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Physical Review A

Third-harmonic generation and multiphoton ionization in rare gases

John C. Miller, R. N. Compton

Published Apr 1, 1982• 102 citations
Publisher

Abstract

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 $6s$, $5s$, and $4s$ 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.

DOI
10.1103/physreva.25.2056

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Physical Review A•Aug 1, 1986•46 citations
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.
Multiphoton ionization (MPI) and third-harmonic generation (THG) in xenon and krypton have been studied via three-photon resonance with the xenon 6s'[1/2]10{[1/2]}_{1}^{0}[1/2]10​ and krypton 5s[(3/2]10{[(3/2]}_{1}^{0}[(3/2]10​ states. A second, tunable dye laser is used to couple these states to higher-lying even-parity states. Observations are reported regarding the third-harmonic related cancellation of the intermediate s-state population and the mechanisms of population of the final states. Effects of the four-photon resonance on THG are detailed. Changes induced by the second laser reduce THG in normally favorable regions and enable it in otherwise forbidden regions. Striking effects in the MPI spectra due to the changes induced in THG are reported.
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.
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.
Multiphoton ionization in the region near a three-photon resonance is treated for focused, plane-polarized Gaussian beams with diffraction-limited beam divergence. In this situation, a third-harmonic field is generated within the laser beam. At, and very near, three-photon resonance the driving rate for the upper-state probability amplitude due to one-photon absorption of third-harmonic light becomes nearly equal to the corresponding three-photon rate due to the laser field, but these effects are 180\ifmmode^\circ\else\textdegree\fi{} out of phase. As a consequence of this cancellation between two pumping terms, the three-photon resonance line essentially disappears at moderate concentrations and the observed ionization has a line shape that is close to the phase-matching curve for third-harmonic generation. The ionization signal, near but not on the resonance, is due almost entirely to absorption of third-harmonic photons plus other laser photons; three-photon resonantly enhanced multiphoton ionization by the laser is much weaker. This is particularly true on the blue side of the three-photon resonance at detunings where phase matching occurs. The problem is treated quite generally with predictions of the full line shape for nnn-photon ionization and third-harmonic light generation near three-photon resonance, including the rather strong influences of positively dispersive buffer gases. We also show that the cancellation between the one-photon and the three-photon process is partially spoiled in the presence of a counterpropagating beam at the same frequency.
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.
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.