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The dehydrogenation of semiconducting boron carbide (B(10)C(2)H(x)) films as well as the three closo-carborane isomers of dicarbadodecaborane (C(2)B(10)H(12)) and two isomers of the corresponding closo-phosphacarborane (PCB(10)H(11)) all appear to be very similar. Photoionization mass spectrometry studies at near-threshold gas phase photoionization indicate that the preferred pathway for dissociation of the parent cation species (C(2)B(10)H(10)(+) or PCB(10)H(9)(+)) is, in all cases, the loss of H(2). Ab initio density functional theory (DFT) calculations indicate that energetically preferred sites for exopolyhedral hydrogen (B-H) bond dissociation are in all cases at B atoms opposite the C atoms in the parent cage molecule. The site of photodissociation of hydrogen from semiconducting boron carbide (B(10)C(2)H(x)) films, fabricated by plasma-enhanced chemical vapor deposition, is a cage boron atom that can bond to nitrogen upon exposure to VUV light in the presence of NH(3). Shifts in core level binding energies due to nitrogen bond formation indicate that B-N bond formation occurs only at B atoms bound to other boron atoms (B-B sites) and not at B-C sites or at C sites, in agreement with gas phase results.

The emission of ultraviolet fluorescence radiation from variable size argon clusters is investigated with high spectral resolution in the Ar 2p-excitation regime. The fluorescence excitation spectra reveal strong fluorescence intensity in the Ar 2p-continuum, but no evidence for the occurrence of discrete low-lying core-exciton states in the near-edge regime. This finding is different from the absorption and photoionization cross section of argon clusters and the solid. The dispersed fluorescence shows a broad molecular band centered near 280 nm. The present work indicates that double and triple ionization via the LMM-Auger decay are required to initiate the fluorescence processes in the Ar 2p-continuum. The present results are consistent with the formation of singly charged, excited moieties within the clusters, which are assigned as sources of the radiative emission in the 280 nm regime. A fast energy transfer process (interatomic Coulombic decay (ICD)), which has been proposed by recent theoretical work, is assigned to be primarily the origin of these singly charged, excited cations besides intra-cluster electron impact ionization by the Auger electrons. Our findings give first possible experimental evidence for ICD in the core level regime.

Following a previous experimental study of the photostability of a series of polycyclic aromatic hydrocarbons (PAHs) this work extends to irregularly shaped PAHs, methyl-substituted PAHs, and related compounds. A photostability index R has been devised, which measures the propensity for H-loss of PAH monocations relative to that for regular catacondensed or pericondensed PAHs containing the same number of atoms. This index is also valid, to a good approximation, for neutral PAHs. Structural effects (presence of a methyl group; dihydro groups; H-H steric hindrance; degree of π conjugation; isomers) and size-dependent effects on R are demonstrated and discussed. Astrophysical applications of the propensity of PAH photodissociation include an estimation of the maximum amount of energy that can be deposited in the nascent parent PAH ion in H I regions, and an evaluation of the lower limit to the size of photostable PAHs in these regions, for various types of PAHs and their derivatives. The results provide a possible explanation for the existence of a limited number of PAHs in the ISM as required by the interstellar carbon budget determined by Snow & Witt. The information obtained is also relevant to interpretation of the presence and formation of PAHs and their methyl derivatives in meteoritic, micrometeoritic, cometary, and interplanetary particle materials.

Three-component aggregates obtained by expansion of boron trifluoride dimethyl etherate and an aromatic compound have been studied by electron impact and photoionization mass spectrometry. The appearance potentials of three-component aggregates and of boron trifluoride dimethyl etherate clusters have been measured by using monochromatized synchrotron radiation. Different fragmentation processes and chemical intra-cluster reactions have been observed. [BF3(O(CH3)2)]+ is unstable but stable ionic aggregates [(BF3)n(O(CH3)2)m]+ were detected if m > n.

The ionization energies of chlorine oxide (ClO) and its dimer (Cl2O2) have been measured using monochromatic synchrotron radiation in the 10−20 eV energy regime in combination with photoionization mass spectrometry. The threshold energy of ClO+ (m/z = 51) is found at 10.85 ± 0.05 eV, whereas the ionization threshold of Cl2O2+ (m/z = 102) occurs at 11.05 ± 0.05 eV. The experimental values are compared to results from ab initio calculations, where three stable isomers of Cl2O2 are considered: dichlorine peroxide (ClOOCl), chloryl chloride (ClClO2), and chlorine chlorite (ClOClO). The results indicate that the experimental threshold energy of Cl2O2+ is due to adiabatic ionization of ClOOCl. The heat of formation of ClOOCl+ is derived from the ionization threshold: ΔfH0298(ClOOCl+) = 1203 ± 12 kJ/mol. Evidence for the occurrence of less stable Cl2O2 isomers, such as ClClO2 and ClOClO, is obtained from appearance energy measurements of the OClO+ (m/z = 67) fragment cation. The shift in ionization energy of C...

