Direct Observation of Hydrogen Tunneling Dynamics in Photoexcited

Jan 12, 2012 - proton tunneling dynamics from tyrosine residues (along the O−H bond of the ... phenol moiety4 (structure shown inset in Figure 1), w...
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Direct Observation of Hydrogen Tunneling Dynamics in Photoexcited Phenol Gareth M. Roberts,† Adam S. Chatterley,† Jamie D. Young, and Vasilios G. Stavros* Department of Chemistry, University of Warwick, Library Road, Coventry CV4 7AL, United Kingdom S Supporting Information *

ABSTRACT: The excited-state dynamics of phenol following ultraviolet (UV) irradiation have received considerable interest in recent years, most notably because they can provide a model for understanding the UV-induced dynamics of the aromatic amino acid tyrosine. Despite this, there has been some debate as to whether hydrogen tunneling dynamics play a significant role in phenol’s excited-state O−H bond fission when UV excitation occurs below the 1ππ*/1πσ* conical intersection (CI). In this Letter, we present direct evidence that 1πσ*mediated O−H bond fission below the 1ππ*/1πσ* CI proceeds exclusively through hydrogen tunneling dynamics. The observation of hydrogen tunneling may have some parallels with proton tunneling dynamics from tyrosine residues (along the O−H bond of the phenol moiety) in a wide range of natural enzymes, potentially adding further justification for utilizing phenols as model systems for investigating tyrosine-based dynamics. SECTION: Dynamics, Clusters, Excited States scission through nonadiabatic crossing at the S2/S0 CI (RO−H ≈ 2.0 Å, Figure 1). This process predominantly yields H atoms in conjunction with ground-state phenoxyl radicals (C6H5O (X̃ )), correlated with the diabatic dissociation asymptote of S2.7,12 Very recently, however, we note that some doubt has been shed on the direct participation of the S1/S2 CI in the dynamics at λ < 248 nm.13 Below the S1/S2 CI (λ > 248 nm), population transfer between the S1 and S2 states is hindered by an energy barrier (see Figure 1). However, after excitation to the zero point energy (ZPE) of S1 (λ = 275.113 nm), frequency domain measurements reveal that O−H bond fission mediated by S2 still persists.7 This remains the case as the excitation energy is incrementally increased to just below the S1/S2 CI. Two schools of thought have been developed to explain these observations: (i) Following population of S1, S1 → S0 internal conversion (IC) is driven by O−H stretch acceptor modes in S0 (hereon, termed S0*), which subsequently promotes predissociation through efficient S0* → S2 coupling at extended RO−H, finally resulting in H elimination on S2.7,22 (ii) More recently, new dynamical calculations,13,15,19 in addition to experiments on substituted phenols (and their H bonded complexes with NH3),23 implicate that O−H fission proceeds through H tunneling under the S1/S2 CI, facilitated by the ν16a motion13 (a ring torsional mode10). Furthermore, selective deuteration of the O−H bond (C6H5OD) leads to an increase in the fluorescence lifetime of the S1 ZPE level from ∼2.4 to ∼13.3 ns,24 and full deuteration (C6D5OD) produces no D atom signature for O−D scission along S2 in Rydberg tagging

