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Physical Review A

Wave-mixing effects in resonance-enhanced multiphoton ionization of xenon in Bessel beams

W. R. Garrett, Viktor E. Peet

Published May 9, 2000• 14 citations
Publisher

Abstract

Three-photon-resonant excitation and subsequent ionization of xenon at high concentrations has been studied utilizing both Bessel and Gaussian beams. Comparisons of analogous results with the two modes of focusing are made. The excitation-ionization profiles can be analyzed on the basis of the well-studied interference effect involving combined contributions of three- and one-photon transition amplitudes in resonant atomic excitation. We show that the principal features of the excitation problem with Bessel beams can be simulated by a simple picture involving crossed plane-wave beams. Using this approach it has been shown that pressure-broadened and shifted lines that result from the interference-based cooperative Lorentzian line shift constitutes the main pressure-dependent peak of off-resonance excitation profiles produced in Bessel beams.

DOI
10.1103/physreva.61.063804

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Journal of the Optical Society of America B•Feb 1, 1985•37 citations
Three-photon-resonant excitation and subsequent ionization of xenon at high concentrations has been studied utilizing both Bessel and Gaussian beams. Comparisons of analogous results with the two modes of focusing are made. The excitation-ionization profiles can be analyzed on the basis of the well-studied interference effect involving combined contributions of three- and one-photon transition amplitudes in resonant atomic excitation. We show that the principal features of the excitation problem with Bessel beams can be simulated by a simple picture involving crossed plane-wave beams. Using this approach it has been shown that pressure-broadened and shifted lines that result from the interference-based cooperative Lorentzian line shift constitutes the main pressure-dependent peak of off-resonance excitation profiles produced in Bessel beams.
Three-photon resonance enhancement of third-harmonic generation and multiphoton ionization have been studied in xenon under excitation by the Bessel and the focused annular laser beams. It has been shown that the transformation of a Gaussian beam into a Bessel or an annular one leads to significant changes of third-harmonic excitation profiles and ionization spectra. These changes result from a principal difference of the beams, where either collinear (Gaussian beams) or noncollinear (Bessel and focused annular beams) excitation geometries are realized. It has been shown that the noncollinear ring-type phase matching shifts the maximum of the generated third harmonic to the longer wavelength toward the atomic resonance. Numerical calculations of third-harmonic excitation spectra have been carried out and compared with experimental results. The efficiency of frequency tripling in collinear and noncollinear excitation geometries have been compared. Cancellation of the three-photon 6s6s6s resonance and the influence of harmonic photons on the excitation of the four-photon 4f4f4f resonance in the Bessel beams are discussed.
Multiphoton ionization profiles in xenon under three-photon excitation by azimuthally segmented conical laser beams have been studied and analyzed on the basis of the interference-based cooperative shift of the atomic resonance line. Segmentation of the conical wave front with various geometrical slits reduces the possible three-photon azimuthal sub-beam combinations in the third order process and a principal (or ``homogeneous'') Lorentzian component can be selected from the overall excitation profile. This Lorentzian is identical to the pressure-broadened atomic line shifted by the cooperative shift. A possibility of controlling the nonlinear response by spatial selection of interacting waves in conical beams is discussed.
Three-photon excitation and associated wave mixing near the 6s6s6s and $6{s}^{\ensuremath{'}}$ resonances of xenon have been studied utilizing resonance-enhanced multiphoton ionization in angled beams with different polarizations. It has been shown that a complete cancellation of three-photon resonant atomic excitation caused by the well-known destructive wave-mixing interference occurs in sss and ppp polarization of angled beams but distinct resonance ionization enhancement is observed when pump beams have orthogonal polarization planes. Pressure-induced evolution of the resonance ionization peak in cross-polarized beams is identical to that observed with counterpropagating beams. The reason for such resonance ionization enhancement is unknown and cannot be explained within the frame work of existing theory. The effect may result from some peculiarities of wave-mixing interference in a multilevel atomic system, where different degenerate magnetic sublevels of the upper atomic state and multiple interfering excitation processes are involved. Another possibility is that the resonance ionization enhancement results from a process where weak counterpropagating light is generated within the excitation region of cross-polarized angled beams.
Two-color excitation by spatially coherent and incoherent laser beams has been used to study three-photon-resonant excitation and subsequent ionization of xenon in conditions, when internally generated sum-frequency field plays an important role in excitation of atomic resonances through interfering one-photon excitation pathway. We show that the incoherence in one of the pumping fields reduces the efficiency of generated sum-frequency field, and thus suppresses the interference between the three- and the one-photon excitation channels. The degree of suppression is controlled by varying the crossing angle between coherent and incoherent laser beams. We show that ionization profiles can be analyzed on the basis of the well-studied interference of one- and three-photon transition amplitudes, but with pumping field decomposed into multiple small-scale uncorrelated domains where coherent process of four-wave mixing occurs. The gain length for a coherent process in these domains depends on the coherence degree and excitation geometry. It gives a possibility of controlling the contribution of coherent processes to the excitation of multiphoton resonances.
Through multiphoton ionization measurements, the polarization effects in destructive quantum interference under three-photon resonant excitation have been studied. Recent observations [V. Peet, Phys. Rev. A 74, 033406 (2006)] have indicated that contrary to the well-known pattern of a total suppression of resonance excitation, the destructive interference becomes incomplete if three-photon transition is driven by crossed beams with orthogonal polarization planes. These observations have been tested for a more general case of two-color excitation and very similar polarization-dependent anomalies in the interference character have been registered. It has been shown that the destructive interference is modified and the resonance excitation does occur if two crossed laser beams have opposite circular polarizations. The pressure-induced evolution of the uncanceled ionization peaks has the ratio of blue shift to width close to 0.5 exactly as it is known for resonance ionization peaks registered under excitation by counterpropagating laser beams.
Two independently tunable, linearly polarized dye lasers have been used to study multiphoton ionization of gaseous xenon. One laser is tuned near three-photon resonance with the allowed 6s[3/2]J=1° resonance, which is not observed with one laser at these pressures because of the destructive interference between the three-photon-excitation and third-harmonic-excitation pathways to the 6s[3/2]J=1° state. The second laser is tuned to resonance with higher allowed np and np′ states of xenon. The missing ionization signal is seen to be dramatically restored by the addition of the second laser beam; however, the signal again disappears as the pressure is increased. These results are discussed in relation to recent experimental and theoretical studies of cancellation effects between three-photon excitation and third-harmonic generation. Spectroscopic information obtained from tuning each dye laser, together with data on the angular distributions resulting from rotating the plane of polarization of one laser relative to that of the second laser, clearly shows that two different ionization mechanisms are operative. In addition, these experiments unambiguously illustrate the importance of dissociative ionization of quasi-dimers of xenon over a rather broad frequency range.