Direct observation of a Feshbach resonance by coincidence detection of ions and electrons in Penning ionization collisions

Abstract

Observation of molecular dynamics with quantum state resolution is one of the major challenges in chemical physics. Complete characterization of collision dynamics leads to the microscopic understanding and unraveling of different quantum phenomena such as scattering resonances. Here we present an experimental approach for observing molecular dynamics involving neutral particles and ions that is capable of providing state-to-state mapping of the dynamics. We use Penning ionization reaction between argon and metastable helium to generate argon ion and ground state helium atom pairs at separation of several angstroms. The energy of an ejected electron carries the information about the initial electronic state of an ion. The coincidence detection of ionic products provides a state resolved description of the post-ionization ion-neutral dynamics. We demonstrate that correlation between the electron and ion energy spectra enables us to directly observe the spin-orbit excited Feshbach resonance state of HeAr +. We measure the lifetime of the quasi-bound HeAr + A 2 state and discuss possible applications of our method.

Low-temperature reaction dynamics of paramagnetic species in the gas phase

“…78 As has already been noted in this section, merged beam methods have facilitated the direct observation of a number of quantum effects, with a near-merged beam set-up (a = 4.51) recently identifying evidence of Feshbach resonances as a result of coincidence measurements of different reaction products. 79 An exceptional amount of detail about the state-to-state dynamics of a Penning ionisation reaction was uncovered by examining the correlation between the energy of an ejected Penning electron and the associated Penning ion product. The coincidence part of the measurement was crucial, as this enabled the ion-electron pairs to be correlated to a selected collision event.…”

“…The additional details provided by the coincidence measurements enabled the initial and final quantum states to be ''mapped'' and a scattering resonance observed, with the detection of spin-orbit excited HeAr + Feshbach molecules. 79 The combination of a merged beam apparatus with a sensitive velocity map imaging set-up has enabled quantum resonances to be resolved in the (elastic) differential cross sections for collisions between metastable He and normal D 2 . The low collision energies and near-threshold photoionisation of the scattered He( 3 P 2 ) products allowed diffraction oscillations to be observed, in addition to resonances causing a preference for backward scattering (and enhanced scattering rates) at selected collision energies.…”

A novel crossed-molecular-beam experiment for investigating reactions of state- and conformationally selected strong-field-seeking molecules

“…[28][29][30][31][32] Moreover, the development of merged-beam experiments lead to an increased understanding of chemi-ionisation reactions of excited rare gas atoms with state-selected and even oriented atoms and molecules at very low collision energies. [33][34][35][36][37][38][39][40] All of these studies exploited the focusing effects obtained by the interaction of an inhomogeneous external field with the electric or magnetic dipole moment of the molecules in weak-field seeking quantum states, i.e., states whose energy increases with increasing field strength, and focused on dior small polyatomic systems. Of additional interest in chemistry are also larger polyatomic molecules featuring different stereoisomers.…”

The F + HD(v = 0, 1; j = 0, 1) reactions: stereodynamical properties of orbiting resonances

“…They are observed at low energies (typically <1 meV), constitute a sensitive probe of the attractive potential between approaching collision partners, [1] and can also play an important role in many dynamical processes. [1,2] They were experimentally observed in elastic scattering in the seventies [3,4] and, more recently, also in inelastic [5][6][7][8][9][10] and reactive [11][12][13] scattering using merged beam techniques [14][15][16]. In collisions involving molecules, rotational excitation can give rise to stereodynamical effects associated with the polarization of the rotational angular momentum in the scattering frame, which is defined by the relative velocity vectors of reactants and products (see, for instance, refs.…”