Measurements of Lifetimes of Excited States of Atoms by the Method of Delayed Coincidences

Abstract

WE have adapted the method of delayed coincidences¹ to the investigation of lifetimes of excited states of atomic helium. Helium gas at a pressure of about 0.01 mm. mercury was excited by a beam of 40 eV. electrons, and the photons emitted in the subsequent de-excitation detected by means of an E.M.I. VX 5031 photo-multiplier, the wave-lengths corresponding to the various transitions being selected by means of Barr and Stroud interference filters. The intensity of the electron beam was modulated by means of a grid which was maintained at a potential sufficiently negative to cut off the beam completely except during the application of a positive voltage pulse of duration 2 × 10⁻⁸ sec. and repetition rate 10 kc./s. The helium atoms were therefore excited by short bursts of electrons. The voltage pulses from the photomultiplier, corresponding to the ejection of single electrons from its cathode, were amplified and applied to one side of a coincidence unit of resolving time 2 × 10⁻⁸ sec. The pulse used for modulating the electron beam was also applied to the other side of the coincidence unit through a variable length of ‘Uniradio 57’ cable. A coincidence was therefore registered only if the delay in this cable was equal to the interval between the electron burst and the subsequent emission of a photon, and for delays longer than the overall resolving time of the apparatus the variation of coincidence-rate with delay should show an exponential decrease, the decay constant being the reciprocal of the mean life of the excited state involved. We are satisfied that for the transitions referred to below, the conditions of our experiment are such that the effects on the apparent lifetime of recombination of ions and of cascade transitions from higher states are negligible.