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Le Journal de Physique Colloques

A.C. STARK SHIFT AS A PROBE TO MEASURE A LASER POWER DENSITY DISTRIBUTION AND ITS APPLICATION TO MULTI-PHOTON IONIZATION

P. Kruit, J. Kimman, Harm Geert Muller, Marnix J. van der Wiel

Published Nov 1, 1982• 1 citation
Publisher

Abstract

In order to perform multi-photon ionization experiments at a particular light intensity, the AC Stark-shift of the xenon 6s level is used as a probe to select that intensity from the range of intensities present in the laser focus. Our analysis provides an accurate, wide-range distribution function of the intensities in our multi-mode laser pulses. Using this method we determined there al order of non-linearity of the resonant ionization process and the probability for additional photon absorption.

DOI
10.1051/jphyscol:1982241

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International Conference on Ultrafast Phenomena•Jan 1, 1992•0 citations
In order to perform multi-photon ionization experiments at a particular light intensity, the AC Stark-shift of the xenon 6s level is used as a probe to select that intensity from the range of intensities present in the laser focus. Our analysis provides an accurate, wide-range distribution function of the intensities in our multi-mode laser pulses. Using this method we determined there al order of non-linearity of the resonant ionization process and the probability for additional photon absorption.
The authors report a high-resolution electron spectroscopic study of three-photon resonant, five-photon ionisation of xenon. The results are analysed in terms of a dynamic resonance of three photons with an AC Stark-shifted intermediate level (6s), followed by a saturated two-photon transition to the continuum. They demonstrate a linear relationship between detuning from the zero-field resonance position and the local momentary light intensity causing the five-photon ionisation. Thus, the AC Stark shift acts as a probe which selects a particular intensity from the wide range present in the laser focus. They determined the AC Stark shift of the 6s level the distribution function of the light intensities and the intensity dependence of the ionisation probability. High-resolution electron spectra are used in arguments to support the validity of their analysis.
Dans le but de réaliser des expériences d'ionisation mutiphotonique à une intensité bien déterminée de la lumière laser, nous avons utilisé le déplacement lumineux du niveau 6s du Xénon, afin de sélectionner la région appropriée au foyer du laser. Ceci permet d'obtenir avec précision une large distribution d'intensités à chaque pulse de notre laser multi-mode. De cette façon, nous avons pu déterminer l'ordre réel de la non-linéarité dans le processus d'ionisation résonnante et la probabilité d'absorption de photons supplémentaires.
Nonresonant ac Stark shifts of Rydberg states in Xe have been investigated by probing an atomic transition with a weak UV light in the presence of an intense YAG-laser pulse. Rydberg atoms produced through two-photon excitation, 5p → np (n = 15–48) and 5p → nf (n = 12–49), are subsequently ionized. IR-laser-induced shifts of the resonant ionization profiles are then measured. The measured shift, 3.9 ± 0.3 cm−1 at 1010 W cm−2, is more than a factor of 2 smaller than that expected from the basis of ponderomotive term, 8.5 cm−1 at 1010 W cm−2. In addition, the shift varies linearly as a function of the IR-laser intensity and does not depend on the principal quantum number of the np and nf series investigated here.
The influence of ac Stark shifts on the three-photon ionization of cesium in a moderate-intensity laser beam is investigated. The ionization process involves a two-photon resonance with the 8d8d8d fine-structure levels. The work consists of two related parts. In the first part, atomic parameters, such as the two-photon Rabi rate and the ac Stark shift, are calculated in the nonrelativistic Hartree-Fock approximation. In the second part, the atomic parameters thus obtained are used as coefficients in a system of coupled differential equations representing the time evolution of the density matrix. Ionization line shapes are generated for a transform-limited pulsed laser of moderate intensity with a single-mode transverse structure. As the time history of the intensity profile is varied from square to smooth, the ionization line shape becomes asymmetric due to the time dependence of the ac Stark shift. The effects of varying the spatial intensity distribution of the laser light to include transverse centering and longitudinal focusing are also studied.
We have measured the energy spectrum of photoelectrons emitted in multiphoton ionization of atoms by a short-pulse, tunable laser. The spectrum shows prominent resonances, confirming the observations of Freeman et al. [Phys. Rev. Lett. 59, 1092 (1987)]. The study of the position and intensity of such resonances, as a function of laser wavelength and intensity, probes the structure of the atom in the presence of the strong electromagnetic field and allows their spectroscopic assignment. A systematic study of the ac Stark shift of one of the resonances, the 7s[3/2]\ifmmode^\circ\else\textdegree\fi{} level of xenon, is presented.
As it is well known now1, electron energy spectra from multiphoton ionization (MPI) and Above-Threshold Ionization (ATI) of atoms by subpicosecond pulses reveal Stark-induced resonances and provide an elegant and unique method to investigate AC-Stark shifts of strongly perturbed excited states. In this work we have systematically studied such spectra and their interpretation in the case of MPI of Rare gases with 100 fs, 617 nm pulses. The intensity ranges from 1013 (for xenon) to 1015 W.cm−2 (for helium). The corresponding shifts range from .3 eV to 30 eV and may be, in principle, larger than the atomic ionization potential. Besides the measurement of AC-Stark shifts, the photoelectron spectra provide valuable information about the ionization process itself in a highly non-perturbative regime.