Ultrafast transient absorption revisited: Phase-flips, spectral fingers, and other dynamical features

Abstract

We rebuild the theory of ultrafast transient-absorption/transmission spectroscopy starting from the optical response of an individual molecule to incident femtosecond pump and probe pulses. The resulting description makes use of pulse propagators and free molecular evolution operators to arrive at compact expressions for the several contributions to a transient-absorption signal. In this alternative description, which is physically equivalent to the conventional response-function formalism, these signal contributions are conveniently expressed as quantum mechanical overlaps between nuclear wave packets that have undergone different sequences of pulse-driven optical transitions and time-evolution on different electronic potential-energy surfaces. Using this setup in application to a simple, multimode model of the light-harvesting chromophores of PC577, we develop wave-packet pictures of certain generic features of ultrafast transient-absorption signals related to the probed-frequency dependence of vibrational quantum beats. These include a Stokes-shifting node at the time-evolving peak emission frequency, antiphasing between vibrational oscillations on opposite sides (i.e., to the red or blue) of this node, and spectral fingering due to vibrational overtones and combinations. Our calculations make a vibrationally abrupt approximation for the incident pump and probe pulses, but properly account for temporal pulse overlap and signal turn-on, rather than neglecting pulse overlap or assuming delta-function excitations, as are sometimes done.

Estimation of damped oscillation associated spectra from ultrafast transient absorption spectra

“…The presence of such phase jumps and sharp nodes at the position of the transitions is of special interest, as these have been reported before. 18,[25][26][27] Our results correspond to those in Fig. 4(c) Finally, let us compare the DOAS features in the experimental, Figure 6, and simulated, Figure 8, data.…”

“…Special are the two reverse DOAS (#32 and #33 in Figures 6(v)-6(x)) which are similar to free induction decays, resulting from a probe pulse that precedes the pump pulse, cf., Figure S 11. 26,59 In the data, these are obvious as low frequency oscillations extending before time zero (Figure 3). They are mainly described by the black DOAS32, with a frequency of 406 cm −1 , and damping rate of 39 ps −1 , which has its largest amplitude above 670 nm (black in Figure 6(w)).…”

“…Vibrations can be assigned to the ground or excited state based on their amplitude and phase profiles, where a node in the amplitude, accompanied by a corresponding phase flip, marks the minimum of the electronic potential energy surface that the wavepacket is propagating on. [5][6][7][25][26][27][28][29] Vibronic coherences are relevant to the study of photosynthetic energy transfer, 25,27,[30][31][32][33][34][35][36][37][38] photosynthetic charge separation, 39 exciton coherence, 40 electa) Electronic mail: i.h.m.van.stokkum@vu.nl. Telephone: +31205987868.…”

“…When structural information is available, the dynamics of complex molecular systems can be studied with quantum mechanical methods, and time resolved spectra can be predicted, which has been done for the photosynthetic antenna complexes PE545 49,50 and PC577. 26 Here no structural information is used, nor quantum mechanical methods. Instead, the data are described by a parametric model, employing the global and target analysis methodology [51][52][53] to describe the spectral evolution of electronically excited states.…”

Correlated Protein Environments Drive Quantum Coherence Lifetimes in Photosynthetic Pigment-Protein Complexes

“…Furthermore, previous work has shown that vibrational modes arise from intermolecular coupling between bacteriochlorophylls. 19 Therefore, after excitation with a short laser pulse, a given exciton will exhibit synchronized nuclear motions across the ensemble; 40 but the motions of distinct excitons can diverge over time. It is therefore not a given that FMO's excited states would exhibit these synchronized vibrational oscillations over a picosecond.…”