Electronic properties and dissociative photoionization of thiocyanates. Part II. Valence and shallow-core (sulfur and chlorine 2p) regions of chloromethyl thiocyanate, CH2ClSCN

A combination of photoelectron spectroscopy and synchrotron based photoelectron photoion coincidence (PEPICO) spectra has been applied to investigate the electronic structure and the dissociative ionization of the CH(2)ClSCN molecule in the valence region. The PES is assigned with the electronic structure calculations at the outer-valence Green's function and symmetry adapted cluster/configuration interaction (SAC-CI) levels offer an explanation of our experimental results. Upon vacuum ultraviolet irradiation the low-lying radical cation, located at 10.39 eV is formed. The molecular ion is observed in the time-of-flight mass spectra, together with the CH(2)SCN(+) and CH(2)Cl(+) daughter ions. The total ion yield spectra have been measured in the S 2p and Cl 2p regions and several channels have been determined in dissociative photoionization events for the core-excited species. Thus, by using time-of-flight mass spectrometry and synchrotron radiation the relative abundances of the ionic fragments and their kinetic energy release values were obtained from both PEPICO and photoelectron photoion photoion coincidence spectra. Possible fragmentation processes are discussed and compared with that found for the related CH(3)SCN species.     

Dissociative photoionization of methoxycarbonylsulfenyl chloride, CH3OC(O)SCl, following sulfur 2p, chlorine 2p, and oxygen 1s excitations

Total ion yield spectra and photoinduced fragmentations following S 2p, Cl 2p, and O 1s inner shell excitations of methoxycarbonylsulfenyl chloride, CH(3)OC(O)SCl, have been studied in the gaseous phase by using synchrotron radiation and multicoincidence techniques, which include photoelectron-photoion coincidence (PEPICO) and photoelectron-photoion-photoion coincidence (PEPIPICO) time-of-flight (TOF) mass spectrometry. According to the analysis of the partial ion yield spectra the S+ ion signal shows a steep enhancement near the S 2p resonance, which could represent an evidence of state-specific fragmentations promoted by electronic excitations from the S 2p to vacant orbitals with strong antibonding character mainly located at the sulfur atom. The dissociation dynamics for selected PEPIPICO islands have been discussed. Fragmentation channels that involve the extrusion of H+ and CH(x)+ (x = 0, 1, 2, 3) fragments have been predominantly observed for dissociation of doubly charged CH(3)OC(O)SCl(2+).     

Evidence of site-specific fragmentation on thioacetic acid, CH3C(O)SH, irradiated with synchrotron radiation around the S 2p and O 1s regions

Site-specific fragmentations following S 2p and O 1s photoexcitation of thioacetic acid, CH3C(O)SH, have been studied by means of synchrotron radiation. Total ion yield (TIY) spectra were measured and multicoincidence techniques, which include photoelectron-photoion coincidence (PEPICO) and photoelectron-photoion-photoion coincidence (PEPIPICO) time-of-flight mass spectrometry, were applied. The equivalent-core approximation was employed in order to estimate ionization transition values, and the observed peaks were tentatively assigned. A site-specific fragmentation is moderately observed by comparing the mass spectra collected at resonant energies around the inner and shallow inner shell S 2p and O 1s ionization edges. Beside H+ ion, the most abundant ions observed at the S 2p edge excitation were CH3CO+, SH+, S+, and CH3+. At the O 1s region the large CH3CO+ fragment was depressed, and small CHx+ (x = 0, 1, 2, 3), S+, and SH+ fragments were dominant. The dissociation dynamic for the main ion-pair production has been discussed. Two- and three-body dissociation channels have been observed in the PEPIPICO spectra, and the dissociation mechanisms were proposed.     

