Vibronic coupling in indole: II. Investigation of the 1La–1Lb interaction using rotationally resolved electronic spectroscopy

Abstract

High-resolution electronic spectra of indole (C₈H₇N) and their detailed analysis are reported. Thirteen low-lying vibronic bands—from the electronic origin transition at 35231.4 cm⁻¹ up to 1000 cm⁻¹ above—are recorded with rotational resolution. Besides inertial parameters and inertial defects these spectra yield detailed information, for each individual band, on the transition-dipole-moment orientations in the molecular inertial frame as well as on the reorientation of that inertial frame upon electronic excitation. The natural lifetimes of the individual vibronic states have also been determined. Strongly varying orientations of the transition-dipole-moments, unexpected positive inertial defects, and decreasing lifetimes, which are only partly related to increased excitation energy, are observed. These results are clear indications of the interaction of the two lowest electronically excited singlet states (¹L_b and ¹L_a). Our experimental findings are strongly supported by, and in excellent agreement with, the theoretical description of the interaction of the two electronic states described in the preceding paper. These results provide clear evidence for strong vibronic coupling of the two electronic states ¹L_b and ¹L_a and for the energetic location of the ¹L_a-state more than 1000 cm⁻¹ above the ¹L_b vibrationless state.