Coulomb explosion of H2+ wave packet in ultrashort XUV laser fields

Abstract

A pump–probe Coulomb imaging method, involving a strong infrared (IR) femtosecond (fs) field and ultrashort (sub-fs) XUV harmonic radiation, is proposed to study the dynamics of a vibrational wave packet of H2+ at the fs and sub-fs time scales. The IR field quickly ionizes the H2 molecule, leaving a non-dissociative H2+ wave packet. Then the XUV probe pulse, applied with a controlled time delay, ionizes the H2+ wave packet. The kinetic energy distribution (KED) of the ejected protons is calculated, for various time delays, solving the time-dependent Schrödinger equation (TDSE). The calculations are performed within the Born–Oppenheimer approximation; they include both the electronic and vibrational motion, in full dimensions (3D). The KED shows the evolution of the wave packet at the fs/sub-fs time scale. The coherent wave packet is reconstructed from the KED with the help of the Coulomb law, and compared with the initial distribution (i.e., the H2+ wave packet density of probability as a function of the internuclear distance R). The TDSE has been also solved within the frozen nuclear approximation; this approach brings a better understanding of the physics underlying the wave packet ionization. The case of a train of two XUV pulses is also investigated; the KED shows a distortion which is strongly dependent on the time delay. The pump–probe technique can be applied to explore the nuclear coherent motion or, provided the initial wave packet is well characterized, to bring information on the pulse characteristics.