Positive and negative signal and line shape in stimulated Raman spectroscopy: Resonance femtosecond Raman spectra of diphenylbutadiene.

Resonance stimulated Raman signal and line shape are evaluated analytically under common electronic/vibrational dephasing and exponential Raman/probe pulse, exp(-|t|/τ). Generally, the signal from a particular state includes contributions from higher and lower electronic states. Thus, with S0 → S1 actinic excitation, the Raman signal consists of 15 Feynman diagrams entering with different signs. The negative sign indicates vibrational coherences in S1 or higher Sn, whereas the positive sign reveals coherences in S0 or Sn via S1 → Sn → Sm (n < m) coupling. The signal complexity is in contrast to spontaneous Raman with its single diagram only. The results are applied to femtosecond stimulated Raman spectra of trans-trans, cis-trans (ct), and cis-cis (cc) 1,4-diphenyl-1,3-butadiene, the ct and cc being reported for the first time. Upon actinic excitation, the Stokes spectra show negative bands from S1 or Sn. When approaching higher resonances Sn → Sm, some Raman bands switch their sign from negative to positive, thus, indicating new coherences in Sn. The results are discussed, and the measured Raman spectra are compared to the computed quantum-chemical spectra.

Photoisomerization pathways and Raman activity of 1,1'-difluorostilbene.

The photoisomerization of 1,1'-difluorostilbene, following S0→S1 optical excitation in solution, was studied with femtosecond broadband transient absorption and stimulated Raman spectroscopy, and by quantum-chemical calculations. In n-hexane, trans-to-cis (t→c) isomerization starts with Franck-Condon relaxation (τ1t = 0.07 ps) followed by nearly barrierless torsion around the ethylenic bond (τ2t ≈ 0.3 ps) to a perpendicular conformation P. About 50% of the excited molecules are trapped in P, while others reach the S1(cis) conformation adiabatically. For the opposite cis-to-trans (c→t) path, the dynamics in n-hexane (τ1c = 0.04 ps, τ2c = 0.7 ps) suggest a 5 kJ/mol barrier between the relaxed S1(cis) and P states. The subsequent P decay with τ3 = 0.4 ps is followed by much slower ground-state recovery (τ4 ≈ 3 ps), indicating an intermediate state X. The t→P and c→P torsion depend on solvent viscosity and polarity, whereas the P→X→S0 relaxation and residual torsion is viscosity-independent but still polarity-dependent. Photoisomerization yields are derived from the transient absorption data and compared to those from actinometric measurements. Low-frequency oscillations in the transient signal are assigned to nuclei motions. Transient and stationary stimulated Raman spectra are compared to calculations. Early Franck-Condon Raman spectra differ from those of the quasistationary trans or cis S1 state. The photoisomerization behavior of stilbene and vinyl-substituted derivatives is compared and the general features are discussed.

Broadband transient absorption spectroscopy with 1- and 2-photon excitations: Relaxation paths and cross sections of a triphenylamine dye in solution.

1-photon (382 nm) and 2-photon (752 nm) excitations to the S1 state are applied to record and compare transient absorption spectra of a push-pull triphenylamine (TrP) dye in solution. After 1-photon excitation, ultrafast vibrational and structural molecular relaxations are detected on a 0.1 ps time scale in nonpolar hexane, while in polar acetonitrile, the spectral evolution is dominated by dipolar solvation. Upon 2-photon excitation, transient spectra in hexane reveal an unexpected growth of stimulated emission (SE) and excited-state absorption (ESA) bands. The behavior is explained by strong population transfer S1 → Sn due to resonant absorption of a third pump photon. Subsequent Sn → S1 internal conversion (with τ1 = 1 ps) prepares a very hot S1 state which cools down with τ2 = 13 ps. The pump pulse energy dependence proves the 2-photon origin of the bleach signal. At the same time, SE and ESA are strongly affected by higher-order pump absorptions that should be taken into account in nonlinear fluorescence applications. The 2-photon excitation cross sections σ(2) = 32 ⋅ 10(-50) cm(4) s at 752 nm are evaluated from the bleach signal.

