Mechanism of Potential Inversion of the Joshi Effect

Abstract

The positive Joshi effect +Δi (i.e., the photoincrease of the current i) increases initially with the exciting ac potential V(<Vm the ``threshold potential'') to a maximum at Vm, decreases precipitously and changes sign (to the negative effect—Δi, i.e., photosuppression of i) at an inversion potential ViI. Beyond ViI, —Δi first increases rapidly and diminishes slowly thereafter. A second inversion, —Δi→+Δi, obtains, in certain systems, at ViII in the region of high V. Oscillographic studies reveal simultaneous occurrence, at V>Vm, of intrinsic (+Δi) and intrinsic (—Δi); the integrating current indicator shows the balance. The initial +Δi at V<Vm, and its enhancement with V, results from photoelectron emission from the (momentary) cathode of field diminished work function. At Vm, consequent to the continuous discharge and an electron residuum from the antecedent half-cycle, largely enhanced emission and space accumulation of negative ions (from attachment of the electrons) lead to incipience of (—Δi) due to inhibition of γ and ηθg processes. Electrons able to cross the barrier cause (+Δi). The observed co-occurrence of (+Δi) and (—Δi) follows. Beyond Vm, increase in emission due to field reduction of surface work function enhances space charge density and therefore (—Δi). Observed diminution in +Δi follows from the above, and the reduction in the number of photoelectrons able to cross the barrier. At ViI, (+Δi)=(—Δi) and therefore, ±Δi=0. Increase of —Δi above ViI also follows from the above considerations. The decrease in attachment probability P=f(p/F), where p is the gas pressure and F the field, leads to subsequent diminution of (—Δi) at large V. Increase in the associated (+Δi) is a contributory factor, and above ViII causes the observed positive effect.