Measured Lifetime of the (3p54s)P13State of Argon

A direct measurement of the natural lifetime for the ($3p^{5}4s$) ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ state of argon has been made using a modified delayed-coincidence method. Argon atoms are repetitively excited by pulses of electrons. For each pulse, the decaying light signal is sampled for a time $\Delta t$ at some delay time $t_1$ after the excitation is terminated. The exponential decay is then reconstructed as the delay time is changed. Collision effects are not important in the pressure region (5.0×${10}^{-6\mathrm{}}$-2.0×${10}^{-4\mathrm{}}$ Torr) of the experiment and the effects due to cascading are eliminated by keeping the energy of the bombarding electrons low enough so that excitation of the $3p^{5}4p$ levels is negligible. The measured lifetime is dependent upon the argon pressure, and this effect is attributed to resonance trapping; a value of 21±2 nsec is obtained for the natural lifetime by using the existing theories of resonance trapping to extrapolate the data to zero pressure.