Direct Observation of Electronic Relaxation Dynamics in Adenine via Time-Resolved Photoelectron Spectroscopy

Abstract

We present femtosecond time-resolved photoelectron spectra of adenine in a molecular beam, recorded at pump wavelengths of 250, 267, and 277 nm. This leads to initial excitation of the bright S2(pipi*). Close to the band origin (277 nm), the lifetime is several picoseconds. Higher vibronic levels (267 and 250 nm excitation) show much shorter lifetimes of t < 50 fs, and we observe strong coupling between S2(pipi*) and S1(npi*). Rapid internal conversion (t < 50 fs) populates the lower lying S1(npi*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(pisigma*) state, previously proposed as an ultrafast relaxation pathway from S2(pipi*).

Electronic Structure of Adenine and Thymine Base Pairs Studied by Femtosecond Electron−Ion Coincidence Spectroscopy

“…The dynamics of A and T monomers are characterized by two time constants τ 1 and τ 2 , which are in agreement with earlier results. 8,15 An additional state with a nanosecond lifetime exists in T 7 but plays no role in our discussion.…”

“…Out-of-plane deformation of the six-membered ring can lead to conical intersections between nπ*, ππ*, and the ground state with very small barriers. 21,[24][25][26] A conical intersection to a quasi-dissociative πσ* may also play a role, 8,11 especially at higher excitation energies or in polarizable environments. 16,[27][28][29][30] Time-resolved photoelectron spectroscopy (TRPES) experiments comparing adenine with 9-methyl-substituted adenine 11 found the same biexponential time constants, 0.1 and 1.2 ps, as ion yield studies.…”

On the Participation of Photoinduced N–H Bond Fission in Aqueous Adenine at 266 and 220 nm: A Combined Ultrafast Transient Electronic and Vibrational Absorption Spectroscopy Study

“…35 In general, excited state population of 9H-Ade decays biexponentially, with time-constants of ≤100 fs and ~1 ps. [36][37][38][39] With specific regard to the participation of the 1 πσ* state in 9H-Ade's photochemistry, a number of studies have been performed to investigate the activity of this channel. [38][39][40][41][42][43][44] Early work in both the time and frequency domains led a number of research groups to infer that ultrafast relaxation via this state may in fact be active following excitation at 267 nm; 38, 41, 42 a much longer wavelength than initially predicted by Domcke and co-workers 14 and later theoretical work.…”

Three-State Conical Intersections in Nucleic Acid Bases

“…[18][19][20] The excited-state lifetimes have been reported recently using many different techniques. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] These studies report lifetimes on the order of femtoseconds and suggest that nonradiative relaxation proceeds on an ultrafast time scale to the ground state, with the extra energy being transformed into heat.…”