Thermal broadening data on the zero-phonon impurity bands in the 4500 Å 1B1 ← 1A1 and 3500 Å 1A1 ← 1A1 absorption systems of 1,3-diazaazulene and azulene in a naphthalene host are reported. For both systems, a marked dependence of the thermal broadening on the final impurity vibron level of the transition is observed. The broadening for all fundamental vibron bands studied is smaller than the broadening of the origin or zero-vibron band. This fact allows one to attribute the above vibron level dependence to a specific anharmonic interaction term (a quartic mixed vibron-phonon term). It is this term which describes the dependence of the quadratic off-diagonal electron-phonon coupling on impurity vibron level. Apparently the vibron-phonon anharmonic terms which lead to temperature dependent vibron relaxation processes are negligible. We report also the unexpected observation of a dependence of zero-phonon band thermal broadening on crystal samples. No correlation between this dependence and residual widths is observed.