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AIP conference proceedings

A study of multiphoton resonances in Kr and Ar under intense laser field

Joe J. Tiee, Monty J. Ferris, G.K. Anderson

Published Jan 1, 1987• 0 citations
Publisher

Abstract

Multiphoton ionization and vacuum ultraviolet (vuv) light generation through 3‐ and 4‐photon resonances in Kr and Ar are investigated as a function of gas pressure and laser fluence. The observed spectral broadening and shifting in the MPI spectra near 3‐ and 4‐photon resonances are interpreted as a result of the ac Stark effect and the competing harmonic generation processes. 4‐photon resonances was observed to enhance and deplete third harmonic generation under certain pressure conditions. In addition, vuv light at wavelength shorter than the third harmonic wavelength was produced when several 4‐photon resonances in Ar were excited.

DOI
10.1063/1.36840

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Multiphoton ionization and vacuum ultraviolet (vuv) light generation through 3‐ and 4‐photon resonances in Kr and Ar are investigated as a function of gas pressure and laser fluence. The observed spectral broadening and shifting in the MPI spectra near 3‐ and 4‐photon resonances are interpreted as a result of the ac Stark effect and the competing harmonic generation processes. 4‐photon resonances was observed to enhance and deplete third harmonic generation under certain pressure conditions. In addition, vuv light at wavelength shorter than the third harmonic wavelength was produced when several 4‐photon resonances in Ar were excited.
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Resonantly enhanced multiphoton ionization signals through the 6s[32]J=16s[\frac{3}{2}]J=16s[23​]J=1 state of Xe rapidly disappear at pressures above 0.30 Torr; concurrently, intense, vacuum-ultra-violet third-harmonic radiation is detected in the forward direction. Laser excitation spectra of the vacuum ultraviolet light exhibit increasing blue shifts and bandwidths with increasing pressure. The observed competition between ionization and third-harmonic generation is discussed in terms of a model involving fast collective emission.
In connection with a recent experiment of vacuum-ultraviolet generation in multiphoton resonant ionization of xenon at increasing pressure, a model is elaborated to account for the simultaneous effect of a laser excitation tuned near a three-photon resonance and a pressure-induced emission from the resonant state to the ground state. Under certain assumptions usual in various super-radiance treatments, a system of Bloch equations is derived and numerically solved. The dependence of the ionization probability and third-harmonic production versus the interaction time, gas pressure, dynamical detuning, and laser intensity is extensively studied. A transition between a high and low ionization rate is observed when the pressure increases simultaneously with the disappearance of Rabi oscillations and a saturation of third-harmonic generation. An order-of-magnitude calculation reveals that such phenomena are accessible to experiment and a qualitative comparison with the above-mentioned results in xenon is found to be satisfactory.
Resonantly enhanced multiphoton ionization (MPI) and third-harmonic generation (THG) have been concurrently studied in the rare gases, xenon, krypton, and argon. The disappearance with increasing pressure of the MPI signals resonant with the three-photon allowed 6s6s6s, 5s5s5s, and 4s4s4s states, respectively, of the rare gases is linked to the appearance of THG. Conversely, THG is shown to be partially quenched at certain wavelengths by one- and two-photon absorption leading to ionization.
Several extensions are made in theoretical studies of multiphoton ionization associated with third-harmonic fields that are generated near three-photon resonances in gaseous media. The treatment includes the strong coherent cancellation effect which occurs between three-photon pumping of a resonant level and pumping by the third-harmonic field that is generated in the same medium. The incomplete spoiling of the cancellation effect with counterpropagating beams is quantitatively described. Also, treatment of the multiphoton-ionization problem involving phase-matched, third-harmonic fields is generalized to include quantitative effects of index changes due to a buffer gas, absorption due to dimers, and collisional dephasing of coherent excitations. Finally, we show how measurements of linewidths, phase-matching positions, and pressure dependences of ionization yields associated with third-harmonic generation and subsequent absorption can be used to determine oscillator strengths of a resonant level, absorption coefficients for a resonant species and a buffer-gas species, and the refractive index of the buffer gas at the third-harmonic frequency.