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 S → S actinic excitation, the Raman signal consists of 15 Feynman diagrams entering with different signs. The negative sign indicates vibrational coherences in S or higher S, whereas the positive sign reveals coherences in S or S via S → S → S (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 S or S. When approaching higher resonances S → S, some Raman bands switch their sign from negative to positive, thus, indicating new coherences in S. The results are discussed, and the measured Raman spectra are compared to the computed quantum-chemical spectra.