Laser spectroscopic investigation of Ca atoms in liquid helium - Comparison between spectral properties for liquid helium-3 and helium-4

Excitation and emission spectra of the 4s(21)S(0) - 4s4p(1)P(1) and 4s4p(3)P(J)(J = 0, 1, 2) - 4s5s(3)S(1) transitions of Ca atoms implanted not only in liquid He-4 but also in liquid He-3 have been measured. It has been found that the excitation spectra for liquid He-3 show considerably smaller widths and peak shifts from transition wavelengths of a free Ca atom, in comparison with those for liquid He-4. These spectral characteristics have been well reproduced by our theoretical calculation based on a vibrating bubble model. This calculation has shown that, due to a significant difference in surface tension between the two liquids, the radius of a bubble formed around Ca in liquid He-3 is larger than the one in He-4, and that this fact as well as the smaller number density of He atoms in liquid He-3 cause weaker perturbation for Ca, resulting in the smaller peak shift and width for liquid He-3.