A direct measurement of the natural lifetime for the (
3p54s)
3P1 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 $\ensuremath{\Delta}t$ at some delay time
t1 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\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$-2.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ 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
3p54p levels is negligible. The measured lifetime is dependent upon the argon pressure, and this effect is attributed to resonance trapping; a value of 21\ifmmode\pm\else\textpm\fi{}2 nsec is obtained for the natural lifetime by using the existing theories of resonance trapping to extrapolate the data to zero pressure.