D. Rolles

Measurements of molecular-frame Auger electron angular distributions at the CO C 1s−1 2π* resonance with high energy resolution

The molecular-frame Auger electron angular distributions (MFAEADs) of resonantly excited CO 1s → π molecules in the gas phase were determined with high energy resolution using a novel experimental approach. We investigated the process of excitation, Auger decay and fragmentation in unprecedented detail. We confirmed theoretical predictions for the different MFAEADs of close lying Auger final states. By examining the dependence of the MFAEADs on the vibrational state of the excitation and on the fragmentation energy we found that the measured MFAEADs can be considered independent of the vibrational excitation and the fragmentation process. The method used to obtain molecular-frame angular distributions of Auger electrons is based on electron–ion coincidence measurements using two high-resolution electron spectrometers with limited acceptance angles mounted at fixed positions.

Photoionization of argon clusters in the Ar 3s → np Rydberg resonance region

The photoionization of argon clusters with an average size of 270 atoms per cluster was investigated in the vicinity of the Ar 3s → np autoionizing resonances using angle-resolved time-of-flight photoelectron spectroscopy. The Ar 3p outer-valence photoelectron spectra were measured as two-dimensional maps that show the resonance profiles for free atoms as well as for the cluster. For the first time, partial photoelectron yields and photoelectron angular distributions for the two spin–orbit components in argon clusters are reported as a function of the photon energy. The angular distributions of cluster photoelectrons differ substantially from the atomic ones and, moreover, allow identification of bulk and surface resonances.

Theoretical and experimental demonstrations of the existence of quadrupole Cooper minima

Calculations and measurements of the Xe 5s and 5p nondipole photoelectron asymmetry parameters are obtained, which present clear evidence of the existence of quadrupole Cooper minima, i.e., minima in quadrupole matrix elements as a function of energy, in the photoionization process. This verifies earlier predictions of quadrupole Cooper minima.

Size effects in van der Waals clusters studied by spin and angle-resolved electron spectroscopy and multi-coincidence ion imaging

We have studied the valence and inner-shell photoionization of free rare-gas clusters by means of angle and spin resolved photoelectron spectroscopy and momentum resolving electron-multi-ion coincidence spectroscopy. The electron measurements probe the evolution of the photoelectron angular distribution and spin polarization parameters as a function of photon energy and cluster size, and reveal a strong cluster size dependence of the photoelectron angular distributions in certain photon energy regions. In contrast, the spin polarization parameter of the cluster photoelectrons is found to be very close to the atomic value for all covered photon energies and cluster sizes. The ion imaging measurements, which probe the fragmentation dynamics of multiply charged van der Waals clusters, also exhibit a pronounced cluster size dependence.

Molecular-frame angular distribution of normal and resonant Auger electrons

Molecular-frame angular distributions (MFADs) of resonant and non-resonant (normal) Auger electrons were determined using angle-resolved electron-ion coincidence spectroscopy. Here, we concentrate on the MFADs of normal N2 KVV Auger electrons. We conclude that their MFADs are independent of both photon energy and light polarization direction, thus confirming that the two-step model is a good approximation. All threshold and shape-resonance phenomena can be attributed to the absorption process and are irrelevant for the Auger decay in the molecular frame. Distinct differences in the MFADs are found as a function of the Auger final state.

Shape resonances in inner-shell photodetachment of transition metal negative ions

Inner-shell photodetachment of transition metal negative ions was studied using photoion spectroscopy. We focused on shape resonances, their behavior, and their decay mechanisms produced by np → (nd+d) (n = 3, 4, 5) photoexcitation for Fe, Ru, Ni, Pd, and Pt transition metal negative ions.

Core-shell and valence-shell photoionization of argon clusters

Core- and valence-shell photoionization of variable size argon clusters were investigated using angle-resolved time-of-flight photoelectron spectroscopy. The Ar 3p outer-valence and Ar 3s inner-valence photoelectron spectra were measured as two formats: individual spectra and two-dimensional maps. The Ar 2p core-shell photoelectron spectra were measured only in individual spectra format. The photoelectron angular distribution parameters of Ar 3p, 3s, and 2p clusters as a function of photon energy and cluster size were determined. In the case of the 2p-shell, measurements were also spin-orbit resolved. Argon cluster with cluster sizes =70 and 210-250 were studied.