MULTIPHOTON IONIZATION IN SUPERINTENSE, HIGH-FREQUENCY LASER FIELDS .2. STABILIZATION OF ATOMIC-HYDROGEN IN LINEARLY POLARIZED FIELDS

This is the second of two papers studying multiphoton ionization (MPI) in superintense, high-frequency laser fields. They are based on a general iteration scheme in increasing powers of the inverse frequency. To lowest order in the frequency, i.e., the high-frequency limit, the atom is stable against decay by MPI, though distorted. To next order in the iteration, an expression for the MPI amplitude was obtained. In our first paper [preceding paper, Phys. Rev. A 44, 2141 (1991)], an alternative expression for the MPI amplitude was obtained for atomic hydrogen, which is substantially simpler, though somewhat less accurate. In the present paper, we study its consequences for the case of atomic hydrogen in superintense, linearly polarized fields with the emphasis on the ground state. Special attention is paid to the case in which the de Broglie wavelength of the photoelectrons is small with respect to the amplitude of oscillation of the (distorted) electronic cloud. This condition defines a radiation regime which yields features in sharp contrast to those obtained in weak fields. Most importantly, the total decay rate decreases with increasing intensity at given (high) frequency ("high-intensity stabilization"). The angular distributions of photoelectrons are found to be characterized by rapid oscillations with the polar angle, arising from a peculiar way in which outgoing electron waves interfere. At the same time, the overall behavior of the photoelectrons is to be ejected in directions nearly perpendicular to the polarization axis. We have solved the limit of extremely high intensities at fixed, but otherwise arbitrarily chosen, frequency analytically. We find that in "ultrastrong fields" the branching ratios for decay by absorption of the various number of photons possible are only weakly dependent on the values of the intensity and frequency of the laser field, yet excess-photon ionization constitutes a sizable part of the decay modes of the atom (typically 30%). At very high intensities, the hydrogen atom tends to stabilize at fixed, but otherwise arbitrarily chosen frequency. The (a priori unexpected) relative stability of the hydrogen atom in ultrastrong fields is explained as a result of "radiative distortion" of the electron cloud and "destructive interference" of outgoing electron waves. Although the lifetime of the atom turns out to be extremely short for values of the intensity around the atomic unit, for low enough frequencies and very high intensities, it can be remarkably long. Finally, we discuss the problem of how the atom subject to these extreme radiation conditions could be observed experimentally.