Oriented Polar Molecules Trapped in Cold Helium Nanodropets: Electrostatic Deflection, Size Separation, and Charge Migration

Abstract

Helium nanodroplets doped with polar molecules are studied by electrostatic deflection. This broadly applicable method allows even polyatomic molecules to attain sub-Kelvin temperatures and nearly full orientation in the field. The resulting intense force from the field gradient strongly deflects even droplets with tens of thousands of atoms, the most massive neutral systems studied by beam "deflectometry." We use the deflections to extract droplet size distributions.Moreover, since each host droplet deflects according to its mass, spatial filtering of the deflected beam translates into size filtering of neutral fragile nanodroplets. As an example, we measure the dopant ionization probability as a function of droplet radius and determine the mean free path for charge hopping through the helium matrix. The technique will enable separation of doped and neat nanodroplets and size-dependent spectroscopic studies. * Present address: Modern Electron, Bellevue, WA 98007, USA Introduction.-If the internal and relative motion of molecules is cooled into the sub-Kelvin range, it becomes possible to observe and steer their reactions with precision, to determine their physical parameters and structures with high accuracy, and to use external fields to finely control their motion and orientation [1-4]. For example, buffer-gas cooling [5] can be employed as an entryway to electrostatic guiding and ultracold trapping [6,7], merged beams enable exploration of chemical reactions in the quantum regime [8], and Stark deflection of small molecules in a supersonic beam can be used to spatially separate their low rotational states and conformers [9].While high level of control has been demonstrated for individual small molecules, pursuing it for larger polyatomic systems becomes increasingly demanding [10]. Their rotational spectra are more congested, their degrees of freedom are less efficiently and uniformly cooled by nozzle expansion [10,11], and their higher masses reduce the deflection.A powerful tool to cool and study molecules of a wide range of sizes is "helium nanodroplet isolation" [12][13][14][15][16]. Molecules are entrapped and transported by a beam of 4 He N nanodroplets generated by expansion of helium gas through a cryogenic nozzle. Nanodroplets cool by evaporation upon exiting the nozzle, reaching an internal temperature of only T 0 =370 mK and turning superfluid. This temperature is set by the surface binding energy of helium atoms [17,18] and has been verified, as has the onset of superfluidity, by rotational spectroscopy of entrapped molecules [12]. When the droplet beam passes through one or more vapor-filled cells, atoms and molecules are readily picked up, cooled by heat transfer to the helium matrix (evaporation of surface helium atoms promptly brings the complex back to T 0 ), and carried along by the droplet beam.This method is unique in being applicable to a variety of molecules and atoms: essentially all that is required for embedding is the availability of ~10 -6 -10 -4 mbar of vapor. Its other key feature...

Strong permanent magnet gradient deflector for Stern–Gerlach-type experiments on molecular beams

“…Helpfully, in this case a reference undeflected profile can be acquired from an undoped, nonmagnetic, droplet beam without needing to switch off the field. This is analogous to recent electric deflection experiments on nanodroplets with embedded polar molecules 12,13 in which undoped nanodroplets were unaffected by passage between high-voltage deflection electrodes.…”

“…For example, as mentioned in the Introduction, the pole shape chosen for this particular unit is based on its intended use for magnetic deflection experiments on ultra-cold paramagnetic atoms and molecules embedded in superfluid helium nanodroplets. In such experiments the variation of droplet masses 12,33 is of greater consequence than the slight variation in the defocusing component of the field.…”

Towards chemistry at absolute zero

“…Reactions can also be studied at temperatures ∼1 K within He nanodroplets. Species incorporated into He nanodroplets are cooled by elastic and inelastic collisions, with the energy dissipated by the evaporation of He atoms from the droplet 39 . As the He nanodroplets are typically chemically inert and spectroscopically transparent, they provide a useful means for studying clusters and reaction complexes at low temperatures, although the He nanodroplet 'solvent' may have some impact on the reaction rate 40,41 .…”

Cold and controlled chemical reaction dynamics

“…The droplets cool by evaporating atoms off the surface, reaching a final temperature of 0.37 K (for nanodroplets containing 4 He, with this temperature determined by the surface binding energy of the He atoms). 158,159 Droplet beams can be directed through one or more regions containing the reactant species of interest, with these dopant species readily picked up by (and incorporated into, or bound on the surface of) the droplets. One can select the experimental conditions-for example, by designing an apparatus with several pick-up cellsto ensure that the reactant species are incorporated into each droplet in a controlled fashion.…”