We present an efficient and robust setup for femtosecond stimulated Raman (FSR) spectroscopy with 60 fs and 10 cm⁻¹ resolution. Raman pulses of 0.5–5 ps are tunable between 450–750 nm with energies1–10 μJ. Experimental features of the setup, signal processing, and data treatment are discussed in detail to be readily reproduced in other labs. The setup is tested by measuring FSR spectra of stilbene in solution.

The ferroelectricity of multivalent codoped Bismuth ferrite (BiFeO₃; BFO) nanoparticles (NPs) is revealed and utilized for photocatalysis, exploiting their narrow electronic bandgap. The photocatalytic activity of ferroelectric photocatalysts BiFe_0.95Mn_0.05O₃ (BFM) NPs and mono‐, di‐, or tri‐valent cations (Ag⁺, Ca²⁺, Dy³⁺; MDT) coincorporated BFM NPs are studied under ultrasonication and in acidic conditions. It is found that such doping enhances the photocatalytic activity of the ferroelectric NPs approximately three times. The correlation of the photocatalytic activity with structural, optical, and electrical properties of the doped NPs is established. The increase of spontaneous polarization by the mono‐ and tri‐valent doping is one of the major factors in enhancing the photocatalytic performance along with other factors such as stronger light absorption in the visible range, low recombination rate of charge carriers, and larger surface area of NPs. A‐site doping of BFO NPs by divalent elements suppresses the polarization, whereas trivalent (Dy³⁺) and monovalent (Ag+) cations provide an increase of polarization. The depolarization field in these single domain NPs acts as a driving force to mitigate recombination of the photoinduced charge carriers.

Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes, is observed and calculated for planar ring-fluorinated stilbenes, trans-2,3,5,6,2,3,5,6-octofluorostilbene (tF2356) and trans-2,3,4,5,6,2,3,4,5,6-decafluorostilbene (tF23456). Due to the fluorination, Franck-Condon states and are dominated by the quasi-degenerate HOMO-1→LUMO and HOMO-2→LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S1 state. After optical excitation of tF2356, one observes an ultrafast (~0.06 ps) evolution, that reflects relaxation from initial nonpolar to long-lived (1.3 ns in n-hexane and 3.4 ns in acetonitrile) polar S1. The polarity of S1 is evidenced by a solvatochromic shift of its fluorescence band. The experimental results provide a sensitive test for quantum-chemical calculations. In particular, our calculations agree with the experiment, and raise concerns about the applicability of the common TDDFT approach to relatively simple stilbenic systems.

Photoinduced electron transfer (ET) in heavily fluorinated benzenes in solution has been studied with broadband transient absorption spectroscopy. Spectrally resolved kinetics exhibit oscillations with amplitude up to 70% of the signal. The oscillation frequency is specific for each probe; in addition, for pentafluorobenzene it markedly depends on solvent, being $86{\mathrm{cm}}^{-1}$ in hexane and $94{\mathrm{cm}}^{-1}$ in acetonitrile. We argue that the observed behavior is not related to vibrational coherences, but originate from coherent electronic motion between an optically excited and an ET state.