Depolarization of Light Scattered by Aligned2S3and2P3Helium Atoms at Resonance

The $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ (metastable) and $2{}_{}{}^3{}_{}{}^{}P_{1,2}^{}{}_{}{}^{}$ excited-state atoms of ${\mathrm{He}}^4$ are aligned by an unpolarized beam of resonance radiation. The alignment is detected by the depolarization of the resonance fluorescence at 1μ, which occurs when either the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ level or the $2{}_{}{}^2{}_{}{}^{}P_{1,2}^{}{}_{}{}^{}$ levels are saturated by a rf magnetic field at the Zeeman frequency. The observed signals are several orders of magnitude larger than the corresponding signals observed by monitoring intensity changes in the transmitted beam. Considerably smaller signals are observed even in the absence of the pumping beam. Expressions for the intensity of the $\pi$ and $\sigma$ components of the scattered radiation are derived in terms of the alignment and density of the metastable atoms. The alignment of the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{} (F=\frac{3}{2})$ and the $2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{} (F=\frac{5}{2})$ levels of ${\mathrm{He}}^3$ is also observed. The excited-state linewidth as a function of pressure leads to a radiative decay time of $\tau \approx 0.96\times {10}^{-7\mathrm{}}$ sec and a mixing cross section of 53×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. The alignment of the $3{}_{}{}^3{}_{}{}^{}P$ level of ${\mathrm{He}}^4$ is also observed by monitoring polarization changes of the 3889 Å line.