Andreas Osterwalder

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.

Observation of orbiting resonances in He(3S1) + NH3 Penning ionization

Resonances are among the clearest quantum mechanical signatures of scattering processes. Previously, shape resonances and Feshbach resonances have been observed in inelastic and reactive collisions involving atoms or diatomic molecules. Structure in the integral cross section has been observed in a handful of elastic collisions involving polyatomic molecules. The present paper presents the observation of shape resonances in the reactive scattering of a polyatomic molecule, NH3. A merged-beam study of the gas phase He((3)S1) + NH3 Penning ionization reaction dynamics is described in the collision energy range 3.3 μeV < Ecoll < 10 meV. In this energy range, the reaction rate is governed by long-range attraction. Peaks in the integral cross section are observed at collision energies of 1.8 meV and 7.3 meV and are assigned to ℓ = 15,16 and ℓ = 20,21 partial wave resonances, respectively. The experimental results are well reproduced by theoretical calculations with the short-range reaction probability Psr = 0.035. No clear signature of the orbiting resonances is visible in the branching ratio between NH3 (+) and NH2 (+) formation.

Cold and Controlled Molecular Beams: Production and Applications

The field of cold molecules has become an important source of new insight in fundamental chemistry and molecular physics. High-resolution spectroscopy benefits from translationally and internally cold molecules by increased interaction times and reduced spectral congestion. Completely new effects in scattering dynamics become accessible with cold and controlled molecules. Many of these experiments use molecular beams as a starting point for the generation of molecular samples. This review gives an overview of methods to produce beams of cold molecules, starting from supersonic expansions or effusive sources, and provides examples of applications in spectroscopy and molecular dynamics studies.

Study of the Ne(3P2) + 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 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)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.

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.

State-selective detection of velocity-filtered ND₃ molecules

Translationally cold and slow ND3 is prepared by filtering the slow molecules from a thermal gas-phase sample using a curved electrostatic hexapole guide. This filter, like the curved quadrupole guide introduced by Rangwala et al. Phys. Rev. A 67 043406 (2003) selects molecules by their forward velocity and effective electric dipole moment. Here we describe two main modifications with respect to previous work: (1) A segmented hexapole guide is used that produces a harmonic potential for the linearly Stark-shifted levels of ND3. A curved guide is combined with a straight hexapole guide, and independent high-voltage supplies are employed to allow for bandpass 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.

A traveling wave decelerator for neutral polar molecules

Recently, a decelerator for neutral polar molecules has been presented that operates on the basis of macroscopic, three-dimensional, traveling electrostatic traps [A. Osterwalder, S. A. Meek, G. Hammer, H. Haak, and G. Meijer, Phys. Rev. A 81, 051401 (2010)]. In the present paper, a complete description of this decelerator is given, with emphasis on the electronics and the mechanical design. Experimental results showing the transverse velocity distributions of guided molecules are shown and compared to trajectory simulations. An assessment of non-adiabatic losses is made by comparing the deceleration signals from (13)CO with those from (12)CO and with simulated signals.

Traveling-wave deceleration of heavy polar molecules in low-field-seeking states

We demonstrate the deceleration of heavy polar molecules in low-field-seeking states by combining a cryogenic source and a traveling-wave Stark decelerator. The cryogenic source provides a high-intensity beam with low speed and temperature, and the traveling-wave decelerator provides large deceleration forces and high phase-space acceptance. We prove these techniques using YbF molecules and find the experimental data to be in excellent agreement with numerical simulations. These methods extend the scope of Stark deceleration to a very wide range of 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.

Communication: Importance of rotationally inelastic processes in low-energy Penning ionization of CHF3

Low energy reaction dynamics can strongly depend on the internal structure of the reactants. The role of rotationally inelastic processes in cold collisions involving polyatomic molecules has not been explored so far. Here we address this problem by performing a merged-beam study of the He((3)S1)+CHF3 Penning ionization reaction in a range of collision energies E/kB = 0.5-120 K. The experimental cross sections are compared with total reaction cross sections calculated within the framework of quantum defect theory. We find that the broad range of collision energies combined with the relatively small rotational constants of CHF3 makes rotationally inelastic collisions a crucial player in the total reaction dynamics. Quantitative agreement between theory and experiment is only obtained if the energy-dependent probability for rotational excitation is included in the calculations, in stark contrast to previous experiments where classical scaling laws were able to describe the results.