Studying unusual molecular clusters and radicals by superfluid Helium Nanodroplet Isolation Spectroscopy (HENDI)

(in collaboration with K.K. Lehmann)


Why are we doing it?


This series of images (counterclockwise, starting from top left corner) shows a rendition (not to scale) of the sequence of events culminating in the formation of a cold molecular complex (in this case the H2O dimer) inside a He droplet.

  1. An H2O molecule approaches a droplet already doped with another H2O molecule.
  2. The second H2O molecule is captured. The solvation energy is released, causing evaporation of some He atoms.
  3. The droplet has returned to its equilibrium temperature (0.38 K).
  4. The binding energy of the two molecules is released. Again this results in He evaporation.
  5. The final equilibrium state, with the two H2O molecules bound into a complex at 0.38 K.

Due to the low temperature and cooling during complexation, very unusual clusters can be synthesized in this way.

Helium droplets are superfluid; they can capture atoms/molecules (even large ones) and cool them to 0.38 K, without impeding their motion (translation and rotation) within the droplet; they can be used to form complexes by multiple capture. Because the release of binding energy tends to destroy the droplets, weakly bound complexes survive, while strongly bound ones are lost from the beam. Most of these facts were unknown just a few years ago, and a good fraction of them was discovered in our laboratory. The two questions we're asking now are:

  1. Can we use spectroscopy to understand the dynamics inside a droplet (superfluidity in particular)?
  2. How far can we push the syntetic ability of droplets? (for example, can we stabilize unusual organic radicals in the same way as we did inorganic ones?)

 Molecular motion and superfluidity in He droplets

Imagine you land on a liquid planet, something like a huge sphere made entirely of water. Gravity pulls you towards the center of the planet and---depending on whether your density is higher or lower than that of water--- you either get inside or sit on a small dimple on the surface of the planet. You also have some kinetic energy, which means you do not stay put, but rather you are ---again depending on your density, either randomly moving near the center of the planet, or floating on its surface.
The same happens to atoms and molecules ("dopants") captured by He droplets, except that van der Waals forces are involved instead of gravity.
In complete analogy to the planet picture, dopants still have the "option" of going inside or remaining on the surface of the droplet. What counts here is no longer the density, but rather the van der Waals interaction of the dopant with He: if it is stronger than the He-He van der Waals interaction, then the dopant will solvate and go inside, otherwise it will remain on the surface. The residual thermal kinetic energy (remember the temperature is low but finite: 0.38 K) will make the dopant rattle inside, or surf outside. So far, only alkali (and some alkaline earth) atoms have been observed to reside on the surface; everything else is solvated inside.
Now, at 0.38 K He is no ordinary liquid: it is superfluid, which in our case means the motion of the dopant is minimally hindered. In particular: (1) molecular rotation is not damped (unlike in any other liquid, rotationally resolved spectra can be observed even for strongly asymmetric molecules) (2) two or more dopants in the same droplet will quickly find each other and form a van der Waals bound complex, whose geometry might be determined by the presence of barriers in the dopant-dopant interaction potential. One of the main goals in the study of the mechanisms of chemical reactions is the stabilization of unstable species. Helium nanodroplets have turned out to be an ideal medium to reach this goal. At the same time, by studying the rotation of molecules in the droplet, we have an excellent opportunity to study superfluidity in a finite-size object, opening a new window on this interesting and a bit misterious phenomenon.

How do we do it?

We produce He clusters by expansion in vacuum of high pressure (50-100 atm) He gas through a small orifice, or nozzle, (10 µm diameter) kept at a temperature of 15-30 K. Under these conditions, a supersonic beam of He is produced; rapid cooling of the gas occurs in the first few hundred microns downstream the nozzle, and He atoms aggregate into clusters of a few thousands molecules each. The clusters fly off into vacuum and cool by evaporation to the temperature of 0.38 K, where their vapor pressure becomes negligible.
The supersonic beam has a speed of 400 m/s, so the clusters survive about a millisecond (the time it takes to travel from the source to the detector). During this time we (1) load them, (2) excite the doped clusters with our favorite tunable radiation source, (3) detect their interaction with radiation.

  1. Loading: is accomplished by passing the clusters through a pick-up cell containing 10-4 torr of the atoms/molecules of interest. Each cluster picks up one or more atoms/molecules, much the same way as flies on the windshield of a car; therefore the probability of having a chosen number of atoms/molecules per cluster follows a Poisson distribution which depends on the pressure in, and length of, the pick-up cell.
  2. Excitation: can be done with any source of tunable radiation: microwaves, infrared, visible, or ultraviolet, as long as the dopant absorbs at some frequency within the tuning range.With powerful radiation sources, or when sensitivity is not an issue, the radiation is made to cross the supersonic beam orthogonally. Otherwise, to maximize the interaction region, either the radiation is propagated collinearly with the beam or multi pass arrangements are used.
  3. Detection: when the excited complexes do emit fluorescence, photon counting (with a photomultiplier) gives the best sensitivity; in addition, the fluorescence can be dispersed with a monochromator and frequency-resolved emission spectra can be collected. This is the standard choice when studying electronic excitations of atoms/molecules in He clusters. With H2 clusters, fluorescence may be quenched, and beam-depletion techniques may be more suitable. The latter are also used in all the cases when the molecule's fluorescence is too slow (infrared, microwaves) and its collection is not an option. Beam depletion detection is based instead on the concept that the energy of the absorbed radiation causes part of the beam to evaporate in random directions, thus decreasing the on-axis flux. Detection is accomplished by sensing the kinetic energy of the beam (with a bolometer), or with any other detector that can probe the intensity of the on-axis helium flux.

