Electronic Stark spectroscopy allows for measuring changes of molecular electric dipole moments (Δμ) upon an electronic transition [1]. To address transient electric interactions of chromophores embedded in a fluctuating polar environment, we introduce ultrafast time-resolved terahertz (THz) Stark spectroscopy [2]: In a temporally and spectrally resolved pump-probe approach, a non-resonant picosecond THz pulse provides an external electric field of up to several MV/cm and a femtosecond optical probe pulse monitors the induced changes of electronic absorption.
Here, we present results for Betaine-30 (B-30) in liquid DMSO and for wild-type bacteriorhodopsin (wt bR). The strong THz field induces a transient broadening of their absorption bands around the unshifted center of the spectrum, indicating a predominant change in the electric dipole moment (Fig. 1a,b). The time evolution of the absorbance changes ΔA follows the THz intensity in time (Fig. 1c,e), without contributions from solvent dynamics and depends linearly on THz intensity (Fig. 1d,f). Using a numerical approach, we extract a S₀-S₁ dipole difference Δμ=7.2 Debye (D) for B-30 in DMSO, in good agreement with electronic structure calculations. For wt bR, we find a surprisingly small Δμ=5-6 D, much less than expected for a neat S₁ character of the excited state. In line with theory [3], our results suggest a strong S₁/S₂ mixing in the bR excited state, which reduces Dµ, favoring a 3-state model for the initial excited-state dynamics.

<i>Fig. 1 THz Stark spectroscopy of B-30 in DMSO (a) and wt bR (b) Spectrally resolved absorbance change ΔA (symbols), simulated absorption changes (grey dashed), steady state absorption spectrum (blue line). ΔA=-log(T/T<sub>0</sub>) is plotted as a function of probe frequency, ν<sub>pr</sub> (T, T<sub>0</sub>: sample transmission with and without THz field). (c),(e) Time dependent absorbance changes ΔA at fixed ν<sub>pr</sub> (symbols) and THz intensity (red lines). (d), (f) ΔA at zero delay versus THz peak intensity.</i>

Fig. 1 THz Stark spectroscopy of B-30 in DMSO (a) and wt bR (b) Spectrally resolved absorbance change ΔA (symbols), simulated absorption changes (grey dashed), steady state absorption spectrum (blue line). ΔA=-log(T/T₀) is plotted as a function of probe frequency, ν_pr (T, T₀: sample transmission with and without THz field). (c),(e) Time dependent absorbance changes ΔA at fixed ν_pr (symbols) and THz intensity (red lines). (d), (f) ΔA at zero delay versus THz peak intensity.

Dieter Engel

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