Wavelength Dependence of Electronic Relaxation in Isolated Adenine Using UV Femtosecond Time-Resolved Photoelectron Spectroscopy

Abstract

Electronic relaxation pathways in photoexcited nucleobases have received much theoretical and experimental attention due to their underlying importance to the UV photostability of these biomolecules. Multiple mechanisms with different energetic onsets have been proposed by ab initio calculations yet the majority of experiments to date have only probed the photophysics at a few selected excitation energies. We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA base adenine in a molecular beam at multiple excitation energies between 4.7-6.2 eV. The two-dimensional TRPES data is fit globally to extract lifetimes and decay associated spectra for unambiguous identification of states participating in the relaxation. Furthermore, the corresponding amplitude ratios are indicative of the relative importance of competing pathways. We adopt the following mechanism for the electronic relaxation of isolated adenine; initially the S(2)(ππ*) state is populated by all excitation wavelengths and decays quickly within 100 fs. For excitation energies below ∼5.2 eV, the S(2)(ππ*)→S(1)(nπ*)→S(0) pathway dominates the deactivation process. The S(1)(nπ*)→S(0) lifetime (1032-700 fs) displays a trend toward shorter time constants with increasing excitation energy. On the basis of relative amplitude ratios, an additional relaxation channel is identified at excitation energies above 5.2 eV.

Ultrafast Dynamics of UV‐Excited Imidazole

“…[44] The optical layout is based on a commercially available femtosecond oscillator/amplifier pair (Coherent Inc., Mira 900, and Legend Elite HE). The amplifier output (1 kHz, 800 nm, 130 fs, 3.5 mJ/pulse) is split to produce UV pump and probe pulses at different wavelengths in subsequent frequency-conversion steps.…”

Surface Hopping Dynamics with DFT Excited States

“…Ground State Population at 1 ps (%) Fig. 6 Ground-state population of adenine gas 1 ps after photoexcitation according to the experiment [182] and surface hopping based on diverse LR-TDDFT simulations. TDDFT data from [35,148].…”

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

“…[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. 18,21 However, high-resolution photofragment translational spectroscopy measurements by Nix et al.…”