HYDROGEN IONIZATION BY SHORT UV LASER PULSES: SPLITTING OF THE ATI

V D RODRÍGUEZ

Freiburg, Germany

Congreso; 25th International Conference on Photonic, electronic and Atomic Collisions; 2007

ICPEAC

 Theoretical  results  employing  a  particular variant of the modified Coulomb-Volkov approximation MCV2-  in order to describe the above threshold ionization (ATI) of hydrogen atoms by short UV laser pulses are presented. Beyond the perturbative casepreviusly discussed , here the focus is on the laser field in resonance with the 1s -> 2p transition. Increased laser intensity and length pulse are examined. The theoretical frame introduced previusly is used, namely a variational approach with Coulomb-Volkov state as the final trial wave function is employed. However, the initial trial function here proposed is the bound state wave function obtained by solving the time dependent Schrödinger for the two resonantly coupled states. This wave function exhibits Rabi oscillations and energy splitting. The theory is referred to as Columb Volkov-two states (CV-TS).     Multiphoton splitting of hydrogen ATI peaks due to resonant intermediate state population has been previously found either with a perturbative analytical procedure for  long laser pulses, or by solving the time dependent Schrödinger equation TDSE.        The theoretical electron spectrum of Hydrogen ionization for three different laser field amplitudes , 0.07 and 0.1 a.u. are obtained. The pulse duration is fixed to 80 cycles and the resonant  1s -> 2p0 condition for the laser frequency is kept. The energy range displayed allows for the observation of up to five ATI broad peaks. Nevertheless, the normal behavior of the ATI peaks around the maximum is replaced by a structure showing clearly two sub-peaks. Notably, this substructure still remains at the five main ATI peaks. The energy splitting of each peak equals to half the Rabi frequency at the middle of the pulse and increases linearly with the field amplitude.     As the field amplitude increases, the main ATI structure shifts towards greater electron energies and becomes slightly broader. The small shift isascribed to increasing ponderomotive potential.      Our purpose is to bring some light on themechanism of this phenomenon by using a quasi-analytical  theory. In this way, it is expected tocomplement full numerical treatments involvingintensive calculations  that sometimes may hide theessential physics of the problem.