Rotational spectra of minor isotopologues of 4He N–N 2O ( N = 3–19) clusters

Abstract

Pure rotational spectra of 4He N – 14N 15NO, 4He N – 15N 14NO, and 4He N – 15N 15NO clusters with N = 3–19 were recorded using a pulsed Fourier transform microwave spectrometer. The clusters were formed in a free jet expansion, and cluster sizes were selected and assigned based on the nozzle temperature and backing pressure dependence of the observed line strengths. Rotational constants were derived from spectra for all three isotopologues, for each cluster size. The rotational constant evolves with cluster size, showing oscillatory behaviour above N = 5. For the 14N containing species, the nuclear quadrupole coupling constants ( χ cc ) were determined from the measured hyperfine splittings. Finally, the coordinates of the outer nitrogen atom were calculated to determine the placement of nonsuperfluid helium density as N increases. The calculated coordinates indicate that the first five helium atoms add in a ring around the equator of the nitrous oxide molecule, close to the monomer centre of mass, followed by an increase in the nonsuperfluid helium density at the oxygen end.