Comments on the effect of the ponderomotive potential in the above-threshold ionization processes

Pan, Liwen; Armstrong, Lloyd; Eberly, J. H.

doi:10.1364/josab.3.001319

Cited by 76 publications

(28 citation statements)

References 9 publications

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“…Second-order perturbation theory [5] for the two states gives 0(1^2) = 1.48 and a{\s4) = 1.28. Now using Eq.…”

Section: (5)mentioning

confidence: 97%

“…It was first postulated that these peaks result from a resonant enhancement of the MPI brought about by the ac Stark effect shifting states in and out of resonance with the laser field. Up to second order in perturbation theory [5], the ac Stark shift of each level relative to the ground state is simply proportional to the laser intensity, and, thus, to the ponderomotive energy aUp. The resonance condition is given by…”

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confidence: 99%

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Verification of the dominant role of resonant enhancement in short-pulse multiphoton ionization

1992

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We report experimental results which imply that observed peaks in the short-pulse photoelectron spectrum of atoms in high fields result only from a resonant enhancement of the multiphoton ionization rate at the peak of the laser pulse, rather than a two-step process of real population transfer and subsequent single-photon ionization. These results consist of the first identification, in a high intensity (> 10'^ W/cm^) short-pulse photoelectron spectrum, of low-lying states of argon with an ac Stark shift significantly different from the ponderomotive energy.PACS numbers: 32.80. Rm, 32.80.Wr Over the past few years, the interpretation of the photoelectron spectra of atoms in strong fields has been marked by several major shifts [l]. One of the most important observations was that in the short-pulse regime (in which the pulse length of the laser is much less than the time it takes an electron to leave the laser focus) the photoelectrons experience no ponderomotive acceleration from the laser field subsequent to ionization and, thus, the spectrum measured at the detector faithfully reproduces the spectrum of emitted electrons in the laser focus [2]. As is well known, the short-pulse spectrum consists of many narrow electron peaks, with the pattern reproduced in energy at intervals corresponding to the photon energy. The original explanation for the observed spectrum suggested that the peaks simply correspond to an enhancement of the multiphoton ionization (MPI) rate brought about by the shifting of excited states into resonance with the laser field by the ac Stark shift [2,3]. More recently [4], it has been suggested that the photoelectron peaks actually result from a two-step process: As intermediate states are brought into resonance with the laser by the ac Stark shift a real population is transferred to the excited state. Subsequently, those excited states ionize by single-photon absorption later in the laser pulse. Unfortunately, these two possibilities predict identical electron spectra for states whose ac Stark shift is equal to the ponderomotive energy. While they predict different results for states with a nonponderomotive ac Stark shift, no such states have yet been identified in actual spectra. Our new result is the identification of such nonponderomotively shifted states in argon irradiated by short-pulse high-intensity 308-nm laser light. This observation requires that the electron peaks in the photoelectron spectrum must result only from a resonant enhancement of the photoionization process.Since we will be considering states with nonponderomotive shifts, we must rederive the equations governing short-pulse MPI for arbitrary ac Stark shifts. The energy of an ionized electron is simply given by the generalized photoelectric formula [l]:where n is the number of absorbed photons, hv'xs the photon energy, IpiQ) is the ionization potential at zero field, and Upil) is the ponderomotive energy, which is a function of the laser intensity. (We have neglected the difference in the ground-state Stark shifts of ...

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“…Second-order perturbation theory [5] for the two states gives 0(1^2) = 1.48 and a{\s4) = 1.28. Now using Eq.…”

Section: (5)mentioning

confidence: 97%

mentioning

confidence: 99%

Verification of the dominant role of resonant enhancement in short-pulse multiphoton ionization

1992

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“…Applying Eq. (4) to electrons that are emitted perpendicular to the light propagation, we find that for a pulse length of 1 ps, less than 5 meV are recovered from the field, while this amount is 142 meV in the case of a 20-ps pulse For neither pulse length is any significant acceleration expected when the electron travels along the light propagation, hence explaining the observed spatial asymmetry. The calculated gain in energy by about 13%%uo corresponds to a gain in velocity of 6.5%, quite in line with our experimental measurement.…”

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confidence: 72%

“…Under these conditions the energy gained by the ponderomotive acceleration exactly balances the increase [4] in the ionization potential by the quiver energy. On the other hand, for a pulse duration that is short as compared to the time that an electron requires to exit from the laser focus, very little conversion can occur, and the electron is left with the nascent translation energy at which it was created.…”

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confidence: 97%

Spatial anisotropy of the velocity of electrons emitted from a short-pulse laser focus

1994

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Conversion of ponderomotive energy into translational energy is investigated for photoelectrons originating in the focus of a short-pulse laser beam of 1and 20-ps duration. For the long pulse a gradual difFerence in the amount of ponderomotive energy gained is observed as a function of angle between the nascent electron velocity and the laser propagation direction. The experimental results are in good agreement with theory.PACS number(s): 32.80.Rm, 32.80.Wr, 42.50.Vk

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“…17 This may have significant implications for determination of the tunnel rates using the structure-based model. In particular, both the ionization potential and the characteristic length may change in the field in comparison with their field-free values.…”

Section: Discussionmentioning

confidence: 99%

Influencing Strong Field Excitation Dynamics through Molecular Structure

2002

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We report the loss of discrete above-threshold ionization photoelectron peaks in the strong-field (800 nm, 60 fs, 3.6 -9.0 × 10 13 W‚cm -2 ) excitation in a series of polyatomic molecules of increasing characteristic length. The molecules, biphenyl (C 12 H 10 ), diphenylmethane (C 13 H 12 ), and diphenylethane (C 14 H 14 ), have two phenyl groups spaced by a bridge of zero, one, or two carbon atoms. The photoelectron spectra correlate with the characteristic lengths, not the number of atoms, of these molecules. The molecules having the smallest and largest numbers of atoms (but similar characteristic lengths) display a broad featureless distribution of photoelectron kinetic energies, peaked at lower kinetic energies and extending to many tens of electronvolts. Diphenylmethane, the molecule with intermediate number of atoms but the smallest characteristic length, displays a photoelectron spectrum containing discrete peaks in this range of laser intensities. The absence of discrete photoelectron peaks in the spectra of biphenyl and diphenylethane is interpreted using an eigenstate lifetime model based on calculated field ionization rates in an intense laser field. Finally, the abundance of photoelectrons with high kinetic energies suggests higher probability of electron rescattering in the polyatomic molecules in comparison with atomic intense field ionization.

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Comments on the effect of the ponderomotive potential in the above-threshold ionization processes

Cited by 76 publications

References 9 publications

Verification of the dominant role of resonant enhancement in short-pulse multiphoton ionization

Verification of the dominant role of resonant enhancement in short-pulse multiphoton ionization

Spatial anisotropy of the velocity of electrons emitted from a short-pulse laser focus

Influencing Strong Field Excitation Dynamics through Molecular Structure

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