T

unneling is a purely quantum mechanical phenomenon (with no inherent classical analogue), where the “wavelike” nature of a particle facilitates passage through an energetic barrier exceeding the particle’s kinetic energy. In this work, we draw our attention to quantum tunneling of hydrogen (H) atoms. H tunneling has been recognized to play a fundamental role in a myriad of chemical1 and biological processes,2 particularly in enzyme activity, such as alcohol dehydrogenases3 and photosystem II.4 In the latter species, the active mechanism is proposed to involve H tunneling from the O−H bond of a phenol moiety4 (structure shown inset in Figure 1), which itself acts as an ultraviolet (UV) chromophore in the aromatic amino acid tyrosine. Here we interrogate isolated phenol, which has been considered to be a model for tyrosine,5 to establish the role of H tunneling in its UV-induced excited-state dynamics. Throughout the past decade, the (electronic) excited-state dynamics of phenol have received considerable attention from both experimental6−13 and theoretical14−19 standpoints. Figure 1 shows calculated 1-D (diabatic) potential energy cuts13 along the O−H coordinate (RO−H) for the first two singlet electronic excited states, S1 (1ππ*) and S2 (1πσ*), together with the S0 (1ππ) ground state. At RO−H ≈ 1.2 Å, the S1 and S2 states cross, resulting in the formation of an S1/S2 conical intersection (CI): a point in nuclear coordinate space at the confluence of the two multidimensional potential energy surfaces.20 Following excitation at wavelengths (λ) < 248 nm (∼5 eV), the role of the S1/S2 CI in the excited state dynamics has been comprehensively investigated. This has been achieved using a variety of techniques, including multimass ion imaging,6,9 high-resolution H-atom Rydberg tagging methods,7,21 and ultrafast timeresolved velocity map ion imaging.12 In combination, the results of these studies implicate that coupling onto the dissociative S2 state, via the S1/S2 CI, causes rapid O−H bond © 2012 American Chemical Society

Received: December 12, 2011 Accepted: January 12, 2012 Published: January 12, 2012 348

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Figure 1. Calculated potential energy cuts along the O−H coordinate in phenol (molecular structure inset), for the first two electronic excited states, S1 (1ππ*) and S2 (1πσ*), together with the S0 (1ππ) ground state. These cuts are adapted from those calculated at the CAS(10,9)/aug(O)-AVTZ level of theory in ref 13. Excitation (hνpump) to S1 is shown for four different excitation wavelengths: 275 (green), 268 (blue), 258 (red), and 253 nm (orange). Shaded regions in the S1 potential represent the energy bandwidth of the femtosecond pump pulses. Excitation of S1 at all wavelengths results in H tunneling under the S1/S2 CI and S2-mediated O−H bond fission, proceeding exclusively from the zero point energy (ZPE) region of S1 (green arrows). H atoms (produced in coincidence with C6H5O (X̃ ) radicals) are subsequently probed using femtosecond probe pulses (hνprobe). The shaded gray area, labeled V(u) − E, represents the potential barrier used in BKW tunneling calculations (see the text for details).

Figure 2. (a) TKER spectra obtained following excitation at 275 (green), 268 (blue), 258 (red), and 253 nm (orange). All spectra were recorded at Δt = 1.2 ns. Horizontal dotted lines correlate to zero signal baselines. Vertical arrows represent predicted TKERmax values at each pump wavelength, whereas the dashed gray line shows a predicted ⟨TKER⟩ for dissociation from the S1 ZPE. (See the text for details.) Inset: raw H+ velocity map image (left) and reconstructed slice through the center of the H+ distribution (right) at 275 nm and Δt = 1.2 ns. The white arrow indicates the polarization of hνpump. (b) H+ signal transients for the high TKER (Gaussian) features at each pump wavelength. Error bars correspond to two standard deviations (two sigma). Solid lines through the data present a predicted kinetic model for H tunneling.

experiments conducted below the S1/S2 CI.7 These observations are consistent with either of the aforementioned mechanisms because the probability for D tunneling is ∼103 less than H atoms13 whereas the O−D stretch is hypothesized to be a less efficient acceptor for S1 → S0* IC.7,22 In this Letter, we provide direct experimental evidence that following excitation to S1 below the S1/S2 CI, O−H bond fission dynamics in phenol are governed purely by H tunneling and not through population transfer to O−H stretch acceptor modes in S0*. The experiments study time scales for excitedstate H elimination dynamics in isolated phenol by utilizing a combination of femtosecond (fs) pump−probe laser spectroscopy25 and gas-phase velocity map ion imaging (VMI) methods.26 This time-resolved VMI arrangement, which has previously been reported in detail,27 provides both temporal and energetic information regarding the O−H dissociation process in question. Following excitation to S1 using broadband (∼500 cm−1) fs pump pulses (hνpump), any H photoproducts are subsequently ionized to form H+, via (2 + 1) resonance enhanced multiphoton ionization (REMPI) with a time delayed fs probe pulse (hνprobe) tuned to 243.1 nm. H+ is then detected using VMI. The inset in Figure 2a is a representative H+ velocity map image recorded after broadband excitation around the S1 ZPE, using a pump wavelength centered at 275 nm and a pump−probe delay (Δt) of 1.2 ns; that is, the pump precedes the probe by 1.2 ns. The signal at the center of the image corresponds to H+ with low amounts of kinetic energy, whereas signal at larger radii is produced from higher kinetic energy H+. The image in Figure 2a is dominated by a single ring at larger radius. This feature possesses a near-isotropic angular distribution (β2 ≈ 0), tentatively suggesting that this H+ signal

