The role of specific normal modes during non‐Born–Oppenheimer dynamics: the S₁–S₀ internal conversion of β‐carotene interrogated on a femtosecond time‐scale with coherent anti‐Stokes Raman scattering

Abstract

The population flow from the first excited singlet state (S 1 ) to the electronic ground state (S 0 ), facilitated through the S 1 -S 0 internal conversion, and the subsequent internal vibrational energy redistribution and vibrational cooling processes are monitored selectively with respect to the different forms of nuclear motion in all-trans-b-carotene. This is realized by exciting the molecule into the second excited singlet state (S 2 ) with an auxiliary pump laser pulse and interrogating the population recovery into the electronic ground state by means of a time-delayed, coherent anti-Stokes Raman (CARS) process. This spectroscopic scheme is referred to as a pump-CARS scheme in analogy to the classical pump-probe scheme in timeresolved spectroscopy. Here, the profound enhancement of the CARS signal intensity in the case of a Raman resonance to a specific vibrational mode is utilized as an intensity filter that amplifies the signal from the vibrational modes of interest, making the contributions from other vibrational modes negligible. This filter allows for the population flow in a specific vibrational mode to be monitored as the radiationless electronic transition between the S 1 and S 0 state takes place. This spectroscopic scheme opens up the possibility of identifying the vibrational motion with a large-amplitude motion in the C C double bond symmetric stretch as the primary acceptor of population from the S 1 state. A mechanism with which the other normal modes are populated is postulated.

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

“…The maximum enhancement of |ρ 12 | results in 1.6, and for ρ 33 + ρ 44 is 2.1. Of relevance, however, is the maximum enhancement factor in |ρ 34 |, which becomes in this case 38.5. This large value is observed close to the resonance of the first vibrational upper state level, with ν 19,365 cm −1 and for two almost symmetric chirp values, namely φ −210 fs 2 and φ 220 fs 2 (see Figure 6).…”

“…This result is partially in accordance with the studies made in [60], where it is shown that excited state vibrational coherences are enhanced predominantly by positively chirped pulses. Figure 4b shows the evolution in time of the coherence |ρ 34 | at the relevant parameter values. It is clear that the two chirped pulses after the interaction of the first half of the pulse are able to retain the |ρ 34 | coherence as compared to the TL pulse.…”

“…Figure 4b shows the evolution in time of the coherence |ρ 34 | at the relevant parameter values. It is clear that the two chirped pulses after the interaction of the first half of the pulse are able to retain the |ρ 34 | coherence as compared to the TL pulse.…”

“…Closed-loop learning algorithms are exploited in CC to generate complex optical phases in order to optimize specific reaction pathways. CC has been used as a tool in the control of dynamic processes [17][18][19][20][21], and it explains how a specific state can be selectively populated, for example, to attain 100% population transfer [22][23][24][25], and recent studies have shown the possibility of having enhancements even higher than 100% [26][27][28][29][30][31][32][33][34][35]. QCS also exploits tailored excitation but differs from CC in its applications [30,36].…”

Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

“…Within the overall pump-probe scheme a wide variety of different techniques has been developed including, e.g. time-resolved resonance Raman [6], pump-CARS (coherent anti-Stokes Raman scattering) [35][36][37], fluorescence upconversion (see, for example, [38]), macrocrystalline time-resolved X-ray scattering [39], photoelectron spectroscopy [40] and -perhaps most commonly applied -time-resolved transient absorption [41].…”