Seminar”Laser spectroscopy of Dy atoms in superfluid helium: Interaction of atomic nano-bubbles with phonons and rotons" by Dr. Petr Moroshkin, RIKEN

Speaker: Dr. Petr Moroshkin, RIKEN, Center for Emergent Matter Science

Venue: C016 (Lab1, Level C)

Date: Dec 4th

Time: 14:00-15:00

Title ”Laser spectroscopy of Dy atoms in superfluid helium: Interaction of atomic nano-bubbles with phonons and rotons"

Abstract:

When a metal atom is injected into superfluid helium, it resides in the center of a nanometer-sized cavity formed in the surrounding quantum fluid. These cavities are known as atomic bubbles (for a review see [1]). Elementary excitations of superfluid He, such as phonons and rotons couple to various electronic properties of the impurity atom via the vibrations of the bubble surface. Of particular interest are optical transitions within the inner electronic shells of the embedded atom that are screened from the surrounding He by the valence electrons. In that case the kick on the bubble interface produced by the sudden change of the impurity electron configuration is rather small and the spectrum typically consists of two parts: a zero-phonon line (ZPL) and a phonon wing (PW).
We present a laser-spectroscopy study of dysprosium atoms injected in cryogenic superfluid He by means of laser ablation. A special attention is paid to the transitions between the electronic configurations 4f106s2 and 4f95d6s2 which display well-resolved PW and ZPL structures. The results are interpreted with the help of the atomic bubble model. The spectral profile of the phonon wing reflects the spectrum of the phonon wavepacket generated by the vibrating atomic bubble. It‘s characteristic feature is the gap between ZPL and PW that results from the peculiar shape of the phonon-roton spectrum of superfluid He. Zero-phonon line arises from the transitions that involve no creation or annihilation of phonons. It is broadened by the elastic scattering of excitations already existing in the liquid which disrupt the phase of the oscillating atomic dipole of Dy. The spectral width of ZPL thus provides a measure of the scattering rate of thermal phonons and rotons.
We show that impurity spectra provide the information about superfluidity on the nanometer scale. Due to their narrow spectral lines and strong electronic transitions, dysprosium and other lanthanide atoms can be also used as tracers of superfluid He flow in the studies of quantum turbulence.
[1] P. Moroshkin, A. Hofer, A. Weis, Physics Reports 469, 1 (2008)