D. P. Seccombe

Fragmentation of the valence states of CF2Cl2+, CF2H2+, and CF2Br2+ studied by threshold photoelectron–photoion coincidence spectroscopy

Using tunable vacuum–ultraviolet radiation from a synchrotron, the decay pathways of the valence electronic states of CF2X2+ (X=Cl, H, Br) in the range 10–25 eV have been determined by threshold photoelectron–photoion coincidence spectroscopy. The ions are separated by a linear time-of-flight mass spectrometer. Coincidence spectra are recorded continuously as a function of energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are recorded with improved time resolution, allowing the mean total translational kinetic energy, 〈KE〉t, into some dissociation channels to be determined. By comparing the 〈KE〉t values for single-bond fragmentations with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. The excited states of all three parent cations show some evidence for isolated-state behavior. With CF2Cl2+ and CF2H2+, this ...

Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

The vacuum–ultraviolet (VUV) absorption and fluorescence spectroscopy of CF2X2 (X=H, Cl, Br) in the range 190–690 nm is reported. Tunable vacuum–UV radiation in the range 8–22 eV from synchrotron sources at either Daresbury, U.K. or BESSY1, Germany is used to excite the titled molecules. Fluorescence excitation spectra, with undispersed detection of the fluorescence, were recorded at Daresbury with a resolution of 0.1 nm. VUV absorption spectra at a resolution of 0.08 nm, and dispersed emission spectra with an optical resolution of 8 nm were recorded at BESSY1. Action spectra, in which the VUV energy is scanned with detection of the fluorescence at a specific wavelength, were also recorded at BESSY1 with a resolution of 0.3 nm; appearance energies for production of a particular emitting state of a fragment are then obtained. Using the single-bunch mode of BESSY1, lifetimes of all emitting states that fall in the range ∼3–80 ns have been measured. The peaks in the VUV absorption spectra of CF2X2 are assign...

The effects of isotopomers in the state-selected photofragmentation of BCl3+, PCl3+ and PBr3+

Analysis of the shapes of time-of-flight (TOF) peaks of singly-charged polyatomic cations produced in photoelectron–photoion coincidence spectra allows a determination of the total mean kinetic energy released into translational motion of the fragment species, 〈KE〉t. In turn, this value can indicate the mechanism of unimolecular photofragmentation. We show that in cases where the daughter ion has more than one isotopomer, allowance must be made for them in analysing the TOF distributions. Otherwise, values for 〈KE〉t are obtained which are too large. Examples are given from recent work by us on state-selected (BCl3+)∗→BCl2++Cl and (PX3+)∗→PX2++X (X=Cl,Br).

Vacuum-UV fluorescence spectroscopy of CCl3F, CCl3H and CCl3Br in the range 8–30 eV

The fluorescence spectroscopy of CCl3X (X=F, H, Br) in the range 200–700 nm is reported, using vacuum-UV radiation in the range 8–30 eV from a synchrotron as a tunable photoexcitation source. Excitation spectra, with undispersed detection of the fluorescence, have been recorded at the Daresbury UK source with a resolution of 0.1 nm, corresponding to an average energy resolution of ca. 0.015 eV. Dispersed emission spectra in the range 200–700 nm have been recorded at the BESSY 1 Germany source with an optical resolution of 8 nm, following photoexcitation at the energies of the peaks in the excitation spectra. Action spectra, in which the vacuum-UV energy is scanned with detection of the fluorescence at a specific wavelength, have also been recorded at BESSY 1 with a resolution of 0.3 nm; thresholds for production of a particular excited state of a fragment are then obtained. Using single-bunch mode, lifetimes of all the emitting states that fall in the range ca. 3–100 ns have been measured. For photon energies in the range 8–12 eV, emission is due to both CCl2 A1B1–1A1 and CXCl A1A″–1A′. These products form by photodissociation of low-lying Rydberg states of CCl3X, and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. It is not possible to say whether the other products form as two halogen atoms or a diatomic molecule. For energies in the range 13–17 eV, emission is due to diatomic fragments; CCl A2Δ, CF B2Δ, CH B2Σ- and A2Δ, CBr A2Δ, and Cl2 D′ 23Πg. From their threshold energies, there is now accumulated evidence that the excited state of CCl or CX forms in association with three isolated atoms. Our results yield no information on whether the three bonds in CCl3X* break simultaneously or sequentially. In the range 13–17 eV, Cl2* almost certainly forms in conjunction with ground-state CX+Cl. This ion-pair state of Cl2 also forms at higher excitation energies around 20 eV, probably with atomic products C+X+Cl. In no cases is emission observed from excited states of either the CCl3 radical or the parent molecular ion, CCl3X+.