is generated on a time scale slower than the rotational period of parent phenol species.28 H+ velocity map images are used to derive desired total kinetic energy release (TKER) spectra with the aid of an image reconstruction algorithm,29 an appropriate Jacobian, and a calibration factor.30 In Figure 2a, TKER spectra recorded at Δt = 1.2 ns are presented for four different pump wavelengths (275, 268, 258, and 253 nm), all of which are beneath the S1/S2 CI. The TKER spectra in fact reveal that there are two features present at all wavelengths: a broad underlying “Boltzmann-like” distribution and a Gaussian signal centered at higher TKER (∼5500−6500 cm−1). Previously, it has been suggested that the Boltzmann-like feature is multicomponent in nature, with contributions from both statistical7 and multiphoton processes,31 whereas the Gaussian component is assigned to O−H fission via S2 forming C6H5O (X̃ ) + H.7 The latter assignment is confirmed by determining the maximum TKER (TKERmax) for the O−H dissociation process at each pump wavelength using the relationship TKERmax = hνpump − D0, where D0 is the adiabatic O−H dissociation energy (previously determined to be 30015 cm−1).7 The predicted TKERmax at each wavelength is indicated by the vertical arrows in Figure 2a. The increase in TKERmax with increasing hνpump is approximately mirrored by the broadening of the Gaussian features toward higher TKER.32 However, the mean TKERs, ⟨TKER⟩, of these high TKER 349

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features are approximately unchanged between the four excitation wavelengths, in line with results from frequency domain studies.7 Within the context of the 1-D cuts through the full multidimensional potential energy surfaces in Figure 1 (i.e., the O−H stretch coordinate only), this provides some initial indication that: (i) after populating S1, O−H dissociation dynamics below the S1/S2 CI are independent of hνpump and proceed from approximately the same vibronic region of S1, nominally around the S1 ZPE (this is based on the observation that TKERmax for 275 nm predicts the observed ⟨TKER⟩ at other wavelengths reasonably well (dashed gray line, Figure 2a)); and (ii) with increasing hνpump, populated vibrational modes in S1 which are orthogonal to the O−H dissociation coordinate represented in Figure 1 (spectator modes) are mapped into vibrational excitation of the C6H5O (X̃ ) cofragment. Figure 2b shows normalized H+ signal transients of the high TKER features out to Δt = 1.2 ns at the four pump wavelengths: 275 (green), 268 (blue), 258 (red), and 253 nm (orange). These are obtained by generating a series of TKER spectra at each Δt and integrating the high TKER feature. In all cases, there is an initial rapid step at Δt = 0, which appears within our instrument response window (∼160 fs) and correlates to the appearance of the underlying Boltzmann-like signal. This is then followed by a slower signal rise due to growth of the high TKER feature, corresponding to S2mediated O−H fission. At all four excitation wavelengths the transients show no indication that the dynamics have terminated by the temporal limit of our experiments, which is currently 1.2 ns. As such, it is tenuous to extract definitive appearance time constants for the high TKER features via kinetic fits. Instead, we conservatively quote a lower limit for the H elimination dynamics of ≥1.2 ns in all cases. At this stage, we critically emphasize that as more excess internal energy is imparted to S1 (increasing hνpump), there is no noticeable increase in the rate of S2 driven H formation but rather an apparent invariance in the H elimination time scales. Such an observation is inconsistent with a model whereby the high TKER features originate through successive S1 → S0* → S2 couplings and the production of C6H5O (X̃ ) + H. Rather, within Fermi’s golden rule, one would anticipate that as the density of vibrational states (orthogonal to the O−H coordinate) in S1 is enhanced with increasing hνpump, the rate of S1 → S0* IC and concomitantly the (predissociative) O−H fission rate should notably increase.33 To confirm this, we have performed time-resolved ion yield experiments on the parent cation (phenol+) using (1 + 1′) REMPI, which provide a measure of S1 population decay (from the Franck−Condon excitation window) through all available relaxation processes (e.g., fluorescence, IC, intersystem crossing, tunneling, etc.). Figure 3 presents two representative parent ion transients for excitation at 275 and 258 nm and probing (ionization) at 243.1 nm. Fits to these transients with a single exponential decay (solid lines) provide time constants of 1.9 and 0.9 ns at 275 and 258 nm, respectively. This is in line with the expectation that the overall lifetime of S1 at the excited Franck−Condon window (containing modes orthogonal to the O−H coordinate) will decrease with increasing hν pump through an enhanced propensity for S1 → S0* IC. Yet, no similar increase in O−H fission rate is observed in the H+ transients, which are exclusively sensitive to decay into H photoproducts along S2, implying that an S1 → S0* → S2 → C6H5O (X̃ ) + H dissociation mechanism is inoperative.

