Antony A. Wills

High-resolution angle-resolved measurements in atoms and molecules using advanced photoelectron spectroscopy at the ALS

Abstract A two-dimensional photoelectron imaging technique using time-of-flight spectrometry and a high-resolution SES-200 electron energy analyzer have been used with photons from the atomic and molecular undulator beamline of the Advanced Light Source to study the structure and dynamics of atoms and molecules. The analyzers are rotatable around the direction of the photon beam, allowing angular-dependent studies to be performed. Results on the photoexcitation and decay mechanisms of doubly excited resonances in neon illustrate the importance of spin-orbit effects for such a light atom, since significant breakdown of LS-coupling was measured for such a light atom . Also, fully vibrationally resolved Auger electron spectra measured under Resonant Raman conditions following the C 1s→π* excitation in a diatomic molecule, CO, is presented which allows us to probe the core excited states, their excitation and de-excitation dynamics with unprecedented resolution.

Auger decay of the C 1s−12π* resonance in carbon monoxide: Vibrationally and angularly resolved spectra

Auger electron spectra from the decay of the ν=0, 1, and 2 levels of the C 1s−12π* state were measured with sufficiently high electron and photon energy resolution to completely resolve the vibrational structure of the final electronic states. The results are compared with ab initio calculations with emphasis on the analysis of the spectator Auger transitions. The anisotropy parameters extracted from angle-resolved spectra show variations within the vibrational envelopes of the participator Auger decay and exhibit a complex oscillating behavior for the spectator Auger transitions. In addition, two-dimensional (2D) imaging of the electron emission across the C 1s−12π* resonance was performed. The ratio of resonant and nonresonant ionization processes is estimated from the 2D map. Strong vibrational lifetime interference effects apparent in the 2D imaging are discussed.

An investigation of dissociative resonant photoionization in HCl and DCl using two-dimensional photoelectron spectroscopy

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.

A study of vibrational selectivity of electronic autoionization processes in using two-dimensional photoelectron spectroscopy

Two-dimensional photoelectron spectroscopy has been used to investigate vibrational selectivity accompanying electronic autoionizing decays of Rydberg states between the and ionization thresholds in . Measurements of electron yield as a function of both electron and photon energy have been carried out using tuneable synchrotron radiation. This study, which is the most comprehensive to date, has enabled the vast majority of the accessible vibrational levels of the ion to be investigated. The experimental resolution was good enough to observe individual vibrational levels and complex vibrational selectivity was evident throughout the two-dimensional spectrum. A significant number of the features in the spectrum could be ascribed to vibrational progressions in the symmetric stretch mode.

The interpretation and application of two-dimensional photoelectron spectroscopy

Abstract The technique of two-dimensional photoelectron spectroscopy, in which photoelectron yield is measured as a function of both electron and photon energy, is explained. The appearance of autoionising resonance features is modelled under a variety of experimental conditions. Although two-dimensional spectra normally provide a large amount of detailed information, it is often necessary to extract one-dimensional spectra in order to perform comparisons with other work. The extraction procedure is considered using the specific example of constant transition energy spectra, which indicate the population of particular ion states as a function of photon energy. In addition, experimental effects that are readily visible in two-dimensional spectra, but which may be hard to identify in one-dimensional measurements, are highlighted. Representative two-dimensional spectra are presented in order to illustrate these points with real examples.

A study of autoionizing decay routes of doubly excited states in helium using two-dimensional photoelectron spectroscopy

The autoionizing decay routes of doubly excited, , Rydberg states in helium have been investigated using two-dimensional photoelectron spectroscopy. Constant ionic state spectra for the (N = 4,5 and 6) states have been extracted from two-dimensional photoelectron spectra obtained in the photon energy region 75.5 - 79 eV. A preference was observed for autoionizing decays in which the change in the principal quantum number, N, of the electron which remains bound was minimized. This is in accordance with propensity rules determined from theoretical studies. The energies of the lower members of the most prominent Rydberg series converging on each of the (N = 5) and (N = 6) thresholds have been determined.

New low-lying mirroring singly and doubly excited resonances in neon

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.

Coincident energy and angular distributions in xenon 4d5/2 inner-shell double photoionization

We report on measurements of the triply differential cross section for the 4d5/2 inner-shell photoionization in xenon followed by N5O2,3O2,3 Auger decay using electron–electron coincidence spectroscopy. The experimental setup made it possible to obtain the first coincident angular distributions for the (1D2) and (3P2) final states at a photon energy of 97.45 eV. Relative amplitudes and phases describing the photoionization were estimated from these angular distributions.

Inner-shell photoionization of molecules using a two-dimensional imaging technique

Angle resolved two-dimensional photoelectron spectroscopy has been used to study resonances corresponding to inner-shell excitation in hydrogen halide molecules. In HBr 3d → σ* excitations have been studied, whilst in HCl and DCl 2p → σ* resonances have been explored. In HBr the photon energy positions of the two 3d5/2,3/2 → σ* resonances have been measured directly. Atomic Br is produced following dissociation of the σ* state and the asymmetric photon energy profile of Br atomic Auger lines is discussed. Angular distribution parameters of Br Auger lines have also been measured. In HCl and DCl the comprehensive nature of the analogous 2DPES reveal unexpected spectral features, the origin of which is considered.

High-resolution photoelectron spectroscopy in atoms and molecules

New features are revealed by critically combining high photon resolution from the Advances Light Source and differential photoelectron spectroscopic techniques. Two LS-forbidden doubly-excited resonances have been observed in the 3p3/2,1/2−1 partial cross-sections of Ar 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)]. We will also discuss recent preliminary measurements on the vibrational structure and partial rates of resonant Auger decay of the N 1s→2π core excitation in nitric oxide.