Experimental molecular hyperpolarizabilities from vibrational spectra in systems with large electron-phonon coupling

Experimental data are presented and discussed on the relaxation contribution (βr and γr) to first- and second-order molecular hyperpolarizabilities of classes of polyconjugated organic materials. These data are obtained from the measure of classical vibrational observables, i.e., vibrational frequencies, infrared intensities and Raman cross sections. It turns out that the values of βr and γr are very similar (both in values and trends) to the electronic molecular hyperpolarizabilities β and γ obtained from traditional direct experiments. It is concluded that for the molecular systems considered, βr and γr may be a good estimate of static, non-resonant β and γ. The justification for such conclusions is that in polyconjugated systems very large electron-phonon coupling takes place such that vibrational displacements are strictly related to and cannot be separated from electronic excitations.