Figure 3. Representative parent ion (phenol+) signal transients. Transients were recorded using (1 + 1′) REMPI following excitation at 275 (green squares) and 258 nm (red diamonds) and probing (ionization) at 243.1 nm. Solid lines correspond to fits with a single exponential decay.

Given our conclusion that O−H stretch acceptor modes in S0* do not promote O−H scission through S1 → S0* → S2 coupling, we instead attempt to reconcile the observed H+ transients in Figure 2b with the alternative H tunneling mechanism. To begin, we focus on H tunneling to S2 from the S1 ZPE (275 nm). Within a 1-D approximation, the tunneling lifetime, τ, at the ZPE of the O−H stretch mode in S1 can be estimated using a semiclassical Brillouin−Kramers−Wentzel (BKW) method:34

⎡ u2 ⎛ τ = ⎢νOH exp⎜ − 2 u1 ⎝ ⎣

∫

−1

⎞⎤ ( V ( u ) E ) d u − ⎟⎥ ⎠⎦ ℏ2

2m

where u is the O−H bond coordinate, νOH is the O−H stretch frequency (3582 cm−1), m is the mass of H, V(u) is the potential barrier (obtained from the S1 and S2 1-D potential energy cuts shown in Figure 1), and E is the kinetic energy of the H atom (in this case the ZPE of the O−H stretch in S1, 1 /2νOH). The potential energy cuts in Figure 1 generate a value of τ = 2.5 ns for H tunneling onto S2 from the ZPE of νOH in S1. This is in excellent agreement with the value for τ empirically derived by Pino et al. (∼2.4 ns).23 A kinetic model (see the Supporting Information for details) of the 275 nm H+ transient, where τ has been fixed to 2.5 ns, is shown in Figure 2b by the solid green line through the experimental data points. The model represents the data exceptionally well and adds weight to an argument that following excitation around the S1 ZPE, H elimination via S2 is promoted by H tunneling from S1. This is in line with predictions by Dixon et al.13 who, at the S1 ZPE, model H tunneling dynamics using 2-D wavepacket calculations in the ν16a and O−H stretch coordinates. These authors reveal that the ν16a (a2) motion is required for vibronically coupling S1 (1B2) and S2 (1B1) within phenol’s nonrigid G4 (isomorphous with C2v) symmetry group (B2 ⊗ a2 ⊗ B1 = A1). We, however, emphasize that our broadband pump−probe measurements presented here cannot provide us with information regarding mode-specific behavior. Similar kinetic models, with H tunneling time constants of 2.5 ns, can also be used to represent the H+ transients resulting from a vibrationally excited S1 state at 268, 258, and 253 nm (solid lines, Figure 2b). In each case, the models fit the experimental data very well. Within the H tunneling mechanism, this suggests that even when excess internal energy is imparted into S1, by populating modes orthogonal to the O− 350

