Spectral Signatures of Ground- and Excited-State Wavepacket Interference after Impulsive Excitation

Impulsive transient absorption spectroscopy is used to track the formation and evolution of vibrational coherences in cresyl violet perchlorate under different excitation conditions. Resonant and off-resonant pump pulses result in the selective formation of excited (S₁)- and ground (S₀)-state wavepackets. Partially resonant and broadband excitation conditions lead to the simultaneous formation of wavepackets in the ground and excited states. The wavepackets are characterized by the phase-flips in the coherent signal associated with wavepacket motion across the absorption and emission maxima and by a red shift of 2-10 cm^-1 in the Raman features of the excited state compared to the ground-state wavepacket. We observe that, when wavepackets are simultaneously excited on the ground- and excited-state surfaces, interference on a picosecond timescale between coherent oscillations in the two wavepackets gives rise to features that cannot be attributed to the passage of a wavepacket through a conical intersection, such as shifting phase-flips and zero-amplitude nodes. Wavepacket filtering using windowed Fourier transforms highlights these interference effects and demonstrates that special care must be taken in order to properly interpret data that have been processed in this manner.