•VUV photoelectron spectrum of CH2F2 and CH2Cl2 recorded with resolution of 0.002 eV.•Franck–Condon simulations of vibrationally-resolved bands mostly very successful.•Effects of anharmonicity in the ground-state band of CH2F2+ are pronounced.•Need highlighted for a PFI-ZEKE spectrum of the first band of CH2Cl2+.The threshold photoelectron spectrum (TPES) of difluoromethane and dichloromethane has been recorded at the Swiss Light Source with a resolution of 2 meV or 16 cm−1. Electronic and vibronic transitions are simulated and assigned with the help of Franck–Condon (FC) calculations based on coupled cluster electronic structure calculations for the equilibrium geometries and harmonic vibrational frequencies of the neutrals, and of the ground and excited electronic states of the cations. Notwithstanding a high-resolution pulsed-field ionisation study on CH2F2 (Forysinski et al., 2010) in which a number of transitions to the X∼+ state have been recorded with unprecedented accuracy, we report the first complete vibrationally resolved overview of the low-lying electronic states of CH2X2+, X = F or Cl. Hydrogen atom loss from CH2F2+ occurs at low energy, making the ground state rather anharmonic and interpretation of the X∼+ band challenging in the harmonic approximation. By Franck–Condon fits, the adiabatic ionisation energies to the A∼+2B2, C∼+2A2 and D∼+2B2 states have been determined as 14.3 ± 0.1, 15.57 ± 0.01 and 18.0 ± 0.1 eV, respectively. The first band in the CH2Cl2 TPES is complex for a different reason, as it is the result of two overlapping ionic states, X∼+2B2 and A∼+2B1, with derived ionisation energies of 11.0 ± 0.2 and 11.317 ± 0.006 eV, and dominated by an extended progression in the CCl2 bend (in X∼+) and a short progression in the CCl2 symmetric stretch (in A∼+), respectively. Furthermore, even though Koopmans’ approximation holds for the vertical ionisations, the X∼+ state of CH2Cl2+ is stabilized by geometry relaxation and corresponds to ionisation from the (HOMO−1) orbital. That is, the first two vertical ionisation energies are in the same order as the negative of the orbital energies of the highest occupied orbitals, but the adiabatic ionisation energy corresponding to electron removal from the (HOMO−1) is lower than the adiabatic ionisation energy corresponding to electron removal from the HOMO. The second band in the spectrum could be analysed to identify the vibrational progressions and determine adiabatic ionisation energies of 12.15 and 12.25 eV for the B∼+2A1 and C∼+2A2 states. A comparison of the assignment of electronic states with the literature is made difficult by the fact that the B1 and B2 irreducible representations in C2v symmetry depend on the principal plane, i.e. whether the CX2 moiety is in the xz or the yz plane, which is often undefined in older papers.Figure optionsDownload full-size imageDownload as PowerPoint slide

Internal energy selected carbon tetrachloride cations have been prepared by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet radiation. The threshold photoelectron spectrum shows a newly observed vibrational progression corresponding to the ν2(e) scissors mode of CCl4+ in the third, 2E band. Ab initio results on the first four doublet and lowest-lying quartet electronic states along the Cl3C+–Cl dissociation coordinate show the state to be strongly bound, and support its relative longevity. The 2T1 and à 2T2 cationic states, on the other hand, are barely bound and dissociate promptly. The 2T2 state may intersystem cross to the quartet ã state, which dissociates to a triplet state of the CCl3+ fragment ion. This path is unique among analogous MX4+ (M = C, Si, Ge; X = F, Cl, Br) systems, among which several have been shown to have long-lived states, which decay by fluorescence. The breakdown diagram, recorded here for the first time for the complete valence photoionisation energy range of CCl4, is interpreted in the context of literature based and CBS-QB3, G4, and W1U computed dissociative photoionisation energies. No Cl2-loss channel is observed in association with the CCl2+ or CCl+ fragments below the 2 or 3 Cl-loss reaction energies, and Cl2 loss is unlikely to be a major channel above them. The breakdown diagram is modelled based on the calculated dissociative photoionisation onsets and assuming a statistical redistribution of the excess energy. The model indicates that dissociation is not impulsive at higher energies, and confirms that the 2T2 state of CCl4+ forms triplet-state CCl3+ fragments with some of the excess energy trapped as electronic excitation energy in CCl3+.

