The Photo-Electric Threshold and the Heat of Dissociation of the Potassium Molecule

The absorption curves and the photo-ionisation experiments agree to give a value for the molecular photo-electric threshold of λ 2555 ± 20 a.u. If our view of the ionisation process is correct, the energy of dissociation is 0·50 5 ∓ ·01 volts. If the picture of the ionisation process is wrong, the energy of dissociation may be a little lower (a few hundredths of a volt). The corresponding heat of dissociation is 11400 calories. In the range of temperature (200° C. to 500° C.) generally used in experiments on potassium vapour, the fraction associated varies from 10 −4 to 10 −2 . An equation for calculating the fraction associated at any temperature is given by the writer in an earlier paper.