A method of extracting speed-dependent vector correlations from 2 + 1 REMPI ion images.

We present analytical expressions for extracting Dixon's bipolar moments in the semi-classical limit from experimental anisotropy parameters of sliced or reconstructed non-sliced images. The current method focuses on images generated by 2 + 1 REMPI (Resonance Enhanced Multi-photon Ionization) and is a necessary extension of our previously published 1 + 1 REMPI equations. Two approaches for applying the new equations, direct inversion and forward convolution, are presented. As demonstration of the new method, bipolar moments were extracted from images of carbonyl sulfide (OCS) photodissociation at 230 nm and NO2 photodissociation at 355 nm, and the results are consistent with previous publications.

Unravelling the mechanisms of vibrational relaxation in solution.

We present a systematic study of the mode-specific vibrational relaxation of NO2 in six weakly-interacting solvents (perfluorohexane, perfluoromethylcyclohexane, perfluorodecalin, carbon tetrachloride, chloroform, and d-chloroform), chosen to elucidate the dominant energy transfer mechanisms in the solution phase. Broadband transient vibrational absorption spectroscopy has allowed us to extract quantum state-resolved relaxation dynamics of the two distinct NO2 fragments produced from the 340 nm photolysis of N2O4 → NO2(X) + NO2(A) and their separate paths to thermal equilibrium. Distinct relaxation pathways are observed for the NO2 bending and stretching modes, even at energies as high as 7000 cm-1 above the potential minimum. Vibrational energy transfer is governed by different interaction mechanisms in the various solvent environments, and proceeds with timescales ranging from 20-1100 ps. NO2 relaxation rates in the perfluorocarbon solvents are identical despite differences in acceptor mode state densities, infrared absorption cross sections, and local solvent structure. Vibrational energy is shown to be transferred to non-vibrational solvent degrees of freedom (V-T) through impulsive collisions with the perfluorocarbon molecules. Conversely, NO2 relaxation in chlorinated solvents is reliant on vibrational resonances (V-V) while V-T energy transfer is inefficient and thermal excitation of the surrounding solvent molecules inhibits faster vibrational relaxation through direct complexation. Intramolecular vibrational redistribution allows the symmetric stretch of NO2 to act as a gateway for antisymmetric stretch energy to exit the molecule. This study establishes an unprecedented level of detail for the cooling dynamics of a solvated small molecule, and provides a benchmark system for future theoretical studies of vibrational relaxation processes in solution.

Translational, rotational and vibrational relaxation dynamics of a solute molecule in a non-interacting solvent.

Spectroscopically observing the translational and rotational motion of solute molecules in liquid solutions is typically impeded by their interactions with the solvent, which conceal spectral detail through linewidth broadening. Here we show that unique insights into solute dynamics can be made with perfluorinated solvents, which interact weakly with solutes and provide a simplified liquid environment that helps to bridge the gap in our understanding of gas- and liquid-phase dynamics. Specifically, we show that in such solvents, the translational and rotational cooling of an energetic CN radical can be observed directly using ultrafast transient absorption spectroscopy. We observe that translational-energy dissipation within these liquids can be modelled through a series of classic collisions, whereas classically simulated rotational-energy dissipation is shown to be distinctly faster than experimentally measured. We also observe the onset of rotational hindering from nearby solvent molecules, which arises as the average rotational energy of the solute falls below the effective barrier to rotation induced by the solvent.

Reaction Dynamics of CN Radicals in Acetonitrile Solutions.

The bimolecular reactions that follow 267 nm ultraviolet photolysis of ICN in acetonitrile solution have been studied using transient absorption spectroscopy on the picosecond time scale. Time-resolved electronic absorption spectroscopy (TEAS) in the ultraviolet and visible spectral regions observes rapid production and loss (with a decay time constant of 0.6 ± 0.1 ps) of the photolytically generated free CN radicals. Some of these radicals convert to a solvated form which decays with a lifetime of 8.5 ± 2.1 ps. Time-resolved vibrational absorption spectroscopy (TVAS) reveals that the free and solvated CN-radicals undergo geminate recombination with I atoms to make ICN and INC, H atom abstraction reactions, and addition reactions to solvent molecules to make C3H3N2 radical species. These radical products have a characteristic absorption band at 2036 cm(-1) that shifts to 2010 cm(-1) when ICN is photolyzed in CD3CN. The HCN yield is low, suggesting the addition pathway competes effectively with H atom abstraction from CH3CN, but the delayed growth of the C3H3N2 radical band is best described by reaction of solvated CN radicals through an unobserved intermediate species. Addition of methanol or tetrahydrofuran as a cosolute promotes H atom abstraction reactions that produce vibrationally hot HCN. The combination of TEAS and TVAS measurements shows that the rate-limiting process for production of ground-state HCN is vibrational cooling, the rate of which is accelerated by the presence of methanol or tetrahydrofuran.

