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IEEE Journal of Quantum Electronics

Two-photon ionization of helium for 23S and 21S states

Ravi Sharma, K. C. Mathur

Published Dec 1, 1979• 1 citation
Publisher

Abstract

Two-photon ionization of He atom from its excited metastable2^{3}Sand2^{1}Sstates is studied within the framework of the perturbation theory. Cross sections and ratios for circularly and linearly polarized laser light are obtained in the wavelength ranges 3180-3950 Å and 3650-5100 Å for2^{3}Sand2^{1}Sstates, respectively. The effects of the quasi-resonant intermediate states and the states omitted in the truncated summation method are analyzed.

DOI
10.1109/jqe.1979.1069938

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Journal of Physics B Atomic Molecular and Optical Physics•Apr 14, 1988•9 citations
Two-photon ionization of He atom from its excited metastable2^{3}Sand2^{1}Sstates is studied within the framework of the perturbation theory. Cross sections and ratios for circularly and linearly polarized laser light are obtained in the wavelength ranges 3180-3950 Å and 3650-5100 Å for2^{3}Sand2^{1}Sstates, respectively. The effects of the quasi-resonant intermediate states and the states omitted in the truncated summation method are analyzed.
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.
The infinite summations over the complete set of unperturbed atomic states appearing in the NNNth-order perturbation theory for multiphoton ionization are performed using the Dalgarno-Lewis method. The relevant transition matrix elements are written in a closed integral form exhibiting all the analytic properties of the amplitude as a function of incident photon energy. The cross sections for two- and three-photon ionization for atomic hydrogen are calculated for both linearly and circularly polarized light with a wide range of photon energy spectrum including the near resonance, and numerical comparison is made with the values obtained by different methods.
The wavelength dependence of resonant two-photon ionisation of metastable He (21S) and He(23S) via the 3P intermediate level is reported. In both the singlet and triplet systems an additional complex, but reproducible, structure is seen in the resonance profile of the ion signal. This occurs for moderate laser intensities (<1 GW cm-2) strong enough to saturate the resonant transition. The physical reason for the structure is not understood, but measurements point to a delicate dependence on the time development of the atomic states and the laser pulse.