Resonant-enhacend above-threshold ionization of atoms by XUV short laser pulses

”, V. D. RODRÍGUEZ, P. A. MACRI, D. G. ARBÓ

Freiburg, Alemania

Conferencia; XXV International Conference on the Photonic, Electronic and Atomic Collisions,; 2007

In recent works, it was established that a simple theoretical approach called CV2-, which is based on Coulomb-Volkov-type (CV) states, can supply reliable predictions of atomic ionization by extreme ultraviolet laser pulses in the subfemtosecond regime when compared with time dependent Schrödinger equation simulations (TDSE) [1]. When photon energies are lower than the ionization potential it has been introduced a modified form of CV2- that aims at accounting for the new features revealed in the electron spectrum by the above-mentioned TDSE calculation [2]. The improving theory MCV2- takes into account a pathway through intermediate bound states. Here we would like to examine Hydrogen ionization by a few-cycles laser when the laser frequency is close to resonance with the first excitation level (2p0). We take the laser amplitude   to keep the problem under perturbative conditions so that the MCV2- theory remains valid. Figure 1 shows the ionization electron spectrum for three different frequencies: below, at, and above the resonant w0=0.375 a.u. case. Both non-resonant cases agree in the background given by the simple CV2-, while differing from MCV2-  by the missing n=2 and n=3 secondary peaks. On the contrary, at resonance the background level is considerably increased and further the ionization peak is considerably underestimated by two order of magnitude in CV2-. The shape of the first peak shows a large width. This shape is characteristic of single photon ionization as given by the first Born approximation. It is without doubt the signature of the first step leading to a secondary peak (here, the excitation of the level n=2) that in this case is coincident with the two photon absorption level. Scrutinizing the case w=0.40 reveals that the first secondary peak is larger than the first principal peak. It is attested by both its position, and the position and height of the principal peak as predicted by CV2-. Thus, the first principal peak manifests itself as a shoulder in the right wing of the secondary peak.  The total ionization probabilities are resonantly enhanced and display nice maxima at the resonant frequency. References   [1]    G. Duchateau, E. Cormier and R. Gayet,                Phys. Rev. A 66 023412 (2002).   [2]    V.D. Rodríguez, E. Cormier and R. Gayet,            Phys. Rev. A 69 053402 (2004).