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Journal of Physics B Atomic Molecular and Optical Physics

Hg-Ar collisions in a strong laser field: competition between intense field and collisional effects

Stephen Michael Radley Young, D Segal, Xuhua Chen, Keith R. Burnett

Published Dec 14, 1990• 0 citations
Publisher

Abstract

The authors present an investigation of the modification caused by an intense laser field to the rates of m J changing collisions for Hg perturbed by argon. They have studied experimentally the competition between radiative and collisional coupling between m J states with a strong laser field tuned to the Hg J=1 to J=1 transition at lambda 435.8 nm. Population is pumped into the strongly dressed manifold from the ground state with a weak UV 'probe' laser detuned slightly from the lambda 253.7 nm intercombination line. The fluorescence observed at lambda 404.7 nm depends critically on whether the excitation proceeds via collisional redistribution or resonance through Stark shifted levels. The authors present data that clearly exhibit the effects of this competition. The data show in particular how one can produce excitation to transient Stark shifted levels in the presence of collisions. This will be critical in future experiments on collisions in strong fields.

DOI
10.1088/0953-4075/23/23/002

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The authors present an investigation of the modification caused by an intense laser field to the rates of m J changing collisions for Hg perturbed by argon. They have studied experimentally the competition between radiative and collisional coupling between m J states with a strong laser field tuned to the Hg J=1 to J=1 transition at lambda 435.8 nm. Population is pumped into the strongly dressed manifold from the ground state with a weak UV 'probe' laser detuned slightly from the lambda 253.7 nm intercombination line. The fluorescence observed at lambda 404.7 nm depends critically on whether the excitation proceeds via collisional redistribution or resonance through Stark shifted levels. The authors present data that clearly exhibit the effects of this competition. The data show in particular how one can produce excitation to transient Stark shifted levels in the presence of collisions. This will be critical in future experiments on collisions in strong fields.
We propose a method for studying the modification of atomic collision rates in the presence of an intense laser field. This method employs a three-level lambda-system driven by two laser pulses: one strong and the other weak. The weak laser is used to populate a particular dressed state produced by the intense laser at some time during the pulse history. We show that this point can be selected by tuning the frequency of the weak laser. The spectrum of the emission produced by the dressed state is then recorded. Measuring this spectrum, both in the presence and in the absence of collisions, enables one to deduce the dependence of the collision rates on laser intensity. The method has other advantages discussed in the text.
We study laser-intensity effects in two-photon collisional redistribution of radiation. Thereby we concentrate on a special case of recent experimental interest, where the transition from the excited to the final state is saturated by a strong laser field. Both exciting lasers are, however, assumed weak in the sense that the dynamics of a collision between the radiator and a perturber is not significantly influenced by the laser fields. In particular, our treatment explains the intensity independence of the polarization dependence of total fluorescence from the final state which has been measured previously even if the transition from the excited to the final state is saturated.
We extend the theory of light scattering from an atom undergoing collisions to the case of strong laser fields, where the expansion in powers of $\ensuremath{\Omega}{\ensuremath{\tau}}_{c}$ that was used formerly in a series of papers by Burnett et al. breaks down. In particular we consider in detail the conditions necessary to relate the observable quantities to intense-field collisional rate constants in a dressed-state basis (such as those calculated by Light and Sz\"oke). We also show (following Rabin and Ben-Reuven) that by studying the spectrum emitted by the atoms in the presence of a strong field one may measure the collisional rates for transfer between the dressed states of atom plus radiation field.
The optogalvanic effect induced in a Hg-Ar discharge by light at 546.1 nm is studied experimentally and theoretically. The plasma and its response to a disturbance in the atomic-level populations are described entirely in terms of electron collision cross sections and other fundamental parameters. In a detailed description of the laser interaction with the atomic system, it was necessary to take into account velocity-changing collisions (VCC). These collisions (VCC) strongly modify the interaction between the Doppler-broadened system and the optical field. In some Doppler-free optogalvanic spectra the dramatic influence of VCC on the velocity distribution of atomic-level population is evident. Measurements of the magnitude of the optogalvanic signal confirm most of the theoretical results. Some anomalous features of the impedance change due to a weak beam were observed when a strong counterpropagating beam was introduced.
Measurements are reported of the fundamental mode radiative decay rate of the Hg 6 1P1^{1}P_{1}1P1​ population under conditions of low collision rate and large optical thickness of the 184.9-nm resonance line. The measured data extend over a wide range of experimental conditions. The measurements were made using two different experimental methods, both employing low-pressure mercury--noble-gas discharges. In the first method the decay rate of the 6 1P1^{1}P_{1}1P1​ population was measured in the afterglow of the discharge using a dye-laser absorption technique. In the second one the decay was measured of the laser-induced fluorescence at the 184.9-nm line created by pulsed optical pumping. Radiative decay rates obtained with the two independent experimental methods for nominally the same conditions agreed within the experimental error of 5%. For zero perturber gas density the radiative decay rate as a function of the Hg density shows a pronounced minimum. This minimum disappears with the addition of an argon density of more than about 3\ifmmode\times\else\texttimes\fi{}1022{10}^{22}1022 ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$, which corresponds with an extra collisional broadening of \ensuremath{\sim}15 MHz. The experimental decay rates differ largely from rates calculated with a complete frequency redistribution theory. Comparison with rates calculated from the partial redistribution theory described in paper I (preceding paper) shows agreement within 25%. The behavior of the experimental rates at the lowest mercury densities could be interpreted as due to a deviation from the natural abundance in the 6 1P1^{1}P_{1}1P1​ excited-state population.
The collisionally broadened optical absorption cross section that is constant at low intensities is predicted to decrease at high intensities. The prediction is based on a nonlinear change of the effective interaction of the atom with the strong incident electromagnetic field and a distortion of the atomic interaction potential by it. An experiment is analyzed here that observes this falloff on the “wing” of the 460.73-nm strontium resonance line, collision broadened by an argon buffer gas. Qualitative agreement is reached between theory and experiment when the degeneracy of the levels is taken into account.