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

Abstract

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 ...

Resonant structures in the low-energy electron continuum for single ionization of atoms in the tunnelling regime

“…Usually, ionization is considered as either a "multiphoton" or "tunnelling" process depending on the value of the so-called Keldysh parameter ponderomotive effects, channel-opening and closing etc. were studied, and good agreement between theory and experiment has been achieved [2][3][4][5][6][7].…”

“…In early studies two scenarios resulting in the intensity-independent ATI peak positions were suggested. First one [4] assumes that ionization from an excited state occurs before the intensity has considerably changed, so that it always proceeds at the intensity required to fulfil the resonance condition and accordingly, at some certain value of the ponderomotive shift. If the peak intensity is higher than the resonant value, the resonance condition can be fulfilled somewhere in the laser focus.…”

Femtosecond Photoelectron Imaging of N2at 410 nm

“…The two pictures have been used to successfully model experimental results for atoms. [10][11][12][13][14][15] Previous studies provide evidence that strong-field ionization of molecules shares many of the characteristics with those of atoms, 1,2 and indeed the two mechanisms have been observed by previous experimental results for molecules. [1][2][3] N2 is one of the two major ingredients of the atmosphere, and its MPI dynamics in intense femtosecond laser fields has been studies extensively.…”

“…2,11 In N2 there are three different low-lying ionization energies (corresponding to the removal of the 3σ g , 1π u , and 2σ u electrons) leading the ionic states X Given 410 nm photons (3.02 eV), a minimum of six-, six-, and seven-photons are necessary for ionization to X (15.58 eV), A (16.70 eV), and B (18.75 eV) ionic final states at zero fields, respectively. When the laser intensity is raised, the electrons acquire significant ponderomotive energy.…”

“…The narrow peaks in the short-pulse regime photoelectron spectrum of atoms in strong fields have been interpreted by two types of mechanisms: the resonant enhancement model [9][10][11][12] and the two-step model. 13 For the former mechanism, the electron peaks result from an enhancement of the MPI rate brought about by the ac Stark effect of excited states into resonance with the laser field at the peak of the laser pulse.…”