Helium droplets

Bartelt A, Close J D, Federmann F, Quaas N and Toennies J P 1996 Cold metal clusters helium droplets as a nanoscale cryostat Phys. Rev. Lett. 77 3525... 

M. Behrens, R. Frochtenicht, M. Hartmann, J. G. Siebers, U. Buck, and F. C. Hagemeister, Vibrational spectroscopy of methanol and acetonitrile clusters in cold helium droplets. J. Chem. Phys. Ill, 2436 2443 (1999). 

Figure 2. Diffraction camera for single-molecule electron diffraction. A Lanthanum hexaboride electron source is used. The laser and associated optics is rotated after each data readout for a new molecular beam orientation. Organic molecules are picked up within liquid helium droplets to form a molecular beam traversing the electron beam.

Theoretical and experimental methods are complementary to each other. For example, computational methods have suggested that the amino groups of nucleic acid bases in the ground state are nonplanar [58], However, experimental evidence for amino group nonplanarity was obtained only recently when Dong and Miller [59] measured the vibrational transition moment angles in adenine and three tautomers of cytosine in helium droplets. 

The ability to dope impurity atoms or molecules into large helium clusters by a pick-up method, pioneered by the groups of Toennies and Scoles, has helped make studies in superfluid helium clusters more accessible. In this method, an expansion through a nozzle produces a beam of helium clusters. Under appropriate conditions, helium droplets comprising up to 10 helium atoms can be formed. These droplets then traverse a collision cell containing a foreign gas at a pressure of 10 -10 Pa. Atoms or... 

Astronomers now believe that helium, with a density nearly twice that of hydrogen, tends to fall inward from the planet s outer atmosphere toward its core. If one were able to travel through the hydrogen-rich soup of the lower atmosphere, it might he possible to detect a "rainfall of helium droplets, which are immiscible in hydrogen, falling from the "sky," the planet s outer atmosphere. 

This model might also explain the observed depletion of neon in the planet s atmosphere. The Galileo probe s Neutral Mass Spectrometer (GPMS) observed an abundance of neon about one tenth that in the solar atmosphere. Perhaps, researchers hypothesize, neon dissolves in helium droplets as they form in the atmosphere and fall into the planet s interior. This phenomenon is possible because helium and neon, unlike helium and hydrogen, are completely miscible. 

Fig. 15. Spectrum of the free C-H stretching region of the HCN polymer chains, showing clusters up to at least the heptamer. A linear heptamer chain is shown as an inset. This spectrum was obtained in the presence of a large electric field used to orient the polar chains within the helium droplet, thus collapsing the rotational band contours into a single peak for each cluster size. ...

This technique provides access to the spectroscopy of high-spin states which are not readily formed in the gas phase, and furthermore it allows these clusters with the helium droplets to be investigated by observing their level shifts. 

F. Madeja, M. Havenith et al., Polar isomer of formic acid dimer formed in helium droplets. J. Chem. Phys. 120, 10554 (2004)... 

K. von Haeften, A. Metzelthin, S. Rudolph, V. Staemmler, M. Havenith, High resolution spectroscopy of NO in helium droplets. Phys. Rev. Lett. 95,215301 (2005)... 

The present review is devoted to a relatively recent development, the use of helium droplets as an ultra-cold gentle nanoreactor for studying elementary chemical processes. These experiments take full advantage of the... 

Fig. 7.1. Schematic diagram illustrating a typical experiment in which a helium droplet serves as an ultra-cold gentle nanoreactor. In some experiments (not shown) the products ejected from the droplet may also be detected. In many experiments the resonant absorption of the pump and/or probe lasers is monitored by observing the depletion of the signal of the droplets measured with a detector at the far end of the apparatus.

Since, as mentioned in the Introduction, helium droplets are definitely liquid they possess the remarkable ability to pick up all molecules which they encounter as they move through the apparatus. Because of their large size the pick up cross sections are of the order of the geometrically predicted area of 10 -10. This, of course, means that the residual vacuum has... 

Fig. 7.4. Dependence of the depletion signals for infra-red bands of (SFe) clusters embedded inside helium droplets with N = 4.1 10 atoms as a function of the SFe pressure in a pickup cell. ° Solid curves are best fits based on a Poisson distribution (Eq. (7.2.1)) for a pick up cross section of 3900 A. The insert shows a schematic representation of the pick-up, coagulation and resulting heating induced evaporation. ...

One of the surprising observations made in connection with the clusters formed inside helium droplets is that they frequently adopt unique structures. This was first observed in the infra-red spectra of clusters of polar molecules such as cyanoacetylene (HCCCN). Once inside the cold droplet each additional molecule is rapidly cooled translationally and subsequently aligned by the electric field of its predecessors and as a result they combine... 

---