Elastic Rescattering in the Strong Field Tunneling Limit.
Elastic Rescattering in the Strong Field Tunneling Limit.
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DOI：
10.1103/physrevlett.77.5031
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发表时间：
1996-12
影响因子：
8.6
通讯作者：
Barry C. Walker;B. Sheehy;K. Kulander;L. DiMauro
中科院分区：
文献类型：
--
作者：
Barry C. Walker;B. Sheehy;K. Kulander;L. DiMauro
High precision measurements of helium photoelectron energy and angular distributions for a broad intensity range reflect the change in the continuum dynamics that occurs as the ionization process evolves into the pure tunneling regime. Elastic rescattering of the laser-driven free electron from its parent ion core leaves a distinct signature on the spectra, providing a direct quantitative test of the various theories of strong field multiphoton ionization. We show that it takes a relatively complete semiclassical rescattering model to accurately reproduce the observed distributions. [S0031-9007(96)01922-9] PACS numbers: 32.80.Rm, 31.90.+ s, 32.80.Fb Strong field photoelectron spectra have attracted considerable attention over the past decade and a half, but a comprehensive understanding of the underlying dynamics which produce these spectra and related phenomena has solidified with the development of high repetitionrate, short pulse lasers which can span the entire intensity range of importance [1]. Keldysh [2] showed that at infrared and visible wavelengths the dynamics of strong field atomic ionization undergoes a change in character as the laser intensity increases. In weak fields electrons are promoted into the continuum by the simultaneous absorption of enough photons to increase their energy above the ionization potential. This is called multiphoton ionization (MPI). However, as the laser intensity increases, a completely separate mode of escape becomes possible. At large distances from the nucleus, the electrostatic attraction of the ion core can be overwhelmed by the laser’s instantaneous electric field, producing a barrier through which a valence electron can tunnel. In this regime a quasistatic tunneling picture becomes appropriate: the laser field varies so slowly compared to the response time of the electron that the ionization rate becomes simply the cycle average of the instantaneous dc tunneling rate. Tunneling becomes important when the ratio of the frequency of the applied field to the tunneling rate becomes smaller than unity. This ratio, known as the Keldysh or adiabaticity parameter, g, is given by p Ipys2Upd, where Ip is the binding energy of the electron and Up › Iy4v 2 is the ponderomotive energy in atomic units of a free electron in the laser field of frequency v and intensity I. The majority of experimental studies on neutral atoms exposed to intense, short‐pulse laser fields have been carried out in the MPI regime ( g$ 1). A few experiments [3 ‐ 6] have extended into the tunneling regime, but these measurements have been limited to observations of total ionization rates or electron energy distributions over a small dynamic range. In this Letter, we report upon the first systematic experimental investigation in the strong field tunneling limit. Our study, by virtue of the enhanced dynamic range accessible with kilohertz laser technology, follows the change in the electron spectrum and angular distributions as the ionization evolves from predominantly MPI to pure tunneling. We find that the electron distributions in the tunneling regime are very different from any previous reports obtained in the MPI or mixed regimes [7 ‐ 9]. We achieve a quantitative description of these spectra using a rescattering picture [9,10] which mimics the time evolution of a tunnel-ionized continuum wave packet in the combined fields of the laser and ion core. Since rescattering events are known to be important in other short-pulse, strong-field emission phenomena (e.g., harmonic generation), this investigation better defines the underlying dynamics of these processes. In the experiments presented here, a 150 fs, 1 kHz repetition rate, titanium sapphire laser operating at 0.78 mm was focused by fy4 optics into an ultrahigh vacuum chamber, producing a maximum intensity of 20 PW/cm 2 . The sample gas was 99.999% helium, which was further scrubbed to ,0.1 ppm for O2 ,H 2 ,H 2O, CO2, and hydrocarbon impurities. A 30 cm long time-of-flight photoelectron (PE) spectrometer provides energy and angular resolution of 0.05 eV and 65 mrad, respectively. Data collection used 1 ns binning of discriminated electron events operating at low event probability (#0.25/ shot), ensuring space charge free conditions. The spectrometer’s energy calibration was obtained by recording the high order (.40 photon absorption), long pulse abovethreshold ionization spectrum of xenon. The intensity was calibrated using measurements of the total ion yield, xenon short-pulse PES resonances, and spot size [11]. The uncertainty in the reported intensities is ,25%. The current PE spectra were recorded between 0.5 and 1.5 times the saturation intensity (Isat › 0.8 PW/cm 2 ) for neutral
