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Physical Review A

Laser-induced gas breakdown: Multipole interference in the two-photon ionization of metastable helium

K. C. Mathur

Published Nov 1, 1978• 11 citations
Publisher

Abstract

The two-photon ionization of the He atom in the $2^{1}S$ state is studied in the electric dipole plus quadrupole approximation. A quadrupole resonance peak in the generalized cross section is obtained at 5043.5 \AA{}A. It is shown that for incident photons having wavelengths between 5042 and 5045 \AA{}A, the quadrupole term gives a significant contribution to the total generalized cross section.

DOI
10.1103/physreva.18.2170

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Journal of Physics B Atomic and Molecular Physics•Oct 1, 1976•24 citations
The two-photon ionization of the He atom in the 21S2^{1}S21S state is studied in the electric dipole plus quadrupole approximation. A quadrupole resonance peak in the generalized cross section is obtained at 5043.5 \AA{}A. It is shown that for incident photons having wavelengths between 5042 and 5045 \AA{}A, the quadrupole term gives a significant contribution to the total generalized cross section.
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{}.
There have been relatively few investigations of multiphoton ionization from metastable helium. Of particular interest has been the work of Haberland et al. 1987 and Haberland and Oschwald 1988. In both the 1987 and 1988 papers they have described the two photon ionization of metastable helium. In each of these studies they have reported the occurrence of unexplained structure along the wings of their resonance profiles. Upon the performance of similar measurements by this study, the unexplained structure is not seen and the agreement of the experiment's measurements with the theoretical shape of the resonance curves has been good. To experimentally verify these resonance effects, we have used a tunable dye laser in conjunction with a time of flight mass spectrometer to create and detect ions from metastable helium by two-photon absorption. The use of a metastable state instead of the ground state is advantageous because of its proximity to the ionization continuum and its extended lifetime. Using a metastable state as a starting point for multiphoton absorption requires fewer photons to reach the ionization threshold. The extended lifetime of the state also makes it easy to access experimentally. For helium the singlet metastable state 2$\sp1$S lies at 20.61 eV above the ground level with a natural lifetime of close to a millisecond. Two photons of 501.7 nm and 504.35 nm are required for the ionization processes in resonance with the 3$\sp1$P and the 3$\sp1$D states. This thesis is the accounting of the experimental process involved in the measurement of the dipole and quadrupole resonances of two photon ionization from singlet metastable helium. The study includes the description of the laser, electron gun assembly for metastable helium creation, and the time of flight mass spectrometer. A discussion of the theory of multiphoton processes is included along with the discussion of the data, its reduction and analysis, and a comparison with theoretical prediction. This study has not only involved the acquisition of definitive atomic spectra data sets, but has also established a facility for resonant ionization spectroscopy at The American University.
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.