Pathway studies in the fragmentation of Ge(CH3)4 involving Ge(3d,3p,3s) and C(1s) inner-shell photoexcitation in the range 49.5–450 eV

The dissociative multiple photoionization of tetramethylgermane (Ge(CH₃)₄) in the valence, and in the Ge(3d,3p,3s) and C(1s) inner-shell regions has been studied by using time-of-flight mass spectrometry coupled to synchrotron radiation in the range 49.5–450 eV. Total and individual photoion yields have been recorded as a function of the incident photon energy. Several discrete resonances over a structureless giant resonance are observed below the Ge(3p), Ge(3s) and C(1s) threshold regions. The structureless giant resonance corresponding to the Ge(3d) presumably arises from the continuum enhancement caused by the 3d→εf transition. Various monocations of H⁺, H₂⁺, CH_n⁺ (n=0–4), C₂H_n⁺ (n=0–5), GeH_n⁺, GeCH_n⁺, GeC₂H_n⁺, and GeC₃H_n⁺ are detected in the whole energy range. Dissociation processes have also been investigated by photoelectron–photoion and photoion–photoion coincidence methods. The dominant dissociation channel is found to be CH_n⁺–GeCH_n⁺ in the whole energy examined. Specific energy dependence of dissociation processes is observed in the Ge(3p) and Ge(3s) regions. With the help of ab initio HF/6-311++G(2df,p) calculation, we roughly estimated the photoabsorption positions and symmetries for the discrete core hole states.     

Photoionization and ionic dissociation of the C3H3NS molecule induced by soft X‐ray near the C1s edge

Time of flight mass spectrometry, electron‐ion coincidence, and ion yield spectroscopy were employed to investigate for the first time the thiazole (C₃H₃NS) molecule in the gas phase excited by synchrotron radiation in the soft X‐ray domain. Total ion yield (TIY) and photoelectron‐photoion coincidence (PEPICO) spectra were recorded as a function of the photon energy in the vicinity of the carbon K edge (C1s). The C1s resonant transitions as well as the core ionization thresholds have been determined from the profile of TIY spectrum, and the features were discussed. The corresponding partial ion yields were determined from the PEPICO spectra for the cation species produced upon the molecular photodissociation. Additional ab initio calculations have also been performed from where relevant structural and electronic configuration parameters were obtained for this molecule.     

Interstellar H3+ and HCS+ ions produced in the dissociative photoionization process of CH3C(O)SCH3 in the proximity of the sulfur 2p, carbon 1s, and oxygen 1s edges

In this work we present a study of the dissociative photoionization of S-methyl thioacetate [CH(3)C(O)SCH(3)] by using multicoincidence time-of-flight mass spectrometry and synchrotron radiation in the S 2p, C 1s, and O 1s edges. Total and partial ion yield spectra together with photoelectron-photoion coincidence (PEPICO) and photoelectron-photoion-photoion coincidence (PEPIPICO) spectra were measured. Fragmentation patterns deduced from PEPICO and PEPIPICO spectra at the various excitation energies show a moderate site-specific fragmentation. The dissociation dynamic for the main ion-pair production is discussed. Two-, three-, and four-body dissociation channels have been observed in the PEPIPICO spectra, and the dissociation mechanisms are proposed. The interstellar HCS(+) and H(3)(+) ions can be observed during the synchrotron experiments reported in the present work.     

VUV photodissociation of thiazole molecule investigated by TOF‐MS and photoelectron photoion coincidence spectroscopy

Photoelectron photoion coincidence measurements have been performed for the thiazole (C₃H₃NS) molecule in gas phase, using time‐of‐flight mass spectrometry in the electron‐ion coincidence mode and vacuum ultraviolet synchrotron radiation. photoelectron photoion coincidence spectra have been recorded as a function of the photon energy covering the valence range from 10 to 21 eV. The resulting photoionization products as well as the dissociation pathways leading to the ionic species were proposed and discussed. We have also performed density functional theory and ab initio calculations for the neutral molecule, its cation and the ion fragments produced in order to determine their electronic and structural parameters. Copyright © 2014 John Wiley & Sons, Ltd.     

