Molecular resolvent-operator method: Electronic and nuclear dynamics in strong-field ionization

We present an extension of the resolvent-operator method (ROM), originally designed for atomic systems, to extract differential photoelectron spectra (in photoelectron- and nuclear-kinetic energy) for diatomic molecules interacting with strong, ultrashort laser fields in the single active electron approximation. The method is applied to the study of H2+ photodissociation and photoionization by femtosecond laser pulses in the XUV-IR frequency range. In particular, the method is tested (i) in the perturbative regime, for few-photon absorption and bound-bound electronic transitions, and (ii) in the strong-field regime, in which multiphoton absorption and tunneling are present. In the latter case, we show how the differential ROM allows one to track the transition between both regimes. We also analyze isotopic effects by comparing the dynamics of H2+ and D2+ ionization for different pulses. © 2014 American Physical Society.This work was accomplished with an allocation of computer time from Mare Nostrum BSC and CCC-UAM and was partially supported by the MICINN Projects No. FIS2010- 15127 and No. CSD 2007-00010, ERA-Chemistry Project No. PIM2010EEC-00751, the European grants No. MC-ITN CORINF and No. MC-RG ATTOTREND, the European COST Action No. CM0702, and European Research Council Advanced Grant No. XCHEM 290853. R.E.F.S. acknowledges a Ph.D. contract from ITN CORINF and Grant No. SFRH/BD/84053/2012 from the Portuguese government. P.R. acknowledges a Juan de la Cierva contract grant from the Spanish MICIN

Correlated electron and nuclear dynamics in strong field photoionization of H2+

We present a theoretical study of H2+ ionization under strong IR femtosecond pulses by using a method designed to extract correlated (2D) photoelectron and proton kinetic energy spectra. The results show two distinct ionization mechanisms—tunnel and multiphoton ionization—in which electrons and nuclei do not share the energy from the field in the same way. Electrons produced in multiphoton ionization share part of their energy with the nuclei, an effect that shows up in the 2D spectra in the form of energy-conservation fringes similar to those observed in weak-field ionization of diatomic molecules. In contrast, tunneling electrons lead to fringes whose position does not depend on the proton kinetic energy. At high intensity, the two processes coexist and the 2D plots show a very rich behavior, suggesting that the correlation between electron and nuclear dynamics in strong field ionization is more complex than one would have anticipatedThis work was accomplished with an allocation of computer time from Mare Nostrum BSC and CCC-UAM, and was partially supported by the MICINN Projects No. FIS2010-15127, No. ACI2008-0777, and No. CSD 2007-00010, the ERA-Chemistry Project No. PIM2010EEC-00751, the European Grants No. MCITN CORINF and No. MC-RG ATTOTREND, the European COST Action CM0702, and the Advanced Grant of the European Research Council, Grant No. XCHEM 290853. R. E. F. S. acknowledges a Ph.D. contract from ITN CORINF. P. R. acknowledges a Juan de la Cierva contract grant from MICIN

Energy- and angle-resolved ionization of H2+ interacting with xuv subfemtosecond laser pulses

We present an extension of the resolvent operator method to extract fully differential ionization probabilities resulting from the interaction of ultrashort laser pulses with H2+ by including all electronic and vibrational (dissociative) degrees of freedom. The wave function from which ionization probabilities are extracted is obtained by solving the time-dependent Schrödinger equation in a grid for the case of H2+ oriented parallel to the polarization direction of the field. The performance of the method is illustrated by using pulses in the xuv domain. Correlated kinetic-energy (CKE) and correlated angular and nuclear kinetic-energy (CAKN) spectra have been evaluated and used to analyze the underlying mechanisms of the photoionization process. In particular, for pulses with a central frequency ω=0.8 a.u., which is smaller than the vertical ionization potential of H2+, we show the opening of the one-photon ionization channel by decreasing the pulse duration down to less than 1 fs. An analysis of the CKE and CAKN spectra allows us to visualize individual contributions from one- and two-photon ionization processes, as well as to study the variation of these contributions with pulse duration. The latter information is difficult to extract when only the kinetic energy release (KER) spectrum is measured. This points out the importance of performing multiple-coincidence measurements for better elucidation of competing ionization mechanisms, such as those arising when ultrashort pulses are usedWe gratefully acknowledge fruitful discussions with Dr. A. Palacios. This work was accomplished with an allocation of computer time from Mare Nostrum BSC and CCCUAM and was partially supported by European Research Council Advanced Grant No. XCHEM 290853, MINECO Project No. FIS2013-42002-R, ERA-Chemistry Project No. PIM2010EEC-00751, European Grant No. MC-ITN CORINF, European COST Action XLIC CM1204, and the CAM project NANOFRONTMAG. H.B. acknowledges support for mobility from ITN CORINF and is grateful for the hospitality of the Universidad Autónoma de Madrid. R.E.F.S. acknowledges FCT - Fundacao para a Ciencia e Tecnologia, Portugal, Grant No. SFRH/BD/84053/201