LASER SPECTROSCOPY OF THULIUM ATOMS IMPLANTED IN LIQUID AND SOLID HE-4

The excitation, emission spectra, and decay curves of the emission int ensity of thulium atoms implanted in liquid and solid helium were obse rved in the presence and absence of an external magnetic field. The ob served narrow line (width similar to 0.1 nm) of the excitation spectru m at 590.60 nm is assigned as a zero-phonon transition from the electr onic ground state 4f(13)(F-2(7/2)o)6s(2), which indicates that the tra nsition between the inner shells is weakly perturbed by surrounding he lium atoms. The pressure dependence of the emission wavelength suggest s that the symmetry of helium atoms distributed around a thulium atom in the solid phase is similar to that in the liquid phase. The emissio n intensity was stable and large in the solid phase since thulium atom s 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+/-0.04 mu s, which was l onger than that of the 4f(12)(H-3(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 st ate 4f(13)(F-2(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+/-3 ms.