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Journal of Physics B Atomic and Molecular Physics

The effect of radiation trapping of high intensity scattered radiation on multiphonon ionisation rates and resonance fluorescence

Jim Salter

Published Dec 28, 1979• 23 citations
Publisher

Abstract

The effects of optical trapping of the high-intensity radiation scattering from a laser beam by a dense vapour are considered. It is found that the effect upon the fluorescence is to make it appear not to saturate while it is possible for the scattered radiation to build up in the immediate vicinity of the laser beam to an intensity sufficient to cause multiphoton ionisation. Several experiments are examined in the light of these findings.

DOI
10.1088/0022-3700/12/24/002

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Journal of Physics B Atomic and Molecular Physics•Feb 21, 1977•148 citations
The effects of optical trapping of the high-intensity radiation scattering from a laser beam by a dense vapour are considered. It is found that the effect upon the fluorescence is to make it appear not to saturate while it is possible for the scattered radiation to build up in the immediate vicinity of the laser beam to an intensity sufficient to cause multiphoton ionisation. Several experiments are examined in the light of these findings.
The behaviour of an optically thick column of barium vapour resonantly 10 -2 illuminated at 553.5m by a 50 ns, 10 Wm pulse from a flashlamp pumped dye 19 21 -3 laser system was investigated for barium number densities in the range 10 -10 m in the presence of He buffer gas at 100 Torr.A variety of optical saturation effects associated with the principal resonance transition were studied.In marked contrast to earlier work, saturation of the intensity of the scattered light near 553.5 nm with increasing laser power was found for a suitable geometry of the collection optics but at a laser intensity typically two orders of magnitude higher than that predicted by simple theory, despite the fact that this theory was found to describe correctly transmission measurements along the length of the vapour column.The apparent contradiction between the fluorescence and transmission results, which indicated that the scattered radiation intensity continued to rise with incident laser power, despite the population of the upper resonance level having reached a limiting saturation value, could be understood in terms of a simple model which took into consideration the spectral distribution of the scattered light and its transfer through the unexcited vapour.However, in common with other similar experiments, saturation of the scattered radiation intensity with laser power could not be totally achieved when a more usual collection optics geometry was employed.This was interpreted as a consequence of laser intensity variation across the face of the laser beam.A number of subsidiary experiments were performed.These included spatially resolved intensity measurements of the scattered radiation and time resolved metastable level population measurements.In addition, evidence for laser induced ionization was sought using two different methods and this enabled an upper limit of 10% ionization to be placed on the experiments in the present work.
The theory of the collisional and radiative relaxation of an atom perturbed by foreign-gas atoms in the presence of an intense monochromatic laser beam is developed. The spatial degeneracy of the levels and the reorientation effects of the collisions are included. The impact approximation is not made, but the Doppler effect is neglected. The spectrum of the fluorescent and scattered light is obtained from an eigenvalue problem in Liouville space using the dressed-atom approach and rotationally irreducible tensors. Minimal symmetry-adapted bases in Liouville space are constructed for linear, circular and elliptical incident polarisation. The method is illustrated for S 0 -P 1 transitions.
The non linear interaction between an intense laser radiation and atoms leads to ionization through the absorption of N photons from the laser radiation via laser-induced virtual states. The multiphoton ionization rate varies as a function of the laser intensity I as IN. We discuss the two most important effects which govern multiphoton ionization processes : resonance effects and laser-coherence effects. In a moderate laser intensity range (107 - 109 W cm-2) corresponding to the two, three or four-photon ionization of atoms, resonance effects play a dominant role while the photon statistics of the laser radiation are relatively unimportant. On the contrary, in the very high intensity range (1012 - 1013 W cm-2) required to observe large Nth - order ionization processes, coherence effects play a dominant role through N! enhancement in the N-photon ionization rate, while resonance effects are completely damped due to the high laser intensity. We conclude by reviewing the possible applications of multiphoton absorption processes in other fields.
In this contribution quantum-effects in the radiation-atom interaction will be reviewed. It is well known that most of the phenomena involving laser light can be described by semi-classical methods however phenomena such as laser noise the spectra and photon statistics of resonance fluorescence and the interaction of atoms in the single-atom maser require a quantization of the radiation field. In this paper we will describe results on the latter two effects. We will start with the discussion of resonance fluorescence. 1. RESONANCE FLUORESCENCE The resonant interaction of laser light with atomic systems has received considerable theoretical and experimental attention over the past decade. Until the advent of the laser light sources for spectroscopy consisted of ordinary spectral lamps excited by DC or RF discharges and produced light having a very broad spectral width and hence very short correlation time and a relatively low intensity. For such fields both the experimental and theoretical results are in general well understood. However the development of the laser made available light sources which are sufficiently intense that an atomic (or molecular) transition can be very easily saturated. In addition the lasers are highly monochromatic having a coherence time much greater than typical natural lifetimes of excited atomic states and finally tunable making it possible to selectively excite particular atomic transitions. The theoretical analysis of this physical situation requires the use of techniques more general than
The general method developed in a previous paper (see ibid., vol.10, p.345 (1977)) is applied to the problem of resonance Raman scattering in very intense laser fields. At very low intensities the spectrum consists of three monochromatic lines (Rayleigh, Raman Stokes and anti-Stokes) which split into a triplet and two doublets when the laser light, tuned on one of the two transitions connecting the upper state to the two lower sublevels, has a sufficiently high intensity to saturate this transition but not the second one. The Raman and Rayleigh lines are completely mixed when the intensity becomes so high that both transitions are saturated. The corresponding evolution of the spectrum is determined. Optical-pumping effects leading to asymmetries in the fluorescence spectrum are taken into account.