Real-time observation of multi-mode vibronic coherence in pentafluoropyridine

The ultrafast dynamics of pentafluoropyridine in the 1 ¹B₂ (ππ*) electronic state excited at λ_pump = 255 nm is investigated by femtosecond time-resolved time-of-flight mass spectrometry and photoelectron imaging spectroscopy. A pronounced, long-lived, and complex periodic modulation of the transient ion yield signal with contributions by four distinct frequency components, 72 cm^-1, 144 cm^-1, 251 cm^-1, and 281 cm^-1, is observed for up to 9 ps. The recorded photoelectron images display a spectral band from the excited 1 ¹B₂ (ππ*) state only in the oscillation maxima; the signal is strongly reduced in the oscillation minima. Supported by electronic structure calculations at the RI-SCS-CC2 and XMCQDPT2 levels of theory, the oscillating components of the signal are identified as frequencies of b₁ symmetry coupling modes in a vibronic coherence of the 1 ¹B₂ (ππ*) and 1 ¹A₂ (πσ*) electronic states. The optical excitation initiates regular and periodic wavepacket motion along those out-of-plane modes. In the distorted molecular structure, the initially excited state acquires substantial πσ* character that modulates the transition dipole moment for ionization and results in the observed oscillations.