Asit B. Raksit

Carbene chemistry of cations: the chemistry of the carbene cation :C3H+ in the gas phase

Preparation de C 3 H + a partir du propene dans un spectrometre de masse SIFT et etude de ses reactions avec divers substrats (H 2 , CO, H 2 O, H 2 S, CO 2 , CH 4 , CH 3 OH, C 2 H 4 )

Reactions of O2+, O4+ and O2+·H2O ions with neutral molecules

Abstract A selected-ion drift chamber mass spectrometer has been used to determine the rate coefficients and product ions for the reactions of O2+, O4+ and O2+·H2O ions with simple inorganic and organic molecules in the gas phase at room temperature. Comparisons are made with FA and SIFT measurements. The observed reactions exhibit a wide variety of different routes including charge transfer, dissociative charge transfer, adduct formation and switching. Reaction efficiencies are discussed by comparison with rate coefficients calculated from Langevin or ADO Collision theory. The results of reaction with some chlorofluoromethanes are also included because of their importance in the stratosphere.

Isotope effects on the populations of metastable H3 and D3 molecules in neutralized ion beams

The relative abundances of metastable H*3 and D*3 molecules in collimated beams of neutralized ions are analyzed by the technique of collisional reionization (charge stripping). The fraction of metastable/dissociative species formed in the electron transfer process H+3(D+3)+K(g) → H*3(D*3)+K+ diminishes if the precursor ions have excess internal energy. The abundance ratio of H*3/D*3 for equivalent 6 keV beam fluxes increases sharply when the time span between neutralization and reionization is increased. The measurements show that a state of D*3 with low rotational quantum numbers (N>0) is decaying on the time scale of the beam transit with a lifetime of about 0.2 μs. Results are interpreted in reference to the predissociation behavior of the 2p 2A2 electronic state of the radical. The disproportionate ratio of IH*3/ID*3 ≫ 1 is a consequence of the relative abundances of precursor levels in H+3 and D+3 that can transform into metastables by electron transfer.

The effect of ion excitation on the formation of metastable states of D316O and D2F in fast charge transfer reactions

Abstract Experimental conditions are described for optimizing the formation of metastable states of D 3 16 O and D 2 F in fast electron transfer reactions of their positively charged ions with sodium atoms. Changes in the ratio of metastable/dissociative states of these radicals were monitored for ion beams generated by chemical ionization of mixtures of D 2 O or DF in N 2 , Ar, CD 4 , and D 2 . Ions from N 2 reagent gas mixtures were most efficiently converted to metastables which increased in abundance sharply as the source pressure was increased. The results show that metastables are formed only from ion distributions with low internal energies. Attempts to stabilize D 3 18 O were unsuccessful but the hydrate cluster, D 5 18 O 2 , was detected as a metastable. We explore the possibility that these radicals may be examples of vibrationally bound molecules.

Reactions of Ar2+ ions with neutral molecules

Abstract A selected-ion drift chamber mass spectrometer was used to measure rate coefficients and product channels for the reactions of Ar 2 + with H 2 , CO, O 2 , CO 2 , N 2 O and SO 2 . In most cases, the reactions were found to proceed by charge transfer. Compared with the reactions of Ar + ions, the Ar 2 + ions react quickly with these molecules, except for H 2 . The rate coefficients for the reactions of Ar 2 + with oxygen were found to be dependent on argon pressures in the drift chamber. The rate coefficients are compared with data from other authors where possible and are discussed with respect to reaction efficiencies.

Direct experimental observations of metastability in the (dideuterio) methyloxonium radical

We report the first experimental evidence for metastability in the methyloxonium radical when the oxygen is bound to two deuterium atoms; optimal conditions for generating the radical by neurtralized ion beam techniques are described.