Rotational Dynamics of HCN−M (M = Na, K, Rb, Cs) van der Waals Complexes Formed on the Surface of Helium Nanodroplets

Abstract

Infrared laser spectroscopy was used to probe the unique rotational dynamics of the HCN-M (M = Na, K, Rb, Cs) complexes formed on the surface of helium droplets. The nu1 CH stretch ro-vibrational spectra were measured revealing what appears to be the P and R contours of a nearly rigid linear rotor. To simulate the linear molecule spectra, given a rotational temperature of 0.37 K, effective moments of inertia, IB, were required to be 10(4)-10(5) amu.A2 larger than the ab initio predicted values. The large moments of inertia were found to be strongly dependent on both the mass of the complex and the size of the helium droplet, consistent with a model where the dopant is located in a dimple site on the surface of the droplet. In this model, the moment of inertia is representative of the rotational motion of the dopant on the surface about an inertial axis through the center of the droplet.

Cold Reactions of Alkali-Metal and Water Clusters inside Helium Nanodroplets

“…Strong reactivity of Cs n with H 2 O in He droplets is even more unexpected since Cs atoms and molecules are located in surface states whereas water molecules are immersed inside the droplets. Apparently, long-range attractive forces bring the Cs and H 2 O clusters together, as previously observed with M atoms and HCN molecules [20].…”

“…1. It is clearly dominated by small clusters Na 1−3 carrying up to 8 water molecules reflecting weakly bound van der Waals-complexes (blue and red lines) [20]. The only compounds originating from the reaction with H 2 O are low-abundant Na hydroxide masses, Na(NaOH) n , the most prominent peak being the one of Na(NaOH) 2 (green line).…”

Efficiency of dopant-induced ignition of helium nanoplasmas

“…A particularly curious dopant system is magnesium atoms which aggregate as a metastable foam-like structure in He droplets [24]. By simultaneously doping different species the location of the individual components can be varied due to the pulling of the submerged dopant to the surface [25] or vice versa [26]. For instance, by co-doping He droplets with calcium (Ca) and xenon (Xe) it was shown that Ca atoms are attracted towards the Xe cluster residing at the droplet center [26].…”

Helium droplets: a chemistry perspective

“…6 compares the infrared spectrum of HCN-Mg with that of HCN-Na in the C-H stretching region: these complexes were formed by sequential pickup of HCN and the metal atoms by HeDs. 39 The HCN-Mg spectrum is rotationally resolved and shows structure which is characteristic of a linear complex. The effective rotational constant deduced from the data analysis is substantially lower than would be expected for the free HCN-Mg complex, which is a common observation for molecules and their complexes when solvated inside HeDs.…”