Deuterium isotope effects on the photofragmentation thresholds of naphthalene monocations are studied by photoion mass spectrometry using variable-energy monochromatized synchrotron radiation to photoionize C 10 H 8 and C 10 D 8 . Kinetic isotope shifts (d-h) in the range 30-780 meV were observed for seven fragmentation reactions in the 15-20-eV region. The physicochemical factors leading to kinetic isotope effects are discussed. The observed kinetic isotope shifts are rationalized, mainly in terms of the RRKM/QET model and the specific reaction mechanism pathways

Deuterium isotope effects on the photofragmentation thresholds of naphthalene monocations are studied by photoion mass spectrometry using variable-energy monochromatized synchrotron radiation to photoionize C 10 H 8 and C 10 D 8 . Kinetic isotope shifts (d-h) in the range 30-780 meV were observed for seven fragmentation reactions in the 15-20-eV region. The physicochemical factors leading to kinetic isotope effects are discussed. The observed kinetic isotope shifts are rationalized, mainly in terms of the RRKM/QET model and the specific reaction mechanism pathways

The photochemical behaviour of gaseous N2O5, HNO3, CINO3 and BrNO3 has been investigated in the energy range 10–20 eV by photoionization mass spectrometry using monochromatized synchrotron radiation. The decay reactions are analyzed using the photoion yield curves of fragment ions and their appearance energies. Hitherto unknown photochemical reactions and thermochemical data have been evaluated.

Ultraviolet-visible fluorescence $(300 \mathrm{nm}l~\ensuremath{\lambda}l~500 \mathrm{nm})$ of argon is investigated in the Ar $2p$ excitation regime $(240 \mathrm{eV}l~El~260 \mathrm{eV})$ using monochromatic synchrotron radiation for primary excitation. Fluorescence excitation spectra and dispersed fluorescence spectra at constant photon energy are reported as well as time-resolved dispersed fluorescence profiles. Various radiative processes are assigned, indicating that these processes are primarily due to the relaxation of ${\mathrm{Ar}}^{+}.$ A distinct state-selective behavior is found for some fluorescence excitation spectra, whereas others show no state selectivity at all. Time-resolved experiments give further insight into this state-selectivity and excited-state lifetimes. The results indicate that excited cation states are either directly formed as a result of primary Ar $2p$ excitation and subsequent electronic relaxation or via subsequent radiative cascades. The present results are consistent with earlier work on resonant Auger spectroscopy, extending the dynamic range of processes occurring in the Ar $2p$ regime into the nanosecond time regime.

The photostability of a series of polycyclic aromatic hydrocarbons has been studied experimentally by determining the internal energy Eint of their monocations at which the dissociation rate is 104 S−1. The results on the hydrogen atom loss reaction, fitted to an RRK model calculation, were then scaled to determine the internal energy Ecrit at the astrophysically critical dissociation rate 102 s−1. Data were also obtained on H2 and C2H2 loss channels. The quasi‐linear dependence of Eint and Ecrit on three PAH size parameters was demonstrated and modeledThe results indicate that in HI regions, photoexcited regular PAHs containing less than 30–40 carbon atoms will dissociate rather than relax by infrared emission, whereas for NC≥30–40, and for analogous PAH photoions of any size, the principal relaxation channel will be infrared emission. Some other implications are discussed concerning the photophysics and photochemistry of PAHs in the interstellar medium.

The ionization potential of CH3SCF3 and the appearance potentials of fragment ions have been measured in the photon energy range 9 -19 eV using synchrotron, radiation. From these results thermochemical data of CH3SCF3, CH3SCF3 + and fragment ions have been evaluated.

The ionization potentials of sulfurtetrafluoride have been measured using threshold photoelectron spectroscopy and com pared with theoretical calculations. The decay of SF4 + has been studied by photoionization mass spectrometry and photoion-photoelectron coincidence. The break-down graphs reveal competing decay processes. Bond dissociation energies in SF4 + have been calculated.

