Interaction of Atomic Hydrogen with  and Femtosecond Laser Pulses
Abstract
This thesis presents a theoretical study of the interaction of atomic hydrogen with coherent laser pulses in the 5 femtosecond to 10 picosecond range, in the weak field limit, and in intense fields. We approach the problem in the weakfield limit by studying the relationship between the Fourier relation of the laser pulse (Delta omegaDeltat) and the Delta EDeltat relation of the atomic Rydberg wave packet generated by the laser pulse. A derivation of the wave packet based on the WKB approximation is given, permitting the quantity Deltat to be derived for the quantum state, with the conclusion that under certain circumstances a transformlimited laser pulse (satisfying DeltaomegaDeltat = 1/2) can generate a transformlimited electron (satisfying DeltaEDeltat/ hbar = 1/2). The interaction of hydrogen with femtosecond pulses is studied at field intensities as high as 2.2 cdot10^{14}W/cm ^2. The full threedimensional Schrodinger equation is numerically integrated at intensities of this order as a guide to the development of theory. In terms of the Fermi golden rule (FGR) formulation of ionization, the results may be summarized as follows: just about every approximation employed in the derivation of FGR breaks down at 10^{14}W/cm ^2. Nevertheless, it was possible to provide straightforward nonperturbative methods to replace the approximations and perturbative methods employed FGR. A populationtrapping effect is found numerically and modeled theoretically. Despite the high field intensities, population representing the excited electron is recaptured from the ionization continuum by bound states during the excitation. Population returns to the atom with just the right phase to strongly inhibit ionization. A theory is presented that models this effect for a variety of laser pulse shapes, with and without the rotatingwave approximation. The numerical integration reveals that a certain amount of abovethreshold ionization (ATI) occurs. A theory similar to the Keldyshtype theories of ATI is developed. The theory differs from the Keldysh theories in that, like Schrodinger's equation, it is invariant under certain gauge transformations. The proposed theory gives far superior agreement with the numerical integration than Keldysh theory. Classical ionization at 2.2cdot 10^{14}W/cm ^2 is studied by numerically integrating Newton's equation on a Monte Carlo ensemble constructed to correspond to the above examples.
 Publication:

Ph.D. Thesis
 Pub Date:
 1990
 Bibcode:
 1990PhDT........54P
 Keywords:

 PICOSECOND;
 Physics: Atomic