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and helpful discussions. A.S.C. and G.M.R thank the Leverhulme trust for doctoral funding and a postdoctoral research fellowship, respectively. V.G.S thanks the EPSRC for equipment grants (EP/E011187 and EP/H003401), the Royal Society for a University Research Fellowship and the University of Warwick for an RDF Award.

H dissociation coordinate, O−H fission dynamics still primarily proceed from the ZPE of the O−H stretch mode in S1. This confers with the following expectations. First, excitation to orthogonal modes in S1 has a minimal impact on the observed tunneling rate, although we concede that we may not observe minor changes to a 2.5 ns tunneling lifetime because dynamics are incomplete by the 1.2 ns limit of our measurements. Second, because the v = 1 level of νOH is not energetically accessible at the wavelengths used here (S1(vOH = 1) ≈ 250 nm),35 either through direct excitation or internal vibrational energy redistribution (IVR) from orthogonal modes, a major change in rate cannot be observed. Rather, H tunneling along the O−H coordinate always proceeds from the ZPE of νOH in S1 over our excitation range, explaining the invariance in τ, whereas we note that the reduction in the overall S1 lifetime with increasing hνpump (Figure 3) likely results from an increase in S1 → S0* IC rate driven from the orthogonal modes excited (and following IVR) within the Franck−Condon window. Moreover, exclusive tunneling from the S1 ZPE region consolidates with our previous hypothesis for why the ⟨TKER⟩ of the Gaussian features in all four TKER spectra (Figure 2a) appear close to the predicted TKERmax for O−H dissociation from the S1 ZPE. We therefore conclude that the H+ signal transients reported here, together with the predicted tunneling rate at the ZPE of νOH in S1 from the BKW method, provide compelling evidence that below the S1/S2 CI O−H bond cleavage via S2 in phenol proceeds purely by H tunneling from the S1 ZPE, reinforcing previous postulates by Sobolewski et al.,15 Dixon et al.,13 and Pino et al.23 Phenol has previously been considered to be an appropriate model for studying the excited-state behavior of the aromatic amino acid tyrosine,5 and the observation of H-tunneling dynamics along phenol’s O−H bond may share some parallels with the projected role of proton tunneling from tyrosine residues in enzymes, such as the oxygen evolving complex within photosystem II.4 Currently, our group has plans to investigate the effects of increasing molecular complexity on these excited-state H tunneling dynamics, using substituted phenols, phenol-related species, and isolated tyrosine itself. This is in tandem with time-resolved vibrationally mediated experiments,11 which, through initial mode-specific excitation in S0, will intimately probe the effects of individual vibrational motions on H tunneling time scales.

