Observation of Resonances in Penning Ionization Reactions at Sub-Kelvin Temperatures in Merged Beams

Abstract

Experiments have lagged theory in exploring chemical interactions at temperatures so low that translational degrees of freedom can no longer be treated classically. The difficulty has been to realize in the laboratory low-enough collisional velocities between neutral reactants to access this regime. We report here the realization of merged neutral supersonic beams and the manifestation of clear nonclassical effects in the resulting reactions. We observed orbiting resonances in the Penning ionization reaction of argon and molecular hydrogen with metastable helium, leading to a sharp absolute ionization rate increase in the energy range corresponding to a few degrees kelvin down to 10 millikelvin. Our method should be widely applicable to many canonical chemical reactions.

Beyond universality: Parametrizing ultracold complex-mediated reactions using statistical assumptions

“…This provides a means to estimate one of the parameters that characterizes ultracold processes from first principles. The increasing availability of cold and ultracold samples of atoms and molecules has sprung great interest in chemical reactions at very low temperatures [1][2][3][4][5]. Although new experimental approaches [5] appear highly promising, advances in the field are hampered by technical problems in producing most molecules at low temperatures and high enough densities.…”

“…The model has been recently applied to a variety of systems [9,16,17]. In particular, for the Penning ionization of Ar by He( 3 S) [5], the rate coefficients have been fitted in a wide range of collision energies using only two parameters [9]. However, the parameters of the model are phenomenological and they had not been determined before from first principles.…”

“…Although new experimental approaches [5] appear highly promising, advances in the field are hampered by technical problems in producing most molecules at low temperatures and high enough densities. In contrast to neutral species, ions can be easily trapped and cooled.…”

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

“…[1][2][3][4][5][6][7][8] In recent years, detailed investigation of scattering processes has attracted considerable interest in particular in the context of cold molecule formation and trapping. 9,10 Recent experimental developments have opened the route to investigations of molecular scattering in supersonic expansions at collision energies substantially below 10 K. 5,[11][12][13][14][15][16][17][18][19][20][21] The collision energy depends on the relative velocity of the reactants. In a molecular beams experiment this is determined not just by individual beam speeds but also by the angle between them.…”

“…Several studies in a) andreas.osterwalder@epfl.ch the past years have demonstrated the power of this approach, addressing different aspects of Penning ionization reactions in collisions of excited rare gas atoms with other atoms, diatomics, or light polyatomic molecules. [13][14][15][17][18][19][20][21] Only a handful of low-energy collision studies of polyatomic molecules have been performed to date. [18][19][20][21] In general, higher mass and increasingly complex internal structure of the colliding molecules open more inelastic and reactive channels.…”