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Journal of Physics B Atomic Molecular and Optical Physics

`Short' pulse MPI of xenon: the ionization channel

H. Rottke, Jan Ludwig, Wolfgang Sandner

Published Apr 28, 1996• 16 citations
Publisher

Abstract

We present photoelectron spectra measured by `short' pulse ( ps) MPI of xenon at 526.5 nm excitation wavelength in the intensity range to . Ionization into and channels is observed and resonance structures are found in both ionization channels. They are induced by the AC Stark shift of bound Xe states within the light pulse which brings them into transient n-photon resonance with the Xe ground state. The resonant states are tentatively identified as d- and s-angular momentum states having AC Stark shifts which differ from the shift of the xenon ionization threshold. In the high-intensity limit, ionization into the channel is completely suppressed.

DOI
10.1088/0953-4075/29/8/012

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We present photoelectron spectra measured by `short' pulse ( ps) MPI of xenon at 526.5 nm excitation wavelength in the intensity range to . Ionization into and channels is observed and resonance structures are found in both ionization channels. They are induced by the AC Stark shift of bound Xe states within the light pulse which brings them into transient n-photon resonance with the Xe ground state. The resonant states are tentatively identified as d- and s-angular momentum states having AC Stark shifts which differ from the shift of the xenon ionization threshold. In the high-intensity limit, ionization into the channel is completely suppressed.
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.
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.
Nonresonant ac Stark shifts of Rydberg states in Xe have been investigated by probing an atomic transition with a weak UV light in the presence of an intense YAG-laser pulse. Rydberg atoms produced through two-photon excitation, 5p → np (n = 15–48) and 5p → nf (n = 12–49), are subsequently ionized. IR-laser-induced shifts of the resonant ionization profiles are then measured. The measured shift, 3.9 ± 0.3 cm−1 at 1010 W cm−2, is more than a factor of 2 smaller than that expected from the basis of ponderomotive term, 8.5 cm−1 at 1010 W cm−2. In addition, the shift varies linearly as a function of the IR-laser intensity and does not depend on the principal quantum number of the np and nf series investigated here.
We present results from a time-dependent two-level-plus-continuum model of multiphoton ionization in order to better understand the short-pulse photoelectron spectrum resulting from multiphoton ionization and the role of intermediate states. Both resonant and nonresonant population transfer to the upper state are found to play an important role in the multiphonon ionization process. The dynamic modeling reveals the potential for broadening and shifting peaks in the photoelectron spectrum. However, we conclude that despite the complex dynamics of the excitation, short-pulse photoelectron spectra can be used to measure ac Stark shifts of both ponderomotively and nonponderomotively shifted states in strong laser fields.
We report the observation of pronounced, spectrally narrow laser-induced continuum structures (LICS) in the two ionization continua of xenon. A nanosecond laser pulse with nearly transform-limited bandwidth induces a LICS by coupling the initially unpopulated 5p59p[1/2]0{5p}^{5}9p[1/2{]}_{0}5p59p[1/2]0​ state to the continuum. The LICS is probed by one-photon ionization of the 5p56p[1/2]0{5p}^{5}6p[1/2{]}_{0}5p56p[1/2]0​ state. This state is populated in an independent excitation step by an additional laser pulse acting prior to the probe and dressing pulses. The two ionization channels that leave the xenon ion in either the 2P1/2{}^{2}{P}_{1/2}2P1/2​ or the 2P3/2{}^{2}{P}_{3/2}2P3/2​ fine-structure state are detected separately by monitoring the kinetic energy of the photoelectrons in a time-of-flight electron spectrometer. A suppression of the photoelectron signal of up to 80% as compared to the far off resonance value is achieved. The line shapes reveal a Fano parameter close to zero. The LICS is identical for the coupling into the two continua. Therefore the branching ratio into the two ionization channels does not vary with the dressing-laser detuning for the coupling scheme implemented here.