Dependence of impurity zero-phonon band thermal broadening and shift on impurity site and vibron level in organic crystals

Experimental results on the thermal broadening and shifts of impurity zero-phonon bands in some organic mixed crystals are reported and interpreted in terms of the electron-phonon interaction. Attention is focused on the variation of the thermal broadening and line shift parameters with (1) impurity site in a system exhibiting several energetically different impurity sites and with (2) the vibron level in a given final electronic state of the impurity. Experiments were conducted over a temperature range which ensures that hot vibron transitions are negligible. Data from the 5600 A ¹B₁ ← ¹A₁ transition of 2-phenyl-l-monoazaazulene in p-terphenyl show that the variations of the above parameters with impurity site are as large as the variations of the same between different chemical systems. No dependence of the thermal broadening parameters on the impurity vibron level is observed for the above system or the 7000 A ¹B₁ ← ¹A₁ transition of azulene in naphthalene. It is shown that, within the adiabatic (electron-nucleus) and harmonic approximations, the zero-phonon band spectral shape function arising from the electron-phonon interaction should be independent of the final vibron level of the impurity transition.