Infrared-optical double resonance in organic solids

We report the first observation of IR-optical double resonance in an organic solid. This double resonance is observed for the single-triplet transition of naphthalene in the 1:2 weak crystalline complex naphthalene: 1,4-diiodotetrafluorobenzene. The 10P14 line of CO₂ laser excites the ∼950-cm^-1 ring-breathing ungerade mode from where the 4880-Å line of an argon ion laser connects to the T₁ level to produce naphthalene phosphorescence. In order to separate the thermal effects from a true IR-optical double resonance, the local temperature is monitored by the anti-Stokes Raman intensities of various vibrations in the same experimental arrangement. Our result indicates that the observed effect is due to a nonthermal IR-optical double resonance. The study of anti-Stokes Raman intensities of various vibrations also reveals that the IR-optical double resonance involves an ungerade vibration and a Franck-Condon-type vibronic interaction.