Alignment and pulse-duration effects in two-photon double ionization ofH2by femtosecond XUV laser pulses

Abstract

We present calculations for the dependence of the two-photon double ionization (DI) of ${\mathrm{H}}_{2}$ on the relative orientation of the linear laser polarization to the internuclear axis and the length of the pulse. We use the fixed-nuclei approximation at the equilibrium distance of 1.4 ${a}_{0}$, where ${a}_{0}=0.529\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ is the Bohr radius. Central photon energies cover the entire direct DI domain from 26.5 to 34.0 eV. In contrast to the parallel geometry studied earlier [X. Guan, K. Bartschat, B. I. Schneider, and L. Koesterke, Phys. Rev. A 83, 043403 (2011)], the effect of the pulse duration is almost negligible for the case when the two axes are perpendicular to each other. This is a consequence of the symmetry rules for dipole excitation in the two cases. In the parallel geometry, doubly excited states of ${}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ symmetry affect the cross section, while in the perpendicular geometry only much longer-lived ${}^{1}{\ensuremath{\Pi}}_{u}$ states are present. This accounts for the different convergence patterns observed in the calculated cross sections as a function of the pulse length. When the photon energy approaches the threshold of sequential DI, a sharp increase of the generalized total cross section (GTCS) with increasing pulse duration is also observed in the perpendicular geometry, very similar to the case of the molecular axis being oriented along the laser polarization direction. Our results differ from those of Colgan et al. [J. Colgan, M. S. Pindzola, and F. Robicheaux, J. Phys. B 41, 121002 (2008)] and Morales et al. [F. Morales, F. Mart\'{\i}n, D. A. Horner, T. N. Rescigno, and C. W. McCurdy, J. Phys. B 42, 134013 (2009)], but are in excellent agreement with the GTCSs of Simonsen et al. [A. S. Simonsen, S. A. S\o{}rng\aa{}rd, R. Nepstad, and M. F\o{}rre, Phys. Rev. A 85, 063404 (2012)] over the entire domain of direct DI.