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Physical Review A

Eight- and nine-photon resonances in multiphoton ionization of xenon

Peter Hansch, M. A. Walker, L. D. Van Woerkom

Published Feb 1, 1998• 33 citations
Publisher

Abstract

We have measured and analyzed the clear emergence of ac-Stark-shifted multiphoton resonances with successive photon orders in xenon. The remarkable quality of our data illustrates the unambiguous evolution through parity-allowed resonances at the eight- and subsequent nine-photon levels. This marks a significant advance in showing that the transient resonance model is valid and strong optical-field ionization remains multiphoton in character at higher intensities. Furthermore, a simple Landau-Zener picture is sufficient to understand the basic principles.

DOI
10.1103/physreva.57.r709

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We have measured and analyzed the clear emergence of ac-Stark-shifted multiphoton resonances with successive photon orders in xenon. The remarkable quality of our data illustrates the unambiguous evolution through parity-allowed resonances at the eight- and subsequent nine-photon levels. This marks a significant advance in showing that the transient resonance model is valid and strong optical-field ionization remains multiphoton in character at higher intensities. Furthermore, a simple Landau-Zener picture is sufficient to understand the basic principles.
The authors report a high-resolution electron spectroscopic study of three-photon resonant, five-photon ionisation of xenon. The results are analysed in terms of a dynamic resonance of three photons with an AC Stark-shifted intermediate level (6s), followed by a saturated two-photon transition to the continuum. They demonstrate a linear relationship between detuning from the zero-field resonance position and the local momentary light intensity causing the five-photon ionisation. Thus, the AC Stark shift acts as a probe which selects a particular intensity from the wide range present in the laser focus. They determined the AC Stark shift of the 6s level the distribution function of the light intensities and the intensity dependence of the ionisation probability. High-resolution electron spectra are used in arguments to support the validity of their analysis.
We have measured the energy spectrum of photoelectrons emitted in multiphoton ionization of atoms by a short-pulse, tunable laser. The spectrum shows prominent resonances, confirming the observations of Freeman et al. [Phys. Rev. Lett. 59, 1092 (1987)]. The study of the position and intensity of such resonances, as a function of laser wavelength and intensity, probes the structure of the atom in the presence of the strong electromagnetic field and allows their spectroscopic assignment. A systematic study of the ac Stark shift of one of the resonances, the 7s[3/2]\ifmmode^\circ\else\textdegree\fi{} level of xenon, is presented.
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As it is well known now1, electron energy spectra from multiphoton ionization (MPI) and Above-Threshold Ionization (ATI) of atoms by subpicosecond pulses reveal Stark-induced resonances and provide an elegant and unique method to investigate AC-Stark shifts of strongly perturbed excited states. In this work we have systematically studied such spectra and their interpretation in the case of MPI of Rare gases with 100 fs, 617 nm pulses. The intensity ranges from 1013 (for xenon) to 1015 W.cm−2 (for helium). The corresponding shifts range from .3 eV to 30 eV and may be, in principle, larger than the atomic ionization potential. Besides the measurement of AC-Stark shifts, the photoelectron spectra provide valuable information about the ionization process itself in a highly non-perturbative regime.
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.