Observation of the forbidden 2
sup3S
sub1 Yields 1
sup1S
sub0 spontaneous emission line from helium and measurement of the transition rate
The extremely forbidden transition $2^{3}S_{1}\ensuremath{\rightarrow}^{1}S_{0}$ at 625.56 \AA{} has been observed in a He afterglow using time-resolved high-resolution spectrophotometric techniques. The spontaneous radiative transition rate was determined by measuring the photon emission rate from a known number of metastables at pressures of 0.05, 0.30, and 0.75 Torr. The value measured is 2.4\ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ with an experimental uncertainty of a factor of 3.
We report measurements of the following spontaneous-emission rates in atomic thallium: $A(6{D}_{\frac{3}{2}}\ensuremath{-}7{P}_{\frac{1}{2}})=(5.97\ifmmode\pm\else\textpm\fi{}0.78)\ifmmode\times\else\texttimes\fi{}{10}^{5}{\mathrm{s}}^{\ensuremath{-}1}$, $A(7{P}_{\frac{1}{2}}\ensuremath{-}7{S}_{\frac{1}{2}})=(1.71\ifmmode\pm\else\textpm\fi{}0.07)\ifmmode\times\else\texttimes\fi{}{10}^{7}{\mathrm{s}}^{\ensuremath{-}1}$, $A(7{P}_{\frac{3}{2}}\ensuremath{-}7{S}_{\frac{1}{2}})=(2.37\ifmmode\pm\else\textpm\fi{}0.09)\ifmmode\times\else\texttimes\fi{}{10}^{7}{\mathrm{s}}^{\ensuremath{-}1}$, and $A(8{P}_{\frac{3}{2}}\ensuremath{\rightarrow}\mathrm{all})=(9.2\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}{10}^{6}{\mathrm{s}}^{\ensuremath{-}1}$. These results are compared to predictions of theoretical models and lend support to calculations of the recently measured parity-nonconserving circular dichroism of the $6{P}_{\frac{1}{2}}\ensuremath{-}7{P}_{\frac{1}{2}}$ forbidden
M1 transition in thallium.
Doppler-free two-photon (DFTP) spectroscopy is a standard technique for precision measurement of transition frequencies of dipole-forbidden transitions. Here, we report the observation of quantum interference (QI) of optical transition pathways in DFTP spectroscopy of the cesium $6S\text{\ensuremath{-}}7D$ transitions chosen as a prototype system. The QI manifests itself as asymmetric line shapes of the hyperfine lines of the
7D states, observed through spontaneous emission following excitation by a narrow-linewidth cw laser. The interference persists despite the lines being spectrally well resolved. Ignoring the effect and fitting the spectrum to a Voigt profile causes large systematic shifts in the determination of the line centers, while accounting for QI resolves the apparent line shift and enables the precise determination of hyperfine splitting in the
7D states. We calculate the spectral line shape including the effect of QI and show that it agrees with the experimental observations. Our results are broadly applicable to other species and have implications for portable secondary optical clocks and precision measurements of hyperfine splittings, isotope shifts, and transition frequencies.