MODE-specific deactivation of adenine at the singlet excited states

The deactivation process of adenine excited near the band origin of the lowest pi pi* state (L-1(b)) is investigated using picosecond (ps) time-resolved photoionization spectroscopy. The transients obtained with a ps pump pulse at the sharp vibronic bands, 36 105 cm(-1) (D) and 36 248 cm(-1) (E), in the resonant two-photon ionization spectrum exhibit a bi-exponential decay with two distinct time constants of tau(1) similar to 2 ps and tau(2) > 100 ps, whereas the transients with the pump at other wavenumbers in this energy region show a single exponential decay with tau = 1-2 ps. We suggest that the tau(1) represents the lifetimes of the 1(n pi*) energy levels near the D and E peaks, which are excited together by the ps pump pulse having a broad spectral bandwidth, and the tau(2) shows the lifetimes of D and E peaks. The long lifetime of D level is attributed to a small barrier for internal conversion from the minimum of the L-1(b) state to the 1(n pi*) state. On the other hand, the long lifetime of E level is ascribed to the nuclear configuration of adenine at this level, which is unfavorable to reach the seam of the conical intersection leading to nearly barrierless deactivation to the electronic ground state. This study shows that the ps time-resolved spectroscopy provides a powerful tool to study mode-and energy-specific deactivation processes occurring in a multi-dimensional potential energy surface. (C) 2013 AIP Publishing LLC.