Path-integral Monte Carlo simulations for excitation spectra of Cs in superfluid He

Path-integral Monte Carlo simulations for Cs atoms immersed in liquid He-4 are performed to investigate the effects of He-exchange on the D-2-excitati on energy which shifts or splits depending on the pressure (P) and temperat ure (T). We find substantial difference in P-dependences of the shift betwe en the cases with T below and above the lambda-transition temperature, whil e weak P-dependences are seen for splitting irrespective of T. Microscopic structures of the liquid around the Cs are investigated through analyses of the He-exchange and the density inhomogeneity.