[en] Recent experimental work has shown that the vibrational distribution of the CH⁺₄ molecular ion produced in C 1s photoionization of methane does not follow the Franck-Condon (FC) principle [1, 2]). Here we demonstrate, by explicitly including the vibrational degrees of freedom of methane, that this is due to the strong variation of the dipole-coupling matrix elements with the CH bond distance in the symmetric stretching mode.

[en] Vibrationally resolved photoelectron angular distributions from randomly oriented and fixed-in-space N₂ and CO molecules have been evaluated by using an extension of the static-exchange density functional theory that includes the nuclear motion. Both K-shell and valence-shell photoionization have been considered. Comparison with the experimental data, only available for randomly oriented molecules, is very good. Our predictions for molecular-frame photoelectron angular distributions of N₂ show the signature of electron confinement and coherent two-centre interferences as those previously found in H₂. For CO, they exhibit diffraction patterns associated with the scattering of the ejected electron by the neighbouring atomic centre. The conclusions reported in this work suggest that vibrationally resolved photoelectron angular distributions can be a useful instrument to determine structure parameters in these simple molecules.

[en] Distinct oscillations in vibrationally resolved cross section ratios for the photoionization of CH₄ from the C 1s orbital at photon energies as high as 1keV are predicted. The oscillations are attributed to the different relative vibrational excitation due to the scattering of the photoelectron by the peripheral hydrogen atoms. The latter effect is also responsible for the well known EXAFS oscillations in the integrated photoelectron spectrum. The calculations are performed with an ab-initio DFT method, as well as with a single-particle semi-analytical model, which incorporate both the effect of the nuclear recoil and of the Coulomb corrections.

[en] Vibrationally resolved cross section for the valence shell photoionization of N₂ and CO has been accurately measured experimentally up to 300 eV. Series of ratios between two vibrational cross sections (v-ratios) are compared to theoretical results obtained from a new ah initio DFT method that includes nuclear motion. It reveals strong oscillations around the Franck-Condon factor which are the fingerprints of double-slit like interferences. To support this point, a simple model (based on the Cohen-Fano formula) will be presented at the conference.

[en] A simple theoretical model is used to interpret recent experimental results for two-photon double ionization (DI) of D₂ at 38 eV. We show that the measured kinetic energy distribution associated with emission of two protons can be interpreted as a sum of two processes: a sequential and an instantaneous absorption of the two incident photons. These processes lead to peaks in different regions of the spaectrum.

[en] We present a study of the vibrationally resolved B 1s photoionization cross section of the BF_3 molecule. A combination of high-resolution photoelectron spectroscopy measurements and of state-of-the-art calculations shows the evolution of the photon energy dependence of the cross section from a complete trapping of the photoelectron wave (low energies) to oscillations due to intramolecular scattering. These diffraction patterns allow to access structural information of both the neutral molecule and the core -hole species generated upon photoabsoption. (paper)

[en] Sequential and direct two-photon double ionization (DI) of D₂ molecule is studied experimentally and theoretically at a photon energy of 38.8 eV. Experimental and theoretical kinetic energy releases of D⁺+D⁺fragments, consisting of the contributions of sequential DI via the D₂⁺(1sσ_g) state and direct DI via a virtual state, agree well with each other.

[en] Two-photon double ionization (TPDI) of D₂ is studied for 38-eV photons at the Free Electron Laser in Hamburg (FLASH). Based on model calculations, instantaneous and sequential absorption pathways are identified as separated peaks in the measured D⁺+D⁺ fragment kinetic energy release (KER) spectra. The instantaneous process appears at high KER, corresponding to ionization at the molecule's equilibrium distance, in contrast to sequential ionization mainly leading to low-KER contributions. Measured fragment angular distributions are in good agreement with theory.

[en] We used a split-mirror setup attached to a reaction microscope at the free-electron laser in Hamburg (FLASH) to perform an XUV-pump-XUV-probe experiment by tracing the ultrafast nuclear wave-packet motion in the D₂⁺(1sσ_g) with <10 fs time resolution. Comparison with time-dependent calculations shows excellent agreement with the measured vibrational period of 22±4 fs in D₂⁺, points to the importance of accurately knowing the internuclear distance-dependent ionization probability, and paves the way to control sequential and nonsequential two-photon double-ionization contributions.

[en] The differential photoionization cross section ratio (ν = 1)/(ν = 0) for the symmetric stretching mode in the C 1s photoionization of CF₄ was studied both theoretically and experimentally. We observed this ratio to differ from the Franck–Condon ratio and to be strongly dependent on the photon energy, even far from the photoionization threshold. The density-functional theory computations show that the ratio is significantly modulated by the diffraction of the photoelectrons by the neighbouring atoms at high photon energies. At lower energies, the interpretation of the first very strong maximum observed about 60 eV above the photoionization threshold required detailed calculations of the absolute partial cross sections, which revealed that the absolute cross section has two maxima at lower energies, which turn into one maximum in the cross section ratio because the maxima appear at slightly different energies in ν = 1 and ν = 0 cross sections. These two strong, low-energy continuum resonances originate from the trapping of the continuum wavefunction in the molecular potential of the surrounding fluorine atoms and from the outgoing electron scattering by them. (paper)