Gary K. Jarvis

Charge transfer from neutral perfluorocarbons to various cations : long-range versus short-range reaction mechanisms

The bimolecular reactions of the high recombination energy cations Ar+, F+, and Ne+ with four fully saturated (CF4, C2F6, C3F8, and n-C4F10) and three unsaturated (C2F4, C3F6, and 2-C4F8) perfluorocarbons (PFCs) are reported. The cation branching ratios obtained from these reactions, and from the reactions with O2+, H2O+, N2O+, O+, CO2+, CO+, N+, and N2+ [reported by us, Jarvis et al., J. Phys. Chem. 100 (1996) 17166], are compared with those determined from the threshold photoelectron–photoion coincidence spectra of the PFCs at the recombination energies of the reagent cations. This comparison provides information that helps to interpret the dynamics of charge transfer, and whether it occurs via a long-range or a short-range mechanism. Energy resonance and good Franck-Condon factors connecting the ground electronic state of a reactant neutral molecule to one of its ionic states, at the recombination energy of the reagent cation, are generally considered to be sufficient criteria for long-range charge transfer to occur. However, the results from this study imply that good Franck-Condon factors are not critical in determining the efficiency of a long-range charge transfer. Instead, the results suggest that, in addition to the requirement for energy resonance, the electron taking part in the charge-transfer process must be removed from a molecular orbital which is unshielded from the approaching reagent cation. This enables the cation to exert an influence on the electron at large impact parameters.

Investigations of low energy electron attachment to ground state group 6B hexafluorides (SF6, SeF6, and TeF6) using an electron-swarm mass spectrometric technique

Abstract Studies of low energy electron attachment to SF6, SeF6, and TeF6 have been carried out in an atmospheric pressure nitrogen buffer gas (number density N) at 300 K. The experiments are conducted under nonthermal electron-swarm conditions, using an instrument that combines an atmospheric pressure drift tube, with a quadrupole mass spectrometer. Details of the design, construction and operation of the drift tube and the associated fast electron gate are presented. Electron drift times can be measured, and mean electron drift velocities in N2 as a function of the density reduced electric field strength E/N are reported. Density normalised electron attachment coefficients, α, and electron attachment rate constants, ka, together with product anion branching ratios (for SeF6 and TeF6) are determined as a function of E/N. The studies presented here cover the range E/N = (0.4–17) × 10−18 V cm2, corresponding to mean electron energies of 0.04–0.6 eV. For all three molecules, ka decreases as E/N increases. SF6 attaches electrons much more rapidly than either SeF6 or TeF6. The ratios ka(SF6):ka(SeF6):ka(TeF6) ≈ 3000:10:1 are found not to vary with E/N. The estimated thermal (300 K) electron attachment rate constants are kth(SF6) ≈ (2.5 ± 0.3) × 10−7 cm3 s−1, kth(SeF6) ≈ (8.0 ± 1.2) × 10−10 cm3 s−1, and kth(TeF6) ≈ (8.2 ± 1.1) × 10−11 cm3 s−1. For all three molecules, attachment is dominated by the capture of near-zero-energy electrons. In each case the dominant anion product is XF6− (X = S, Se, Te), accompanied by XF5−. No other anion products directly arising from electron attachment to XF6 are observed. Extrapolation of the relative product anion intensities to zero attaching gas concentration yields the following branching ratios for attachment under swarm conditions: SeF6–SeF5− (20%), SeF6− (80%); and TeF6–TeF5− (3%), TeF6− (97%). These ratios are found to be independent of E/N. The observation of SeF6− and TeF6− as the dominant anions from SeF6 and TeF6 is ascribed to stabilisation of the initial anion formed by electron capture through collisions with the nitrogen buffer gas. For SF6, the observed proportion of SF5− decreases from 8% to 1% over the E/N range of this study, whereas an increase in the SF5− branching ratio with E/N is anticipated from previous low-pressure, electron beam investigations.

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).

On the adiabatic ionisation energy of the CF3 free radical

Abstract The different methods that have been used to measure the adiabatic ionisation energy of the CF 3 free radical in the gas phase are reviewed, with values covering the wide range 8.6–9.1 eV. In a study of vacuum-UV photofragmentation of perfluoropropane, C 3 F 8 , using the threshold photoelectron–photoion coincidence technique, our value for the appearance energy of CF 3 + at 298 K, 13.0±0.1 eV, leads to an upper limit for the adiabatic ionisation energy of CF 3 of 8.8±0.2 eV, in good agreement both with that deduced from a low-temperature photoionisation mass spectrometric (PIMS) study of CF 3 Br and with values obtained from three ion–molecule bimolecular studies. It is lower, however, than the value obtained indirectly from a PIMS study of C 2 F 4 and from ab initio calculations.

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.

Threshold photoelectron spectroscopy of PX3 (X=Cl, Br), and fragmentation of the valence electronic states of PX3+ studied by TPEPICO spectroscopy

The valence threshold photoelectron spectra (TPES) of phosphorous trichloride (PCl3) and phosphorous tribromide (PBr3) are presented for the first time. Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has also been used to measure, state selectively, the decay pathways of the valence states of PX3+ (X=Cl, Br) in the range 10–25 eV. Vacuum-UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules, electrons and ions being detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions to be obtained. The ground and first excited states of both molecular parent ions are stable with respect to dissociation to PX2+, whereas the B2E, C2E, D2A1 and E2E states dissociate exclusively to PX2+. The first experimental observation of the F2A1 state of PBr3+ is presented, which we deduce to have a very small cross-section to photoionisation. Fragmentation of the F2A1 states of both parent ions leads to exclusive production of PX+. Kinetic-energy (KE) releases into PX2++X have been measured at the Franck–Condon maxima of the B2E, C2E, D2A1 and E2E states of PCl3+ and the B2E, C2E and E2E states of PBr3+. For the first time, the method used to calculate the KE releases allows for the full range of daughter ion isotopes to be accommodated.

