Unified explanation for linear and nonlinear optical responses inβ-carotene: A sub-20−fsdegenerate four-wave mixing spectroscopic study

Abstract

The four-wave mixing signal of ␤-carotene measured under the resonant excitation is reported. A clear coherent oscillation with a period of a few tens of femtoseconds was observed. We have estimated the line broadening function required to simulate this oscillation behavior. The parameters, including the solvation effect, which are essential for calculating the optical signals have also been determined. The validity of our simulation has been evaluated by comparing the theoretically calculated linear and nonlinear optical signals with the experimental results. It was found that in addition to the C u C and C v C stretching modes the methyl in-plane rocking mode significantly contributes to the optical responses of ␤-carotene. Calculations based on the Brownian oscillator model were performed under the impulsive excitation limit, and we find that the memory of the vibronic coherence generated in the S 2 state is lost via relaxation processes, which include the S 1 state. Comparison between the simulation and experiment revealed that the two-photon absorption process plays an important role in the very early optical process taking place in ␤-carotene. The vibronic decoherent time of the system is estimated to be 1 ps, which is about five times longer than the population lifetime of the S 2 state determined in the previous studies. The possible relationship between the lifetime of the vibronic coherence and the efficient energy transfer in light-harvesting antenna complexes is discussed.

Ground‐ and Excited‐State Vibrational Coherence Dynamics in Bacteriorhodopsin Probed With Degenerate Four‐Wave‐Mixing Experiments

“…[32][33][43][44][45] In this regard, on the one hand, the highly nonlinear dependence of the DFWM signal intensity on the transition dipole moment (m 8 ) and on the population (N 2 ) results in a high degree of sensitivity regarding spectrally overlapping signal dynamics. This poses a clear advantage of DFWM in contrast to the linear population (N) and nonlinear dipole moment (m 4 ) dependence of the signal intensity in, for example, transient absorption experiments.…”

Spectrally‐resolved transient grating signals from β‐carotene in benzene solution

“…The well-known pronounced structure with an energy spacing of about 1300 cm -1 is due to the combination of several vibrational modes. The absorption spectrum is red-shifted by about 10 nm compared to that measured in THF solution as has been reported in our previous paper [10]. Figure 2 shows the TG signals measured at various excitation wavelengths.…”

“…The rise constituent is less prominent on decreasing the excitation frequency. The coherent oscillations were clearly observed when the excitation wavelength is set to the isosbestic point (curve B) which is the excitation condition of our previous studies [10][11][12]. The coherent oscillations and slowly-varying background become weak when the excitation wavelength is on the longer wavelength side (curves C to D).…”

“…A home-build noncollinear optical parametric amplifier (NOPA) pumped by a femtosecond Ti:sapphire regenerative amplifier (Spectra Physics, Hurricane) was used as the excitation light source for the TG measurements. Details of the NOPA together with the optical setup have been reported elsewhere [10]. All the optical measurements were performed at room temperature.…”

Ultrafast coherent vibronic oscillations in regioregular poly(3‐alkylthiophene)

“…As light source required for ultrafast optical measurements, a noncollinear optical parametric amplifier (NOPA) was used. The details of the NOPA system were previously described [7]. Initially, a femtosecond Ti:sapphire regenerative amplifier (Spectra Physics, Hurricane) was used to generate 100 fs pulses at 800 nm at a 1 kHz repetition rate.…”