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(1) Bell, R. P. The Tunnel Effect in Chemistry; Chapman & Hall: New York, 1980. (2) Kohen, A.; Klinman, J. P. Hydrogen Tunneling in Biology. Chem. Biol. 1999, 6, R191−R198. (3) Kohen, A.; Cannio, R.; Bartolucci, S.; Klinman, J. P. Enzyme Dynamics and Hydrogen Tunnelling in a Thermophilic Alcohol Dehydrogenase. Nature 1999, 399, 496−499. (4) Reece, S. Y.; Hodgkiss, J. M.; Stubbe, J.; Nocera, D. G. ProtonCoupled Electron Transfer: The Mechanistic Underpinning for Radical Transport and Catalysis in Biology. Philos. Trans. R. Soc., B 2006, 361, 1351−1364. (5) Iqbal, A.; Stavros, V. G. Active Participation of 1πσ* States in the Photodissociation of Tyrosine and Its Subunits. J. Phys. Chem. Lett. 2010, 1, 2274−2278. (6) Tseng, C. M.; Lee, Y. T.; Ni, C. K. H Atom Elimination from the πσ* State in the Photodissociation of Phenol. J. Chem. Phys. 2004, 121, 2459−2461. (7) Nix, M. G. D.; Devine, A. L.; Cronin, B.; Dixon, R. N.; Ashfold, M. N. R. High Resolution Photofragment Translational Spectroscopy Studies of the Near Ultraviolet Photolysis of Phenol. J. Chem. Phys. 2006, 125, 133318. (8) Ashfold, M. N. R.; Cronin, B.; Devine, A. L.; Dixon, R. N.; Nix, M. G. D. The Role of πσ* Excited States in the Photodissociation of Heteroaromatic Molecules. Science 2006, 312, 1637−1640. (9) Tseng, C. M.; Lee, Y. T.; Lin, M. F.; Ni, C. K.; Liu, S. Y.; Lee, Y. P.; Xu, Z. F.; Lin, M. C. Photodissociation Dynamics of Phenol. J. Phys. Chem. A 2007, 111, 9463−9470. (10) Ashfold, M. N. R.; Devine, A. L.; Dixon, R. N.; King, G. A.; Nix, M. G. D.; Oliver, T. A. A. Exploring Nuclear Motion Through Conical Intersections in the UV Photodissociation of Phenols and Thiophenol. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 12701−12706. (11) Hause, M. L.; Yoon, Y. H.; Case, A. S.; Crim, F. F. Dynamics at Conical Intersections: The Influence of O-H Stretching Vibrations on the Photodissociation of Phenol. J. Chem. Phys. 2008, 128, 104307. (12) Iqbal, A.; Cheung, M. S. Y.; Nix, M. G. D.; Stavros, V. G. Exploring the Time-scales of H-atom Detachment from Photoexcited Phenol-h6 and Phenol-d5: Statistical vs Nonstatistical Decay. J. Phys. Chem. A 2009, 113, 8157−8163. (13) Dixon, R. N.; Oliver, T. A. A.; Ashfold, M. N. R. Tunnelling Under a Conical Intersection: Application to the Product Vibrational State Distributions in the UV Photodissociation of Phenols. J. Chem. Phys. 2011, 134, 194303. (14) Sobolewski, A. L.; Domcke, W. Photoinduced Electron and Proton Transfer in Phenol and its Clusters with Water and Ammonia. J. Phys. Chem. A 2001, 105, 9275−9283. (15) Sobolewski, A. L.; Domcke, W.; Dedonder-Lardeux, C.; Jouvet, C. Excited-State Hydrogen Detachment and Hydrogen Transfer Driven by Repulsive 1πσ* States: A New Paradigm for Nonradiative Decay in Aromatic Biomolecules. Phys. Chem. Chem. Phys. 2002, 4, 1093−1100. (16) Lan, Z. G.; Domcke, W.; Vallet, V.; Sobolewski, A. L.; Mahapatra, S. Time-Dependent Quantum Wave-Packet Description of the 1πσ* Photochemistry of Phenol. J. Chem. Phys. 2005, 122, 224315. (17) Abe, M.; Ohtsuki, Y.; Fujimura, Y.; Lan, Z. G.; Domcke, W. Geometric Phase Effects in the Coherent Control of the Branching Ratio of Photodissociation Products of Phenol. J. Chem. Phys. 2006, 124, 224316. (18) Nix, M. G. D.; Devine, A. L.; Dixon, R. N.; Ashfold, M. N. R. Observation of Geometric Phase Effect Induced Photodissociation Dynamics in Phenol. Chem. Phys. Lett. 2008, 463, 305−308.

