New features are revealed in the low-energy photoionization spectrum of Ar by critically combining high photon resolution and differential photoelectron spectroscopic techniques. Two LS-forbidden doubly excited resonances are seen in the 3p(-1)(3/2, 1/2) partial cross sections which exhibit mirroring profiles, resulting in complete cancellation in the total photoionization cross section, as was predicted by Liu and Starace [Phys. Rev. A 59, R1731 (1999)]. These results demonstrate that a new class of weakly spin-orbit induced, mirroring resonances should be observable in partial, but not in total, collisional cross sections involving atoms, molecules, and solids in general.

Photoelectron spectra in the vicinity of the 2p3/2,1/2 → σ* dissociative resonances in HCl and DCl have been recorded using two-dimensional photoelectron spectroscopy. The comprehensive data suggest that the explanation for the negative spectral contribution observed in an atomic line, which arises from autoionization following dissociation, might be more complex than the one proposed by Feifel et al (2000 Phys. Rev. Lett. 85 3133). These authors attributed this feature solely to continuum–continuum interference between resonant atomic and molecular Auger contributions, the latter occurring prior to complete dissociation, but resulting in an electron with the same kinetic energy as one produced in the atomic decay. The new data reveal a similar spectral feature in a region of the spectrum where this type of interference cannot occur. Consequently, an interference mechanism that does not require the energy of the resonant Auger electron be the same whether it is emitted before or after the molecule dissociates is indicated, such as the one discussed by Pahl et al (1998 Phys. Rev. Lett. 80 1865). Alternatively, the two-dimensional resonant enhancements, observed in the region of the negative spectral contribution, could be attributed to Auger decay to multiple final ion states, before the dissociating molecule has fragmented completely.

Many new resonances in neon have been discovered by virtue of their LS-forbidden mirroring partial cross section profiles. In particular, all six of the previously undetected 2s22p4(3P)3s([2,4]P)3p(3[S, P, D] 1) resonances have been observed. These results confirm our earlier prediction (Canton-Rogan et al 2000 Phys. Rev. Lett. 85 3113) that mirroring triplet resonances are observable in partial cross sections where continuum spin–orbit effects exist, and suggest that future experimental studies of other atoms will also reveal new resonances.

The triplet character of doubly excited 2lnl' resonances close to the He+ (N = 2) ionization threshold has been seen to affect the angular distribution of photoelectrons associated with the He+ (1s) state: a peak has been observed at a photon energy just below the N = 2 threshold when the photoelectrons are detected at 90° with respect to the photon polarization direction. This is forbidden in pure LS-coupling and therefore the observation immediately indicates the presence of triplet configurations due to spin–orbit interactions. Such weak interaction effects are usually overwhelmed by LS-allowed processes, particularly in such a low-Z atom, but the choice of observation angle in this case desensitizes the measurements to the dominant ionization processes. The results have been reproduced by R-matrix MQDT calculations which also indicate that all seven relativistically allowed Rydberg series are present.

Angle-resolved Auger and valence photoelectron spectra were measured over a 14-eV photon energy range across the Cl 2 2p ionization thresholds. The measurements were carried out using highly efficient time-offlight spectrometers coupled with photons from the Atomic and Molecular undulator beamline of the Advanced Light Source and an advanced data-acquisition system. Auger-electron spectra of 2p→* and 2p→nl resonances were analyzed and the evolution of the resonant Auger to the normal Auger decay distorted by postcollision interaction was examined. We find that valence photoionization channels do not resonate strongly at the photon energies of the core-to-Rydberg excitation, in contrast to the strongly resonating ones observed in the HCl molecule. Auger decay spectra of the 2p Ϫ1 * resonances showed no evidence of atomic transitions in Cl*, also in contrast to HCl. In addition, angular distribution of the photoelectron and Auger-electron lines was derived.