The reactions of twenty one gas-phase cations with C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 have been studied in a selected ion flow tube at 298 K. The cations are both atomic and molecular with recombination energies in the range 6–22 eV, and the kinetics and branching ratios into product ions are revealed for all the reactions. These data, together with that from an earlier study of reactions of CxFy+ with these four fluorinated ethenes (J. Phys. Chem. A., 2012, 116, 8119), are compared with the reactions of these ions with C2H4, where available. Nearly all the reactions have a rate coefficient close to the collisional value calculated by either Langevin or modified average dipole orientation theories. The products of the reactions of N+ and N2+ with C2H4 are found to be anomalous, compared to their reactions with the four fluorinated ethenes. The branching ratios into product cations are compared with those from a high resolution (ca. 0.002 eV) photoionisation (hν = 10–22 eV) study of C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 (Phys. Chem. Chem. Phys., 2012, 14, 3935) in order to gauge the importance of electron transfer in ion–molecule reactions. The higher the recombination energy of the cation, the better the agreement between the two sets of product branching ratios. Where there is disagreement at lower recombination energies, it appears that there is more fragmentation of the products in the photoionisation experiment compared to the ion–molecule reactions.

The dissociative photoionization mechanism of internal energy selected C2H3F+, 1,1-C2H2F2+, C2HF3+ and C2F4+ cations has been studied in the 13–20 eV photon energy range using imaging photoelectron photoion coincidence spectroscopy. Five predominant channels have been found; HF loss, statistical and non-statistical F loss, cleavage of the C–C bond post H or F-atom migration, and cleavage of the CC bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E0, of the daughter ions have been determined. Both C2H3F+ and 1,1-C2H2F2+ are veritable time bombs with respect to dissociation viaHF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for C2HF3 and C2F4 involves an atom migration across the CC bond, giving CF–CHF2+ and CF–CF3+, respectively, which then dissociate to form CHF2+, CF+ and CF3+. The nature of the F-loss pathway has been found to be bimodal for C2H3F and 1,1-C2H2F2, switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C2F4 is found to be comprised of two regimes. At low internal energies, CF+, CF3+ and CF2+ are formed in statistical processes. At high internal energies, a long-lived excited electronic state is formed, which loses an F atom in a non-statistical process and undergoes statistical redistribution of energy among the nuclear degrees of freedom. This is followed by a subsequent dissociation. In other words only the ground electronic state phase space stays inaccessible. The accurate E0 of CF3+ and CF+ formation from C2F4 together with the now well established ΔfHo of C2F4 yield self-consistent enthalpies of formation for the CF3, CF, CF3+ and CF+ species.

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF+, CF2+, CF3+, and C2F4+) with ethene (C2H4), monofluoroethene (C2H3F), 1,1-difluoroethene (CH2CF2), and trifluoroethene (C2HF3) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF2+ exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF+, CF3+, and C2F4+ is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF+ and CF3+ may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF3+ and C2F4+ are significantly slower than those of CF+ and CF2+, with adducts being formed with the former cations. The reactions of C2F4+ with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for ΔfH°298(C3F2H3+, with structure CF2═CH—CH2+) of 628 kJ mol–1 and a lower limit for ΔfH°298(C2F2H+, with structure CF2═CH+) of 845 kJ mol–1 are determined.

Internal energy selected halomethane cations CH3Cl+, CH2Cl2+, CHCl3+, CH3F+, CH2F2+, CHClF2+, and CBrClF2+ were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH3F+, CH2F2+, and CH3Cl+, chlorine atom loss for CH2Cl2+, CHCl3+, and CHClF2+, and bromine atom loss for CBrClF2+. Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl3+, CHClF2+, and CBrClF2+ do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep-well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 ± 0.01 eV, 12.30 ± 0.02 eV, and 11.23 ± 0.03 eV for CHCl3, CHClF2, and CBrClF2, respectively. The updated 0 K enthalpies of formation, ΔfHo0K(g) for the ions CH2F+, CHF2+, CHCl2+, CCl3+, CCl2F+, and CClF2+ have been derived to be 844.4 ± 2.1, 601.6 ± 2.7, 890.3 ± 2.2, 849.8 ± 3.2, 701.2 ± 3.3, and 552.2 ± 3.4 kJ mol–1, respectively. The ΔfHo0K(g) values for the neutrals CCl4, CBrClF2, CClF3, CCl2F2, and CCl3F and have been determined to be −94.0 ± 3.2, −446.6 ± 2.7, −702.1 ± 3.5, −487.8 ± 3.4, and −285.2 ± 3.2 kJ mol–1, respectively.

