Photochemical processes play an integral role in our day-to-day lives. Nature has carefully chosen our molecular building blocks so that the potentially.

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  • Photochemical processes play an integral role in our day-to-day lives. Nature has carefully chosen our molecular building blocks so that the potentially devastating effects of ultraviolet (UV) radiation absorption are by-passed. For example, adenine readily absorbs UV radiation, however its spectroscopy is characterized by low fluorescence quantum yields [1]. The high rate of non- radiative decay in this and other similar hetero-aromatic systems minimizes the lifetimes of the lowest excited states and as a result reduces the risk of photochemical damage. Ab initio [2] electronic structure calculations indicate that there may be a very simple radiationless decay pathway that governs the high rates of non-radiative decay in aromatic and heterocyclic biomolecules. Upon UV irradiation, photon energy is deposited into the molecule through excitation to an optically bright 1 * state. In these molecules an excited state of 1 * character intersects both the initially excited 1 * state and the electronic ground state along the X-H stretch coordinate where X is typically O or N. Non-radiative decay along this pathway is predicted to be highly efficient due to the repulsive nature of the 1 * state. Since the prediction of the 1 * relaxation pathway by ab initio calculations an increasing number of ultrafast, femtosecond experiments have been designed to search for their spectroscopic evidence. Figure 1: Molecular structures of some of the molecules of interest: Tyrosine (a), tryptophan (b) and adenine (c). The dissociative X-H bond attributed to 1 * is highlighted in yellow. (c) Introduction Experimental setup Molecular beam Laser beams Velocity focusing lens Phosphor screen MCP detector Drift tube The optical setup uses a commercial femtosecond laser system which is split into three beams of equal intensity. The first beam is used to generate the 4 th harmonic (pump) at 200 nm through frequency quadrupling. The remaining two beams are used to pump two OPAs. One of the outputs is set at 243.1 nm to probe neutral H atoms via (2+1) REMPI. The pump/probe beams are combined collinearly at a beam- splitter and sent into the interaction region of a velocity map ion imaging (VMI) spectrometer which intercepts a molecular beam of sample. The molecular beam is produced by seeding a vapour pressure of target molecule in He, admitted into vacuum using an Even-Lavie pulsed solenoid valve. Source chamber Interaction chamber Linear TOF-MS VMI setup Prototypical system-ammonia Using a combination of pump/probe spectroscopy and VMI, we have studied the multichannel photodissociation of ammonia with the ultimate goal of carrying forward our knowledge of this simple system to more complex biomolecules displaying similar non-adiabatic dynamics [6]. We have been able to clock the real-time dissociation of the N-H bond, following excitation of the 2 = 4 umbrella mode in the A-state (Fig. 4). In doing so, we have partially untangled the various dissociation paths which lead to different final states of the NH 2 fragment (Figs. 5 and 6). Dissociation lifetimes of the H-atoms with the least KEs suggests the onset of adiabatic dissociation effectively competing with the non- adiabatic pathway. Figure 4: Cut through the A- and X-state potential curves in ammonia, taken from Bach et al. [5] Figure 5: H + transient as a function of H-atom KE. H-atom kinetic energy / cm -1 H + signal t=0 Pump/probe (1 ps) 0400080001200016000 Probe/pump (0.25 ps) + 200 nm with KE 0 2 4 6 8 10 v 2 (NH 2 X-state) Lifetime () / fs 0 100 200 300 -0.5 0 0.5 1 1.5 2.0 t=59 fs (v 2 =1) Time / ps H + signal t=173 fs (v 2 =6) t=250 fs (v 2 =9) By probing the H-atoms following UV excitation at 200 nm, we recently showed that H-atom elimination along the dissociative 1 * PES in phenol occurs within 103 30 fs (Fig. 8) [7], indicative of very efficient coupling at the S 1 /S 2 and S 0 /S 2 CIs (Fig. 9 orange path). This non-statistical route is in competition with the statistical pathway (unimolecular decay) which was not observed on the timescale of these measurements (< 200 ps). Phenol-the chromophore of tyrosine Slow H Fast H Phenol-h6 Phenol-d5 Work in progress.... References [1] Crespo-Hernndez, C. E.; Cohen, B.; Hare, P. M.; Kohler, B. Chem. Rev. 2004, 104, 1977 [2] Sobolewski, A. L.; Domcke, W., Chem. Phys. 2000, 259, 181 [3] Eppink, A. T. J. B.; Parker, D. H., Rev. Sci. Instrum. 