Core‐valence doubly ionized states: General aspects, examples, production mechanisms

Single and double ionization of the camphor molecule excited around the C 1s edge

RATIONALE

An interesting class of volatile compounds, the monoterpenes, is present in some plants although their functions are not yet fully understood. We have studied the interaction of the camphor molecule with monochromatic high-energy photons (synchrotron radiation) using time-of-flight mass spectrometry and coincidence techniques.

METHODS

A commercial sample of S-camphor was admitted into the vacuum chamber, without purification, through an inlet system. Monochromatic light with energy around the C 1s edge was generated by the TGM beamline at the Brazilian Synchrotron Facility. A Wiley-McLaren mass spectrometer was used to characterize and detect the ions formed by the camphor photoionization. The data analysis was supported by energy calculations.

RESULTS

Although the fragmentation patterns were basically the same at 270 eV and 330 eV, it was observed that above the C 1s edge the contribution to the spectrum from lower mass/charge fragment ions increased, pointing to a higher degree of dissociation of the molecule. Projections of the PEPIPICO spectra demonstrated the existence of unstable doubly charged species. The Gibbs free energy was calculated using the Møller-Plesset perturbation theory (MP2) for the neutral, singly and doubly excited camphor molecule.

CONCLUSIONS

Our PEPIPICO spectrum clearly demonstrated the formation of doubly ionic dissociative species. From a slope analysis, we propose a secondary decay after a deferred charge separation mechanism in which, after a few steps, the camphor dication dissociates into C2 H3 (+) and C3 H5 (+) . This is the main relaxation route observed at 270 eV and 330 eV. The large energy difference between the mono and the dication (of the order of 258.2 kcal/mol) may explain the experimentally observed absence of stable dications in the spectra, because their formation is disadvantaged energetically.

Auger electron angular distribution of double core-hole states in the molecular reference frame

The Linac Coherent Light Source free electron laser is a source of high brightness x rays, 2×10(11) photons in a ∼5 fs pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.

Double core-hole production in N2: beating the Auger clock

We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a time scale shorter than the core-hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.

Multielectron coincidence spectroscopy for core-valence doubly ionized states of CO

Double photoionization into states which have holes in one core and one valence orbitals has been observed in CO using a state-of-the-art multielectron coincidence method. The core-valence CO2+ structures exhibited on the electron coincidence spectra are assigned by comparison with the available calculation [H. Schulte et al., J. Chem. Phys. 105, 11108 (1996)]. Features of the spectrum confirm that the properties of the CO2+ states are characterized by the interaction between the localized valence holes and the core holes.

Effects of nuclear dynamics in the low-kinetic-energy Auger spectra of CO and CO2

The CO and CO(2) carbon and oxygen Auger spectra have been measured by electron impact and compared with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. The calculations for CO were previously published [L. S. Cederbaum et al., J. Chem. Phys. 95, 6634 (1991)], while for CO(2) they are new and presented here for the first time. For both molecules, particular attention has been paid to the low-kinetic-energy region of the spectra, which corresponds to doubly charged ion states with the two holes mainly localized in the inner valence region. New bands have been observed. It is shown that a proper consideration of the vibrational broadening and shift of the bands due to the dynamics of the nuclei is needed to assign these features. For CO, very large energy shifts between corresponding features in the C 1s and O 1s spectra have been observed, confirming the theoretical predictions of 1991. The new computed spectra of CO(2) allow a very accurate analysis of the experiments over the whole energy range.

Experimental investigation of core-valence double photoionization

Core-valence double photoionization has been observed in Ne atoms and N2 molecules using a magnetic-bottle time of flight spectrometer. The multielectron coincidence data sets give complete information on the energy correlations between all emitted electrons, which supports a detailed description of the core-valence double photoionization processes including direct spectroscopy of the core-valence doubly ionized states, the final states populated by their Auger decay and details of the dynamics of core-valence double photoionization for selected states.