High intensity above-threshold ionization of He

Abstract

This is the first high resolution measurement of the electron energy spectrum from high intensity above-threshold ionization (ponderomotive energy » ionization potential ^>hv) at intensities up to 7x 10 15 W/cm 2 . In contrast to previous work, we have used a short pulse and a large focal spot along with low gas densities to minimize the effect of ponderomotive forces and collisions. Existing models do not agree well with electron energy spectra for either linearly or circularly polarized light. The electron temperature is too hot for the implementation of 820 nm multiphoton ionized recombination type XUV laser schemes. PACS numbers: 32.80.RmWe report the first high resolution measurement of the electron energy distribution from high intensity abovethreshold ionization (ATI) at intensities up to 7xl0 15 W/cm 2 with a short pulse. In the short pulse regime [1-4] the pulse is over before the electrons have been disturbed by the ponderomotive force: The electrons are therefore characteristic of the ionization intensity and not of the excitation geometry. Previous measurements of electron spectra in the so-called "long pulse regime" were dominated by the ponderomotive acceleration of the electrons, which is their quiver response to the oscillating electromagnetic field in the intensity gradient at the laser focus [1-6]. The measurements reported here are at intensities 30 times more intense than those previously published and allow us to test models of ATI in the high intensity limit when ATI models should be most valid, viz., ponderomotive energy »ionization potentialshv. ATI electron energy spectra from both linearly and circularly polarized light were measured. While the threshold intensities for multiphoton ionization agree with models of ATI [7-9], the electron spectra from linear or circular polarization differ markedly from theory. The predicted preponderance of slow electrons from linearly polarized ATI is not observed. This result is particularly noteworthy in light of recently proposed x-ray laser schemes that use multiphoton ionization to produce high ion stages in a relatively cold electron distribution [10-12].These measurements are valuable because the ATI electron energy distributions contain minimal ponderomotive force contributions, despite the fact that the ponderomotive energy is much higher than the ionization potential of the parent atom. To achieve this, we require both a short pulse and a large focus. Our laser source is a colliding-pulse mode-locked laser [13] centered at 820 nm, amplified in a Ti-sapphire chirped-pulse amplifier [14]. It is capable of delivering 50 mJ pulses of 180 fs duration at 5 Hz. Laser pulses are focused with a 1 m lens into a vacuum chamber to a Gaussian waist of 0)0 = 35 jum. After the lens the laser beam propagates almost entirely in vacuum. The electron energy spectra are measured with a field-free time-of-flight (TOF) spectrometer with a detection cone angle of 2° (3.8 x 10 ~3 sr collection solid angle) at the focus. Electrons are accelerated at the end of the TOF ...

Rescattering of Ultralow-Energy Electrons for Single Ionization of Ne in the Tunneling Regime

“…Mohideen et al [18] "in disagreement" with all theoretical predictions and with results of other measurements [19][20][21][22] Essentially "en passant" very recent experiments [3,4,[7][8][9], originally devoted to measure electron spectra for double ionization, reported on low-4 energy single ionization electron spectra (E e < 150 eV). All of them at least find increasing yields for decreasing electron energies or, some of them, even a maximum at zero energy [3,4,7,8], as expected.…”

“…and, hence, the sharp minimum was not visible at all. As mentioned before, structures at low energies and maybe a minimum at P e|| = 0 have only once been observed in the tunneling regime to the best of our knowledge by Mohideen et al [18] under much less controlled experimental conditions and, accordingly, have not been discussed since then.…”

“…[9][10][11][12][13][14][15][16]). Surprisingly, however, at PW/cm 2 intensities, where recollision is now being generally accepted [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] to dominate non-sequential double ionization, only few differential electron emission studies can be found in the literature for single ionization by femtosecond pulses in the optical frequency regime [17][18][19][20][21][22]. Here, at typical Keldysh [23] parameters γ = (I P /2U P )½ << 1 (I P : ionization potential; U P = I/(2ω) 2 : ponderomotive potential with I the laser intensity and ω its frequency; atomic units are used throughout), the photoelectron spectra become increasingly smooth and unstructured, whereas they display a confusingly rich structure at lower intensities for γ ≥ 1.…”

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

“…Experiments performed at lower γ either revealed a smooth decrease of the photoelectron spectra, with changes of the slope in the plateau region [8,11], or observed in addition some hardly resolved structure below 20 eV [6,18]. Very recently precise measurements of ultra-low energy electron emission found a clear double-peak structure around zero [19] in the electron momentum distribution parallel to the laser polarization for Ne at the intensity 1 .…”

Time-resolved X-ray spectroscopy of optical-field-ionized plasmas

“…"Standard" initial conditions are defined by an electron density Ne 0 as derived from the measured gas density and an electron ' temperature T e 0 given by ATI heating, an assumption recently verified by Thomson scattering ' measurements (Offenberger et al 1993, Glover et al 1994. Note, however, that a higher electron temperature has been measured using time-of-flight spectroscopy (Mohideen et al 1993). These "standard" initial conditions are Ne 0 = 2 x 1018 cm-3, Te 0 = 40 eV for helium ' ' and Ne o = 1Q19 cm-3, Teo= 20 eV for nitrogen.…”