We apply the Monte Carlo Wave Packet (MCWP) approach to investigate the kinetic energy release (KER) spectra of the protons following double ionization in H2 when interacting with laser pulses with central wavelengths ranging from the near-IR (800 nm) to the mid-IR (6400 nm) regions and with durations of 3-21 laser cycles. We uncover the physical origins of the peaks in the nuclear KER spectra and ascribe them to mechanisms such as ionization following a resonant dipole transition, charge-resonance-enhanced ionization (CREI) and ionization in the dissociative limit of large internuclear distances. For relatively large pulse durations, i.e., for 15 or more laser cycles at 3200 nm and 10 or more at 6400 nm, it is possible for the nuclear wave packet in H + 2 to reach very large separations. Ionization of this part of the wave packet results in peaks in the KER spectra with very low energies. These peaks give direct information about the dissociative energy in the 2pσu potential energy curve of H + 2 at the one-and three-photon resonances between the 2pσu and 1sσg curves in H + 2 . With the MCWP approach, we perform a trajectory analysis of the contributions to the KER peaks and identify the dominant ionization pathways. Finally, we consider a pump-probe scheme by applying two delayed pulses to track the nuclear dynamics in a time-resolved setting. Low-energy peaks appear for large delays and these are used to obtain the 2pσu dissociative energy values at the one-photon resonance between the 2pσu and 1sσg curves in H + 2 for different wavelengths.

Recent research interests have been raised in uncovering and controlling ultrafast dynamics in excited neutral molecules. In this work we generalize the Monte Carlo wave packet (MCWP) approach to XUV-pump-IR-probe schemes to simulate the process of dissociative double ionization of H 2 where singly excited states in H 2 are involved. The XUV pulse is chosen to resonantly excite the initial ground state of H 2 to the lowest excited electronic state of the MCWP approach, a sampling method is proposed to determine in time the dominant ionization events from H 2 to H 2 + . By conducting a trajectory analysis, which is a unique possibility within the MCWP approach, the origins of the characteristic features in the nuclear kinetic energy release spectra are identified for delays ranging from 0 to 140 fs and the nuclear dynamics in the singly excited states in H 2 is mapped out.

We apply the Monte Carlo wave-packet approach to study the interatomic Coulombic decay (ICD) dynamics of neon dimers after removing a 2s electron from one of the Ne atoms by one-photon absorption from an XUV pulse. This method reproduces well both the lifetime for the 2s inner-valence vacancy in Ne 2 + (2s −1) and the kinetic energy release (KER) spectra for the coincident Ne + and Ne 2+ fragments following triple ionization, i.e., two photoionizations and one ionization via ICD, of Ne 2 measured in an XUV-pump-XUV-probe experiment [K. Schnorr et al., Phys. Rev. Lett. 111, 093402 (2013)]. Comparisons between the calculated and measured nuclear KER spectra give physical insights in the considered process. For example, an analysis of the ratios between the low-and high-energy peaks in the nuclear KER spectra for large delays provides an estimate of the photoionization cross sections for removing a 2p electron from the excited states in Ne 2 + (2s −1). Such comparisons also allow an estimate for the ICD rates for the 2s inner-valence vacancy in the single-site two hole state in Ne 2 2+ (2s −1 2p −1). Finally, the influence of photon statistics of the free electron laser pulses on the nuclear KER spectra is considered.

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