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Journal of Physics B Atomic Molecular and Optical Physics

Two-photon ionisation of 21S metastable helium

Richard B. Kay, B H Houston, J P Czechanski

Published Mar 28, 1989• 1 citation
Publisher

Abstract

The authors report on two-photon REMPI (resonantly enhanced multiphoton ionisation) in helium from the 2 1 S level via the 3 1 P level. Resonance profiles are reported at intensities of 5 MW cm -2 and 200 kW cm -2 . The profiles are fitted with Lorentzians with Rabi half-widths of Gamma R =pE/. The 2 1 S-3 1 D quadrupole resonance has also been observed. These results are compared with other recent experimental measurements.

DOI
10.1088/0953-4075/22/6/002

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The authors report on two-photon REMPI (resonantly enhanced multiphoton ionisation) in helium from the 2 1 S level via the 3 1 P level. Resonance profiles are reported at intensities of 5 MW cm -2 and 200 kW cm -2 . The profiles are fitted with Lorentzians with Rabi half-widths of Gamma R =pE/. The 2 1 S-3 1 D quadrupole resonance has also been observed. These results are compared with other recent experimental measurements.
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.
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 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.
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.
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).