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH3X (X = F, Cl, Br). The anions X−, H−, CX−, CHX− and CH2X− are observed, and their ion yields recorded in the range 8–35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + hν → A− + B+ (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F− from both SF6 and CF4. The cross sections for formation of X− + CH3+ are much greater than for formation of CH2X− + H+. In common with many quadrupoles, the spectra of m/z 1 (H−) anions show contributions from all anions, and only for CH3Br is it possible to perform the necessary subtraction to obtain the true H− spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X− and CH2X− are used to calculate upper limits to 298 K bond dissociation energies for Do(H3C–X) and Do(XH2C–H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F− from CH3F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition.

With use of vacuum-UV radiation from a synchrotron, gas-phase negative ions are detected by mass spectrometry following photoexcitation of SF5Cl. F−, Cl−, and SF5− are observed, and their ion yields recorded in the range 8−30 eV. F− and Cl− show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written AB + hν → C− + D+ (+ neutral(s)). F− is the strongest signal, and absolute cross sections are determined by calibrating the signal intensity with that of F− from SF6 and CF4. Resonances are observed and assigned to transitions to Rydberg states of SF5Cl. The Cl− signal is much weaker, despite the S−Cl bond being significantly weaker than the S−F bond. Appearance energies for F− and Cl− of 12.7 ± 0.2 and 10.6 ± 0.2 eV are determined. The spectra suggest that these ions form indirectly by crossing of Rydberg states of SF5Cl onto an ion-pair continuum.

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C5F8 with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H2O+ for which the reaction efficiency is only 55%. Apart from H2O+ and N+, the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion–molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of c-C5F8 measured by TPEPICO spectroscopy occurs at 12.25 ± 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 ± 0.10 eV), He (I) photoionisation (11.30 ± 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 ± 0.002 eV). The ground electronic state of c-C5F8+ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C5F8 recorded from 12–23 eV shows detection of the parent ion and the daughter ions C4F6+ and C5F7+, with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of c-C5F8 at 298 K has been determined to be −1495 kJ mol−1.

A complementary study of the interaction of SF5Cl in the gas phase with vacuum-UV photons and low-energy electrons from the onset of ionisation, ca. 12 eV, up to 20 eV is presented. The photon-induced experiments have used tunable vacuum-UV radiation from a synchrotron and threshold photoelectron photoion coincidence spectroscopy for product ion detection, the electron-induced experiments a trochoidal electron monochromator and a quadrupole mass spectrometer. The strengths and limitations of both experiments are contrasted, the main difference being the absence of state selectivity in the electron-induced study. The parent cation is not observed in either study, suggesting that its ground electronic state is repulsive following Franck–Condon vertical excitation. The fragment cations SF5+, SF4Cl+, SF4+ and SF3+ have been observed in both studies, with reasonable agreement in their threshold appearance energies. Using a variant of threshold photoelectron photoion coincidence spectroscopy applicable when the ground state of the parent cation is repulsive, the first dissociative ionisation energy of SF5Cl is determined to be 12.3 ± 0.2 eV, leading to a value for the adiabatic ionisation energy for the SF5 radical of 9.92 ± 0.28 eV. The electron-induced experiment is sensitive to ion-pair production, and onsets for F+ and Cl+ production have been observed which are only possible energetically if the accompanying fragments are the anions SF4Cl− and SF5−, respectively. A lower limit for the electron affinity of the SF4Cl radical of 4.88 eV is determined, a value confirmed by ab initio calculations. The electron-induced experiment is very sensitive to gas impurities, and the effects of minute quantities of SF4, FCl, Cl2 and possibly SF2 in the gas sample are observed.

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron and threshold photoelectron photoion coincidence (TPEPICO) spectra of cyclic-C4F8 in the range 11–25 eV have been recorded. The parent ion is observed very weakly at threshold, 11.60 eV, and is most likely to have cyclic geometry. Ion yield curves and branching ratios have been determined for five fragments. Above threshold, the first ion observed is C3F+5, at slightly higher energy C2F+4, then successively CF+, CF+2 and CF+3 are formed. The dominant ions are C3F+5 and C2F+4, with the data suggesting the presence of a barrier in the exit channel to production of C3F+5 whilst no barrier to production of C2F+4. In complementary experiments, the product branching ratios and rate coefficients have been measured in a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C4F8 with a large number of atomic and small molecular cations. Below the energy where charge transfer becomes energetically allowed, only one of the ions, CF+2, reacts. Above this energy, all but one of the remaining ions react. Experimental rate coefficients are consistently greater than the collisional values calculated from modified average dipole orientation theory. The inclusion of an additional ion–quadrupole interaction has allowed better agreement to be achieved. With the exception of N+, a comparison of the fragment ion branching ratios from the TPEPICO and SIFT data suggest that long-range charge transfer is the dominate mechanism for reactions of ions with recombination energy between 12.9 and 15.8 eV. For all other ions, either short-range charge transfer or a chemical reaction, involving cleavage and making of new bond(s), is the dominant mechanism.

