Photoionization of HBr and DBr near threshold

Photoionization is observed in HBr (at 300 K) below the adiabatic threshold. The photoion yield curve has structure, and is independent of both pressure and electric field over a large range. The peaks can be simulated rather well by a model which assumes rotational autoionization, with ΔN≂−4. This model does not exclude concomitant processes with ΔN=−1,−2,−3. A formal theory is also presented, which describes ΔN=−4 as occurring through successive quadrupole transitions, in second-order perturbation theory. A tentative conclusion is drawn, based on preliminary studies of other molecules, that a type of rotational autoionization can occur in heteronuclear diatomic molecules without an electric field, and in homonuclear diatomic molecules with such a field. The photoionization of DBr has been studied with similar conditions. A corresponding simulation is in good agreement with the observed structure below the adiabatic threshold. In addition, one peak in a triad observed in HBr above threshold, and predicted by an MQDT calculation to be absent in DBr, is still observed.