Angular dependent interaction of metastable helium molecules on the surfaces of helium nanodroplets

Raghuram Petluri, University of Rhode Island


A metastable helium molecule is produced in a helium nanodroplet by electron bombardment. The molecule, identified as the long lived triplet state ( a3S+u ), is found to reside on the surface of the droplet. Upon absorbing an infrared photon, it becomes detached from the surface of the droplet. The molecule is subsequently detected when it releases a secondary electron by striking a metal surface in the detector region. The photo-induced detachment process, which we call “Molecular Detachment Spectroscopy”, is used to study the ro-vibrational structure of c3S+g←a3 S+u electronic transition in 910–985 nm region. ^ The high resolution spectrum exhibits a highly non-thermal rotational state distribution. The rotational lines are found to be blue shifted from their gas values by ∼2.6 cm−1, indicating that the molecule is weakly bound to the droplet. In addition, all the lines are sharp, well resolved, and highly structured as a result of the molecule's interaction with droplet surface. ^ We develop a model that explains semi-quantitatively the experimentally observed spectral shifts and line shapes. The interaction potential between the metastable molecule and the droplet strongly depends on the orientation of the molecule for the final state ( c3S+g ). For the nearly isotropic initial state ( a3S+u ), however, orientation has no effect. Spectral shifts are obtained from these angular dependent surface potentials. Intensities of the spectral lines are explained using Franck-Condon principle. ^

Subject Area

Chemistry, Physical|Physics, Molecular|Physics, Condensed Matter

Recommended Citation

Raghuram Petluri, "Angular dependent interaction of metastable helium molecules on the surfaces of helium nanodroplets" (2005). Dissertations and Master's Theses (Campus Access). Paper AAI3186917.