The threshold photoelectron spectrum and threshold photoelectron–photoion coincidence spectra of CHCl2F, CHClF2 and CH2ClF are reported in the range 11.3–24.8 eV. Tunable photoionizing radiation with a resolution of 0.3 nm is provided from a synchrotron source with a vacuum-UV monochromator. The coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Energetic comparisons suggest that the major products of the titled molecules dissociate in a similar manner at low photon energy, with the parent and first fragment ion, corresponding to cleavage of the weakest bond, appearing at their thermochemical thresholds. The second major ion, corresponding to cleavage of the second weakest bond, is formed ca. 1 eV higher than its predicted threshold, this disparity implying state-selected dissociation. CHCl2F and CHClF2 fragment in a similar manner at higher photon energies, with minor ions formed by the cleavage of three bonds possessing lower appearance energies than fragment ions formed by the cleavage of two bonds. CH2ClF displays the more expected behaviour, namely sequential bond cleavage as the photon energy increases. These observations can be rationalised in terms of the height of the barrier on the exit channel, as determined by the steric bulk of the leaving group. For the three titled molecules, mean translational kinetic energy releases have also been measured into the channels involving C–F or C–Cl bond fission. These data infer that impulsive dissociations occur at lower energy, with a trend towards statistical behaviour with increasing photon energy. Competition between statistical and impulsive processes is observed, for example C–Cl vs. C–F bond cleavage in CHCl2F+ and CHClF+2.

The design and performance of a new normal incidence monochromator at the Daresbury Synchrotron Radiation Source, optimised for experiments requiring high flux of vacuum-UV radiation, are described. The re-developed beamline 3.1, based on the Wadsworth design of monochromator, is the source of tunable vacuum-UV photons in the range 4–31 eV, providing over two orders of magnitude more flux than the vacuum-UV, Seya monochromator in its previous manifestation. The undispersed and dispersed fluorescence spectra resulting from photoexcitation of N2, CO2, CF4 and C6F6 are presented. Emitting species observed were N2+B2Σu+–X2Σg+, CO2+A˜2Πu–X˜2Πg and B˜2Σu+–X˜2Πg, CF4+C˜2T2–X˜2T1 and C˜2T2–A˜2T2, CF3∗2A2′–2A2, and C6F6+B˜2A2u–X˜2E1g. A CCD multi-channel detector has significantly reduced the time period needed to record dispersed fluorescence spectra with a comparable signal-to-noise ratio.

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).

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).

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.

Experiments are described in which UV and visible fluorescence is observed following vacuum-UV photoexcitation of a range of polyatomic molecules in the gas phase. Tunable radiation in the energy range 8–25 eV from synchrotron sources at Daresbury, UK and BESSY 1, Germany is used as the photoexcitation source. Non-dispersed fluorescence excitation spectra and dispersed fluorescence spectra are recorded at Daresbury and BESSY 1, respectively. The experiments are sensitive to Rydberg states of molecules that photodissociate to an excited state of a fragment that fluoresces, and to valence states of the parent molecular ion that fluoresces. Using single-bunch mode of BESSY 1, lifetimes of the emitters in the range ca. 3–100 ns are measured. Examples are taken from work either published or in press in SiF4 and GeF4, PF3, MCl4 (M=C, Si, Ge), and CXCl3 (X=H, F, Br).

The dissociative photoionization mechanism of internal energy selected C2H3F+, 1,1-C2H2F2+, C2HF3+ and C2F4+ cations has been studied in the 13–20 eV photon energy range using imaging photoelectron photoion coincidence spectroscopy. Five predominant channels have been found; HF loss, statistical and non-statistical F loss, cleavage of the C–C bond post H or F-atom migration, and cleavage of the CC bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E0, of the daughter ions have been determined. Both C2H3F+ and 1,1-C2H2F2+ are veritable time bombs with respect to dissociation viaHF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for C2HF3 and C2F4 involves an atom migration across the CC bond, giving CF–CHF2+ and CF–CF3+, respectively, which then dissociate to form CHF2+, CF+ and CF3+. The nature of the F-loss pathway has been found to be bimodal for C2H3F and 1,1-C2H2F2, switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C2F4 is found to be comprised of two regimes. At low internal energies, CF+, CF3+ and CF2+ are formed in statistical processes. At high internal energies, a long-lived excited electronic state is formed, which loses an F atom in a non-statistical process and undergoes statistical redistribution of energy among the nuclear degrees of freedom. This is followed by a subsequent dissociation. In other words only the ground electronic state phase space stays inaccessible. The accurate E0 of CF3+ and CF+ formation from C2F4 together with the now well established ΔfHo of C2F4 yield self-consistent enthalpies of formation for the CF3, CF, CF3+ and CF+ species.