Recombination, Solvation and Reaction of CN Radicals Following Ultraviolet Photolysis of ICN in Organic Solvents.

The fates of CN radicals produced by ultraviolet (UV) photolysis of ICN in various organic solvents have been examined by transient electronic and vibrational absorption spectroscopy (TEAS and TVAS). Near-UV and visible bands in the TEAS measurement enable direct observation of the CN radicals and their complexes with the solvent molecules. Complementary TVAS measurements probe the products of CN-radical reactions. Geminate recombination to form ICN and INC is a minor pathway on the 150 fs -1300 ps time scales of our experiments in the chosen organic solvents; nonetheless, large infrared transition dipole moments permit direct observation of INC that is vibrationally excited in the C≡N stretching mode. The time constants for INC vibrational cooling range from 30 ps in tetrahydrofuran (THF) to 1400 ps in more weakly interacting solvents such as chloroform. The major channel for CN removal in the organic solvents is reaction with solvent molecules, as revealed by depletion of solvent absorption bands and growth of product bands in the TVA spectra. HCN is a reaction product of hydrogen atom abstraction in most of the photoexcited solutions, and forms with vibrational excitation in both the C-H and C≡N stretching modes. The vibrational cooling rate of the C≡N stretch in HCN depends on the solvent, and follows the same trend as the cooling rate of the C≡N stretch in INC. However, in acetonitrile solution an additional reaction pathway produces C3H3N2(•) radicals, which release HCN on a much longer time scale.

Ultraviolet Absorption Induces Hydrogen-Atom Transfer in G⋠C Watson-Crick DNA Base Pairs in Solution.

Ultrafast deactivation pathways bestow photostability on nucleobases and hence preserve the structural integrity of DNA following absorption of ultraviolet (UV) radiation. One controversial recovery mechanism proposed to account for this photostability involves electron-driven proton transfer (EDPT) in Watson-Crick base pairs. The first direct observation is reported of the EDPT process after UV excitation of individual guanine-cytosine (G⋅C) Watson-Crick base pairs by ultrafast time-resolved UV/visible and mid-infrared spectroscopy. The formation of an intermediate biradical species (G[-H]⋅C[+H]) with a lifetime of 2.9 ps was tracked. The majority of these biradicals return to the original G⋅C Watson-Crick pairs, but up to 10% of the initially excited molecules instead form a stable photoproduct G*⋅C* that has undergone double hydrogen-atom transfer. The observation of these sequential EDPT mechanisms across intermolecular hydrogen bonds confirms an important and long debated pathway for the deactivation of photoexcited base pairs, with possible implications for the UV photochemistry of DNA.

Tracking a Paternò-Büchi reaction in real time using transient electronic and vibrational spectroscopies.

A detailed mechanistic investigation of the early stages of the Paternò-Büchi reaction following 267 nm excitation of benzaldehyde in cyclohexene has been completed using ultrafast, broadband transient UV-visible and IR absorption spectroscopies. Absorption due to electronically excited triplet state benzaldehyde decays on a 80 ps time scale via reaction with cyclohexene. The growth and subsequent decay of the biradical intermediate produced following C-O bond formation is followed by transient vibrational spectroscopy. The biradical decays by ring closure to an oxetane or by dissociating, reforming the ground state reactants. Detailed kinetic analysis allowed derivation of quantum yields and rate constants for these competing biradical decay processes, ϕ(oxetane) = 0.53, ϕ(diss) = 0.47, koxetane = 0.27 ± 0.09 ns(-1) and k(diss) = 0.24 ± 0.09 ns(-1). This study provides a striking illustration of the ways in which contemporary ultrafast transient absorption spectroscopy methods can be used to dissect the mechanism and kinetics of a classic photoreaction.

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.

