We present high resolution photoelectron energy spectra of noble gas atoms from high intensity above-threshold ionization (ATI) at midinfrared wavelengths. An unexpected structure at the very low-energy portion of the spectra, in striking contrast to the prediction of the simple-man theory, has been revealed. A semiclassical model calculation is able to reproduce the experimental feature and suggests the prominent role of the Coulomb interaction of the outgoing electron with the parent ion in producing the peculiar structure in long wavelength ATI spectra.

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The acceleration mechanism of electrons in combined strong axial magnetic fields and circularly polarized laser pulse fields is investigated by solving the dynamical equations for relativistic electrons both numerically and analytically. We find that the electron acceleration depends not only on the laser intensity, but also on the ratio between electron Larmor frequency and laser frequency. As the ratio approaches unity, a clear resonance peak is observed, corresponding to the laser-magnetic resonance acceleration. Away from the resonance regime, the strong magnetic fields still affect the electron acceleration dramatically. We derive an approximate analytical solution of the relativistic electron energy in adiabatic limit, which provides a full understanding of this phenomenon. Application of our theory to fast ignition of inertial confinement fusion is discussed.

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