Structure, Rotational Dynamics, and Superfluidity of Small OCS-Doped He Clusters

Abstract

The structural and dynamical properties of OCS molecules solvated in Helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n = 3 − 20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system, and provide a more complex and detailed microscopic picture of this system than inferred from experiments. 61.46.+w, 67.40.Yv, 36.40.Mr, 02.70.Ss Solvation of atoms and molecules in He nanodroplets provides a way to study their properties in an ultra-cold matrix, and also offers a unique opportunity to probe the physics of quantum fluids in confined geometries. Research in this field has been recently reviewed in Ref.[1], with emphasis on experiment and hydrodynamic modeling, and in Ref.[2], with emphasis on computer simulations.Carbonyl sulfide (OCS) is one of the most widely studied dopants of He clusters (OCS@He n ) [3,4], both because of its strong optical activity in the infrared and microwave spectral regions, and also because the OCS@He 1 complex is spectroscopically well characterized [5,6], thus providing a solid benchmark for the atom-molecule interaction potential which is the key ingredient of any further theoretical investigation.In the quest of fingerprints of superfluidity in the spectra of small-and intermediate-size He clusters, Tang et al. have recently determined the vibrational and rotational spectra of OCS@He n at high resolution for n = 2 − 8 [3]. The main results of that investigation are: i) the rotational constant of the solvated molecule roughly equals the nanodroplet (largen) limit [7] at n = 5, but then undershoots this asymptotic value up to the maximum cluster size (n = 8) attained in that work; ii) the centrifugal distortion constant has a minimum at n = 5, thus indicating that the complex is more rigid at this size; iii) the fundamental vibrational frequency of OCS is not a monotonic function of the number of He atoms, but it displays a maximum at n = 5, again suggesting a stronger rigidity at this size. Findings ii) and iii) suggest-and our study of the rotational spectra confirms-that n = 5 is a magic size related to the structure of solvent atoms around the solvated molecule. Finding i) implies the existence of a (yet to be determined) minimum in the rotational constant as a function of the cluster size. The occurrence of this minimum was interpreted as due to quantum exchanges which would decrease the effective inertia of the first solvation shell and would thus be a signature of the onset of superfluidity in this finite system [3].Quantum simulations are complementary to the experiment for understanding the properties of matter at the atomic scale. In fact, while being limited by our incomplete knowledge of the inter-atomic interactions and by the size of the systems one can afford to examine, simulations provide a wealth of detailed information ...

Rotational fluctuation of molecules in quantum clusters. II. Molecular rotation and superfluidity in OCS-doped helium-4 clusters

“…Figure 2 shows the estimated effective rotational constant B eff as a function of N, together with the experimental data 7,32 and the reported ground-state quantum Monte Carlo values. 15 The effective rotational constant by the present study decreases with increasing N up to 9 and then shows a turnover. We find a second ͑weak͒ minimum at N = 15 corresponding to the second maximum in the size dependence on C͑␤ /2͒.…”

“…14,15 The calculations predicted a minimum in the N-dependent rotational constant at N =8-9 ͑Ref. 15͒ or N =6 ͑Ref.…”

“…3,4,[11][12][13][14][15][16][17][18][19][20] Among them, a carbonyl sulfide ͑OCS͒ molecule is one of the most widely studied dopants in the helium nanodroplets. Recently, Tang et al 7 have determined the vibrational and rotational spectra of the OCS͑He͒ N clusters in a small-to-medium size regime, N =2-8.…”

Rotational fluctuation of molecules in quantum clusters. I. Path integral hybrid Monte Carlo algorithm

“…Then, the molecular rotation in the helium-4 clusters at the ground-state was studied by the QMC techniques. [20][21][22][23][24] For finite temperature calculations corresponding to the experimental condition, we developed a path integral hybrid Monte Carlo ͑PIHMC͒ method to handle the molecular rotation quantum mechanically, and reported preliminary results on the OCS molecule in a helium-4 cluster. 25 Independently, Zillich et al 26 and Blinov and Roy 27 extended the PIMC technique to rotating molecules in superfluids.…”

Rotational dynamics of CO solvated in small He clusters: A quantum Monte Carlo study

“…6,7,8 The rotational spectrum of OCS@He N has been studied along these lines in Refs. [9,10]. Among the many different flavors of quantum Monte Carlo available in the literature, we adopt Reptation Quantum Monte Carlo 6,7 which we believe presents distinctive advantages in the present case and which will be briefly introduced in Sec.…”