Rotational spectroscopic study of hydrogen cyanide embedded in small 4He clusters

High resolution microwave spectra of the a-type, J = 1-0, transitions of HeN = 1-6-(HCN)-C-12-N-14, HeN = 1-6-(HCN)-C-13-N-14, HeN = 1-6-(HCN)-C-12-N-15, HeN = 1-7-(DCN)-C-12-N-14, and HeN = 1-6-(DCN)-C-13-N-14 clusters produced in a supersonic jet expansion were measured and analyzed. The resulting effective rotational constants, B-eff, initially decrease with the number of the attached helium atoms before reaching a minimum at N = 3 helium atoms for all isotopologues. The subsequent increase in B-eff for N >= 4 is indicative of the onset of microscopic superfluidity. Comparison of our experimental B-eff constants with those from quantum Monte Carlo simulations [A. A. Mikosz, J. A. Ramilowski, and D. Farrelly, J. Chem. Phys. 125, 014312 (2006)] reveals a nearly congruent trend in B-eff for N up to 6. Analysis of the hyperfine structure of the N-14 containing isotopologues yielded a gradual incremental increase in the magnitude of chi(aa) and < P-2(cos theta)> for N = 1-6, which suggests the internal rotation of the HCN molecule is becoming increasingly hindered. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4762862]