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Journal of Physics B Atomic Molecular and Optical Physics

A measurement of the two-photon ionization of atomic hydrogen H(2s) near the resonance with the 3p state

Philippe Antoine, Nour-Eddin Essarroukh, Jozo J. Jureta, Xavier Urbain, Freddy Brouillard

Published Nov 28, 1996• 9 citations
Publisher

Abstract

We describe a new experiment dealing with the multiphoton ionization of metastable atomic hydrogen H(2s). The ion yield has been measured in a relative way but it provides absolute photoionization rates. In the present contribution, the wavelength is tuned in the vicinity of the 2s - 3p transition and the laser intensity is in the range. Experimental photoionization rates and resonance width are found to be in agreement with theoretical predictions.

DOI
10.1088/0953-4075/29/22/014

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We describe a new experiment dealing with the multiphoton ionization of metastable atomic hydrogen H(2s). The ion yield has been measured in a relative way but it provides absolute photoionization rates. In the present contribution, the wavelength is tuned in the vicinity of the 2s - 3p transition and the laser intensity is in the range. Experimental photoionization rates and resonance width are found to be in agreement with theoretical predictions.
Theoretical calculations for the 1S → 2S two- photon transition rate in atomic hydrogen are reviewed. They are compared with two experiments via the saturation effects that are observed when three-photon ionization is enhanced by two- photon resonance with the metastable 2S state. There is a significant disagreement between theoretical and experimental results. The disagreement is illustrated by determining the transition rate as a function of intensity for experimental laser profiles: (1) using parameters that give rates in agreement with experimental data, and (2) using theoretical values for the parameters. Finally, experiments that would involve direct observation of the 2S metastable state are considered. It is shown that single-longitudinal-mode laser experiments would provide an accurate measurement of the Rabi frequency for this transition and yield a definitive value for the rate. A Monte Carlo simulation method is used to consider practical effects associated with such an experiment, for example, chirp and ASE noise in the laser. Also, simulations of the multiphoton ionization process in multimode and wide band laser fields are presented.
We have measured the resonance-enhanced four-photon ionization of atomic hydrogen. The degenerate four-photon process occurs via a three-photon resonance between the 1s and the 2p levels, with subsequent one-photon ionization near threshold. We have studied the highly asymmetric resonance-enhanced profile, i.e., the photoion yield as a function of laser detuning from three-photon resonance between the 1s and 2p levels. In particular, we have determined the width, shift, peak, and asymmetry of the profile as a function of laser intensity. The experimental results are compared to theoretical models. These models involve both the properties of the atom in an intense near-resonant radiative field, and a detailed model for the multimode laser field, particularly the field fluctuations due to mode beating. The asymmetric resonance-enhanced photoionization profile is well reproduced by both a random-phase and a chaotic-light model.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
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.