DETERMINATION OF THE FIRST DISSOCIATIVE IONISATION ENERGY OF POLYATOMIC MOLECULES BY THRESHOLD PHOTOELECTRON PHOTOION COINCIDENCE (TPEPICO) SPECTROSCOPY: APPLICATION TO CF4, SF6, SeF6, TeF6 and SF5CF3

A recent paper (Science2000, 289, 611) has suggested that an anthropogenic greenhouse gas, SF5CF3, recently detected in the atmosphere, has the highest radiative forcing of any gas-phase molecule. The ground state of is repulsive in the Franck–Condon region, the parent ion is not observed, and the onset of ionisation can only give an upper limit to the energy of the first dissociative ionisation pathway of . Using a variation of threshold photoelectron photoion spectroscopy, we have determined the kinetic energy released into the two fragments over a range of energies. Using an impulsive model, the data has been extrapolated to zero kinetic energy to obtain a value for the first dissociative ionisation energy for SF5CF3 of 12.9 ± 0.4 eV. The enthalpy of formation at 0 K of SF5CF3 is determined to be -1770 ± 47 kJ mol-1, and the dissociation energy of the SF5–CF3 bond at 0 K to be 392 ± 48 kJ mol-1 or 4.06 ± 0.45 eV. The implication of this bond strength is that SF5CF3 is very unlikely to be broken down by UV radiation in the stratosphere. A similar experiment for , , and yielded values for their dissociative ionisation energies of 14.45 ± 0.20, 13.6 ± 0.1, 14.1 ± 0.5 and 14.5 ± 0.6 eV, respectively. The first two results agree with previous data on the CF3 and SF5 free radicals. The final two results yield 0 K enthalpies of formation of and to be 166 ± 52 and 4 ± 62 kJ mol-1, respectively.

Vacuum-UV fluorescence spectroscopy of PX3 (X = Cl, Br) in the range 9–25 eV

The vacuum-UV (VUV) and visible spectroscopy of PCl3 and PBr3 using fluorescence excitation and dispersed emission techniques is reported. Non-dispersed fluorescence excitation spectra have been recorded following photoexcitation with monochromatised synchrotron radiation from the Daresbury, UK source in the VUV energy range 9–25 eV, with an average resolution ofca. 0.02 eV. The use of optical filters shows that fluorescence is due to both resonant excitation of Rydberg states of PX3 (X = Cl,Br) which photodissociate to a fluorescing state of a fragment (e.g. PX2, X2) and to non-resonant excitation of excited electronic states of PX3+. Dispersed emission spectra in the UV–VIS have been recorded, with an optical resolution of ca. 4–8 nm, at the BESSY 1, Germany synchrotron source at the energies of the peaks in the excitation spectra. Four different decay channels areobserved. (a) PCl2 2B2–2B1 fluorescence in the range 350–700 nm for photon energies in the range 10–12 eV; the analogousemission in PBr2 falls outside the sensitivity range of the detection systems. (b) Cl2 and Br2 D′ 2 3Πg–A′ 2 3Πu emission at ca. 260 and 290 nm, respectively, for photon energies in the range 13–18 eV. (c) Emission from the Ẽ2E state of PCl3+ and PBr3+ for photon energies in excess of the adiabatic ionisation energies of these states of 14.6 and 13.9 eV, respectively. (d) Atomic emission from excited states of the P atom for photon energies in the range 19–25 eV. Action spectra have also been recorded at BESSY 1, in which the energy of the VUV radiation is scanned with detection of the fluorescence at a fixed, dispersive wavelength. Energy thresholds for production of the emitters can then be determined. The 2B2 state of PCl2 has a threshold energy which lies well above the thermodynamic energy needed to produce PCl2 2B2 + Cl. We conclude that it forms via a one-step P–Cl bond cleavage of a Rydberg state of PCl3. From the threshold energy for X2 D′ 2 3Πg production, we conclude that this excited ion-pair state of X2 forms in combination with the ground electronic state of PX, and not with isolated P + X atoms. Theseproducts form via a single-step photodissociation of a Rydberg state of PX3 which must now pass through a tightly constrainedtransition state. By contrast, excited states of the phosphorus atom are formed by a sequential, multi-step photodissociation of PX3*. Now the thresholds for these emissions correspond to the thermodynamic energies to form the emitting P* atom in combination with three halogen atoms.

Vacuum-UV photofragmentation of polyatomic ions using coincidence techniques

Abstract We describe an apparatus for performing three types of coincidence experiment to study state-selected photofragmentation of polyatomic ions in the gas phase; threshold photoelectron–photoion coincidences (TPEPICO), photoion–fluorescence coincidences (PIFCO), and threshold photoelectron–fluorescence coincidences (TPEFCO). Photoionisation in the vacuum-UV, 10–25 eV, is obtained using tunable radiation from the Daresbury, UK Synchrotron Radiation Source. Examples are taken from published work in CF4+, PCl3+ and PBr3+, and a range of perfluorocarbon cations CxFy+.