Analytical potential energy surface for the NH3+HNH2+H2 reaction: Application of variational transition-state theory and analysis of the equilibrium constants and kinetic isotope effects using curvilinear and rectilinear coordinates J. Chem. Phys. 106, 4013 (1997) product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, E coll , in the range 8 μeV < E coll < 20 meV. The measured rate constant exhibits a curvature in a log(k)-log(E coll ) plot from which it is concluded that the Langevin capture model does not properly describe the Ne * + NH 3 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, =, is relatively constant, ≈ 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 * + NH 3 and Ne * + ND 3 reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions. © 2014 AIP Publishing LLC.

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 µeV-4.8 meV (150 mK-56 K) collision energy range, using a neutral-neutral merged beam setup. Relative cross sections have been measured for three Ne( 3 P 2 )+ CH 3 F reaction channels by counting the number of CH 3 F + , CH 2 F + , and CH + 3 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 CH 3 F molecule, as seen by Ne( 3 P 2 ) atom, appears not to change as the collision energy is varied, in contrast to related Ne( 3 P J ) + CH 3 X (X=Cl and Br) reactions at higher collision energies.

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.

Translationally cold and slow ND3 is prepared by filtering the slow molecules from a thermal gasphase sample using a curved electrostatic hexapole guide. This filter, like the curved quadrupole guide introduced by Rempe et al. in 2003[1], selects molecules by their forward velocity and effective electric dipole moment. Here we describe two main modifications with respect to previous work: 1. A hexapole guide is used instead of a quadrupole, thus producing a harmonic potential for the linearly Stark-shifted levels of ND3. The curved guide is combined with a straight hexapole guide with independent high-voltage supplies to allow for band-pass velocity filtering. 2. State-selective laser ionization is used to obtain time-and state selective detection of the guided molecules. This enables the experimental determination of the rotational state population of the guided 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.

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.

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