Phase Separation in Cold Para-H_{2} D_{2} Clusters.

Low temperature phase separation in mixtures of ^{3}He and ^{4}He isotopes is a unique property of quantum fluids. Hydrogen has long been considered as another potential quantum liquid and has been predicted to be superfluid at T≤1 K, well below freezing temperature of ≈14 K. Phase separation has also been predicted in mixtures of para-H_{2} and D_{2} at temperatures ≤3 K. To defer the freezing, we produced clusters containing para-H_{2} and D_{2} at an estimated temperature of ≈2 K whose state was studied by vibrational Raman spectroscopy. The results indicate that the clusters are liquid and show the phase separation of the isotopes. The phase separation is further corroborated by the quantum molecular dynamics simulation.

Ro-vibrational Spectra of (para-H2 )N -CH4 in He Droplets.

In this work, we report on the infrared spectroscopic study of clusters of CH4 molecules with up to N=80 para-hydrogen molecules assembled inside He droplets. Upon increase of the number of the added para-hydrogen molecules up to about N=12, both the rotational constant, B, and the origin frequency of the υ3 band of CH4 decrease gradually. In the range of 6 ≤N≤12, the spectra indicate some abrupt changes of B and υ3 with both values being approximately constant at N≥12. The origin of this effect is discussed. Comparison of the spectra of methane molecules in para-hydrogen clusters to that in solid para-hydrogen is also presented.

Infrared spectra in the 3 µm region of ethane and ethane clusters in helium droplets.

Ethane and ethane clusters (N ≈ 10(2)-10(4)) were studied inside helium droplets with infrared laser spectroscopy. The spectra were measured in the 2880-3000 cm(-1) range, which covers the ν5, ν(8+11), and ν7 vibrational bands of ethane. Partially resolved rotational fine structure in the spectrum of the monomer reveals solvent-induced band origin blue shifts that are each approximately 1 cm(-1). The effective B(He) and A(He) rotational constants were found to be reduced by 52% and 16% in comparison to their gas phase values, respectively. Spectra of the clusters show the same three bands shifted toward low frequency by approximately 10 cm(-1) because of intermolecular interactions in the clusters. The spectra of the ethane clusters are dominated by the ν7 band, whereas the relative intensities of the ν5 and ν(8+11) bands are about a factor of 5 weaker than for single molecules or for solid ethane, the spectrum of which is also reported here.

Fast nuclear spin conversion in water clusters and ices: a matrix isolation study.

Single water molecules have been isolated in solid Ar matrices at 4 K and studied by rovibrational spectroscopy using FTIR in the regions of the ν(1), ν(2), and ν(3) modes. Upon nuclear spin conversion at 4 K, essentially pure para-H(2)O was prepared, followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the condensed water upon annealing to T ≥ 250 K indicate fast reconversion of nuclear spin to equilibrium conditions. Our results indicate that nuclear spin conversion is fast in water dimers and larger clusters, which preclude preparation of concentrated samples of para-H(2)O, such as in ice or vapor.

Sizes of large He droplets.

Helium droplets spanning a wide size range, N(He) = 10(3)-10(10), were formed in a continuous-nozzle beam expansion at different nozzle temperatures and a constant stagnation pressure of 20 bars. The average sizes of the droplets have been obtained by attenuation of the droplet beam through collisions with argon and helium gases at room temperature. The results obtained are in good agreement with previous measurements in the size range N(He) = 10(5)-10(7). Moreover, the measurements give the average sizes in the previously uncharacterized range of very large droplets of 10(7)-10(10) atoms. The droplet sizes and beam flux increase rapidly at nozzle temperatures below 6 K, which is ascribed to the formation of droplets within the nozzle interior. The mass spectra of the droplet beam upon electron impact ionization have also been obtained. The spectra show a large increase in the intensity of the He(4) (+) signal upon increase of the droplet size, an effect which can be used as a secondary size standard in the droplet size range N(He) = 10(4)-10(9) atoms.

Temperature dependence of the Raman spectra of liquid parahydrogen.

Vibrational and rotational spectra of liquid para-H(2) at temperature T=14-26 K and of solid at T=6-13 K have been obtained using coherent anti-Stokes Raman scattering technique. The vibrational frequency in the liquid increases with temperature by about 2 cm(-1) and the shift scales with the square of the sample's density. An extrapolation of the vibrational frequency in the metastable para-H(2) liquid below the freezing point is discussed. The results indicate that the vibron hopping between the molecules is active in the liquid, similar to that previously found in the solid.

Interchange-tunneling splitting in HCl dimer in helium nanodroplets.

Midinfrared spectra of HCl dimers have been obtained in helium nanodroplets. The interchange-tunneling (IT) splitting in the vibrationally excited state of the bonded H-Cl stretching band (nu(2)) in (H(35)Cl-H(37)Cl) dimers was measured to be 2.7+/-0.2 cm(-1), as compared to 3.7 cm(-1) in free dimer. From the splitting, the strength of the IT coupling in liquid helium of 0.85+/-0.15 cm(-1) was obtained, which is about a factor of 2 smaller than in the free dimer. The results are compared with the previous spectroscopic study of (HF)(2) in He droplets as well as the theoretical study of (HF)(2) and (HCl)(2) dimers in small He clusters.