Laser intensity dependence of secondary peaks in above threshold ionization spectrum

V. D. RODRÍGUEZ; E. CORMIER; R. GAYET

Rosario, Argentina

Conferencia; 24th International Conference on the Physics of Electronic, Photonic and Atomic Collision (XXIV ICPEAC) Rosario, Argentina (2005); 2005

LASER INTENSITY DEPENDENCE OF SECONDARY PEAKS IN ABOVE THRESHOLD IONIZATION SPECTRUM V. D. Rodríguez1, E. Cormier,2 and R. Gayet2 1) Departamento de Física, FCEyN, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina. 2) CELIA, Université Bordeaux 1, 351 Cours de la Libération, 33405 Talence Cedex, France.      The interaction between an atom and a short VUV laser pulse has been studied in a perturbative regime [1]. The aim of this work is to investigate non-perturbative situations The standard case addressed the ionization of Hydrogen atoms initially in their ground state. The use of a numerical solution of the time-dependent Schrödinger equation, called TDSE [2], allows to show that some series of secondary peaks appear in the electron ionization spectrum when the photon energy is lower than the ionization potential. These new structures are traced back to transient population of intermediate states due to a large bandwidth field followed by subsequent multiphoton absorption. A new theory based on Coulomb-Volkov-type states was proposed to explain these features. The theory, called Modified Coulomb-Volkov MCV2, is a modification of a previous one [3]. MCV2 fully reproduces the main features in the electron spectrum. Moreover, the above threshold ionization ATI peaks are enhanced by the intermediate state contributions in good agreement with the numerical experiments. MCV2 makes good predictions for photon energies as lower as half the ionization threshold.    It has been stated that the number and height of secondary peaks increase with the pulse duration. Here, we analyze the dependence of the secondary peaks on the laser intensity. Unlike previous applications, we use MCV2 beyond the perturbative regime. The laser pulse is featured by a linearly polarized electric field with a sine-square envelope. Ionization of Hydrogen atoms H(1s) is studied by solving numerically the TDSE and by using MCV2 [1]. In fig. 1, MCV2 energy spectra of ejected electrons are compared to TDSE predictions, for  a.u. (ionization potential) and two field amplitudes F0 . Four main ATI peaks are observed as well as up to four secondary peaks arising from n=2 intermediate level population. Further peaks fall down below the background probability. For the highest F0, MCV2 predicts an additional side peak. Although the MCV2 theory increasingly underestimates the primary peaks, it succeeds in reproducing the secondary peaks. Figure 1. Theoretical spectra for the ionization of H(1s) by two 20-cycle laser pulses. Solid line, TDSE; dashed line, MCV2. References [1] V. D. Rodríguez, E. Cormier and R. Gayet, Phys. Rev. A   69,  053402 (2004). [2] E. Cormier and P. Lambropoulos, J. Phys. B  30, 77  (1997). [3] G. Duchateau, E. Cormier and R. Gayet, Phys. Rev. A   66,  023412 (2002).