Deuterium fractionation in gas-phase reactions measured in the laboratory

Abstract

In order to understand quantitatively the enrichment of deuterated molecules observed in non-equilibrium environments such as interstellar clouds, one has to know in detail many state-to-state cross sections. For detailed models, one has to treat both chemical processes such as formation and destruction of the isotopomers and physical processes such as collisional excitation and radiative transitions of the relevant molecular states. This contribution gives a short summary of experimental techniques in the field of low-energy gas-phase collisions such as flow techniques, traps, and beam methods with special emphasis on those methods which can be used to study the dynamics of H–D scrambling. The situation is illustrated with the astrophysically important H ++H 2 collision system in several isotopic variants. This fundamental system is well understood, and most experimental results are in good accordance with predictions from a dynamically biased statistical model. Less well understood are the different isotopic combinations of H 3 + colliding with H 2. The H 3 ++HD variant is discussed in a separate paper in this special issue (H 3 ++HD↔H 2D ++H 2:low-temperature laboratory measurements and interstellar implications, Planet. Space Sci., this volume) while, in this contribution, the situation is illustrated with rate coefficients for isotopic exchange in D 3 ++H 2 collisions. Other examples include the study of deuteration of hydrocarbon ions in a trap at a nominal temperature of 10 K . In particular, the rate coefficients for sequential deuteration of C 2H 2 + in collisions with HD have been measured to be 7.5×10 −10 cm 3 s −1 and 7.0×10 −10 cm 3 s −1 . Another example refers to the reactions that occur when CH 3 + is stored in para-hydrogen ( p-H 2) or normal-hydrogen ( n-H 2) containing the natural abundance of HD. At the low densities used, H–D exchange competes only with radiative association. Some hints to the next generation of experiments are given. One of the aims is to study the role of H atoms and D atoms in low-temperature gas-phase chemistry. Another aim is to combine laser and trapping techniques in order to get both state specific rate coefficients and spectroscopic information.