Excited-state Raman spectroscopy with and without actinic excitation: S1 Raman spectra of trans-azobenzene.

We show that femtosecond stimulated Raman spectroscopy can record excited-state spectra in the absence of actinic excitation, if the Raman pump is in resonance with an electronic transition. The approach is illustrated by recording S1 and S0 spectra of trans-azobenzene in n-hexane. The S1 spectra were also measured conventionally, upon nπ* (S0 → S1) actinic excitation. The results are discussed and compared to earlier reports.

Communication: Optical cooling of trans-stilbene.

Trans-stilbene in n-hexane is excited with excess vibrational energy in the range 0-7000 cm(-1). In the excited electronic state, the Raman linewidth of the ethylenic C=C stretching mode at 1570 cm(-1) is followed with ~100 fs time resolution. Upon excitation with substantial excess energy, the width of the peak is initially broad and then narrows within a few picoseconds, as observed previously by Iwata and Hamaguchi [Chem. Phys. Lett. 196, 462 (1992)]. This narrowing is understood as being caused by cooling of the initially hot molecule, by the surrounding solvent. In this Communication, we report that upon excitation without excess energy, the width is initially relatively narrow and then broadens on a picosecond time scale. The broadening is attributed to heating of the molecule by solvent collisions. It follows that the nascent population in the excited electronic state is cold as compared with the solvent. Such reduction of the initial vibrational energy may affect the rate for the subsequent photoreaction, especially in the absence of the solvent.

Femtosecond Raman spectra of cis-stilbene and trans-stilbene with isotopomers in solution.

Femtosecond stimulated Raman spectra of trans-stilbene (D0), its isotopomers D2, D10, D12, (13)C2 and of cis-stilbene in hexane are measured in the ground (S(0)) and excited (S(1)) electronic states. The ground (13)C2 and excited D12 spectra are presented for the first time; the excited cis-spectra differ substantially from previously published ones. S(1) Raman bands of trans-stilbene are 20 cm(-1) wide corresponding to ~1 ps vibrational dephasing. For cis-stilbene the bands are broadened to 40 cm(-1) reflecting a short excited-state lifetime of 0.3 ps, in agreement with transient absorption data. From a dynamic shift of the 1569 cm(-1) band, pump-induced intramolecular cooling is estimated to be less than 20 K. Many S(1) Raman lines are detected for the first time. Vibrational spectra are calculated at MP2/cc-pVTZ (for S(0)) and XMCQDPT2/cc-pVTZ (for S(1)) levels of theory. Experimental and computational results can be used for a re-evalution of Rice-Ramsberger-Kassel-Marcus (RRKM) predictions for this famous photoisomeration reaction.

Lineshapes for resonant impulsive stimulated Raman scattering with chirped pump and supercontinuum probe pulses.

Molecular vibrational coherence from impulsive stimulated Raman (SR) scattering, as observed by broadband transient absorption spectroscopy, is treated within the well-known third-order perturbation formalism. Shaped femtosecond optical pulses are used for the pump and supercontinuum probe fields. Dephasing is assumed to be homogeneous in the Bloch approximation. A key step requires threefold time integration over response functions and electric fields. For well-separated pulses the triple integral can be solved analytically, resulting in lineshape functions. These allow to describe the SR signal through absorption/emission/dispersion profiles which are associated with the inherent contributions. A clear physical interpretation of the amplitude and phase of the oscillatory signal is thereby obtained, and a direct connection with the vibronic structure of the molecular system is provided. Calculations for model molecular systems illustrate the spectral dependence of the vibrational coherence seen, for example, with perylene in cyclohexane. The nonoscillatory and oscillatory parts of the transient absorption spectra are compared to each other. Observed mode beatings are explained.

Rotamer-Specific Photoisomerization of Difluorostilbenes from Transient Absorption and Transient Raman Spectroscopy.