What we have done

Since 1992, when we detected the first infrared spectrum of a molecule (SF6) in He droplets, our group has carried out a substantial amount of research using HENDI spectroscopy. For sake of simplicity, we will divide the work into two fields: (a) electronic spectroscopy of metal oligomers (1-3 atoms) and (b) rotational/vibrational spectroscopy of molecules.

Electronic spectra of metal atoms

We have collected the spectra of several electronic transitions of Li, Na, K, Mg, Al atoms on He droplets. The shifts (wrt the gas phase transitions) and widths of the spectra tell us about the location of the dopant atom. Li, Na, K stay on the surface; Mg and Al are fully solvated inside. We have calculated the He-dopant interaction potentials and we are able to justify our observations in terms of the energetics of solvation.
We have also observed and characterized Li2, Na2, K2, NaK, Na3, K3 and found that on helium droplets, high spin molecules (e.g. triplet Na2 and quartet Na3, all parallel-spin species) are selectively stabilized over their low spin counterparts (singlet Na2 and doublet Na3). This happens because the latter species liberate much more energy upon formation and destroy the dropletss. Most of the spectra were observed for the first time, because the high-spin species are either very difficult (dimers) or next to impossible (trimers) to form in the gas phase. We have also recorded dispersed fluorescence spectra of most of the above transitions. The spectra showed that intriguing and unexpected events were taking place, such as the formation and desorption of Na*He and K*He excimers (never observed before), and the formation of singlet Na2 and K2 after excitation of the triplet dimers and the quartet trimers. The latter process is due to an intersystem crossing from the triplet (quartet) to the singlet (doublet) manifold. Since a van der Waals bond is converted into a covalent bond, this can be considered as a prototype laser-initiated chemical reaction.
Finally, we have collected the time-resolved fluorescence arising from some of those transitions, which allowed us to determine the timescale for the spin-flip processes mentioned above (... ps).

Rotational/vibrational spectra of molecules

In the last three years we have built and operated a resonant cavity coupled IR laser spectrometer capable of producing rotationally resolved overtone spectra of molecules containing the CH stretch chromophore, solvated in He droplets. The instrument (which uses bolometric detection) is also capable of producing MW and MW-MW double resonance spectra. We have found that while the molecules are indeed free to rotate in the superfluid medium with very little damping, their moment of inertia is increased by the motion of the liquid around them, by an amount that depends on both the molecule's asymmetry and the strength of its interaction with helium. We have devised (in collaboration with F. Dalfovo) a simple hydrodynamic model which seems to account fairly well for the increase in moment of inertia experienced by the molecules when they become solvated by superfluid liquid helium. In addition, from MW-MW and MW-IR double resonance experiments (the latter in collaboration with R.E. Miller) we have learned about the dynamics of these molecules inside a He droplet, and in particular, we have determined the rotational relaxation timescale: 1-10 ns. This relaxation time is extremely long (× 103) compared to the relaxation in a normal liquid.

What plans do we have for the near future?

We want to switch from chemically stable molecules to radicals: initially we will just show that radicals can be produced and loaded on helium droplets (for which we need a clean and relatively cold discharge source). At a later stage, we will put more than one dopant onto each droplet, selecting species for which a small (a few units of kT at 0.38 K) barrier to recombination is believed to exist, and we will initiate the recombination process with a suitably chosen pulse of radiation.
This will probe the nature of the barrier (via the absorbed photon), as well as the reaction products (via the emitted photon). A prototype of this experiment has been already demonstrated with Na3 (quartet) as the precursor, and Na2(singlet)+Na as the products.

Two important series of experiments will then be tried. In the first we will try to detect high-spin radicals formed by capturing in the droplets two "species", such as two CºN radicals. The stable molecule is of course NºC–CºN, but the dipole-dipole interaction may lead to the NºC···NºC van der Waals complex, even if the spin state of the complex may be favorable to the ricombination. In the second series of experiments we will use a newly acquired powerful uv laser (one of the very first of its kind, to be delivered in October 2000) to dissociate a molecule inside the droplet and study the fragment so formed. Are these going to be trapped by the cluster and eventually recombine? If this is the case, can the fragments' spin be manipulated to hinder the recombination?

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