Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube cation-molecule reaction studies of cyclic-C4F8

Abstract

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 C3F5+, at slightly higher energy C2F4+, then successively CF+, CF2+ and CF3+ are formed. The dominant ions are C3F5+ and C2F4+, with the data suggesting the presence of a barrier in the exit channel to production of C3F5+ whilst no barrier to production of C2F4+. 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, CF2+, 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.

Vacuum-UV and electron dissociative ionisation studies of SF5Cl

“…The upper limit arises due to the possible presence of an exit-channel barrier and/or a kinetic shift; if both are zero, the conversion is exact. This procedure was developed for interpretation of photoionisation yields rather than state-selected TPEPICO yields, but we have shown that the procedure is equally valid for both experiments [29,30]. For the vibrational frequencies of SF 5 + and SF 4 Cl + needed for these conversions, values for the isoelectronic neutral molecules PF 5 and PF 4 Cl were used [31,32]; the differences of the vibrational frequencies due to a change in reduced mass of the sulphur and phosphorus analogues make negligible difference to these conversions.…”

Selected ion flow tube cation–molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic-C5F8

“…The only ion which shows a significant variation from the TPEPICO branching ratios is N + (RE = 14.53 eV), as observed in previous work. 1,14 It appears that reaction with N + causes much 'softer' ionisation ( i.e. less fragmentation)…”

“…1 The other investigated negative ion formation via electron attachment. 2 c-C 4 F 8 is used extensively in the dry etching of SiO 2 , but it has a high global warming potential.…”

The photoionization dynamics of the three structural isomers of dichloroethene

“…Large isotope effects were observed for the reactions of CF 3 + , CF + and H 3 O + , cations with recombination energy (RE) less than the ionisation energy (IE) of C 2 H 2 Cl 2 , but small isotope effects were also observed for ions with RE values above the IE (C 2 H 2 Cl 2 ). We also compared the yields for each reaction to the ion yields from a photon-induced threshold photoelectron photoion coincidence (TPEPICO) study to determine the importance of long-range charge transfer in such reactions [3,4]. We have previously studied the reactions of these three isomers with small anions [5].…”