictions.
We present a systematic theoretical analysis of two-photon ionization of cesium in the perturbation-theory regime. Matrix elements from diverse sources, such as quantum-defect theory and model potentials, as well as from experimental oscillator strengths, have been used in the calculation. The results are discussed in the context of recent measurements. In addition to total generalized cross sections (gcs) for linearly polarized light, ratios of gcs for linear to circular polarization, as well as photoelectron spin polarization, have been calculated. The various sets of matrix elements give results which are consistent, given variations that should be expected from the diversity of the sources. The disagreement with experiment, however, persists.