Vacuum-UV fluorescence spectroscopy of SiF4 in the range 10–30 eV

The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2 a 3B1–X 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2 A 1B1–X 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the D 2A1 state of SiF4+ predomin...

The use of threshold photoelectron–photoion coincidence spectroscopy to probe the spectroscopic and dynamic properties of the valence states of CCl3F+, CCl3H+ and CCl3Br+

Using tunable vacuum-ultraviolet radiation from a synchrotron source in the range 10–25 eV, threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to determine the decay pathways of the valence electronic states of CCl3X+ (X=F, H, Br). TPEPICO spectra are recorded continuously as a function of photon energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are also recorded with improved time resolution, allowing total mean translational kinetic energy releases, 〈KE〉t , into some dissociation channels to be determined. By comparing 〈KE〉t values for single bond-fission processes (i.e. cleavage of a C–Cl or C–X bond) with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. Excited states of all three parent cations show evidence for isolated-state behaviour, and the 〈KE〉t values suggest a relationship between the part of the molecule where ionisation occurs and the bond that breaks to form daughter ion plus neutral atom products; impulsive values of 〈KE〉t are more likely to be obtained when the breaking bond lies close to the part of the molecule where ionisation occurs, statistical values when ionisation occurs further away from the breaking bond. At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond. With CCl3F and CCl3Br, the appearance energies of the daughter ions are close to the thermochemical energy for production of that ion with isolated neutral atoms, suggesting strongly that these ions form by bond-fission processes only. With CCl3H, at certain energies some fragment ions can only form with molecular neutral fragments (e.g. CCl2++HCl), involving bond-breaking and bond-making processes. It is suggested that this phenomenon is related to the small size of the hydrogen atom, and hence less steric hindrance in a tightly constrained transition state along the reaction coordinate.

Vacuum-UV fluorescence spectroscopy of PF3 in the range 9–20 eV

The vacuum-UV and visible spectroscopy of PF3 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following photoexcitation with monochromatized synchrotron radiation from the Daresbury, UK source in the energy range 9–20 eV with an average resolution of ∼0.015 eV. Transitions to the three lowest-energy bands in the Rydberg spectra show resolved vibrational structure, they are assigned to transitions to the (8a1)−1 4p, 5p, and 6p Rydberg states of PF3, and fluorescence is due to valence transitions in the PF2 radical. From a Franck–Condon analysis of the vibrational structure, it is shown that the FPF bond angle in PF3 increases by ∼14±1° upon photoexcitation. The use of optical filters shows that at least three excited electronic states of PF2 are responsible for the induced emission. Dispersed emission spectra in the UV/visible region have been recorded with an optical resolution of 8 nm at the BESSY 1, Germany synchrotron s...

Fragmentation of valence electronic states of CHF2CF3+ studied by threshold photoelectron–photoion coincidence (TPEPICO) techniques in the photon energy range 12–25 eV

Abstract Vacuum ultraviolet synchrotron radiation and threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy have been used to study the decay dynamics of the valence electronic states of CHF2CF3+. The threshold photoelectron spectrum (TPES) and ion yield curves of the observed fragments have been recorded in the photon energy range 12–25 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight (TOF) mass spectrometry, respectively. Using a combination of the measured TPES and ab initio molecular orbital calculations, we conclude that the CHF2CF3+ cation adopts a staggered Cs geometry, the X 2 A ′ ground state being formed by electron removal from the 18a′ σ-bonding orbital of CHF2CF3. Upon ionisation, large geometry changes and broad spectral bands are both predicted and observed. The next outer-valence orbitals of CHF2CF3, 17a′ and 11a′′, are predominantly associated with fluorine 2p orbitals located on the CHF2 group. Translational kinetic energy releases are determined from fixed-energy TPEPICO-TOF spectra. The ground state of CHF2CF3+ dissociates through C–C bond cleavage with a relatively small release of energy. By contrast the A and B states dissociate rapidly by an impulsive mechanism with a larger fractional release of energy to CHFCF3++F. Upper limits for the ionisation threshold of CHF2CF3 (12.70±0.05 eV) and the enthalpy of formation of CHFCF3+ at 298 K ( 299±7 kJ mol −1 ) are determined.

