Charles R. Herd

Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H‐atom production in the dissociative recombination of some protonated ions

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H‐atom contributions, fH, to the product distributions for the dissociative recombination of a series of protonated ions (N2H+, HCO+, HCO+2, N2OH+, OCSH+, H2CN+, H3O+, H3S+, NH+4, and CH+5 ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H‐atom number density using vuv absorption spectroscopy at the Lα (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO2→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of fH varying from ∼0.2 (for OCSH+ ) to 1.2 (for CH+5 ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO+2/OCSH+ and H3O+/H3S+ showe...

OH production in the dissociation recombination of H3O+, HCO2+, and N2OH+: comparison with theory and interstellar implications

Les pourcentages de radicals OH produits dans les reactions de recombinaison dissociative de H 3 O + , HCO 2 + et N 2 OH + avec des electrons a 3000 K, determines en laboratoire, sont presentes

Studies of dissociative electron attachment to the free radicals CCl3 and CCl2Br using the FALP apparatus

Abstract Measurements are reported of the rate coefficients, β, for the (secondary) dissociative attachment reactions of electrons with the unstable molecular radical species CCl 3 and CCl 2 Br formed in the primary dissociative attachment reactions of electrons with CCl 4 and CCl 3 Br molecules. This was achieved using the flowing afterglow/Langmuir probe (FALP) apparatus. The values of β for both the primary (β 1 ) and the secondary (β 2 ) attachment reactions are large, indicating that each CCl 4 and CCl 3 Br molecule introduced into a sufficiently dense thermalized plasma will result in the rapid loss of two free electrons to form two negative halide ions. To our knowledge, these are the first measurements of β for unstable molecular radical species.

Formation of Br−2 in the reactions of thermal electrons with some bromomethanes and bromoethanes

Abstract Following the recent report that Br−2 ions (together with Br− ions) are observed as products in the dissociative attachment reaction of electrons with CF2Br2 as studied using the krypton photoionisation method, we have studied the attachment reactions of CF2Br2, CFBr3, CF2BrCF2Br, CH2BrCH2Br and CH2ClCH2Br using the flowing afterglow/Langmuir probe technique at 298, 380 and 475 K with a helium bath gas pressure of about 1 torr. It was confirmed that Br−2 is indeed a product of the CF2Br2 reaction and that it is also a significant product of the CFBr3, CF2BrCF2Br and CH2BrCH2Br reactions along with the major product ion Br−. The production of Br−2 is briefly discussed and it is speculated that the Br−2:Br− product ratio may be larger at higher bath gas pressures which are used in electron capture detectors.

Development of the flowing afterglow/Langmuir probe technique for studying the neutral products of dissociative recombination using spectroscopic techniques: OH production in the HCO+2+e reaction

The flowing afterglow/Langmuir probe (FALP) technique has been extended to enable the neutral products of electron–ion dissociative recombination in thermalized afterglows to be identified by spectroscopic methods. Absolute number densities of H atoms in the afterglow have been determined using vacuum ultraviolet (VUV) absorption at the Lα wavelength. By exploiting the reaction H+NO2→OH+NO, all of the H atoms can be incorporated into OH molecules and thus observation of the intensity of laser induced fluorescence (LIF) If, obtained by exciting the (1,0) band of OH(A 2Σ←X 2Π), allows a calibration to be obtained of If against the known number density of OH X 2Π(ν‘=0) in the afterglow. Following this procedure, a recombining HCO+2 /electron afterglow was probed for production of ground state OH X 2Π(ν‘=0) using LIF and it was established that OH(ν‘=0) resulted from 17% of the recombining ground state HCO+2 ions. It was also established that a further 17% of the recombinations resulted in OH(ν‘>0), i.e., tha...

