Pump-degenerate four wave mixing as a technique for analyzing structural and electronic evolution: Multidimensional time-resolved dynamics near a conical intersection

Pump-degenerate four wave mixing (pump-DFWM) is used to simultaneously study the early events in structural and electronic population dynamics of the non-adiabatic passage between two excited electronic states. After the precursor state S₂ is populated by an initial pump beam, a DFWM sequence is set resonant with the S₁ → S_n transition on the successor state S₁. The information obtained by pump-DFWM is two-fold: by scanning the delay between the initial pump and the DFWM sequence, the evolution of the individual excited-state modes is observed with a temporal resolution of 20 fs and a spectral resolution of 10 cm^-1. Additionally, pump-DFWM yields information on electronic population dynamics, resulting in a comprehensive description of the S₂ → S ₁ internal conversion. As a system in which the interplay between structural and electronic evolution is of great interest, all-trans-β- carotene in solution was chosen. The pump-DFWM signal is analyzed for different detection wavelengths, yielding results on the ultrafast dynamics between 1B_u⁺ (S₂) and 2A_g^- (S₁). The process of vibrational cooling on S₁ is discussed in detail. Furthermore, a low-lying vibrationally hot state is excited and characterized in its spectroscopic properties. The combination of highly resolved vibrational dynamics and simultaneously detected ultrafast electronic state spectroscopy gives a complete picture of the dynamics near a conical intersection. Because pump-DFWM is a pure time domain technique, it offers the prospect of coherent control of excited-state dynamics on an ultrafast time scale.