Lifetimes and Total Transition Probabilities for NH, SiH, and SiD

Radiative lifetimes have been measured for the NH ${\mathrm{(d}}^1{\mathrm{\Sigma –c}}^1{\mathrm{\Pi ),\; (c}}^1{\mathrm{\Pi –a}}^1{\mathrm{\Delta ),\; (c}}^1{\mathrm{\Pi –b}}^1\mathrm{\Sigma )}$ , and ${\mathrm{(A}}^3{\mathrm{\Pi –X}}^3\mathrm{\Sigma )}$ systems, and for the SiD and SiH ${\mathrm{(A}}^2{\mathrm{\Delta –X}}^2\mathrm{\Pi )}$ transitions. The phase‐shift technique yielded lifetimes ranging from 10 nsec for the NH ${\mathrm{(d}}^1{\mathrm{\Sigma –c}}^1\mathrm{\Pi )}$ transition to 700 nsec for the SiH ${\mathrm{(A}}^2{\mathrm{\Delta –X}}^2\mathrm{\Pi )}$ band. All the lifetimes of upper states measured showed the effects of radiative cascading. In particular, the $c^1\Pi$ state of NH shows radiative cascading originating at least in part from the observed $d^1\Sigma$ state. The measured $d^1\Sigma$ state lifetime is near the nominal lifetime computed for the state cascading into the $c^1\Pi$ state and is considerably shorter than the $c^1\Pi$ state lifetime. The NH ${\mathrm{(A}}^3{\mathrm{\Pi –X}}^3\mathrm{\Sigma )}$ and the SiD and SiH ${\mathrm{(A}}^2{\mathrm{\Delta –X}}^2\mathrm{\Pi )}$ transitions showed cascading that arises from states which have not been spectroscopically identified. All the systems studied, with the exception of the NH ${\mathrm{(d}}^1{\mathrm{\Sigma –c}}^1\mathrm{\Pi )}$ transition, showed cascading in which the cascading state lifetime was shorter than the measured state lifetime. This situation has not been completely discussed previously and is given consideration as to the effect on themeasured phase shifts and calculated radiative lifetimes.