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

Abstract

High resolution microwave spectra of the a-type, J = 1-0, transitions of He(N = 1-6)-H(12)C(14)N, He(N = 1-6)-H(13)C(14)N, He(N = 1-6)-H(12)C(15)N, He(N = 1-7)-D(12)C(14)N, and He(N = 1-6)-D(13)C(14)N 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 (14)N containing isotopologues yielded a gradual incremental increase in the magnitude of χ(aa) and <P(2)(cos θ)> for N = 1-6, which suggests the internal rotation of the HCN molecule is becoming increasingly hindered.