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Journal of Physics B Atomic and Molecular Physics

Multiphoton ionisation and light statistics: application to the caesium atom

Yves Gontier, M. Trahin

Published Jul 14, 1979• 15 citations
Publisher

Abstract

The consequences of photon correlation in resonant multiple absorption are discussed. As a result of the authors' accurate treatment of three- and four-photon ionisation of the caesium atom, it is found that the resonance peaks obtained with chaotic light are lowered, shifted and broadened with regard to those corresponding to a coherent light with the same average intensity.

DOI
10.1088/0022-3700/12/13/008

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Journal of Physics B Atomic and Molecular Physics•Nov 14, 1980•13 citations
The consequences of photon correlation in resonant multiple absorption are discussed. As a result of the authors' accurate treatment of three- and four-photon ionisation of the caesium atom, it is found that the resonance peaks obtained with chaotic light are lowered, shifted and broadened with regard to those corresponding to a coherent light with the same average intensity.
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.
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.
The resonantly enhanced multiphoton ionization of atomic hydrogen is studied for the case of four-photon ionization (three-photon resonant). The time-dependent probabilities, the shape of the resonance profile at different intensities, and the nonlinear index are computed for coherent light at frequencies around the resonance (365 nm). The systematic dependence of the cross section as a function of the principal quantum number of the resonant state (as this tends towards the Rydberg states) is investigated at zero detuning. The probabilities of absorption of one photon more than necessary for the four-photon ionization process (resonantly enhanced above-threshold ionization) is computed and the relevant branching ratios are examined. Then it is shown that the effect of the spatio-temporal structures of the laser pulse is to red shift the maximum of the resonance profile, whereas that of an ideal chaotic light is to blue shift and broaden the resonance peak. The shape, shift, and width of the profile resulting from both these effects are in very fair agreement with the experimental ones.
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.
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 .