Quantum-State Control and Manipulation of Paramagnetic Molecules with Magnetic Fields

Abstract

Since external magnetic fields were first employed to deflect paramagnetic atoms in 1921, a range of magnetic field–based methods have been introduced to state-selectively manipulate paramagnetic species. These methods include magnetic guides, which selectively filter paramagnetic species from all other components of a beam, and magnetic traps, where paramagnetic species can be spatially confined for extended periods of time. However, many of these techniques were developed for atomic—rather than molecular—paramagnetic species. It has proven challenging to apply some of these experimental methods developed for atoms to paramagnetic molecules. Thanks to the emergence of new experimental approaches and new combinations of existing techniques, the past decade has seen significant progress toward the manipulation and control of paramagnetic molecules. This review identifies the key methods that have been implemented for the state-selective manipulation of paramagnetic molecules—discussing the motivation, state of the art, and future prospects of the field. Key applications include the ability to control chemical interactions, undertake precise spectroscopic measurements, and challenge our understanding of chemical reactivity at a fundamental level. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 72 is April 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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

“…Many book chapters and review articles on gas-phase chemical dynamics feature reactions involving radical species. [14][15][16][17][18][19]22,[31][32][33][34][35][36] And yet, many decades after the first experimental studies of radical reaction dynamics, there are still unanswered questions about how paramagnetic species react-both in terms of the general behaviour exhibited, and when considering specific reaction systems. For example, the F + H 2 reaction system has been extensively and actively studied, both experimentally and theoretically, for more than 50 years.…”

“…This is primarily due to the challenges associated with traps for neutral species (such as radicals), which are typically much shallower than those for ionic species. 33,34 Cryogenic environments and ultra-high vacuum conditions can extend the trap lifetime of radical species (up to 10s of seconds)-providing sufficient time for reactions to be examined. 155,156 When reactions with trapped radicals do occur, only rarely can the neutral reaction products also be trapped (as they often do not possess the necessary properties to be confined by the fields, or may be formed with more kinetic energy than the depth of the trap).…”

“…For example, plans are already underway to combine a magnetic radical filter with a liquid surface apparatus, for the study of radical-liquid surface collisions with state-selected and velocity-controlled radical species. 34,176,187 This would represent a significant advancement beyond current capabilities and is an exciting future prospect.…”

“…With ever-improving detection sensitivity and the optimisation of Zeeman deceleration methods—in addition to innovative combinations of experimental techniques—the study of reactions with Zeeman-decelerated radicals is now becoming feasible. 34,71,147–149 To this end, as discussed in the following sub-section, Zeeman (and Stark) decelerators have been successfully combined with a range of traps—facilitating the confinement of radical species for an extended period of time (up to 10s of seconds). Travelling-wave (or moving magnetic trap) Zeeman decelerators, where paramagnetic species are confined in three dimensions and progressively decelerated as they travel along a series of spatially overlapped quadrupole traps, are particularly suited to being combined with traps (due to their improved transverse stability compared to conventional Zeeman decelerators).…”

High-Resolution Imaging of C + He Collisions using Zeeman Deceleration and Vacuum-Ultraviolet Detection

“… 7 − 10 One of the most stringent tests for the involved PESs can be found in measurements of angular scattering distributions, which directly reflect the differential cross sections (DCSs). 11 − 15 In this regard, the high resolution afforded by the combination of Stark 16 , 17 or Zeeman 16 , 18 , 19 deceleration to control collision partners and velocity map imaging (VMI) to probe collision products has enabled the observation of delicate features in angular scattering distributions. 20 − 23 However, the sparse availability of efficient near-threshold resonance-enhanced multiphoton ionization (REMPI) schemes, a prerequisite for obtaining high-resolution scattering images, still limits the number of systems for which the full potential of this approach can be exploited.…”