Effect of blackbody radiation on the threshold for two-electron photodetachment ofH−

The threshold law of the titled process, which is of the same form as that for electron-impact ionization of H, can experimentally be contaminated by simultaneous single photodetachment and Rydberg excitation of the residual atom, followed by ionization of the Rydberg atom by (room-temperature) blackbody photons. This gives a signal indistinguishable from direct double photodetachment. The correction is here calculated and shown to be finite at the double photodetachment threshold, and to have, possibly, an implication about the form of the threshold law itself. However, at room temperature its contribution is found to be small as it pertains to a recently initiated experiment at the Los Alamos Meson Physics Facility (LAMPF) for available energies greater than a millivolt above threshold. The temperature dependence of the correction is shown to be large ${\left(k_{B}T\right)}^{\frac{7}{2}}$; thus at higher temperatures the effect may be observable.