PUB - Publikationen an der Universität Bielefeld

It is widely believed that intense-field ionization occurs either though a 'multiphoton' mechanism (for Keldysh parameter gamma > 1) or through a tunnelling mechanism (or 'field ionization'), for gamma < 1. Recently, high-resolution measurements of the parallel-momentum distributions of electrons emitted from He and other atoms in intense laser fields have been reported in this journal and elsewhere. They revealed a remarkable central minimum with two 'horn-like' ends and a double sequence of sharp peaks in the tunnel regime (gamma < 0.5). They are thus found to contradict the smooth distributions observed under lower resolution in the past, which had been widely interpreted to arise from the field-ionization (or tunnelling) mechanism. In this letter, we present a discrete photon analysis of the parallel-momentum distributions for the case of He atom. Our analysis is based on the well-known KFR (Keldysh-Faisal-Reiss) theory of intense-field processes that explicitly take account of discrete absorptions of photons; it is also modified to take account of the asymptotic Coulomb potential. Results are obtained for both constant-amplitude fields and short Gaussian pulses. Comparison of the results shows that the prominent features in the calculated parallel-momentum distributions arise from a discrete photon absorption mechanism in the 'tunnel regime' and agree qualitatively well with observed distributions.