Studies of dissociative electron attachment to some haloethanes using the falp apparatus: comparisons with data obtained using non-thermal techniques

Abstract Measurements are reported of the rate coefficients, β, for the dissociative attachment reactions with electrons of the four haloethanes CH3CCl3, CH2ClCHCl2, CF3CCl3 and CF2ClCFCl2, obtained under truly thermal equilibrium conditions at 298, 385 and 470 K using the flowing afterglow/Langmuir probe (FALP) technique. In all four reactions the only product ion observed was Cl−. These values of β are compared with the equivalent β values obtained by the non-thermal swarm and krypton photoionization techniques for which β values were deduced as a function of mean electron/molecule interaction energy, E , at a fixed reactant molecule temperature of ∼ 298 K. The different senses of the variations of β with temperature and with E are discussed and are rationalized in terms of the thermal and non-thermal nature of the different experiments and not as “discrepancies” as described previously.

FALP studies of electron attachment reactions of C6F5Cl, C6F5Br and C6F5I

Abstract Electron attachment reactions of thermal electrons with C 6 F 5 Cl, C 6 F 5 Br and C 6 F 5 I molecules have been studied at 300 and 450 K using the flowing afterglow/Langmuir probe (FALP) technique. The rate coefficients at both temperatures are large and increase with increasing temperature. Different ionic products are obtained in the reactions: C 6 F 5 Cl − only for C 6 F 5 Cl, predominantly C 6 F − 5 for C 6 F 5 I, and predominantly C 6 F 5 Br − at 300 K for C 6 F 5 Br with the percentage of Br − increasing with increasing temperature and constituting ca. 40% of the total products at 450 K. Comparison of these “high pressure” FALP data with previous “low pressure” data obtained from an ion beam time-of-flight experiment, has enabled the autodetachment lifetime of the excited intermediate ion (C 6 F 5 I − ) * to be bracketed between 10 −7 and 10 −6 s. Also an upper limit to the CBr bond dissociation energy in C 6 F 5 Br of ca. 84 kcal mol −1 has been determined from a consideration of the kinetics of the C 6 F 5 Br reaction. In a parallel study, the reactions of C 6 F − 5 ions with several molecular gases have been studied using a SIFT apparatus and these results are reported in an Appendix.

A study of dissociative electron attachment to CF3SO3CH3 and CF3SO3C2H5 triflate esters using the FALP apparatus

Abstract The rate coefficients, β, for dissociative electron attachment to the triflate esters CF3SO3CH3 and CF3SO3C2H5, have been determined at 300, 385 and 475 K using the FALP apparatus. The β for both reactions are small (∼10−10 cm3 s−1) at 300 K, in contrast to the β for triflic acid, CF3SO3H, which is very large (∼ 10−7 cm3 s−1) at 300 K. In a parallel study, the reactions of F−, Cl−, Br− and I− with the two esters have been studied at 300 K using the SIFT technique. These ion/molecule reactions proceed at or near their collisional rates, again producing the triflate anion CF3SO−3, and reveal that the dissociative electron attachment reactions of these esters are exothermic.

Determinations of the products of dissociative recombination reactions

Product distributions for the electron‐ion dissociative recombination of HCO+, O2H+, HCO2+, N2OH+ and H3O+ have been determined at 300 K using a flowing afterglow/Langmuir probe apparatus. Electron (and thus ion) densities were determined using the Langmuir probe and the OH products of the reactions were detected using laser induced fluorescence spectroscopy. The OH fluorescence was quantitatively related to the OH number density by using the calibration reaction H+NO2→OH+NO and by determining the H‐atom number density using vacuum ultraviolet absorption spectroscopy. Preliminary determinations of the H atom and O atom contributions to the product distributions are also described.

Recent Falp Studies of Dissociative Recombination and Electron Attachment

The Flowing Afterglow/Langmuir Probe (FALP) apparatus was designed to study plasma reaction processes at thermal energies. It has been used to study several such processes including positive ion/negative ion mutual neutralization (Smith et al., 1978; Smith and Adams, 1983), positive ion/electron dissociative recombination (Adams et al., 1984; Adams and Smith, 1988) and electron attachment (Smith et al., 1984; Adams et al., 1986), and the rate coefficients for many such reactions have been determined, including some over appreciable temperature ranges.