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Journal of Physics B Atomic and Molecular Physics

Two-photon ionisation of metastable helium

Hellmut Haberland, Michael Oschwald, John T. Broad

Published Jul 28, 1987• 16 citations
Publisher

Abstract

An experimental and theoretical study of two-photon ionisation of He(2 1 S) and He(2 3 S) by a pulsed laser strong enough to saturate the ionisation process through the 3P intermediate state and to display the quadrupole transition through the 3D intermediate state is presented. A pronounced dip sensitive to the laser pulse shape in the profile of the 2 1 S-3 1 P resonance is shown to stem from a competition between relaxation and ionisation. Further complex but reproducible unexpected structure, which is not yet understood, is seen in both the singlet and triplet 2S-3P resonances.

DOI
10.1088/0022-3700/20/14/010

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Two-photon ionisation of metastable helium: complex structure in the 2S-3P resonance profiles

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Journal of Physics B Atomic Molecular and Optical Physics•Feb 28, 1988•3 citations
An experimental and theoretical study of two-photon ionisation of He(2 1 S) and He(2 3 S) by a pulsed laser strong enough to saturate the ionisation process through the 3P intermediate state and to display the quadrupole transition through the 3D intermediate state is presented. A pronounced dip sensitive to the laser pulse shape in the profile of the 2 1 S-3 1 P resonance is shown to stem from a competition between relaxation and ionisation. Further complex but reproducible unexpected structure, which is not yet understood, is seen in both the singlet and triplet 2S-3P resonances.
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.
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.
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.