Quantum rotation of HCN and DCN in4He

Abstract

We present calculations of rotational absorption spectra of the molecules HCN and DCN in superfluid helium-4, using a combination of the Diffusion Monte Carlo method for ground state properties and an analytic many-body method (Correlated Basis Function theory) for the excited states. Our results agree with the experimentally determined effective moment of inertia which has been obtained from the J = 0 → 1 spectral transition. The correlated basis function analysis shows that, unlike heavy rotors such as OCS, the J = 2 and higher rotational excitations of HCN and DCN have high enough energy to strongly couple to rotons, leading to large shifts of the lines and accordingly to anomalous large spectroscopic distortion constants, to the emergence of roton-maxon bands, and to secondary peaks in the absorption spectra for J = 2 and J = 3. In microwave helium nanodroplet isolation spectroscopy experiments, Conjusteau et. al. [1] have measured the rotational excitation energy J = 0 → 1 of HCN and DCN embedded in 4 He clusters. Their results show a reduction of this excitation energy by factors of 0.815 and 0.827 with respect to gas phase HCN and DCN, respectively. Infrared spectroscopy experiments of HCN by Nauta et. al. [2] yield similar results from analysis of the ro-vibrational excitation of the C-H stretching mode, namely a reduction of 0.795 in the J = 0 → 1 energy. These fractional reductions are considerably smaller than those observed for heavier molecules such as SF 6 and OCS, where reductions by factors of ∼ 0.3 are seen [3]. The gas phase rotational constants, B = 1.478222 cm −1 for HCN and B = 1.207780 cm −1 for DCN, are also much larger than the corresponding values for the heavier molecules (e.g. B = 0.2029 cm −1 and 0.0911 cm −1 for OCS and SF 6 respectively). The widely observed reduction in B is understood to be due to the interaction of the molecule with the surrounding 4 He atoms [4]. For the heavier molecules it has been found that calculations based on the microscopic 2-fluid theory [5] can reproduce the effective rotational constant B eff [4, 6]. For some heavy linear rotors, a semiclassical hydrodynamical analysis that combines a classical treatment of the molecular rotation with a quantum calculation of helium solvation density approximately reproduced the moment of inertia increase measured in experiments (see table I in Ref. 7), although no agreement is found for the octahedral SF 6 molecule [4,8,9]. The hydrodynamic contribution to the effective moment of inertia is found to be considerably decreased when the molecular rotation is treated quantum mechanically [10].These models for heavier molecules are based on analysis of partial or complete adiabatic following of the molecular rotational motion by helium and cannot describe the dynamics of light rotors like HCN and DCN in helium for which adiabatic following does not hold [11]. Furthermore, infrared spectra of HCN [2] and acetylene, C 2 H 2 [12], and other light molecules show a small splitting of the ro-vibrational R(0)-line which cannot be...

Electronically excited rubidium atom in a helium cluster or film

“…Also shown in the table are the average distance ͗r͘ of Rb and Rb ‫ء‬ from the tional theory, 20 which reduces the many-body problem of the 4 He environment to an effective one-body problem that allows calculation of dynamical properties. We are planning to study the coupled dynamics of Rb and the 4 He surface upon excitation to 1 2 ⌸ 1/2 using correlated basis function theory, 61 taking into account also energy dissipation via collective excitations of helium.…”

OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules

“…The values for R and ϑ calculated by DMC and reported in Table V also show a very good agreement with the corresponding experimentally determined quantities. A complete analysis of the ccQA-DMC results for N =1 was previously reported [31].…”

“…We have previously applied this approach to analysis of small OCS( 4 He) N clusters [30] and the OCS(p-H 2 ) complex [31].…”

Spectroscopy and dynamics in helium nanodroplets

“…Zillich and Whaley have developed the DMC/CBF method for treating the interaction of molecular rotation excitation with the phonons and rotons of helium and applied it to the case of HCN [153] and C 2 H 2 [151], an important yet challenging problem. This model treats the phonons of the droplet as if they are continuous and have the same form as for bulk helium and then decomposes the density anisotropy that rotates with the molecule in terms of these phonon modes.…”