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Journal of Physics B Atomic and Molecular Physics

Laser light statistics and bandwidth effects in resonant multiphoton ionisation of caesium atoms at 1.059 μm

Louis A. Lompre, G. Mainfray, C. Manus, J. P. Marinier

Published Nov 28, 1981• 40 citations
Publisher

Abstract

Laser temporal-coherence effects are investigated in four-photon ionisation of caesium atoms using a tunable wavelength Nd-glass laser pulse and tuning the frequency onto the resonant three-photon transition 6S to 6F. The laser can be operated in either single or multimode. In the latter case, the laser bandwidth can be varied between 3*10 -2 and 1.5 cm -1 . The resonance peaks obtained with incoherent laser pulses are enhanced, shifted and broadened with respect to those induced by coherent pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses is statistically enhanced by a factor of 1.8+or-0.2 compared with the shift induced by coherent pulses, in good agreement with calculations. The resonant multiphonon ionisation rate induced by multimode pulses depends both on intensity and bandwidth effects. It is enhanced by a factor of 2.5 for pulses with bandwidths less than 0.2 cm -1 , while it becomes smaller than that induced with a coherent pulse of the same intensity when the bandwidth becomes larger than 1 cm -1 . Finally, the resonance width depends both on intensity an bandwidth effects. At low intensity (2*10 7 W cm -2 ) it is governed only by the laser bandwidth, while at high intensity (10 8 W cm -2 ) it varies linearly with the intensity and is statistically enhanced.

DOI
10.1088/0022-3700/14/22/015

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Laser temporal-coherence effects are investigated in four-photon ionisation of caesium atoms using a tunable wavelength Nd-glass laser pulse and tuning the frequency onto the resonant three-photon transition 6S to 6F. The laser can be operated in either single or multimode. In the latter case, the laser bandwidth can be varied between 310 -2 and 1.5 cm -1 . The resonance peaks obtained with incoherent laser pulses are enhanced, shifted and broadened with respect to those induced by coherent pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses is statistically enhanced by a factor of 1.8+or-0.2 compared with the shift induced by coherent pulses, in good agreement with calculations. The resonant multiphonon ionisation rate induced by multimode pulses depends both on intensity and bandwidth effects. It is enhanced by a factor of 2.5 for pulses with bandwidths less than 0.2 cm -1 , while it becomes smaller than that induced with a coherent pulse of the same intensity when the bandwidth becomes larger than 1 cm -1 . Finally, the resonance width depends both on intensity an bandwidth effects. At low intensity (210 7 W cm -2 ) it is governed only by the laser bandwidth, while at high intensity (10 8 W cm -2 ) it varies linearly with the intensity and is statistically enhanced.
This paper reports the four-photon ionization of caesium atoms when the laser frequency is tuned through the resonant three-photon transition 6S → 6F. This experiment was performed by using a tunable-wavelength bandwidth-limited subnanosecond laser pulse at 1.06 μm, in the 108-109 W.cm-2 laser intensity range. Pulse widths of 1.5 ns, 50 ps, and 15 ps were used. The resonant character of the multiphoton ionization process was observed, even with the shortest pulse of 15 ps. Nevertheless the influence of a temporal effect is demonstrated according to theoretical predictions. The resonance shift ΔE of the 6S → 6F transition energy was found to be linear with the laser intensity I within the range 108-109 W.cm-2. ΔE = α I, with α = 2 cm-1/GW.cm-2. This result confirms previous measurements performed with single-mode 35 ns laser pulses and is in very good agreement with calculated resonance shifts.
The time evolution of a resonance peak is studied for three- and four-photon ionisation of the caesium atom with square and Gaussian laser pulses. In the domain 10 -13 -10 -7 s two different regimes are defined according to whether the time is smaller or larger than a characteristic time t c . For t<t c the ionisation probability varies as t 3 whereas it is proportional to t when t>t c . The corresponding resonance width varies as t -1 in the first case becomes intensity independent in the second. The effects of the spatial distribution of the intensity in the focal volume are discussed and an excellent agreement is found between the theoretical and experimental values of the order of non-linearity, the resonance width and the absolute magnitude of the ion signal.
Presents a theoretical analysis of the effect of laser light statistics on two-photon resonant three-photon ionisation of sodium. The ionisation probability and the population of the atom in the resonant state are compared in the presence of an intensity stabilised laser with a diffusing phase (phase diffusion model) and light undergoing Gaussian amplitude fluctuations (chaotic field). In the limit of large bandwidth fields the resonant two-photon excitation is statistically decoupled from the following ionisation step. A decrease of the bandwidth results in a partial correlation between these two steps manifesting itself in an enhancement of the ionisation probability in the chaotic field. With increasing intensity and the onset of saturation of the bound-bound transition, the ionisation probability and resonant excited-state population in the chaotic field may become smaller than the one in the phase diffusing light. In the large-bandwidth limit the Stark shift in the chaotic field is enhanced, which has a pronounced effect on the effective power-law dependence of the ionisation probability.
A single-transverse-mode QQQ-switched Nd-glass laser, which can operate over a number of longitudinal modes variable from 1 to 100, has been used to investigate laser temporal-coherence effects on the 11-photon ionization of xenon atoms. When the laser pulse has an over-all bandwidth of 4 ${\mathrm{cm}}^{\ensuremath{-}1}$, the number of ions formed is enhanced by ${10}^{6.9\ifmmode\pm\else\textpm\fi{}0.3}$ over that produced by a monomode laser pulse with the same average laser intensity. The observed enhancement in the number of ions due to the change in temporal coherence of the pulse is in good agreement with recent theoretical prediction.
Non-resonant k-photon ionisation cross sections may be calculated by kth order perturbative treatment. This method is generally considered to be good when the detuning from the closest resonance is much larger than the expected shift and width of this resonance. The aim of this paper is to discuss the validity of this criterion for several realistic cases. By diagonalising an effective Hamiltonian built on a few quasi-resonant states, we have calculated multiphoton ionisation cross sections for ground state caesium by linearly polarised light in frequency ranges containing the tuning range of the ruby laser, the Nd glass laser and its first harmonic. The study is carried out up to an intensity of 10 11 W cm -2 .