A combination of ultrafast transient electronic absorption spectroscopy (TEAS) and transient vibrational absorption spectroscopy (TVAS) is used to investigate whether photoinduced N­H bond fission, mediated by a dissociative 1πσ(*) state, is active in aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visible and IR spectral signatures of the adeninyl radical (Ade[-H]), formed as a result of N­H bond fission, TEAS and TVAS are performed on Ade in D2O under basic conditions (pD = 12.5), which forms Ade[-H](-) anions via deprotonation at the N7 or N9 sites of Ade's 7H and 9H tautomers. At 220 nm we observe one-photon detachment of an electron from Ade[-H](-), which generates solvated electrons (eaq(-)) together with Ade[-H] radicals, with clear signatures in both TEAS and TVAS. Additional wavelength dependent TEAS measurements between 240­260 nm identify a threshold of 4.9 ± 0.1 eV (∼250 nm) for this photodetachment process in D2O. Analogous TEAS experiments on aqueous Ade at pD = 7.4 generate a similar photoproduct signal together with eaq(-) after excitation at 266 and 220 nm. These eaq(-) are born from ionization of Ade, together with Ade(+) cations, which are indistinguishable from Ade[-H] radicals in TEAS. Ade(+) and Ade[-H] are found to have different signatures in TVAS and we verify that the pD = 7.4 photoproduct signal observed in TEAS following 220 nm excitation is solely due to Ade(+) cations. Based on these observations, we conclude that: (i) N­H bond fission in aqueous Ade is inactive at wavelengths ≥220 nm; and (ii) if such a channel exists in aqueous solution, its threshold is strongly blue-shifted relative to the onset of the same process in gas phase 9H-Ade (≤233 nm). In addition, we extract excited state lifetimes and vibrational cooling dynamics for 9H-Ade and Ade[-H](-). In both cases, excited state lifetimes of <500 fs are identified, while vibrational cooling occurs within a time frame of 4­5 ps. In contrast, 7H-Ade is confirmed to have a longer excited state lifetime of ∼5­10 ps through both TEAS and TVAS.

Transient UV pump-IR probe investigation of heterocyclic ring-opening dynamics in the solution phase: the role played by nσ* states in the photoinduced reactions of thiophenone and furanone.

The heterocyclic ring-opening dynamics of thiophenone and furanone dissolved in CH3CN have been probed by ultrafast transient infrared spectroscopy. Following irradiation at 267 nm (thiophenone) or 225 nm (furanone), prompt (τ < 1 ps) ring-opening is confirmed by the appearance of a characteristic antisymmetric ketene stretching feature around 2150 cm(-1). The ring-opened product molecules are formed highly vibrationally excited, and cool subsequently on a ∼6.7 ps timescale. By monitoring the recovery of the parent (S0) bleach, it is found that ∼60% of the initially photoexcited thiophenone molecules reform the parent molecule, in stark contrast with the case in furanone where there is less than 10% parent bleach recovery. Complementary ab initio calculations of potential energy cuts along the S-C([double bond, length as m-dash]O) and O-C([double bond, length as m-dash]O) ring-opening coordinate reveals insights into the reaction mechanism, and the important role played by dissociative (n/π)σ* states in the UV-induced photochemistry of such heterocyclic systems.

KOALA: a program for the processing and decomposition of transient spectra.

Extracting meaningful kinetic traces from time-resolved absorption spectra is a non-trivial task, particularly for solution phase spectra where solvent interactions can substantially broaden and shift the transition frequencies. Typically, each spectrum is composed of signal from a number of molecular species (e.g., excited states, intermediate complexes, product species) with overlapping spectral features. Additionally, the profiles of these spectral features may evolve in time (i.e., signal nonlinearity), further complicating the decomposition process. Here, we present a new program for decomposing mixed transient spectra into their individual component spectra and extracting the corresponding kinetic traces: KOALA (Kinetics Observed After Light Absorption). The software combines spectral target analysis with brute-force linear least squares fitting, which is computationally efficient because of the small nonlinear parameter space of most spectral features. Within, we demonstrate the application of KOALA to two sets of experimental transient absorption spectra with multiple mixed spectral components. Although designed for decomposing solution-phase transient absorption data, KOALA may in principle be applied to any time-evolving spectra with multiple components.

Experimental and theoretical investigation of correlated fine structure branching ratios arising from state-selected predissociation of BrO (A2Π3/2).

We present results for the v'-dependent predissociation dynamics of the BrO (A(2)Π3/2) state using velocity map ion imaging. Correlated fine structure branching ratios, Br((2)P(J)) + O((3)P(J)), have been measured for v' = 5-16 states. The experimental branching ratios are non-statistical and strongly dependent on the initial vibronic state. The current measurements represent an extensive dataset containing rich information about the predissociation dynamics of this system and should provide a stringent test for modern theory. New high level ab initio excited state potentials are presented and have been optimized using experimental v'-dependent predissociation lifetimes and calculated coupling constants. Comparisons between the experimental branching ratios and the predictions based on diabatic and adiabatic limiting models are presented. We find that the adiabatic model is most consistent with the observed trends in the correlated branching ratios, in contrast to previous studies on the related ClO system.

