Vacuum-UV negative photoion spectroscopy of CH4

Using synchrotron radiation in the range 12-35 eV, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of methane. Ion yields for H$^-$, CH$^-$ and CH$_2^-$ are recorded, the spectra of CH$^-$ and CH$_2^-$ for the first time. All ions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F$^-$ from SF$_6$ and CF$_4$. Following normalisation to total vacuum-UV absorption cross sections, quantum yields for anion production are reported. There is a major discrepancy in the H$^-$ cross section with an earlier measurement, which remains unresolved. The anions arise from both direct and indirect ion-pair mechanisms. For a generic polyatomic molecule AB, the former is defined as AB $\rightarrow$ A$^-$ + B$^+$ (+ neutrals), the latter as the predissociative crossing of an initially-excited Rydberg state of AB by an ion-pair state. In a separate experiment, the threshold photoelectron spectrum of the second valence band of CH$_4$, ionisation to CH$_4^+$ A $^2$A$_1$ at 22.4 eV, is recorded with an instrumental resolution of 0.004 eV; many of the Rydberg states observed in indirect ion-pair formation converge to this state. The widths of the peaks are lifetime limited, increasing with increasing $v$ in the $v_1$ (a$_1$) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions

Vacuum-UV chemical physics in the gas phase using synchrotron radiation

Charged particles travelling in a circle at velocities close to the speed of light have the fundamental property that they emit continuous electromagnetic radiation, widely called synchrotron radiation (SR). Initially discovered in the 1950s by high-energy physicists and regarded as a hindrance, SR is now used worldwide by physicists, chemists and biologists for fundamental and applied research. The physics of such sources is well understood. Briefly, the total energy emitted by a charged particle is proportional to z$^2$E$^4$/m$^4$R$^2$, where R is the radius of curvature, and m, z and E are the mass, charge and energy of the charged particle, respectively. In most SR sources the electron is used for the charged particle, because it is easy to generate and has a high z/m ratio. Nearly all the experiments described here use the Daresbury SR source, run by the Council of the Central Laboratory of the Research Councils on behalf of the UK Government

Selected ion flow tube study of the reactions between gas phase cations and CHCl2F, CHClF2 and CH2ClF

The branching ratios and rate coefficients have been measured at 298 K for the reactions between CHCl$_2$F, CHClF$_2$ and CH$_2$ClF and the following cations (with recombination energies in the range 6.3 - 21.6 eV); H$_3$O$^+$, SF$_x$$^+$ (x = 1 - 5), CF$_y$$^+$ (y = 1  3), NO$^+$, NO$_2$$^+$, O$_2$$^+$, Xe$^+$, N$_2$O$^+$, O$^+$, CO$_2$$^+$, Kr$^+$, CO$^+$, N$^+$, N$_2$$^+$, Ar$^+$, F$^+$ and Ne$^+$. The majority of the reactions proceed at the calculated collisional rate, but the reagent ions SF$_3$$^+$, NO$^+$, NO$_2$$^+$ and SF$_2$$^+$ do not react. Surprisingly, although all of the observed product channels are calculated to be endothermic, H$_3$O$^+$ does react with CHCl$_2$F. On thermochemical grounds, Xe$^+$ appears to react with these molecules only when it is in its higher-energy $^2$P$_{1/2}$ spin-orbit state. In general, most of the reactions form products by dissociative charge transfer, but some of the reactions of CH$_2$ClF with the lower-energy cations produce the parent cation in significant abundance. The branching ratios produced in this study and by threshold photoelectron-photoion coincidence spectroscopy (preceding paper) agree reasonably well over the energy range 11 - 22 eV. In about one fifth of the large number of reactions studied the branching ratios are in excellent agreement and appreciable energy resonance between an excited state and the ground state of the ionized neutral exists, suggesting that these reactions proceed exclusively by a long-range charge transfer mechanism. Upper limits for the enthalpy of formation at 298 K of SF$_4$Cl (-637 kJ mol$^{-1}$), SClF (-28 kJ mol$^{-1}$) and SHF (-7 kJ mol$^{-1}$) are determined

A study of the reactions of trifluoromethyl sulphur pentafluoride SF5CF3 with several positive ions of atmospheric interest

An investigation of the bimolecular reactions of several positive ions of atmospheric interest, H$_3$O$^+$, NO$^+$, NO$_2^+$, O$_2^+$, H$_2$O $^+$, N$_2$O$^+$, O$^+$, CO$_2^+$, CO$^+$, N$^+$, and N$_2^+$, with the greenhouse gas SF$_5$CF$_3$ is reported. The thermal rate coefficients and ion product distributions have been determined at 300 K using a selected ion flow tube. H$_3$O$^+$, NO$_2^+$, and NO$^+$ are found to be unreactive. The reaction with O$_2^+$ proceeds with a rate coefficient significantly less than the capture value via chemical routes, in which bonds are broken and formed. The other reagent ions, H$_2$O $^+$, N$_2$O$^+$, O$^+$, CO$_2^+$, CO$^+$, N$^+$, and N$_2^+$ react with SF$_5$CF$_3$ with reaction rate coefficients close to or at the capture values. With the exception of the reaction with H$_2$O$^+$, all these efficient reactions occur by dissociative charge transfer, with CF$_3^+$ and SF$_3^+$ being the dominant product ions. CF$_3^+$ forms by direct cleavage of the S-C bond in SF$_5$CF$_3^+$, and SF$_3^+$ probably from dissociation of (SF4+)* following intramolecular rearrangement within the lifetime of (SF$_5$CF$_3^+$)*. For H$_2$O$^+$, the observed ion branching ratios suggest that the reaction proceeds via a chemical pathway. The reactions of SF$_5$CF$_3$ with cations will destroy this molecule in the upper atmosphere