Photoisomerization of 2,2'-, 3,3'-, and 4,4'-difluorostilbene (F2, F3, F4, respectively) in n-hexane, perfluoro-n-hexane, and acetonitrile is studied with broadband transient absorption (TA) and femtosecond stimulated Raman (FSR) spectroscopy and by DFT/TDDFT calculations. F2 and F3 possess three rotamers (rotational isomers) each, while F4 has one single conformation only. These differences are reflected in TA and FSR spectra. Thus F4 reveals a monoexponential decay of TA with τ1 = 172 ps in n-hexane, as expected for a single species. For F2 and F3, the decays are biexponential in all solvents, corresponding to two distinctly discerned rotamers or rotamer fractions. Specifically, for F2 in n-hexane, τ1 = 357 ps (83%) and τ2 = 62 ps (17%), and for F3 in the same solvent, τ1 = 222 ps (57%), and τ2 = 81 ps (43%). The weights in brackets agree with theoretically estimated ground-state abundances of the rotamers. Furthermore, a global fit of the TA and FSR data allows us to extract the spectra of the pure rotamers. The Raman spectra of S0 and S1 are in qualitative agreement with calculations.

Two-Photon-Induced versus One-Photon-Induced Isomerization Dynamics of a Bistable Azobenzene Derivative in Solution.

We report on a bistable azobenzene derivative with sufficiently high 2-photon absorption to induce its photochemical isomerization and measurable excited state dynamics. Broadband transient absorption spectra were recorded and compared upon 1-photon (331 nm) and 2-photon (640 nm) excitation of the S0 → S2 transition. The spectra are different at early (t ∼ 1 ps) and late (t ∼ 100 ps) time but show similar photoisomerization behavior on a 10 ps time scale. With 2-photon excitation, strong population transfer S2 → Sn occurs due to resonance absorption of a third pump photon. Subsequent internal conversion Sn → S1 results in a very hot S1 population causing extra-broadening of the transient spectra. The resonance pump absorption is common with nonlinear excitation and should be taken into account when considering photochemical applications. The 2-photon excitation cross-section σ((2)) at 640 nm was measured to be 7 GM for the specific tetra-ortho-fluorinated azobenzene derivative and 1 GM for unsubstituted parent azobenzene. The direct 2-photon induced trans-to-cis isomerization, described herein, provides an unprecedented potential for spatially addressing P-type (bistable) azobenzene photoswitches in 3D.

Photoisomerization dynamics of stiff-stilbene in solution.

Photoinduced isomerization of 1,1'-bis-indanyliden (stiff-stilbene) in solution was studied with broadband transient absorption and femtosecond Raman spectroscopies, and by quantum-chemical calculations. Trans-to-cis S1 isomerization proceeds over a 600 and 400 cm(-1) barrier in n-hexane and acetonitrile, respectively. The reaction develops on multiple time scales with fast (0.3-0.4 ps) viscosity-independent and slower (2-26 ps) viscosity-dependent components. In the course of intramolecular torsion (which should be the main reaction coordinate) some excited molecules pass through the perpendicular conformation P and reach the cis geometry, to be temporarily trapped there. Subsequently they relax back to P and further to the ground state S0. The cis-to-trans isomerization reveals ultrafast (0.06 ps) oscillatory relaxation followed by 13 ps decay in n-hexane and 2 ps decay in acetonitrile, corresponding to barriers of 800 and 400 cm(-1), respectively. Raman S0 and S1 spectra are reported and discussed. The perpendicular conformation P was not detected, possibly due to its low oscillator strength and short lifetime, or because of strong overlap with hot product spectra. XMCQDPT2 calculations locate a stationary S1 point on the cis side and two perpendicular-pyramidalized stationary points, to be reached from the former over 300 and 680 cm(-1) barrier. Implications for parent stilbene are discussed; in this case we also see evidence for the trans-to-cis adiabatic path, as in stiff-stilbene. Very similar viscosity dependence for the two compounds supports the common isomerization pathway: torsion about the central double bond.

