Anomalously sharp phonon excitations in ³He droplets

Max-Planck-Institut für Dynamik und Selbstorganisation - Bunsenstraße 10, D-37077 Göttingen, Germany, EU

Corresponding author: jtoenni@gwdg.de

Received: 20 July 2009
Accepted: 9 October 2009

Abstract

The absorption spectra of the S₁ S₀ electronic transition of a glyoxal molecule (C₂H₂O₂) embedded inside a pure Fermionic ³He droplet shows a sharp zero-phonon excitation line followed by a slowly decreasing wing extending to higher frequencies. Superimposed upon the wing, a small but sharp peak is observed at = 0.99 meV. The wing is shown to be due to the continuum of particle-hole pair excitations while the sharp peak results from the excitation of localized phonons of the snowball shell of ³He atoms surrounding the chromophore molecule. The exceptional sharpness of the observed phonon peak (at least 10 times narrower than the ordinary bulk phonon peak at the same energy) is attributed to the localization associated with Friedel-like oscillations of the ³ He liquid density.