Direct and indirect methods for studying the energetics and dynamics of the Auger Doppler effect in femtosecond ultra-fast dissociation

Ultrafast dissociation of ammonia: Auger Doppler effect and redistribution of the internal energy

We study vibrationally-resolved resonant Auger (RAS) spectra of ammonia recorded in coincidence with the NH₂⁺ fragment, which is produced in the course of dissociation either in the core-excited 1s^-14a11 intermediate state or the first spectator 3a^-24a11 final state. Correlation of the NH₂⁺ ion flight times with electron kinetic energies allows directly observing the Auger-Doppler dispersion for each vibrational state of the fragment. The median distribution of the kinetic energy release E_KER, derived from the coincidence data, shows three distinct branches as a function of Auger electron kinetic energy E_e: E_e + 1.75E_KER = const for the molecular band; E_KER = const for the fragment band; and E_e + E_KER = const for the region preceding the fragment band. The deviation of the molecular band dispersion from E_e + E_KER = const is attributed to the redistribution of the available energy to the dissociation energy and excitation of the internal degrees of freedom in the molecular fragment. We found that for each vibrational line the dispersive behavior of E_KERvs. E_e is very sensitive to the instrumental uncertainty in the determination of E_KER causing the competition between the Raman (E_KER + E_e = const) and Auger (E_e = const) dispersions: increase in the broadening of the finite kinetic energy release resolution leads to a change of the dispersion from the Raman to the Auger one.

Double-core-hole spectroscopy for chemical analysis with an intense X-ray femtosecond laser

Theory predicts that double-core-hole (DCH) spectroscopy can provide a new powerful means of differentiating between similar chemical systems with a sensitivity not hitherto possible. Although DCH ionization on a single site in molecules was recently measured with double- and single-photon absorption, double-core holes with single vacancies on two different sites, allowing unambiguous chemical analysis, have remained elusive. Here we report that direct observation of double-core holes with single vacancies on two different sites produced via sequential two-photon absorption, using short, intense X-ray pulses from the Linac Coherent Light Source free-electron laser and compare it with theoretical modeling. The observation of DCH states, which exhibit a unique signature, and agreement with theory proves the feasibility of the method. Our findings exploit the ultrashort pulse duration of the free-electron laser to eject two core electrons on a time scale comparable to that of Auger decay and demonstrate possible future X-ray control of physical inner-shell processes.

Dissociation of core-valence doubly excited states in NO followed by atomic Auger decay

The decay processes of core-valence doubly excited states near the N K edge of NO have been studied using electron spectroscopy. Electron yields measured as a function of photon energy and kinetic energy enable the clear identification of atomic Auger lines associated with the dissociation of doubly excited states. The atomic Auger lines exhibit Doppler profiles, allowing the entire reaction scheme of such dissociation processes to be determined.

Core excitation in O3 localized to one of two symmetry-equivalent chemical bonds: molecular alignment through vibronic coupling

Core excitation from terminal oxygen OT in O3 is shown to be an excitation from a localized core orbital to a localized valence orbital. The valence orbital is localized to one of the two equivalent chemical bonds. We experimentally demonstrate this with the Auger-Doppler effect which is observable when O3 is core excited to the highly dissociative OT1s(-1)7a1 1 state. Auger electrons emitted from the atomic oxygen fragment carry information about the molecular orientation relative to the electromagnetic-field vector at the moment of excitation. The data together with analytical functions for the electron-peak profiles give clear evidence that the preferred molecular orientation for excitation only depends on the orientation of one bond, not on the total molecular orientation. The localization of the valence orbital "7a1" is caused by mixing of the valence orbital "5b2" through vibronic coupling of antisymmetric stretching mode with b2 symmetry. To the best of our knowledge, it is the first discussion of the localization of a core excitation of O3. This result explains the success of the widely used assumption of localized core excitation in adsorbates and large molecules.

Mass spectrometry study of the fragmentation of valence and core-shell (Cl 2p) excited CHCl3 and CDCl3 molecules

The dissociative photoionization of the chloroform and chloroform-d molecules has been studied in the valence region and around the chlorine 2p edge. Time-of-flight mass spectrometry in the coincidence mode-namely, photoelectron-photoion coincidence (PEPICO)-was employed. He I lamp and tunable synchrotron radiation were used as light sources. Total and partial ion yields have been recorded as a function of the photon energy. Singly, doubly, and triply ionized species have been observed below (195 eV), on (201 eV), and above (230 eV) the Cl 2p resonances. A definite degree of site-selective fragmentation was observed at the Cl 2p resonance as the relative contributions of several ionic species were seen to go through a maximum at 201 eV. At the same time all stable doubly charged ions were also observed at 198 eV (below the 2p resonances), resulting from direct ionization processes. Isotopic substitution is shown to provide a very efficient means of improving the mass resolution and assignment of unresolved peaks in spectra of CHCl(3), particularly for those fragments differing by a hydrogen atom. It is suggested that ultrafast fragmentation of the system following 2p excitation to a strongly antibonding state contributes to the large amount of Cl(+) observed in the PEPICO spectrum measured at 201 eV. Kinetic energy distributions were determined for the H(+), D(+), and Cl(+) fragments.