Fragmentation pathways of H+(H2O)2 after extreme ultraviolet photoionization.

Photofragmentation of the protonated water dimer H+(H2O)_{2}, a fundamental system both in aqueous solutions and gas-phase water clusters, has been studied at 13.8 nm using the Free Electron Laser FLASH in Hamburg. In a crossed-beam experiment using time-resolved, single-molecule fragment imaging, the two-body breakup into H2O++H3O+ was found as a prominent fragmentation channel with a kinetic energy release of up to 10 eV. This channel was observed with at least a similar yield as events with stronger fragmentation, producing protons together with neutral fragments and showing an absolute cross section of (0.5 ± 0.2) × 10(-18) cm2.

Combining high mass resolution and velocity imaging in a time-of-flight ion spectrometer using pulsed fields and an electrostatic lens.

We describe a momentum resolving time-of-flight ion mass spectrometer that combines a high mass resolution, a velocity focusing condition for improved momentum resolution, and field-free conditions in the source region for high resolution electron detection. It is used in electron-ion coincidence experiments to record multiple ionic fragments produced in breakup reactions of small to medium sized molecules, such as F(3)SiCH(2)CH(2)Si(CH(3))(3). These breakup reactions are caused by soft x rays or intense laser fields. The ion spectrometer uses pulsed extraction fields, an electrostatic lens, and a delay line detector to resolve the position. Additionally, we describe a simple analytical method for calculating the momentum from the measured hit position and the time of flight of the ions.

Performance of a short "magnetic bottle" electron spectrometer.

In this article, a newly constructed electron spectrometer of the magnetic bottle type is described. The instrument is part of an apparatus for measuring the electron spectra of free clusters using synchrotron radiation. Argon and helium outer valence photoelectron spectra have been recorded in order to investigate the characteristic features of the spectrometer. The energy resolution (E/ΔE) has been found to be ∼30. Using electrostatic retardation of the electrons, it can be increased to at least 110. The transmission as a function of kinetic energy is flat, and is not impaired much by retardation with up to 80% of the initial kinetic energy. We have measured a detection efficiency of most probably 0.6(-0.1) (+0.05), but at least of 0.4. Results from testing the alignment of the magnet, and from trajectory simulations, are also discussed.

Separating the vibrationally resolved Auger decay channels for a CO core hole state.

The K-VV Auger spectrum of carbon monoxide (CO) excited by C 1s photoionization has been investigated with a novel electron-electron coincidence setup. The energy resolution is sufficiently high to resolve the vibrational energy levels of the core-ionized intermediate state and of most dicationic final states in the two-dimensional electron energy map. We demonstrate how the influence of vibrational states on a molecular Auger spectrum can be accessed experimentally without the constraint of averaging over all intermediate state energies.

Study of the dissociation of nitrous oxide following resonant excitation of the nitrogen and oxygen K-shells.

A photochemistry study on nitrous oxide making use of site-selective excitation of terminal nitrogen, central nitrogen, and oxygen 1s-->3pi excitations is presented. The resonant Auger decay which takes place following excitation can lead to dissociation of the N2O+ ion. To elucidate the nuclear dynamics, energy-resolved Auger electrons were detected in coincidence with the ionic dissociation products, and a strong dependence of the fragmentation pathways on the core-hole site was observed in the binding energy region of the first satellite states. A description based on the molecular orbitals as well as the correlation between the thermodynamical thresholds of ion formation and the first electronic states of N2O+ has been used to qualitatively explain the observed fragmentation patterns.

Ionization of linear alcohols by strong optical fields.

We have experimentally probed the strong-field ionization dynamics of gas-phase linear alcohols, methanol, ethanol, and 1-propanol, by irradiating them with intense, femtosecond-duration laser pulses of 800 and 400 nm wavelength. Specifically, we make high resolution measurements of the energies of electrons that are ionized by the action of the optical field. Our electron spectroscopy measurements enable us to bifurcate the dynamics into multiphoton ionization and tunneling ionization regimes. In the case of 800 nm irradiation, such bifurcation into different ionization regimes is reasonably rationalized within the framework of the adiabaticity parameter based on the original Keldysh-Faisal-Reiss model of atomic ionization, without recourse to any structure-dependent modifications to the theory. In that sense, our 800 nm spectroscopy indicates that the linear alcohols exhibit atom-like properties as far as strong field ionization dynamics in the multiphoton ionization and tunneling regimes are concerned. We also explore the limitations of this atom-like picture by making measurements with 400 nm photons wherein the ponderomotive potential experienced by the ionized electrons is much less than the photon energy; effects that are purely molecular then appear to influence the strong field dynamics.

Charging mechanisms of trapped element-selectively excited nanoparticles exposed to soft x rays.

Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.

Experimental evidence of interatomic coulombic decay from the auger final states in argon dimers.

Interatomic Coulombic decay (ICD) from an Auger-final dicationic state is observed in the Ar dimer. A 2p inner-shell vacancy created by photoionization is replaced with 3s and 3p vacancies via intra-atomic Auger decay. The Auger-final dicationic state is subject to ICD in which one of the 3p electrons in the same Ar atom fills the 3s vacancy while one of the 3p electrons from the neighboring Ar atom is emitted as an ICD electron. This ICD process is unambiguously identified by electron-ion-ion coincidence spectroscopy in which the kinetic energy of the ICD electron and the kinetic energy release between Ar+ and Ar2+ are measured in coincidence.

Symmetry-dependent vibrational excitation in N 1s photoionization of N2: experiment and theory.

We have measured the vibrational structures of the N 1s photoelectron mainline and satellites of the gaseous N2 molecule with the resolution better than 75 meV. The gerade and ungerade symmetries of the core-ionized (mainline) states are resolved energetically, and symmetry-dependent angular distributions for the satellite emission allow us to resolve the Sigma and Pi symmetries of the shake-up (satellite) states. Symmetry-adapted cluster-expansion configuration-interaction calculations of the potential energy curves for the mainline and satellite states along with a Franck-Condon analysis well reproduce the observed vibrational excitation of the bands, illustrating that the theoretical calculations well predict the symmetry-dependent geometry relaxation effects. The energies of both mainline states and satellite states, as well as the splitting between the mainline gerade and ungerade states, are also well reproduced by the calculation: the splitting between the satellite gerade and ungerade states is calculated to be smaller than the experimental detection limit.

Asymmetry in photoelectron emission from chiral molecules induced by circularly polarized light.

In photoionization of free, unoriented chiral molecules with circularly polarized radiation, a significant circular dichroism, i.e., an asymmetry in the forward-backward electron emission, has been observed in the photoelectron angular distribution. This leads also to an asymmetry in the momentum transfer to the photoions. The spectra for the left- and right-handed enantiomers of bromocamphor exhibit asymmetries up to several percent which vary as a function of orbital binding energy. This enantioselective effect can similarly occur for biomolecules with handedness, like amino acids, and may thus be a contributing factor related to the origin of the terrestrial biomolecular homochirality.