Laser spectroscopy of thulium atoms implanted in liquid and solid4He

Abstract

The excitation, emission spectra, and decay curves of the emission intensity of thulium atoms implanted in liquid and solid helium were observed in the presence and absence of an external magnetic field. The observed narrow line (width $\ensuremath{\sim}0.1$ nm) of the excitation spectrum at 590.60 nm is assigned as a zero-phonon transition from the electronic ground state ${4f}^{13}{(}^{2}{F}_{7/2}^{o}{)6s}^{2}$, which indicates that the transition between the inner shells is weakly perturbed by surrounding helium atoms. The pressure dependence of the emission wavelength suggests that the symmetry of helium atoms distributed around a thulium atom in the solid phase is similar to that in the liquid phase. The emission intensity was stable and large in the solid phase since thulium atoms were trapped at a density of 10${}^{10}$--10${}^{11}$ atoms/cm${}^{3}$. The lifetime of the excited state was measured to be $7.09\ifmmode\pm\else\textpm\fi{}0.04$ $\ensuremath{\mu}$s, which was longer than that of the ${4f}^{12}{(}^{3}{H}_{6}{)5d}_{5/2}{6s}^{2}$ ${(6,5/2)}_{7/2}$ state of free thulium atoms. The excited state is expected to be a mixed state of the ${4f}^{12}{5d6s}^{2}$ and ${4f}^{13}6s6p$ configurations. The metastable state ${4f}^{13}{(}^{2}{F}_{5/2}^{o}{)6s}^{2}$ is populated by a radiative transition from this excited state and relaxes to the ground state through a magnetic dipole transition. The lifetime of the metastable state of a neutral thulium atom was measured to be $75\ifmmode\pm\else\textpm\fi{}3$ ms.