David M. Hanson

Single-event, energy-resolved, Auger--electron, multiple-ion coincidence mass spectroscopy

An experimental method is described that is directed at monitoring the creation and decay of core hole states in molecules from state‐to‐state. Data in the form of a coincidence map are presented and analyzed for the case of nitrous oxide.

The origin of Stark shifts and splittings in molecular crystal spectra. I. The effective molecular polarizability and local electric field: Durene and naphthalene

The origin of the linear Stark splittings observed in spectra of polar molecules dissolved in nonpolar host crystals is described. In order to obtain molecular dipole moment parameters from such splittings, the local electric field, among other things, must be known. The effective molecular polarizability and the local electric field are not determined uniquely by the crystal geometry and electric susceptibility for the general case of two or more anisotropic molecules per unit cell. The complete infinite set of solutions to the equations relating these quantities is reported for a crystal with two molecules per unit cell. Consequently, the variation or uncertainty in local field and effective molecular polarizability can be examined for specific cases. The cases of durene and naphthalene crystals are examined here, and it is found that the local field may differ by as much as a factor of two from values used previously in the analysis of Stark splittings.

Angular dependence of photofragments from K-shell excited N2 and O2 : photoabsorption anisotropy and excited state symmetry

The symmetry of excited states reached by photoexcitation of K‐shell electrons of N2 and O2 is identified from the fragmentation anisotropy. For each molecule, ion yield excitation spectra were obtained at 90°, 54.7°, and 0° relative to the axis of soft x‐ray polarization. The degree of polarization of the synchrotron radiation was determined experimentally. From this determination and the angular dependence of the ion yield, the photoabsorption anisotropy parameter β is obtained as a function of photon energy. Variations in the β spectra are discussed in terms of previously assigned spectral features and are compared with other experimental results. A comparison also is made with theoretical calculations, which are available for the case of N2.

Stark effects on the phosphorescent states of l-indanone, l-tetralone, and 2,4,5-Trimethylbenzaldehyde

Abstract The differences in dipole moments between the ground and the phosphorescent states of l-indanone, l-tetralone, and 2,4,5-trimethylbenzaldehyde using durene and hexamethylbenzene host crystals are determined from spectral line splittings in a static electric field and from intensity modulation in an alternating field. It is concluded that the magnitude of the vector difference in dipole moments characteristic of a 3 nπ * excited state for l-indanone is −2.2 ± 0.1 D and for l-tetralone is −1.8 ± 0.1 D while the difference characteristic of a 3 ππ * excited state for trimethylbenzaldehyde is ± 1.1 ±0.1 D.

The effective molecular polarizability and local electric field in molecular crystals

Abstract The problem of calculating the effective molecular polarizabilities and the local electric fields in molecular crystals is considered. It is shown that in a crystal with two or more anisotropic molecules per unit cell, these quantities cannot be found from the crystal structure and the dielectric tensor alone.

Core‐hole excited states of H2O: Symmetries and relative oscillator strengths

Tunable and polarized soft x‐ray radiation was used to excite core electrons of H2O. Angle‐resolved proton yields were measured by retarding potential analyzers at 0° and 90° with respect to the polarization vector of the radiation. The ability of angle‐resolved detection to reveal hidden resonances in rich molecular spectra is demonstrated, and the anisotropy parameter obtained from the two ion yields identifies the symmetries and the relative oscillator strengths of the core‐hole excited states in the near‐edge region of the spectrum. The analysis of the data substantiates the validity of the axial recoil approximation and provides evidence for ultrafast dissociation. Comparisons with the results of theoretical calculations are made.

Evidence for valence hole localization in the Auger decay and fragmentation of carbon and silicon tetrafluorides

Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and fluorine atoms in carbon tetrafluoride and silicon and fluorine in silicon tetrafluoride. The fragmentation processes were examined using time‐of‐flight mass spectroscopy. The mass spectra show the distribution of ions collected in coincidence with low and high energy electrons. Distinct changes in the mass spectra with atomic site of excitation and photon energy are observed. The observation of F2+ ions following fluorine 1s excitation in SiF4 provides significant evidence for a ‘‘valence bond depopulation’’ mechanism involving the formation of a localized, two‐hole final state that persists on the time scale of fragmentation. In contrast, no F2+ was observed for CF4, which indicates that fragmentation for this molecule is more characteristic of a delocalized two‐hole state.

The ionic states of molecular crystals

Abstract The electronic states generally characterize solids as insulators, semiconductors, or metals,1 but in some respects organic molecular crystals are an exception to ths scheme. To describe the lower energy states of these solids, the concepts of a valence band and a conduction band are not very useful because the electrons are hghly localized. These states are described better by the Frenkel tight-binding model in which molecular states are the progenitors of crystal states.2 Generally at higher energies, crystal states not present in the free molecule are expected.3 These states, the ionic states, consist of an electron and an ionized molecule. Such states may be similar to the exciton and conducting states of other solids or may be somewhat unique to the molecular solid, e.g., ion pair states in which an electron and hole are localized on neighboring lattice sites. In spite of the apparent importance of the ionic states in understanding electron and energy transfer phenomena in molecular aggregat...

Dipole moments of the lowest excited singlet and triplet states of 2,4,5‐trimethylbenzaldehyde in a durene host crystal

From Stark spectra, the differences in dipole moments between the ground state and the lowest singlet and triplet excited states of 2,4,5‐trimethylbenzaldehyde isolated in a durene host crystal are determined within a precise Lorentz local field approximation to be 1.65 ± 0.09 D and 1.05 ± 0.06 D, respectively, for the most prominent sites. Neither dipole moment change is parallel to the carbonyl axis, but the change for the singlet state has the largest component along this axis while that for the triplet state does not. These data are consistent with the assignment of these states as primarily 1nπ* and 3ππ*. The dipole moment of the ground state was determined to be 3.53 ± 0.05 D from dielectric constants of benzene solutions.

Optical luminescence excitation spectra of molecular oxygen in the soft x-ray region

The observation of an anomaly in the optical luminescence excitation spectrum of oxygen in the region of the oxygen K edge is reported. Dispersed luminescence spectra were obtained for x‐ray excitation at the pi and sigma resonances, at the anomaly, and in the continuum. These spectra indicate enhanced production of O2+2 ions at both the sigma resonance and at the anomaly. The anomaly thus is attributed to a shake‐up or shake‐off state associated with an antibonding sigma molecular orbital of oxygen. This work also demonstrates that optical luminesence spectra provide state‐specific information about the products of core hole excitation and relaxation.