Dynamics of gas phase Ne(*) + NH3 and Ne(*) + ND3 Penning ionisation at low temperatures.

Two isotopic chemical reactions, Ne(*) + NH3, and Ne(*) + ND3, have been studied at low collision energies by means of a merged beams technique. Partial cross sections have been recorded for the two reactive channels, namely, Ne(*) + NH3 → Ne + NH3(+) + e(-), and Ne(*) + NH3 → Ne + NH2(+) + H + e(-), by detecting the NH3(+) and NH2(+) product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, Ecoll, in the range 8 µeV < Ecoll < 20 meV. The measured rate constant exhibits a curvature in a log(k)-log(Ecoll) plot from which it is concluded that the Langevin capture model does not properly describe the Ne(*) + NH3 reaction in the entire range of collision energies covered here. Calculations based on multichannel quantum defect theory were performed to reproduce and interpret the experimental results. Good agreement was obtained by including long range van der Waals interactions combined with a 6-12 Lennard-Jones potential. The branching ratio between the two reactive channels, Γ = [NH2(+)]/[NH2(+)] + [NH3(+)], is relatively constant, Γ ≈ 0.3, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalized in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the Ne(*) + NH3 and Ne(*) + ND3 reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions.

Low-temperature collisions between neutral molecules in merged molecular beams.

We have developed an experiment for the investigation of neutral molecular collisions in the gas phase at temperatures as low as 100 mK. These low temperatures are obtained by merging two supersonic expansions, using an electric and a magnetic guide, and by matching the velocities of the beams. Since the energy available for the collisions, or the temperature, is determined only by the relative velocity of the reaction partners this enables the study of chemical processes at very low temperatures without the need to prepare slow molecules in the laboratory frame of reference. This paper describes the method and presents results on the Ne((3)P2)+NH3 Penning ionization.

Study of the Ne((3)P2) + CH3F electron-transfer reaction below 1 K.

Relatively little is known about the dynamics of electron-transfer reactions at low collision energy. We present a study of Penning ionization of ground-state methyl fluoride molecules by electronically excited neon atoms in the 13 µeV­4.8 meV (150 mK­56 K) collision energy range, using a neutral­neutral merged beam setup. Relative cross sections have been measured for three Ne((3)P2) + CH3F reaction channels by counting the number of CH3F(+), CH2F(+), and CH3(+) product ions as a function of relative velocity between the neon and methyl fluoride molecular beams. Experimental cross sections markedly deviate from the Langevin capture model at collision energies above 20 K. The branching ratios are constant. In other words, the chemical shape of the CH3F molecule, as seen by the Ne((3)P2) atom, appears not to change as the collision energy is varied, in contrast to related Ne((3)PJ) + CH3X (X = Cl and Br) reactions at higher collision energies.

Preparation of translationally cold neutral molecules.

Efforts at EPFL to obtain translationally cold neutral molecules are described. Active deceleration of polar molecules is performed by confining the molecules in moving three-dimensional electrostatic traps, and by appropriately choosing the velocity of those traps. Alternatively, cold molecules can be obtained by velocity filtering. Here, the velocity of the molecules is not changed, but instead the cold molecules are extracted from a thermal sample by using the competition between the electrostatic force and the centrifugal force inside a bent electrostatic guide for polar molecules.

Dynamics of individual rotational states in an electrostatic guide for neutral molecules.

The guiding properties of individual rotational states of deuterated ammonia inside an electrostatic hexapole guide are presented. The guide is combined with resonance enhanced multiphoton ionization detection to assess the guiding probabilities and velocity distributions as a function of the rotational quantum numbers J and K. Due to the differences in the effective dipole moment these states are prepared at significantly different translational temperatures. A model is presented that describes the velocity-distribution for individual M-sublevels, and this model is also used to determine a rotational-state dependent translational temperature. Furthermore, the hexapole field has been replaced by a dipole field in order to obtain a band-pass velocity filter. However, the resulting change in the final velocity distribution is similar to that obtained from a hexapole guide but with increased backing pressure, leading to collisional acceleration of the slow molecules.