Energy and Radiative Lifetime of the5d96s2D522State in Hg II by Doppler-Free Two-Photon Laser Spectroscopy

The Doppler-free, two-photon $5d^{10}6s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-5\mathrm{}{d}^{9}6s^2{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ transition in singly ionized Hg, attractive as an optical-frequency standard, has been observed for the first time on a small number of ${}_{}{}^{198}{}_{}{}^{}\mathrm{Hg}_{}^{+}{}_{}{}^{}$ ions confined in a radio-frequency trap. The radiative lifetime of the ${}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ state and the absolute wave number of the two-photon transition were measured to be 0.090(15) s and 17 757.152(3) ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. Optical amplitude-modulation sidebands, induced by the secular (thermal) motion of the harmonically bound ions, were observed also for the first time.