Threshold photoelectron photoion coincidence study of the fragmentation of valence states of CF3-CH3+ and CHF2-CH2F+ in the range 12-24 eV

Using tunable vacuum-ultraviolet radiation from a synchrotron source, threshold photoelectron photoion coincidence spectroscopy has studied the unimolecular decay dynamics of the valence electronic states of CF3–CH3+ and CHF2–CH2F+. Threshold photoelectron spectra and fragment ion yield curves of CF3–CH3 and CHF2–CH2F have been recorded in the range 12–24 eV, electrons and ions being detected by a threshold electron analyser and a linear time-of-flight mass spectrometer, respectively. For the dissociation products of (CF3–CH3+)* and (CHF2–CH2F+)* formed via cleavage of a single covalent bond, the mean translation kinetic energy releases have been measured and compared with the predictions of statistical and impulsive mechanisms. Ab initio G2 calculations have determined the minimum energies of CF3–CH3 and CHF2–CH2F and their cations, with their geometries optimised at the MP2(full)/6-31G(d) level of theory. The nature of the valence orbitals of both neutral molecules has also been deduced. Enthalpies of formation of both titled molecules and all fragment ions and neutrals observed by dissociative photoionisation have also been calculated. Combining all experimental and theoretical data, the fragmentation mechanisms of the ground and excited states of CF3–CH3+ and CHF2–CH2F+ are discussed. The ground state of both ions, formed by electron removal from the C–C σ-bonding highest occupied molecular orbital, is stable only over a narrow range of energies in the Franck–Condon region; it dissociates by C–C bond cleavage with a small fractional translational energy release. Low-lying excited states of both ions, produced by electron removal from F 2pπ nonbonding orbitals, show some evidence for isolated-state behaviour, with impulsive dissociation by cleavage of a C–F bond and a larger fractional translational energy release into the two fragments. For energies above ca. 16 eV smaller fragment ions, often resulting from cleavage of multiple bonds and HF elimination, are observed; for both molecules with hν > 18 eV, CF–CH2+ is the dominant fragment ion. New experimental values are determined for the enthalpy of formation at 298 K of CF3–CH3 (−751 ± 10 kJ mol−1) and CHF2–CH2F (−671 ± 12 kJ mol−1), with upper limits being determined for CF2–CH3+ (≤546 ± 11 kJ mol−1) and CHF–CH2F+ (≤663 ± 13 kJ mol−1).

Fragmentation of valence electronic states of CF3–CH2F+ and CHF2–CHF2+ in the range 12–25 eV

Tunable vacuum-ultraviolet radiation from a synchrotron source and threshold photoelectron–photoion coincidence spectroscopy have been used to study the decay dynamics of the valence electronic states of CF3–CH2F+ and CHF2–CHF2+. The threshold photoelectron spectra, fragment ion yield curves, and breakdown diagrams of CF3–CH2F and CHF2–CHF2 have been obtained in the photon energy range 12–25 eV, the electrons and fragment ions being detected by a threshold electron analyser and a linear time-of-flight mass spectrometer, respectively. For the dissociation products of (CF3–CH2F+)* and (CHF2–CHF2+)* formed via a single-bond cleavage, the mean translational kinetic energy releases have been measured and compared with the predictions of statistical and pure-impulsive mechanisms. Ab initio G2 calculations have determined the minimum-energy geometries of CF3–CH2F and CHF2–CHF2 and their cations, and deduced the nature of the high-lying valence orbitals of both neutral molecules. Furthermore, enthalpies of formation at 298 K of both neutral molecules, and all the neutral and fragment ions observed by dissociative photoionisation have been calculated. Combining all experimental and theoretical data, the decay mechanisms of the ground and excited valence states of CF3–CH2F+ and CHF2–CHF2+ are discussed. The first and second excited states of both ions show some evidence for isolated-state behaviour, with fast dissociation by cleavage of a C–F or C–H bond and a relatively large translational energy released in the two fragments. The ground state of both ions dissociate by cleavage of the central C–C bond, with a much smaller translational energy release. Several fragment ions are observed which form via H-atom migration across the C–C bond; for hν > 18 eV, CH2F+ is even the dominant ion from dissociative photoionisation of CHF2–CHF2. New experimental values are determined for the enthalpy of formation at 298 K of CF3–CH2F (−905 ± 5 kJ mol−1) and CHF2–CHF2 (−861 ± 5 kJ mol−1), with upper limits being obtained for CF2–CH2F+ (⩽485 ± 7 kJ mol−1), CF2–CHF2+ (⩽324 ± 7 kJ mol−1) and CHF–CHF2+ (⩽469 ± 7 kJ mol−1).