1997, 68, 3477 [4] Roberts, G. M; Nixon, J. L.; Lecointre, J.; Wrede, E.; Verlet, J. R. R., Rev. Sci. Instrum. 2009, 80, 053104 [5] Bach, A.; Hutchison, J. M.; Holiday, R. J.; Crim, F. F. J. Phys. Chem. A. 2003, 107, 10490 [6] Wells, K. L.; Perriam, G.; Stavros, V. G., J. Chem. Phys. 2009, 130, 074308 [7] Iqbal, A. J.; Pegg, L. G.; Stavros, V. G., J. Phys. Chem. A. 2008, 112, 9531 [8] Iqbal, A. J.; Cheung, M. S. Y.; Nix, M. G. D.; Stavros, V. G., J. Phys. Chem. A. 2009, 113, 8157 [9] Wells, K. L.; Roberts, G. A.; Stavros, V. G., Chem. Phys. Lett., 2007, 446, 20 [10] Bisgaard, S.Z.; Satzger, H.; Ullrich, S.; Stolow, A., ChemPhysChem. 2009, 10, 101 Acknowledgements The interaction chamber contains the VMI detector, replicating the setup by Eppink and Parker to detect H-atoms [3]. A timed voltage pulse extracts H + ions towards the detector consisting of a 40 mm diameter Chevron microchannel plate assembly coupled to a phosphor screen (Photek). Deconvolution of the raw images is done using the polar ion peeling method [4]. Velocity map ion imaging (VMI) Statistical vs. non-statistical decay has been an area of increasing interest in the literature. Very recently [8], we have shown using a combination of pump/probe spectroscopy and VMI, that H-atom elimination in phenol-d5 giving low KE H- atoms occurs in < 150 fs (Figs. 10-central spot and 11 (a)), in sharp contrast to what one expects from a statistical decay process, which would also give low KE H-atoms. This implies that these H-atoms are formed through a non- statistical dynamic route and not through a statistical route as previously thought. Figure 6: Integrated H + signal corresponding to NH 2 fragments coming off with different quanta of 2 (bend). Figure 7: Observed lifetime () vs. quanta of 2 bend in NH 2. In our efforts to understand the role of the 1 * in the excited state dynamics of larger, more complex systems, our research is currently looking at the photochemistry of imidazole (en route to adenine [9, 10]) and derivatives there-of in addition to tyrosine. Figure 10: Raw images for H + following photodissociation at 200 nm. Only half of each image is shown for illustrative purposes. The probe pulse is delayed 2 ps relative to the pump. Figure 12: Extended H + transients. Circles and squares represent high and low KE H-atoms. Adenine Imidazole part Thanks go to Lara-Jane Pegg for experimental assistance. We would like to thank Dr Jan Verlet and Mr Gareth Roberts for use of their polar onion peeling program and discussions about VMI. We would also like to thank Dr Mike Nix for modelling the data and valuable discussions and Professor Mike Ashfold for valuable discussions. Figure 9 (right): Cut along R O-H for the S 0, S 1 and S 2 PES of phenol-h6 Figure 8 (left): Total integrated H + transient as a function of pump (200 nm)/probe(243.1 nm) delay Figure 11: H + transients as a function of pump (200 nm)/probe (243.1 nm) delay for the low and high KE H-atoms in phenol-d5. Figure 3: LHS shows a schematic of the VMI setup. RHS depicts a raw image of H + ions following photodissociation of HBr at 200 nm and REMPI of the H-atoms with 243.1 nm. The dominant channel at 200 nm gives Br. Figure 2: Schematic of experimental setup (RHS) and a picture of our molecular beam machine (LHS). Dynamics of X-H dissociation in biomolecules Imidazole2-methyl imidazole 4-methyl imidazole (a) (b) Kym Wells, Azhar Janjuah, Gareth Perriam, Michelle Cheung, David Hadden and Vasilios Stavros Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK Br* + H ( 0) Br + H ( -1) HBr 200 nm Br Br* Dissociation leaves the NH 2 excited vibrationally, the extent of excitation determining the KE in the H-atom fragment. Indole part Tryptophan t=90 fs (imidazole) t=140 fs (4-methylimidazole) t=160 fs (2-methylimidazole) -0.5 0 0.5 1 1.5 2.0 2.5 Time / ps H + signal Indole Financial support Figure 15: Raw half- images for H + following photodissociation of indole at 200 nm. The probe pulse is delayed (as indicated) relative to the pump pulse. Figure 13 (top) : Total integrated H + transient as a function of pump (200 nm)/probe(243.1 nm) delay for imidazole and its derivatives Figure 14 (bot): Cut along R N-H for S 0 and S 1 PES of imidazole/derivatives 243 nm200 nm 4 5 6 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 R O-H A O PE / eV imidazole 2,4-methylimidazole 2-methylimidazole 4-methylimidazole


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