Three-photon excitation of autoionizing states of atomic krypton between the ${}_{}{}^2{}_{}{}^{}\mathrm{°}_{3/2}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{°}_{1/2}^{}{}_{}{}^{}$ fine-structure thresholds

The nonresonant, three-photon ionization spectrum of Kr between the ${\mathrm{Kr}}^{+}$ ${}_{}{}^2{}_{}{}^{}\mathrm{°}_{3/2}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{°}_{1/2}^{}{}_{}{}^{}$ fine-structure limits is reported. Members of three Rydberg series converging to the ${\mathrm{Kr}}^{+}$ ${}_{}{}^2{}_{}{}^{}\mathrm{°}_{1/2}^{}{}_{}{}^{}$ limit, corresponding to ${(}^2$P${\mathrm{°}}_{1/2}$)nd’[1/2]${\mathrm{\equiv }}_1$, ${(}^2$${\mathit{P}}_{1/2}$)nd’[3/2]${\mathrm{°}}_{,}$ and ${(}^2$P${\mathrm{°}}_{1/2}$)ng’[7/2]${\mathrm{°}}_3$, are observed. A fourth allowed series, ${(}^2$P${\mathrm{°}}_{1/2}$nd’[5/2]${\mathrm{°}}_3$, is not observed. Photoelectron angular distributions are reported for the 9s’, 7d’, and 6g’ members of the observed series. Comparison of the present results with analogous measurements in Xe indicate that transitions to J=3 final states are significantly weaker in Kr.