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Our research focuses on chemistry and physics in ultra-cold superfluid helium droplets.
Our research focuses on chemistry and physics in ultra-cold superfluid helium droplets.
In He droplet study, we pursue two paths:
In He droplet study, we pursue two paths:
i) droplets as a gentle matrix for study of molecules, ions as well as molecular and ionic clusters.
i) droplets as a gentle matrix for study of molecules, ions as well as molecular and ionic clusters.
ii) droplets as the object of the low temperature physical study of superfluidity in finite systems.
ii) droplets as the object of the low temperature physical study of superfluidity in finite systems.
Our experimental techniques range from infrared spectroscopy in the lab to femtosecond x-ray diffraction at free electron lasers (EFL).
Spectroscopy of molecular ions in He droplets. From JPCA 2020.
He droplets are produced from a cold pulsed nozzle in the source chamber and capture molecules in the pickup chamber. The droplets are ionized in the external ionizer and interact with the counter propagating infrared laser beam once in the ion region of the probe ionizer. The released ions are mass selected by the quadrupole mass spectrometer.
Spectrum of Zundel H5O2+ in He droplets (red), compared with the spectra of He-H5O2+ (blue) and Ar-H5O2+ (black). From JPCA 2020.
The spectra in He droplets appear to have about a factor of 10 narrower bands and similar matrix shifts as compared to those obtained via tagging with He and Ar atoms. The results confirm the calculated structure of the free Zundel ion, where the proton is equidistant from the two water units.
X-ray coherent diffraction experiment with He droplets. From Science 2014.
Helium droplets are produced via nozzle expansion (A) and are collimated by a skimmer (B). They pass through the pickup chamber (C) and are doped with Xe before interacting with the FEL beam (D). Scattering images are collected on a pnCCD detector (E).
Diffraction patterns from Xe-doped rotating droplets with various shapes. From PRL 2020.
The rotation of isolated superfluid 4He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns (a1-c1) provide simultaneous access to the shape of the droplets and the vortex arrays they host. In a spheroidal droplet (a2) vortices form a triangular lattices that extend throughout the entire droplet volume. Vortices are arrange along elliptical contours in an ellipsoidal droplet ( b2), whereas in a capsule-shaped droplet (c2), vortices form a distorted triangular lattice. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity.
Spectra of HCl – H2O clusters. From JPC Letters 2010.
He droplets are unique hosts for obtaining and interrogating molecular clusters. Molecules embedded in sequence move freely inside the superfluid droplets and combine into clusters. The structures of the clusters as shown in the inserts were obtained by comparison with calculations and electric dipole measurements.
Spectra of OCS molecules in superfluid 4He and normal-fluid 3He droplets. From Science 1998.
High resolution infrared spectra demonstrated that molecules rotate freely in superfluid 4He droplets. The analysis of the spectra revealed that moments of inertia of the embedded molecules are enlarged compared to the gas phase. The precise increase factor characterizes interaction of the molecule with its superfluid environment. The rotational structure in the spectra is lacking for molecules in non-superfluid 3He droplets. Experiments with mixed 4He :3He droplets give evidence that 4He clusters of less than 100 atoms are superfluid
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