The lifetime and hyperfine structure of the $(6{s}^{2}6p7s) ^{3}{P}_{1}^{\ensuremath{\circ}}$ excited state in lead have been investigated by the technique of level-crossing spectroscopy. The lifetime of the state was determined to be $\ensuremath{\tau}=5.75(20)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec. The linewidth of the level-crossing signal was observed to saturate at a "coherence-narrowed" value in agreement with theoretical predictions. The magnitude of this narrowing corresponds to a 27(3)% branching ratio to the ground state. When the branching ratio and the lifetime are combined with the
f(3639A˚):f(4058A˚) oscillator-strength ratio measured by Khokhlov, one may infer the absolute
f values:
f(2833A˚)=0.169(17),
f(3639A˚)=0.040(8),
f(4058A˚)=0.155(31). The high-field level crossing in the hyperfine structure of the first $^{3}{P}_{1}^{\ensuremath{\circ}}$ state of
Pb207 was found to occur at a magnetic field
Hc=4661.1(4) G, which corresponds to a magnetic-dipole-interaction constant $A=8.811(17)\ifmmode\times\else\texttimes\fi{}{10}^{9}$ cps.
The oscillator strength of the (6s6p
)3P1-(6s7s
)1S0 transition and the lifetime of the (6s7s
)1S0 state in
174Yb have been measured to be (8.0\ifmmode\pm\else\textpm\fi{}1.6)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ and 45.8\ifmmode\pm\else\textpm\fi{}1.0 nsec, respectively. The former value was obtained by use of coherent optical techniques (optical nutation and ac Stark splitting), while the latter was obtained by monitoring cascade fluorescence following pulsed excitation.
The lifetime, coherence narrowing, and collision broadening of the
72S21 excited state of atomic thallium has been studied by the technique of zero-field level-crossing spectroscopy. The lifetime of the
72S21 state is determined to be 7.55 (0.08) nsec. The full coherence-narrowing effect gave a branching ratio, from
F=0 of
72S21 to the ground state, of 43%. The cross section for thallium-thallium resonance-broadening collisions is ${\ensuremath{\sigma}}_{\mathrm{T}\mathrm{l}\ensuremath{-}\mathrm{T}\mathrm{l}}=\frac{5.38(0.49)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}}{\overline{v}}$
cm2. The effect of 426-torr helium on the thallium lifetime, coherence narrowing, and resonance broadening was also investigated. From the low-thallium-density region the cross section for helium depolarization of the thallium
72S state is ${\ensuremath{\sigma}}_{\mathrm{T}\mathrm{l}\ensuremath{-}\mathrm{H}\mathrm{e}}\ensuremath{\cong}\frac{7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}}{\overline{v}}$
cm2. There is also indication of a buildup of population in the metastable
62P23 thallium state. From the data, the cross section for the process $\mathrm{Tl}(6^{2}P_{\frac{3}{2}})\ensuremath{\rightarrow}\mathrm{Tl}(6^{2}P_{\frac{1}{2}})$ by helium collisions is $\ensuremath{\sigma}\ensuremath{\simeq}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$
cm2. The Tl branching ratio in the presence of helium is 39%. It was also found that the thallium-thallium resonance-broadening cross section is ${\ensuremath{\sigma}}_{\mathrm{T}\mathrm{l}\ensuremath{-}\mathrm{T}\mathrm{l}}=\frac{4.04(0.32)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}}{\overline{v}}$
cm2 in the presence of 426 torr of helium. In the presence of 113 torr of He, the resonance-broadening cross section was found to be ${\ensuremath{\sigma}}_{\mathrm{T}\mathrm{l}\ensuremath{-}\mathrm{T}\mathrm{l}}=\frac{5.22(0.13)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}}{\overline{v}}$
cm2. In the case of Ar as the buffer gas, ${\ensuremath{\sigma}}_{\mathrm{T}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{r}}=\frac{8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}}{\overline{v}}$
cm2.
The radiative lifetime of the
21P1 and
31P1 state of atomic helium has been measured by the level-crossing technique. In the experiment, a beam of
21S0 metastable atoms was first produced and then excited to the
21P1 or
31P1 state with resonance radiation. The lifetimes were measured by observing the zero-field level-crossing signals in the detected 584- or 537-\AA{} fluorescent radiation when these
1P1 states decayed to the
11S0 ground state. The results are $\ensuremath{\tau} (2^{1}P_{1})=(5.57\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and $\ensuremath{\tau} (3^{1}P_{1})=(1.73\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec. These values are in excellent agreement with theory.
The lifetimes of the
5p2P23 and
5p2P21 states of
Sr+ and the
6p2P23 and
6p2P21 states of
Ba+ have been measured by the Hanle-effect technique with optical excitation from the ion ground state. (The lifetimes of the
Mg+ 3p2P23 and
Ca+ 4p2P23 states were obtained by the same technique and reported previously.) These
Ca+,
Sr+, and
Ba+ states each decay to a
2D23,25 doublet as well as to the ground
2S21 state, and the branching ratios for these transitions have been measured. The ions were produced by introducing traces of the atomic vapor into an argon discharge. Consequently, the cross sections for collisional depolarization of these
2PJ states due to collisions with argon were also obtained from the measurements. The experimental results are given in Tables I and II. A discussion of the ion-atom collisional depolarization cross sections is included.
The oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the
21P state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of
21S metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the
21S atoms, which are excited to the
21P state. This state then primarily decays to the ground
11S state with the emission of resonance (584 \AA{}) radiation. The lifetime of the
21P state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.