Heats of formation of HCCl3, HCCl2Br, HCClBr2, HCBr3, and their fragment ions studied by threshold photoelectron photoion coincidence

The dissociative photoionization onsets for Cl and Br loss reactions were measured for HCCl3, HCCl2Br, HCClBr2, and HCBr3 by threshold photoelectron photoion coincidence (TPEPICO) in order to establish the heats of formation of the mixed halides as well as the following fragment ions: HCCl2(+), HCClBr(+), HCBr2(+). The first zero Kelvin onsets were measured with a precision of 10 meV. The second onsets, which are in competition with the lower energy onsets, were established with a precision of 60 meV. Because both the chloroform and bromoform have relatively well established heats of formation, these measurements provide a route for establishing the heats of formation of the mixed halomethanes within uncertainties of less than 5 kJ mol(-1).     

Photoelectron spectroscopy and thermochemistry of tert-butylisocyanide-substituted cobalt tricarbonyl nitrosyl

A new organometallic complex, Co(CO)2NOtBuNC, was synthesized and investigated by photoelectron spectroscopy (PES) and threshold photoelectron photoion coincidence (TPEPICO) spectrometry in order to determine its ionization energy as well as the bond energies in the ionic forms. The assignment of the nine peaks in the PES was based on Kohn-Sham molecular orbital energies, and an adiabatic ionization energy of 7.30 +/- 0.05 eV was determined. In the TPEPICO experiment, the following 0 K onsets were determined for the various fragment ions: CoCONOtBuNC+ (8.17 +/- 0.05 eV); CoNOtBuNC+ (9.01 +/- 0.05 eV); and CotBuNC+ (10.42 +/- 0.05 eV). Because the photon source did not extend above 14 eV, we could not observe the bare Co+ ion in the experiment. The heat of formation of the CotBuNC+ ion was estimated by ab initio and DFT calculations of the CoL+ + tBuNC --> CotBuNC+ + L (L = CO, NO, NH3, H2O, PMe3) substitution enthalpies.     

He I photoelectron spectra and valence synchrotron photoionization for XC(O)SCl (X = F, Cl) compounds

Small penta-atomic molecules like FC(O)SCl and ClC(O)SCl have been analyzed by using both photoelectron spectroscopy (PES) and results derived from the use of synchrotron radiation in the same energy range. For this second experiment total ion yield (TIY), photoelectron photoion coincidence (PEPICO), and partial ion yield (PIY) spectra have been recorded. This set of data together with results obtained by computational chemistry allow us to study electronic properties and the ionization channels of both species. Thus, whereas the photodissociation behavior of FC(O)SCl can be divided into three well-defined energy regions, the fragmentation dynamics of ClC(O)SCl seems to be more complex. Nevertheless, simultaneous evaluation of the PES and valence synchrotron photoionization studies helps to clarify the molecular ionization processes.     

Probing the pyrolysis of methyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH₃OC(O)H, were elucidated to be CH₃OH + CO, 2 CH₂O and CH₄ + CO₂ as in part distinct from the dissociation of the radical cation (CH₃OH^+• + CO and CH₂OH⁺ + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH₂O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.     

Inner-valence photoionization of O((1)D): experimental evidence for the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition

Photoionization and autoionization of electronically excited atomic oxygen O((1)D) are investigated in the energy range between 12 and 26 eV using tunable laser-produced plasma radiation in combination with time-of-flight mass spectrometry. A broad, asymmetric, and intense feature is observed that is peaking at 20.53+/-0.05 eV. It is assigned to the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition, which subsequently autoionizes by a Coster-Kronig transition, as predicted by the previous theoretical work [K. L. Bell et al., J. Phys. B 22, 3197 (1989)]. Specifically, the energy of the unperturbed transition occurs at 20.35+/-0.07 eV. Its shape is described by a Fano profile revealing a q parameter of 4.25+/-0.8 and a width of gamma=2.2+/-0.15 eV. Absolute photoionization cross section sigma is derived, yielding sigma=22.5+/-2.3 Mb at the maximum of the resonance. In addition, weak contributions to the O((1)D) yield from dissociative ionization originating from molecular singlet oxygen [O(2)((1)Delta(g))] are identified as well. Possible applications of the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition as a state-selective and sensitive probe of excited oxygen in combination with photoionization mass spectrometry are briefly discussed.     

