Lifetime of the2S13state in heliumlike sulphur and chlorine

Beam-foil measurements of the lifetimes of the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state in heliumlike sulphur and chlorine are reported. A Doppler-tuned x-ray spectrometer was used to detect the x rays emitted from the $1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\leftarrow 2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ transition. Time-of-flight techniques were used to trace the decay profile of the emitted x rays for 38-55-MeV sulphur and chlorine beams over flight paths of 56-861 cm. For foil-to-detector flight times greater than one half the theoretical lifetimes of 698 nsec (sulphur) and 374 nsec (chlorine), the respective experimental lifetimes of 706 ± 86 and 354 ± 24 nsec are in agreement with theory. For shorter flight times the experimental decay profiles are not single exponentials. This experimental effect is the source of the previously reported discrepancy between experimental and theoretical lifetimes of the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state in heliumlike chlorine.