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Journal of Physics B Atomic and Molecular Physics

Off-resonance photoexcitation and photoionization of a gas

R. Papoular

Published Feb 1, 1969• 0 citations
Publisher

Abstract

Even in the frame of first-order perturbation theory a molecule can be excited by off-resonance photons if it is subjected to a line-broadening mechanism. The case of collision-broadening is considered here. When a beam of coherent monochromatic off-resonance photons is passed through a gas, the competing processes of excitation and de-excitation lead to an equilibrium concentration of excited molecules, which is proportional to the beam intensity. This concentration is computed through the successive use of a classical and of a semi-quantum-mechanical treatment. Ionization is considered as a special case of excitation. The probability of ionization deduced from this theory in the case of medium pressures, is larger than that of multiphoton ionization.

DOI
10.1088/0022-3700/2/2/305

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Even in the frame of first-order perturbation theory a molecule can be excited by off-resonance photons if it is subjected to a line-broadening mechanism. The case of collision-broadening is considered here. When a beam of coherent monochromatic off-resonance photons is passed through a gas, the competing processes of excitation and de-excitation lead to an equilibrium concentration of excited molecules, which is proportional to the beam intensity. This concentration is computed through the successive use of a classical and of a semi-quantum-mechanical treatment. Ionization is considered as a special case of excitation. The probability of ionization deduced from this theory in the case of medium pressures, is larger than that of multiphoton ionization.
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The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.
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A semi-classical theory describing the quenching and depolarization of resonance radiation by collisions with foreign gas molecules is presented. The total depolarizing probability is divided into two parts to allow for adiabatic and nonadiabatic depolarization. The equations for polarization are generalized in order to apply to the case in which the observed polarization results from several lines belonging to a hyperfine group. The effect of a magnetic field applied along the direction of observation is also included in the treatment. Experimental results on the polarization of the $\ensuremath{\lambda}2537\mathrm{A}$ mercury line as a function of gas pressure and as a function of magnetic field at various fixed gas pressures show good agreement with the theory. Cross sections for quenching and depolarization have been calculated for the gases investigated and are presented in Table VI.
A two-state atom in a perturber gas is irradiated by intense light with an arbitrary spectral profile. The authors derive a general expression for the fluorescence spectrum in terms of the conventional low-intensity absorption profile and the spectral density of the incident light. This result, which contains previous results as special limiting cases, describes the transition from the three-line fluorescence spectrum excited by intense monochromatic light and the power-broadened Lorentzian spectrum produced by broad-band excitation.