Pulsed field ionization-photoelectron photoion coincidence spectroscopy with synchrotron radiation: the heat of formation of the C2H5+ ion

Pulsed field ionization photoelectron (PFI-PE) spectroscopy combined with ion coincidence detection has been used with multi-bunch synchrotron radiation at the Advance Light Source (ALS) to energy select ions and to measure their breakdown diagram. The resolution for ion state selection achieved with Ar+ (2P3/2, 1/2) employing this PFI-PE-photoion coincidence apparatus is 0.6 meV (full width at half maximum). The production of C2H5+ from C2H5Br was investigated near the dissociative photoionization limit with this pulsed field ionization-threshold photoelectron photoion coincidence (PFI-PEPICO) scheme. Although the PFI-PE spectra of C2H5Br, C2H5I, and benzene show that the production of ions in the Franck-Condon gap regions is quite low, the selectivity for PFI-PE detection and the suppression of prompt electrons is such that we can detect 1 PFI-PE out of 25,000 total electrons s-1. The derived C2H5+ heat of formation from the analysis of the C2H5Br+ breakdown diagram and a critical analysis of other results is 900.5 +/- 2.0 kJ mol-1 at 298 K, or 913.2 +/- 2.0 kJ mol-1 at 0 K. This leads to an ethylene proton affinity at 298 K of 682.0 kJ mol-1. The measured IE of C2H5Br is 10.307 eV.     

HeI photoelectron and valence synchrotron photoionization studies of the thioester molecule CH3C(O)SCH3: evidence of vibronic structure

One of the simplest thioester molecules, S-methyl thioacetate, CH 3C(O)SCH 3, has been investigated by HeI photoelectron spectroscopy (PES) and valence photoionization studies using synchrotron radiation in the same energy range. In the second series of experiments, total ion yield (TIY), photoelectron photoion coincidence (PEPICO), and partial ion yield (PIY) spectra were recorded. It was found that the photodissociation behavior of CH 3C(O)SCH 3 can be divided into three well-defined energy regions. Vibronic structure was observed in the valence synchrotron photoionization process, being associated with wavenumbers of 912, 671, 1288, 1690, and 1409 cm (-1) for the bands at 12.82, 13.27, 15.66, 15.72, and 17.42 eV, respectively. Evaluation of the PE spectrum in concert with the synchrotron photoionization measurements and complemented by high-level ab initio calculations thus provides unusually detailed insights into the valence ionization processes of this molecule.     

Dissociative photoionization and thermochemistry of dihalomethane compounds studied by threshold photoelectron photoion coincidence spectroscopy

The dissociative photoionization studies have been performed for a set of dihalomethane CH(2)XY (X,Y = Cl, Br, and I) molecules employing the threshold photoelectron photoion coincidence (TPEPICO) technique. Accurate dissociation onsets for the first and second dissociation limits have been recorded in the 10-13 eV energy range, and ionization potentials have been measured for these compounds. By using our experimental dissociation onsets and the known heat of formation of CH(2)Cl(2) molecule, it has been possible to derive the 0 and 298 K heats of formation of all six neutral dihalomethanes as well as their ionic fragments, CH(2)Cl(+), CH(2)Br(+), and CH(2)I(+), to a precision better than 3 kJ/mol. These new measurements serve to fill the lack of reliable experimental thermochemical information on these molecules, correct the old literature values by up to 19 kJ/mol, and reduce their uncertainties. From our thermochemical results it has also been possible to derive a consistent set of bond dissociation energies for the dihalomethanes.     