In inner-shell photoionization of an atom followed by Auger decay, the Auger electron initially screens the ionic Coulomb field experienced by the photoelectron. A sudden change in screening occurs when the Auger electron overtakes the photoelectron, causing energy transfer from the latter to the former. According to both semiclassical and quantum theories of post-collision interaction, the photoelectron can thus be recaptured by the atom from which it was emitted. This phenomenon has been measured directly with synchrotron radiation, as a function of incident-photon energy, and is found to be in accord with predictions.

The absorption spectra, photoion spectra, and resonance photoelectron spectra of CF3Cl, CF2Cl2, and CFCl3 have been measured in the energy range from 9 eV to 25 eV, - Absolute cross sections of absorption are given. The stability of the parent ions decreases with increasing symmetry of the parent molecules. The fragment ions have been analyzed. Besides photocations, atomic chlorine and fluorine, fluoride and chloride are formed. The decay mechanisms are discussed, spectra are influenced by autoionization. The resonance photoelectron spectra reveal some new ionization potentials at high excitation energies. Es wurden die Absorptions-, Photoionen- und Resonanzphotoelektronenspektren von CF3Cl, CF2Cl2 und CFCl3 im Energiebereich von 925 eV gemessen.—Fur die Absorptionsspektren werden absolute Wirkungsquerschnitte angegeben. Die Stabilitat der Molekulionen nimmt mit steigender Symmetrie der neutralen Molekule ab. Die Fragmentionen wurden analysiert; neben verschiedenen Photokationen, atomarem Chlor und Fluor werden Fluorid und Chlorid gebildet. Die Zerfallsmechanismen werden diskutiert, der Einflus von Autoionisation auf die Spektren kann nachgewiesen werden. Die Resonanzphotoelektronenspektren zeigen einige bisher nicht beobachtete lonisierungs-potentiale bei hohen Anregungsenergien.

Aus Gleichgewichtsmessungen an dem System SSF2 ⇌ FSSF wird ein ⊲H = 11,3 ± 1,5 kJ/mol abgeleitet. Bildungsenthalpien und Bindungsenergien von SSF2, FSSF, SF3SF und SF3SSF werden aus photoionisations-massenspektroskopisch gemessenen Auftrittspotentialen berechnet. Lower Chalcogen Fluorides. X. Thermochemical Data and Photoionization Mass Spectra of SSF2, FSSF, SF3SF, and SF3SSF From measurements of the equilibrium of the system SSF2 ⇌ FSSF a reaction enthalpy ⊲H = 11.3 ± 1.5 kJ/mol is evaluated. Enthalpies of formation and bond energies of SSF2, FSSF, SF3SF, and SF3SSF have been obtained from appearence potentials which have been measured by photoionization mass spectrometry.

Investigations of the fragmentation processes of acetaldehyde were performed by photoionization mass spectrometry of its deuterium labeled species CH 3 CDO and CD 3 CHO. Intramolecular exchange of hydrogen atoms (hydrogen scrambling) was observed. Obviously this process is accompanied by predissociation of the parent ion. Results are compared with previous work on acetaldehyde CH 3 CHO.

The formation of N + ( 3 P), O + ( 4 S 0 ), NO + and excited nitrogen atoms from NO is studied by means of synchrotron radiation in the wavelength region from 550 A to 650 A. The cross section for each of the products is strongly influenced by the Rydberg states (2sσ *−1 ) n dπ and (2sσ *−1 ) n p π σ of the NO molecules. The molecular ion state c 3 Π pre-dissociates into N + + O but not into O + + N.

The appearance potentials of fragment ions from SF5Cl have been measured in the energy range 12 - 20 eV by means of photoionization mass spectrometry. From these data, the ionization potential of SF5 comes to 9.65 eV.

The photoionization mass spectrum of SBr2 has been m easured and the (S−Br)-bond energy has been calculated from experimental data.

The dependence of the relative cross section for the process N2O(x 1 Sigma +(0.0.0)) +h nu to N2O+(A/sup / 2 Sigma +(0,0,0))+e- on the incident photon energy has been measured between 16.4 and 20.3 eV via the fluorescent radiation of the transition N2O+(A/sup / 2 Sigma +(0,0,0) to X/sup / 2 Pi i(0,0,0)). It is strongly influenced by Tanaka's apparent emission series R IX and the Rydberg series R VI and R VII.

The dissociation of NO molecules to excited atomic states by dispersed synchrotron radiation (700 A – 800 A) is investigated by observing vuv-fluorescence (1050 A – 1400 A) of the fragments. The atomic levels are populated by predissociation of high-lying molecular states.