Photoisomerization dynamics and pathways of trans- and cis-azobenzene in solution from broadband femtosecond spectroscopies and calculations.

The photoisomerization of azobenzene in solution was studied experimentally and by calculations. trans-to-cis and cis-to-trans dynamics are described through broadband transient absorption, fluorescence, and stimulated Raman spectroscopy. Transient absorption was extended to cover not only the nπ* band but also the ππ* band in the ultraviolet. Isomerization yields are used for a quantitative comparison of trans and cis transient spectra under different excitation. For the trans-to-cis path upon nπ*(S(1)) excitation, the evolution develops with 0.3, 3, and 16 ps. The first two times reflect population relaxation to a local minimum S(1t )(L) and subsequent transition to a dark intermediate S(1t)(D) over an 8 kJ/mol barrier. The existence of stationary points S(1t)(L) and S(1t)(D) is confirmed by quantum-chemical calculations. The third time corresponds to S(1t) (D) → S0 relaxation to the ground state via an S1/S0 conical intersection over a 12 kJ/mol barrier. Thus, the 16 ps time constant is attributed to the isomerization process and not to vibrational cooling, contrary to the current view and in line with the previous interpretation by Lednev et al. (J. Phys. Chem. 1996, 100, 13338). The decay of the long-lived intermediate S(1t)(D) is consistent with the hula twist rather than with the inversion mechanism. For the cis-totrans reaction following nπ* excitation, signal decay is strongly nonexponential, with 0.1 and 1 ps. The latter (1 ps) is much shorter than the 16 ps decay of the trans isomer, implying different S1/S0 conical intersections and relaxation paths for the cis-totrans and trans-to-cis reaction. New results are also obtained with ππ*(Sn) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S1 region, which is not accessible under nπ* excitation. For cis-azobenzene, up to 30% of the excited species isomerize to trans via an Sn/S1 intersection, resulting in a mixed cis/trans S1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.

An efficient setup for femtosecond stimulated Raman spectroscopy.

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 energies 1-10 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.

Femtosecond pump/supercontinuum-probe spectroscopy: optimized setup and signal analysis for single-shot spectral referencing.

A setup for pump/supercontinuum-probe spectroscopy is described which (i) is optimized to cancel fluctuations of the probe light by single-shot referencing, and (ii) extends the probe range into the near-uv (1000-270 nm). Reflective optics allow 50 µm spot size in the sample and upon entry into two separate spectrographs. The correlation γ(same) between sample and reference readings of probe light level at every pixel exceeds 0.99, compared to γ(consec)<0.92 reported for consecutive referencing. Statistical analysis provides the confidence interval of the induced optical density, ΔOD. For demonstration we first examine a dye (Hoechst 33258) bound in the minor groove of double-stranded DNA. A weak 1.1 ps spectral oscillation in the fluorescence region, assigned to DNA breathing, is shown to be significant. A second example concerns the weak vibrational structure around t=0 which reflects stimulated Raman processes. With 1% fluctuations of probe power, baseline noise for a transient absorption spectrum becomes 25 µOD rms in 1 s at 1 kHz, allowing to record resonance Raman spectra of flavine adenine dinucleotide in the S(0) and S(1) state.

Detecting electronic coherence in excited-state electron transfer in fluorinated benzenes.

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 cm(-1) in hexane and 94 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.

Coherent and sequential contributions to femtosecond transient absorption spectra of a rhodamine dye in solution.

A unified description is presented of sequential and coherent contributions to optical transient absorption measured by femtosecond pump-supercontinuum probe spectroscopy. All inherent transient terms are taken into account. The "coherence spike" seen during pump-probe overlap is thereby decomposed into its components. The method is demonstrated with rhodamine 110 in methanol. Pure homogeneous dephasing times are obtained from a simultaneous fit of all pertinent measurements. Vibronic structure in the coherence spectrum is assigned to stimulated Raman scattering between vibrational levels in the first excited electronic state. The time-zero spectrum for stimulated emission and the solvation relaxation function are also obtained.