Elastic Rescattering in the Strong Field Tunneling Limit.

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