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Production of thermal negative ion (X-) from a polyatomic molecule (ZX) incident upon a heated metal surface is investigated theoretically from the viewpoint of thermochemistry. Quantitative expressions of emitted ion current i- (X-) as a function of surface temperature (T) are derived for the two extreme cases as to the degree of dissociation (γ) of ZX on the surface.Namely, i- (X-) ≅ K1 exp [A (X) -φ] for γ ≅ 1.0K2T-1/4exp [A (X) -φ-D (ZX) /2/kT] for γ << 1.0Here A (X), φ and D (ZX) are the electron affinity of X, the work function of the surface and the dissociation energy of ZX, respectively, and K's are virtually independent of T under a usual experimental condition. For the intermediate value of γ, however, i- (X-) cannot be expressed in such a simple form. These results indicate that neither A (X) nor D (ZX) can be determined accurately by measuring temperature dependence of i- (X-) if the condition γ ≅ 1.0 or γ << 1.0 is not satisfied in the temperature range investigated.

Thermochemical Study of Thermal Negative Ion Production through Dissociation of Polyatomic Molecules