Quantum Monte Carlo study of helium clusters doped with nitrous oxide: Quantum solvation and rotational dynamics

Abstract

Abstract The infrared spectra of small molecules embedded in helium nanodroplets display sharp rotational lines suggesting a decoupling of the molecular rotation from the motion of the helium atoms. The spectra observed for several different molecules have been explained in earlier reptation QMC calculationsa giving moments of inertia and thus effective rotational constants B as a function of the number N of helium atoms in a cluster. This paper reports such calculations for the nitrous oxide molecule NNO which has been observed to have a turnaround in the evolution of B with N. The value of B first decreases to a minimum at N = 6 to 7, then increases slightly to a maximum at N = 11, followed by another minimum and a slow increase. The reptation QMC calculations provided ground-state energies, density profiles, and correlation functions for angular momenta. The results gave excellent agreement with the experimentally determined values of B. The variations of B with N were reproduced quantitively, and the behavior was explained completely with the aid of the helium density profiles showing the accumulation of density at various positions surrounding the NNO molecule as N is increased. Additional calculations using finite-temperature path integral Monte Carlo (PIMC) gave excellent agreement with the energetics and structural properties given by the ground-state calculations. The rotational constants, obtained with Boltzmann statistics in the PIMC calculations, showed no turnaround and no oscillations, thereby indicating that the experimentally observed behavior is the result of exchange effects not included in the PIMC calculations.