It is widely believed that intense-field ionization occurs either though a 'multiphoton' mechanism (for Keldysh parameter γ > 1) or through a tunnelling mechanism (or 'field ionization'), for γ < 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 (γ < 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.
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文献种类:期刊
期刊名称: Journal of Physics B: Atomic, Molecular and Optical Physics
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Correlated Multielectron Dynamics in Ultrafast Laser Pulse Interactions with Atoms
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