Lifetime of the2SState of Atomic Hydrogen

Abstract

A sensitive test for any mixing of the $2S_{\frac{1}{2}}$ and $2P_{\frac{1}{2}}$ state of atomic hydrogen is the measurement of the rate for single-quantum decay of the $2S$ atom to the ground state. A new upper limit of this decay rate has been determined. A section along a beam of $2S$ atoms, produced by electron excitation of a ground-state atom beam, was viewed by an iodine-vapor-filled ultraviolet photon counter, which responds to the Lymanalpha radiation of the single-quantum decay process. From the counts observed when an electrostatic quenching field was superposed on the counter's field of view, the necessary experimental parameter (product of $2S$ atom current and counter efficiency) was determined. With the field removed, a portion of the remaining counts could be ascribed to quenching on collision of the $2S$ atoms with residual gases in the vacuum chamber, the quenching cross sections for which were measured. The decay rate not ascribable to known quenching effects was 420 ${\sec }^{-1\mathrm{}}$. Since unknown quenching effects may have been operative, this figure must be considered only as an upper limit for the natural single-quantum decay rate.