Exploring the Time Scales of H-Atom Elimination from Photoexcited Indole Azhar Iqbal and Vasilios G. Stavros* Department of Chemistry, UniVersity of Warwick, CoVentry CV4 7AL, United Kingdom ReceiVed: August 25, 2009; ReVised Manuscript ReceiVed: October 26, 2009 Recent spectroscopic measurements have shown that following excitation of indole molecules above the 1 ππ*- 1 πσ* conical intersection, photoinduced N-H bond cleavage results in a range of H-atom kinetic energy release. H-atoms with large amounts of kinetic energy were attributed to direct dissociation whereas those with low kinetic energy were attributed to indirect pathways such as statistical unimolecular decay. With use of a combination of femtosecond pump-probe spectroscopy and velocity map ion-imaging techniques, both energy and time-resolved photoinduced H-atom elimination at 200 nm has been measured. The results show that H-atoms with both high and low kinetic energies are generated on an ultrafast time scale, <200 fs, suggesting that on the time frame of our measurements (<200 ps) there appears to be a direct route to H-atom formation yielding H-atoms with low kinetic energies. Introduction The photochemistry of biological chromophores, for example, nucleic bases and amino acids, is controlled by the various relaxation mechanisms after initial ultraviolet (UV) excitation. 1-5 The photophysics of indole, the chromophore of the amino acid tryptophan, has been a topic of recent interest as it is a precursor to unraveling the relaxation pathways of tryptophan. Identifying these pathways in the chromophore and relating them to its electronic structure will undoubtedly assist in the transition from the micro (chromophore) to the macro (solvated amino acid and beyond) and to eventually understanding the photochemistry of these biomolecules in vivo. Indole has two absorption bands in the near UV and these are assigned as π* r π transitions. These two excited states are traditionally labeled as 1 L a and 1 L b states. The 1 L b state has a strong transition at 283.78 nm 6 and lies about 1400 cm -1 below the 1 L a state. 7-10 Sobolewski et al. 11 recently showed that the lowest singlet 1 πσ* state has repulsive character along the N-H coordinate intersecting the bound 1 L a and 1 L b ( 1 ππ*) states and the ground (S 0 ) state, providing a route for nonradiative decay. Their ab initio calculations have shown that the low fluorescence quantum yield of indole after excitation above the 1 ππ*- 1 πσ* conical intersection (CI) is due to the 1 πσ* state that can lead to the elimination of neutral hydrogen as a nonradiative product. 11 Because of the small energy gap between 1 L a and 1 L b states, the vibronic coupling is predicted to be very strong 12 and it is assumed that both these states can be excited simultaneously. Owing to the weak 1 πσ* r S 0 transition, the 1 πσ* state is not excited directly (optically dark) and is populated by radiationless transfer from the optically bright 1 ππ* state via the 1 ππ*- 1 πσ* CI. 13 Once on the 1 πσ* state, its repulsive nature along the N-H coordinate means that excited indole molecules evolve toward the 1 πσ*-S 0 CI, resulting in either direct photodissociation to yield H-atoms with large amounts of kinetic energy (KE) or population of vibrationally excited indole (S 0 ) molecules. These “hot” ground-state molecules can undergo statistical unimolecular decay resulting in further H-atoms, albeit with much less KE. Time scales of H-atom elimination from these systems are of obvious importance to develop a better understanding about how potential energy surfaces (PES) couple with one another and the role of the CIs. This paper focuses on the application of time-resolved velocity map ion imaging (TRVMI) which enables one to establish a time constant for H-atom elimination as a function of KE release. Because of the repulsive nature of the 1 πσ* state, direct dissociation via the 1 ππ*- 1 πσ* and 1 πσ*-S 0 CIs is known to produce H-atoms with large amounts of KE, whereas indirect dissociation via hot ground-state molecules leads to H-atoms with much less KE. VMI enables one to disentangle these two pathways because H-atoms with varying amounts of KE can be separated from the ion-image obtained. The work described herein reveals that for all KE ranges, N-H bond dissociation occurs on an ultrafast (<200 fs) time scale within the time frame of our measurements (<200 ps), in stark contrast to the time scale expected for low KE H-atoms born from a statistical unimolecular decay pathway. This suggests that although vibrationally excited indole (S 0 ) molecules may undergo unimolecular decay on a much longer time scale, as recently reported through multimass ion-imaging measurements, 14 an additional route to low KE H-atoms is also operative, which is direct in nature. The photoinduced H-atom elimination from indole through the 1 πσ* state was first reported by Lin et al. 14 using multimass ion imaging. Following photoexcitation at 248 and 193 nm, the H-atom elimination was suggested to occur through two pathways: The H-atoms with high KE were attributed to direct coupling of the 1 ππ* and ground state with the 1 πσ* state at the 1 ππ*- 1 πσ* and 1 πσ*-S 0 CIs, respectively. The second pathway resulted in low KE H-atoms and the dissociation was assumed to occur after internal conversion to ground (S 0 ) state. According to these authors, 80% of indole dissociated from the electronically excited state after photoexcitation at 248 nm, whereas photoexcitation at 193 nm resulted in 54% of dissocia- tion occurring from an electronically excited state. As the excitation energy increased, the rate of internal conversion to the ground state increased, apparently because of the increase in the density of states at higher energies, thus enhancing the low kinetic energy H-atom elimination channel. They established * To whom correspondence should be addressed. E-mail: v.stavros@ warwick.ac.uk. J. Phys. Chem. A 2010, 114, 68–72 68 10.1021/jp908195k  2010 American Chemical Society Published on Web 11/20/2009