Quenching of the2S01Metastable State of Helium by an Electric Field

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Abstract
A time-of-flight technique has been used to measure the quenching of the 2S01 metastable state of helium by a static electric field. Neutral ground-state helium atoms effusing from a cooled source slit are immediately excited to the 2S01 and 2S13 metastable states by a pulse of antiparallel magnetically focused electrons. The metastable beam is collimated before passing through a uniform electric field region 0.5 m long and is then preferentially detected at the end of the time-of-flight region, 1.825 m from the electron gun. The time-of-flight distribution for the 2S01 state is separated from that of the 2S13 state by illuminating the beam before the electric field region with an rf-discharge helium lamp. The 2S01 state is quenched by resonant absorption of a 20 581-Å photon, raising the atom to the 2P11 state, which then decays to the S01 ground state; the 2S13 state remains unaffected because it is the ground state for the triplet system. The 2S01 time-of-flight distribution is therefore obtained from the difference between the full beam and the quenched beam. The number of 2S01 atoms arriving at the detector in specific velocity intervals with the electric field off is compared to the number with the field on to determine the quenching rate (=kE2). The result for the quenching constant k for both He3 and He4 with E in kV/cm is k=0.933±0.005; this value is in good agreement with theory and with an earlier less accurate experiment. The error in the present experiment arises from a 0.5% uncertainty in the effective length of the electric field region.