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

Abstract

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 (S 1 )-and ground (S 0 )-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 groundstate 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.

Electronic State-Resolved Multimode-Coupled Vibrational Wavepackets in Oxazine 720 by Two-Dimensional Electronic Spectroscopy

“…It is noted that except the π-phase changes at 15400 and 16,540 cm –1 , there are two noticeable phase shifts appearing at 14,570 and 17,000 cm –1 , which coincidentally correspond to the known vibronic transitions of oxazine 720 marked as sticks in Figure d. Because of strong mixing of excited- and ground-state wavepackets in the probed spectral region, ,, the origin of vibrational wavepackets and the underlying coupling among these vibronic modes become complicated and thus cannot be disentangled through such one-dimensional (1D) spectra, although we have identified an 595 cm –1 excited-state wavepacket according to the node at fluorescence maximum (15,400 cm –1 ).…”

Tracking ultrafast reactions in organic materials through vibrational coherence: vibronic coupling mechanisms in singlet fission

“…The combination of bandwidth and overlap is key -if a suitably broadband pulse only partially overlaps with the ground-state absorption (the middle of Figure 2(e)), any resulting high-frequency VC is likely to belong to the ground state. Even in optimal conditions, though, the generation of ground-state VC cannot be avoided, and isolating pure excited-state VC from the entangled oscillation signals that result has long been regarded as one of the chief issues facing time-domain Raman methods [65].…”

Population and coherence dynamics in large conjugated porphyrin nanorings

“…33,34 This picture can be further complicated by nonresonant contributions from solvent Raman active modes. 31,35 Multidimensional spectroscopic techniques (in which information is spread over two, or more, frequency dimensions) 33,[36][37][38][39] provide a rigorous means of discriminating between ground and excited state vibrational coherences, but at the price of a more challenging experimental implementation and longer acquisition times. A feature which helps to differentiate between ground and excited state vibrations in conventional (one-dimensional) fs transient spectroscopies is the amplitude dip (or "node") appearing at probe wavelengths corresponding to the maxima of the steady-state electronic absorption (for ground state nuclear wavepackets) or fluorescence (for excited state nuclear wavepackets) spectra.…”