Observation of Subnatural Linewidths by Two-Step Resonant Scattering inI2Vapor

Observation of linewidths narrower than the natural width with use of a two-step resonant scattering scheme in ${\mathrm{I}}_2$ is reported. The 80-kHz-wide hyperfine lines obtained for the $B{0_{\mathrm{u}}}^{+}\to X{{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{g}}^{}{}_{}{}^{}}^{+}\mathrm{}(43,11)\mathrm{}$ $R\left(15\right)\mathrm{}$ transition are used and as high-resolution probes for collisional studies. Applications to frequency standards are considered. Also included are high-resolution measurements of the ac Stark effect in a folded Doppler-broadened system in ${\mathrm{I}}_2$.