ASSOCIATED CONTENT

S Supporting Information *

Experimental details and kinetic model details. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

These authors contributed equally to the work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank Dave Hadden and Craig Williams for experimental assistance, Dr. Mike Nix for helpful discussions, and Dr. Thomas Oliver, Prof. Mike Ashfold and Prof. Richard Dixon for both the use of their calculated potential energy cuts 351

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(19) An, H.; Baeck, K. K. Quantum Wave Packet Propagation Study of the Photochemistry of Phenol: Isotope Effects (Ph-OD) and the Direct Excitation to the 1πσ* State. J. Phys. Chem. A 2011, 115, 13309−13315. (20) Yarkony, D. R. Diabolical Conical Intersections. Rev. Mod. Phys. 1996, 68, 985−1013. (21) King, G. A.; Oliver, T. A. A.; Nix, M. G. D.; Ashfold, M. N. R. High Resolution Photofragment Translational Spectroscopy Studies of the Ultraviolet Photolysis of Phenol-d5. J. Phys. Chem. A 2009, 113, 7984−7993. (22) Sur, A.; Johnson, P. M. Radiationless Transitions in Gas-Phase Phenol and the Effects of Hydrogen-Bonding. J. Chem. Phys. 1986, 84, 1206−1209. (23) Pino, G. A.; Oldani, A. N.; Marceca, E.; Fujii, M.; Ishiuchi, S. I.; Miyazaki, M.; Broquier, M.; Dedonder, C.; Jouvet, C. Excited State Hydrogen Transfer Dynamics in Substituted Phenols and their Complexes with Ammonia: ππ*-πσ* Energy Gap Propensity and Ortho-Substitution Effect. J. Chem. Phys. 2010, 133, 124313. (24) Ratzer, C.; Kupper, J.; Spangenberg, D.; Schmitt, M. The Structure of Phenol in the S1-State Determined by High Resolution UV-Spectroscopy. Chem. Phys. 2002, 283, 153−169. (25) Zewail, A. H. Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond. J. Phys. Chem. A 2000, 104, 5660−5694. (26) Eppink, A. T. J. B.; Parker, D. H. Velocity Map Imaging of Ions and Electrons using Electrostatic Lenses: Application in Photoelectron and Photofragment Ion Imaging of Molecular Oxygen. Rev. Sci. Instrum. 1997, 68, 3477−3484. (27) Wells, K. L.; Perriam, G.; Stavros, V. G. Time-resolved Velocity Map Ion Imaging Study of NH3 Photodissociation. J. Chem. Phys. 2009, 130, 074304. (28) Zare, R. N. Angular Momentum: Understanding Spatial Aspects in Chemistry and Physics; Wiley: New York, 1988. (29) Roberts, G. M.; Nixon, J. L.; Lecointre, J.; Wrede, E.; Verlet, J. R. R. Toward Real-Time Charged-Particle Image Reconstruction using Polar Onion-Peeling. Rev. Sci. Instrum. 2009, 80, 053104. (30) Williams, C. A.; Roberts, G. M.; Yu, H.; Evans, N. L.; Ullrich, S.; Stavros, V. G. Exploring Ultrafast H-Atom Elimination versus Photofragmentation Pathways in Pyrazole Following 200 nm Excitation. J. Phys. Chem. A 2011, DOI: 10.1021/jp2053212. (31) Schick, C. P.; Weber, P. M. Ultrafast Dynamics in Superexcited States of Phenol. J. Phys. Chem. A 2001, 105, 3725−3734. (32) We highlight that the lack of absolute agreement between the predicted TKERmax and the experimentally observed TKERmax of the high TKER Gaussian features arise because of our restricted energy resolution below 7000 cm−1 (ΔE/E ≈ 15−20%). (33) Beddard, G. S.; Fleming, G. R.; Gijzeman, O. L.; Porter, G. Internal Conversion from Vibrationally Excited Levels. Chem. Phys. Lett. 1973, 18, 481−487. (34) Le Roy, R. J.; Liu, W. K. Energies and Widths of Quasibound Levels (Orbiting Resonances) for Spherical Potentials. J. Chem. Phys. 1978, 69, 3622−3631. (35) Time-resolved experiments following excitation at λ < 253 nm and, in particular, around 250 nm (S1(vOH = 1)) were attempted. However, because of the highly reduced absorption cross-section for excitation to S1 below 253 nm, these time-resolved measurements were unfortunately not possible.

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