Laboratory Transition-rate Measurement of the Coronal Intercombination Line of Ar xv by Time-resolved Laser Spectroscopy

The extreme-ultraviolet emission line (424 Å) of the intercombination 1s22s21S0–1s22s2p3P1 transition of Ar xv can potentially characterize the electron temperature of astrophysical plasma. Various theoretical studies have investigated the intercombination transition rate, which is essential for the plasma diagnostics; however, experimental difficulties have prevented its measurement. We present here measurement of the lifetime of the 3P1 excited state of Ar xv, providing the experimental value of the intercombination transition rate. Employing time-resolved plasma-assisted laser spectroscopy, a method we recently demonstrated, enables us to measure this submicrosecond lifetime. The experimental result exhibits a 25%–43% higher transition rate than theoretical predictions.