High-Resolution and Dispersed Fluorescence Examination of Vibronic Bands of Tryptamine: Spectroscopic Signatures for La/Lb Mixing near a Conical Intersection

The vibronic spectrum of tryptamine has been studied in a molecular beam up to an energy of 930 cm(-1) above the S-0-S-1 electronic origin. Rotationally resolved electronic spectra reveal a rotation of the transition dipole moment direction from L-1(b) to L-1(a) beginning about 400 cm(-1) above the L-1(b) origin. In this region, vibronic bands which appear as single bands at low resolution contain rotational structure from more than one vibronic transition. The number of these transitions closely tracks the total vibrational state density in the L-1(b) electronic state as a function of internal energy. Dispersed fluorescence spectra show distinct spectroscopic signatures attributable to the L-1(b) and L-1(a) character of the mixed excited-state wave functions. The data set is used to extrapolate to a L-1(a) origin about 400 cm(-1) above the L-1(b) origin. DFT-MRCI calculations locate a conical intersection between these two states at about 900 cm(-1) above the L-a origin, whose structure is located along a tuning coordinate which is close to a linear interpolation between the two excited-state geometries. Along the branching coordinate, there is no barrier from L-1(a) to L-1(b). A two-tier model for the vibronic coupling is proposed.