Trends in angle-resolved molecular photoelectron spectroscopy.

The field of angle-resolved molecular photoelectron spectroscopy is reviewed, with emphasis on foundations and most recent applications in different regimes of light-matter interaction. The basic formalism underlying one-photon electron angular distributions is presented, from the primary molecular frame (MF) photoemission i.e. emission from fully oriented molecules to laboratory frame (LF) observables produced from randomly oriented targets, extensions to multiphoton and strong field processes being briefly described, followed by a survey of current quantum mechanical computational approaches. The description of experimental developments is focused on the advancements in two major instrumentation fields for angle-resolved PES of molecules in the last two decades, namely charged-particle imaging spectrometers and adiabatically or impulsively laser-induced molecular alignment, together with their interplay with the remarkable characteristics achieved nowadays by the ionizing light sources and the challenging control of complex molecules in the gas phase. Aspects and applications of LF angular observables from unoriented targets are presented, with contemporary applications, especially as probes of the target electronic structure, including higher angular observables, in particular photoelectron circular dichroism (PECD) from chiral molecules, which is confirmed as a powerful chiral technique, and higher terms arising from multiphoton or non-dipole terms. Molecular frame photoelectron angular distributions (MFPADs), which stand out as the most complete observables of molecular photoionization stereodynamics in different excitation regimes, now broadly extended to characterize molecular structure and dynamics, are then discussed stemming from fully oriented molecules tackled by electron-ion momentum coincidence techniques, or from laser aligned samples. Finally, novel developments and challenging perspectives, notably the implementation of PAD in time-resolved schemes at ultrashort time scales, high energy, and high intensity regimes are drawn.

Isotope Effects in the Predissociation of Excited States of N2 + Produced by Photoionization of 14N2 and 15N2 at Energies Between 24.2 and 25.6 eV.

Photoelectron/photoion imaging spectrometry employing dispersed VUV radiation from the SOLEIL synchrotron has been used to study the predissociation of N2 + states located up to 1.3 eV above the ion's first dissociation limit. Branching ratios for unimolecular decay into either N2 + or N+ were obtained by measuring coincidences between threshold electrons and mass-selected product ions, using a supersonic beam of either 14N2 or 15N2 as photoionization target. The results confirm that predissociation of the C 2 Σ u + state of 14N2 + is faster than emission to the electronic ground-state by a factor 10 or more for all vibrational levels v' ≥ 3, while for 15N2 + the two decay modes have comparable probabilities for the levels v' = 3, 4, and 5. In contrast, no significant isotope effect could be observed for the other states of N2 + identified in the photoelectron spectrum. For both 14N2 + and 15N2 + isotopologues all vibrational levels of these other states decay to an extent of at least 95% by predissociation.

Evidence of depolarization and ellipticity of high harmonics driven by ultrashort bichromatic circularly polarized fields.

High harmonics generated by counter-rotating laser fields at the fundamental and second harmonic frequencies have raised important interest as a table-top source of circularly polarized ultrashort extreme-ultraviolet light. However, this emission has not yet been fully characterized: in particular it was assumed to be fully polarized, leading to an uncertainty on the effective harmonic ellipticity. Here we show, through simulations, that ultrashort driving fields and ultrafast medium ionization lead to a breaking of the dynamical symmetry of the interaction, and consequently to deviations from perfectly circular and fully polarized harmonics, already at the single-atom level. We perform the complete experimental characterization of the polarization state of high harmonics generated along that scheme, giving direct access to the ellipticity absolute value and sign, as well as the degree of polarization of individual harmonic orders. This study allows defining optimal generation conditions of fully circularly polarized harmonics for advanced studies of ultrafast dichroisms.