Quantum damping of the resonance lines in ionized gas

A.P. Gavriluk, N. Ya. Shaparev

Joe J. Tiee, Monty J. Ferris

A competition between resonant multiphoton ionization (MPI) and third harmonic generation (THG) involving three-photon resonances was first reported by Miller et al.1 Recently, a similar competing process involving four-photon resonances was observed in the multiphoton ionization of Hg2. In this paper we report a similar observation in the three- and four-photon excitation spectra of Kr and Ar. Both the multiphoton ionization and the VUV radiation generated in the focal volume are measured over a broad range of gas pressures (0.01-100 Torr) and laser fluences. The 5s, 5f, and 6p′ levels of Kr and the 4s, 4p′, and 7p levels of Ar are investigated. The spectral lines are observed to broaden and to shift from line center as a function of increasing gas pressure and laser intensity. The effect observed in three-photon resonant lines is explained by the occurrence of a competition between MPI and THG.1 A similar effect observed in the four-photon resonances is interpreted as the result of THG enhanced by the four-photon levels—a five-photon process. However, it is not clear why the four-photon spectral line-width becomes narrower when the gas pressure is further increased; this is discussed.

Alexey E. Kazakov, Vyacheslav P. Makarov, Mikhail Vladimirovich Fedorov

M. R. Woodworth

Photon-echo relaxation rates are measured in weakly ionized plasmas of binary mixtures of noble gases, in which the photon echo generated on a transition in one noble-gas species is damped primarily by atoms of the second species. The special cases of pure krypton and xenon are extensions of previous work in pure helium, neon, and argon. With the exception of mixtures with helium as the perturber, measured relaxation rates are consistent with collisional line-broadening calculations.

O. Nédélec, J.C. Pébay-Peyroula

R. Papoular

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.