Dissociation dynamics of positive-ion and negative-ion fragments of gaseous and condensed Si(CH(3))(2)Cl(2) via Si 2p, Cl 2p, and Cl 1s core-level excitations

The state-selective positive-ion and negative-ion dissociation pathways of gaseous and condensed Si(CH(3))(2)Cl(2) following Cl 2p, Cl 1s, and Si 2p core-level excitations have been characterized. The excitations to a specific antibonding state (15a(1) (*) state) of gaseous Si(CH(3))(2)Cl(2) at the Cl 2p, Cl 1s, and Si 2p edges produce significant enhancement of fragment ions. This ion enhancement at specific core-excited states correlates closely with the ion kinetic energy distribution. The results deduced from ion kinetic energy distribution are consistent with results of quantum-chemical calculations on Si(CH(3))(2)Cl(2) using the ADF package. The Cl(-) desorption yields for Si(CH(3))(2)Cl(2)Si(100) at approximately 90 K are notably enhanced at the 15a(1) (*) resonance at both Cl 2p and Si 2p edges. The resonant enhancement of Cl(-) yield occurs through the formation of highly excited states of the adsorbed molecules. These results provide insight into the state-selective ionic fragmentation of molecules via core-level excitation.     

Construction and evaluation of a miniature electron ion coincidence analyzer mounted on a conflat flange with an outer diameter of 114 mm.

We have developed a miniature electron ion coincidence (EICO) analyzer mounted on a conflat flange with an outer diameter of 114 mm. It consists of a cylindrical mirror analyzer (CMA), a time-of-flight ion mass spectrometer (TOF-MS), a commercially available linear motion feed through, and a tilt adjustment mechanism. Each sample surface was irradiated by synchrotron radiation, and the energies of emitted electrons were analyzed and detected by the CMA, while desorbed ions were collected by the TOF-MS in coincidence with the electrons. The performance of the EICO analyzer was tested by measuring the Auger-electron H+ photoion coincidence spectrum of condensed water at 4a1 <-- O 1s resonance.     

VUV photoionization of acetamide studied by electron/ion coincidence spectroscopy in the 8-24 eV photon energy range

A VUV photoionization study of acetamide was carried out over the 8-24 eV photon energy range using synchrotron radiation and photoelectron/photoion coincidence (PEPICO) spectroscopy. Threshold photoelectron photoion coincidence (TPEPICO) measurements were also made. Photoion yield curves and branching ratios were measured for the parent ion and six fragment ions. The adiabatic ionization energy of acetamide was determined as I.E. (1²A′) = (9.71 ± 0.02) eV, in agreement with an earlier reported photoionization mass spectrometry (PIMS) value. The adiabatic energy of the first excited state of the ion, 1²A″, was determined to be ≈10.1 eV. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made. The neutral species lost in the principal dissociative photoionization processes are CH₃, NH₂, NH₃, CO, HCCO and NH₂CO. Heats of formation are derived for all ions detected and are compared with literature values. Some astrophysical implications of these results are discussed.     

Ionized o-, m-, and p-difluorobenzene dissociate through ring-opened intermediates: A TPEPICO investigation

Threshold photoelectron photoion coincidence (TPEPICO) experiments have shown that o-, m-, and p-difluorobenzene ions dissociate via a common, ring-opened intermediate and not via ionized p-difluorobenzene. Rice-Ramsperger-Kassel-Marcus (RRKM) modeling of the experimental breakdown curves yields activation energies for the initial isomerization of 4.48 +/- 0.05, 4.55 +/- 0.05, and 4.68 +/- 0.05 eV for o-, m-, and p-difluorobenzene, respectively. These values place each ion at a similar absolute energy and thus similar transition states. A large positive DeltaS(double dagger) for each ion (ca 100 J K(-1) mol(-1)) suggests a ring-opened structure for these transition states.     

O1s photoionization dynamics in oriented NO2

We have performed extensive density functional theory (DFT) calculations, partial cross sections, dipole prepared continuum orbitals, dipole amplitudes and phase shifts, asymmetry parameters β, and molecular frame photoelectron angular distributions, to elucidate the O1s photoionization dynamics of NO(2) molecule with emphasis on the shape resonances in the O1s ionization continuum. In the shape resonance region, the β parameters and photoelectron angular distributions have been compared with our experimental results. Fairly good agreement between the theory and experiment has confirmed that the DFT level calculations can well describe the photoionization dynamics of the simple molecule such as NO(2). Interference due to equivalent atom photoionization is theoretically considered, and the possibility of detection of the effect in the two degenerate channels with different combinations of light polarization and photoemission direction is discussed.