Pump−Deplete−Probe Spectroscopy and the Puzzle of Carotenoid Dark States

Abstract

Pump−deplete−probe and transient absorption spectroscopy are applied to carotenoids with N = 11 conjugated double bonds in solution to study the origin of recently observed transient features that have been previously assigned to new electronic states. The depletion pulse pumps the transient near-IR band, whose lifetime coincides with the fluorescence lifetime, and is hence attributed to the S2 state. The subsequent signal of any lower-lying dark excited-state populated by internal conversion from S2 should be affected by the depletion pulse. Correspondingly, the signal in the S1 deactivation channel is diminished by the depleted excited population. In contrast, the Ssol* signal, purportedly reflecting an intermediate state on a competing deactivation pathway, is not affected by the depletion pulse. When comparing our results with literature data for other carotenoids, we find that the Ssol* lifetime is constant at 6.2 ± 0.4 ps for any N ≥ 11 carotenoid; for shorter chain lengths, it is equal to the S1 lifetime. To explain this puzzle, Ssol* is identified as a vibrationally excited ground state (Ssol* = hot S0), populated by a combination of impulsive Raman scattering of the pump pulse and internal conversion (S1−S0), and decaying by vibrational relaxation. The Ssol* state is not identical to the ST* state, which appears in the same spectral region when the carotenoid is embedded in light-harvesting complexes.

Quantum Control of Population Transfer and Vibrational States via Chirped Pulses in Four Level Density Matrix Equations

“…A central aspect in both approaches, CC and QCS, is the control of population transfer between the ground and excited states, as well as the generation of vibrational coherence in both potential surfaces. By controlling the population transfer or by suppressing specific molecular vibrational coherences, a photochemical reaction channel can be selectively chosen [27,29,32,[37][38][39][40][41][42][43][44][45][46][47][48][49], or a certain mode in a multidimensional time-resolved signal can be suppressed [37,[50][51][52][53][54][55].…”

Reaction Pathways of Photoexcited Retinal in Proteorhodopsin Studied by Pump−Dump−Probe Spectroscopy

“…The pump-dump-probe experimental technique consists of the administration of an additional (dump) pulse to the sample, in the spectral region of the stimulated emission, delayed with respect to the excitation (i.e., pump) pulse. [17][18][19][20][21] This pulse causes depletion of the excited state population, sending part of it back to the ground state. During the delay time between the pump and the dump pulses, the excited state population evolves on the reaction surface, and after dumping the population evolves on the ground state potential energy surface.…”

Charge Transfer Initializes Photoexcited YOYO-1 Intramolecular Rotation and Fluorescent Quenching

“…In summary, the bands centered at 2.54 eV and 2.71 eV is the ground state depletion after excitation. [30][31][32] . The positive correlation between 1.7 eV and 3.4 eV group them to the S1 electrons, and the both positive and negative correlation between 2.54 eV and 2.24 eV split the 2.24 eV to from both S0* and S1 electrons.…”