Interchange-tunneling splitting in HCl dimer in helium nanodroplets

High resolution infrared spectroscopy of (HCl)2 and (DCl)2 isolated in solid parahydrogen: Interchange-tunneling in a quantum solid

Infrared spectroscopic studies of weakly bound clusters isolated in solid parahydrogen (pH₂) that exhibit large-amplitude tunneling motions are needed to probe how quantum solvation perturbs these types of coherent dynamics. We report high resolution Fourier transform infrared absorption spectra of (HCl)₂, HCl-DCl, and (DCl)₂ isolated in solid pH₂ in the 2.4-4.8 K temperature range. The (HCl)₂ spectra show a remarkable amount of fine structures that can be rigorously assigned to vibration-rotation-tunneling transitions of (HCl)₂ trapped in double substitution sites in the pH₂ matrix where end-over-end rotation of the cluster is quenched. The spectra are assigned using a combination of isotopically (H/D and ³⁵Cl/³⁷Cl) enriched samples, polarized IR absorption measurements, and four-line combination differences. The interchange-tunneling (IT) splitting in the ground vibrational state for in-plane and out-of-plane H³⁵Cl-H³⁷Cl dimers is 6.026(1) and 6.950(1) cm^-1, respectively, which are factors of 2.565 and 2.224 smaller than in the gas phase dimer. In contrast, the (DCl)₂ results show larger perturbations where the ground vibrational state IT splitting in D³⁵Cl-D³⁷Cl is 1.141(1) cm^-1, which is a factor of 5.223 smaller than in the gas phase, and the tunneling motion is quenched in excited intramolecular vibrational states. The results are compared to similar measurements on (HCl)₂ made in liquid helium nanodroplets to illustrate the similarities and differences in how both these quantum solvents interact with large amplitude tunneling motions of an embedded chromophore.

Direct detection of polar structure formation in helium nanodroplets by beam deflection measurements

Long-range intermolecular forces are able to steer polar molecules submerged in superfluid helium nanodroplets into highly polar metastable configurations. We demonstrate that the presence of such special structures can be identified, in a direct and determinative way, by electrostatic deflection of the doped nanodroplet beam. The measurement also establishes the structures' electric dipole moments. In consequence, the introduced approach is complementary to spectroscopic studies of low-temperature molecular assembly reactions. It is enabled by the fact that within the cold superfluid matrix the molecular dipoles become nearly completely oriented by the applied electric field. As a result, the massive (tens of thousands of helium atoms) nanodroplets undergo significant deflections. The method is illustrated here by an application to dimers and trimers of dimethyl sulfoxide (DMSO) molecules. We interpret the experimental results with ab initio theory, mapping the potential energy surface of DMSO complexes and simulating their low temperature aggregation dynamics.

Rotation of methane molecules in dimers and small clusters

This work reports on the study of the internal rotation of methane molecules in small clusters containing up to about five molecules. The clusters were assembled in helium droplets at T = 0.38 K by successive capture of single methane molecules and studied by infrared laser spectroscopy of the fundamental CH4 ν3 vibration around 3030 cm(-1). The spectra demonstrate well resolved structure due to internal rotation of the constituent molecules in the clusters. The most resolved spectrum for the dimers shows characteristic splitting of the lines due to anisotropic intermolecular interaction. The magnitude of the splitting is found to be in a good quantitative agreement with the recent theoretical anisotropic intermolecular potentials.