Internal energy selected carbon tetrachloride cations have been prepared by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet radiation. The threshold photoelectron spectrum shows a newly observed vibrational progression corresponding to the ν2(e) scissors mode of CCl4+ in the third, 2E band. Ab initio results on the first four doublet and lowest-lying quartet electronic states along the Cl3C+–Cl dissociation coordinate show the state to be strongly bound, and support its relative longevity. The 2T1 and à 2T2 cationic states, on the other hand, are barely bound and dissociate promptly. The 2T2 state may intersystem cross to the quartet ã state, which dissociates to a triplet state of the CCl3+ fragment ion. This path is unique among analogous MX4+ (M = C, Si, Ge; X = F, Cl, Br) systems, among which several have been shown to have long-lived states, which decay by fluorescence. The breakdown diagram, recorded here for the first time for the complete valence photoionisation energy range of CCl4, is interpreted in the context of literature based and CBS-QB3, G4, and W1U computed dissociative photoionisation energies. No Cl2-loss channel is observed in association with the CCl2+ or CCl+ fragments below the 2 or 3 Cl-loss reaction energies, and Cl2 loss is unlikely to be a major channel above them. The breakdown diagram is modelled based on the calculated dissociative photoionisation onsets and assuming a statistical redistribution of the excess energy. The model indicates that dissociation is not impulsive at higher energies, and confirms that the 2T2 state of CCl4+ forms triplet-state CCl3+ fragments with some of the excess energy trapped as electronic excitation energy in CCl3+.

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C5F8 with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H2O+ for which the reaction efficiency is only 55%. Apart from H2O+ and N+, the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion–molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of c-C5F8 measured by TPEPICO spectroscopy occurs at 12.25 ± 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 ± 0.10 eV), He (I) photoionisation (11.30 ± 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 ± 0.002 eV). The ground electronic state of c-C5F8+ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C5F8 recorded from 12–23 eV shows detection of the parent ion and the daughter ions C4F6+ and C5F7+, with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of c-C5F8 at 298 K has been determined to be −1495 kJ mol−1.

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH3X (X = F, Cl, Br). The anions X−, H−, CX−, CHX− and CH2X− are observed, and their ion yields recorded in the range 8–35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + hν → A− + B+ (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F− from both SF6 and CF4. The cross sections for formation of X− + CH3+ are much greater than for formation of CH2X− + H+. In common with many quadrupoles, the spectra of m/z 1 (H−) anions show contributions from all anions, and only for CH3Br is it possible to perform the necessary subtraction to obtain the true H− spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X− and CH2X− are used to calculate upper limits to 298 K bond dissociation energies for Do(H3C–X) and Do(XH2C–H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F− from CH3F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition.

The reactions of twenty one gas-phase cations with C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 have been studied in a selected ion flow tube at 298 K. The cations are both atomic and molecular with recombination energies in the range 6–22 eV, and the kinetics and branching ratios into product ions are revealed for all the reactions. These data, together with that from an earlier study of reactions of CxFy+ with these four fluorinated ethenes (J. Phys. Chem. A., 2012, 116, 8119), are compared with the reactions of these ions with C2H4, where available. Nearly all the reactions have a rate coefficient close to the collisional value calculated by either Langevin or modified average dipole orientation theories. The products of the reactions of N+ and N2+ with C2H4 are found to be anomalous, compared to their reactions with the four fluorinated ethenes. The branching ratios into product cations are compared with those from a high resolution (ca. 0.002 eV) photoionisation (hν = 10–22 eV) study of C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 (Phys. Chem. Chem. Phys., 2012, 14, 3935) in order to gauge the importance of electron transfer in ion–molecule reactions. The higher the recombination energy of the cation, the better the agreement between the two sets of product branching ratios. Where there is disagreement at lower recombination energies, it appears that there is more fragmentation of the products in the photoionisation experiment compared to the ion–molecule reactions.