Absorption Band Spectra of Silver Bromide and Silver Iodide Vapors

Vibrational quantum analysis.---The absorption spectra of the vapors of AgBr and AgI have been photographed with a 21 foot concave grating. Band heads were measured in the region 3165-3501A for AgBr and 3016-3556A for AgI. Continuous absorption below 3350A, with its maximum at about 3170A, was observed for both molecules. A vibrational analysis similar to that already carried out for AgCl gives the following equations for ${{ν}={{ν}}ₑ+{{{ω}}ₑ}^{{'}}{u}^{{'}}{-}{{{ω}}ₑ}^{{'}}{{x}ₑ}^{{'}}{u}^{{'}2}+{{{ω}}ₑ}^{{'}}{{y}ₑ}^{{'}}{u}^{{'}3}{-}{{{ω}}ₑ}^{{'}{'}}{u}^{{'}{'}}+{{{ω}}ₑ}^{{'}{'}}{{x}ₑ}^{{'}{'}}{u}^{{'}{'}2}}{AgBr: {ν}=,31280.43+180.8{u}^{{'}}{-}4.45{u}^{{'}2}{-}0.060{u}^{{'}3}{-}247.72{u}^{{'}{'}}+0.6795{u}^{{'}{'}2}}{AgI: {ν}=,31190.87+131.3{u}^{{'}}{-}5.175{u}^{{'}2}{-}0.050{u}^{{'}3}{-}206.18{u}^{{'}{'}}+0.4327{u}^{{'}{'}2}}$ where $u=v+1/2$. Rotational structure is not sufficiently resolved for analysis. The band heads are single. The electron jump is not identified, but the most likely possibility is $¹{Σ}{-}¹{Σ}$.Heats of dissociation.---Heats of dissociation were calculated by extrapolation of vibration frequencies. For the normal states the values are 2.77 volts for AgBr and 3.02 volts for AgI. For the excited states the heats of dissociation are 0.21 volt for AgBr and 0.10 volt for AgI. With the available data it was not possible to decide whether the products of photochemical dissociation of the silver halides are metastable halogen and normal silver, or metastable halogen and excited silver.Isotope effects of bromine and silver.---AgBr proves to be an excellent example of the vibrational isotope effect in band spectra. The bands show in most cases four well-separated heads of practically equal intensity. Agreement of observed and calculated displacements confirm the existence of two silver isotopes and two bromine isotopes. The vibrational isotope effect of silver was observed also in the AgI spectrum. No evidence for an isotope of iodine was found.