We have measured the cross section for the quenching (i.e., depopulation) of the $2s$ (metastable) state of atomic hydrogen by collisions with atoms of helium and argon. The energy range covered was 0.25-30.0 keV. The experimental technique was to measure the attenuation of a beam passing through the various gases, the detector being a Lyman-$\alpha$ counter viewing a section of the beam where a dc electric field mixed $2s$ and $2p$ states, which essentially converted metastable atoms into Lyman-$\alpha$ photons. For helium as target gas, the cross section was also calculated using a pseudopotential and the eikonal approximation. For helium, the measured cross section falls rapidly as the energy increases from 0.25 to 2.5 keV and remains roughly constant at $7.5\pi a_0^2$ from 2.5 to 30 keV. The theoretical curve is about a factor of 4 below the measured value. Implications of this discrepancy are discussed in the text. For argon, the measured cross section increases from $20\pi a_0^2$ to $27\pi a_0^2$ as the energy increases from 1 to 30 keV.

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