Comparing molecular photofragmentation dynamics in the gas and liquid phases.

This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by ~4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement.

Stereodynamics of multistate roaming.

We present a molecular level description of NO3→ NO + O2 photodissociation for both of the experimentally observed reaction pathways using the results of ion imaging experiments and recent theoretical studies. Vector correlation and Λ doublet propensity measurements have been performed on state-selected NO fragments in order to further characterize the stereodynamics of this reaction. Previous measurements (Grubb et al., Science, 2012, 1075-1078) of relative Λ doublet propensities along with ab initio calculations revealed that both pathways arise from roaming-type mechanisms, but each pathway arises from roaming on a different electronic potential. This model, however, assumes that NO3 dissociation takes place in the molecular plane. In the present paper, we have confirmed this assumption through speed-dependent vector correlation measurements. Strong perpendicular correlations between the velocity vector v and the angular momentum vector j are observed in the NO fragment originating from both pathways, in agreement with a constrained planar dissociation. These results are discussed in light of the absence of vector correlations in other roaming systems, which have previously been characterized by an unconstrained intra-molecular abstraction. We show that geometrical constraints should in fact be quite prevalent in roaming dynamics, and are analogous to the geometrical constraints of the corresponding bimolecular abstraction reaction.

No straight path: roaming in both ground- and excited-state photolytic channels of NO3 → NO + O2.

Roaming mechanisms have recently been observed in several chemical reactions alongside trajectories that pass through a traditional transition state. Here, we demonstrate that the visible light-induced reaction NO(3) → NO + O(2) proceeds exclusively by roaming. High-level ab initio calculations predict specific NO Λ doublet propensities (orientations of the unpaired electron with respect to the molecular rotation plane) for this mechanism, which we discern experimentally by ion imaging. The data provide direct evidence for roaming pathways in two different electronic states, corresponding to both previously documented photolysis channels that produce NO + O(2). More broadly, the results raise intriguing questions about the overall prevalence of this unusual reaction mechanism.

A method for the determination of speed-dependent semi-classical vector correlations from sliced image anisotropies.

We present analytical expressions relating the bipolar moment ß(Q)(K)(k(1)k(2)) parameters of Dixon to the measured anisotropy parameters of different pump/probe geometry sliced ion images. In the semi-classical limit, when there is no significant coherent contribution from multiple excited states to fragment angular momentum polarization, the anisotropy of the images alone is sufficient to extract the ß(Q)(K)(k(1)k(2)) parameters with no need to reference relative image intensities. The analysis of sliced images is advantageous since the anisotropy can be directly obtained from the image at any radius without the need for 3D-deconvolution, which is not applicable for most pump/probe geometries. This method is therefore ideally suited for systems which result in a broad distribution of fragment velocities. The bipolar moment parameters are obtained for NO(2) dissociation at 355 nm using these equations, and are compared to the bipolar moment parameters obtained from a proven iterative fitting technique for crushed ion images. Additionally, the utility of these equations in extracting speed-dependent bipolar moments is demonstrated on the recently investigated NO(3) system.

Ion imaging study of NO3 radical photodissociation dynamics: characterization of multiple reaction pathways.

The photodissociation of NO(3) has been studied using velocity map ion imaging. Measurements of the NO(2) + O channel reveal statistical branching ratios of the O((3)P(J)) fine-structure states, isotropic angular distributions, and low product translational energy consistent with barrierless dissociation along the ground state potential surface. There is clear evidence for two distinct pathways to the formation of NO + O(2) products. The dominant pathway (>70% yield) is characterized by vibrationally excited O(2)((3)Σ(g)(-), v = 5-10) and rotationally cold NO((2)Π), while the second pathway is characterized by O(2)((3)Σ(g)(-), v = 0-4) and rotationally hotter NO((2)Π) fragments. We speculate the first pathway has many similarities to the "roaming" dynamics recently implicated in several systems. The rotational angular momentum of the molecular fragments is positively correlated for this channel, suggesting geometric constraints in the dissociation. The second pathway results in almost exclusive formation of NO((2)Π, v = 0). Although product state correlations support dissociation via an as yet unidentified three-center transition state, theoretical confirmation is needed.

Correlated fine structure branching ratios arising from state-selected predissociation of ClO (A 2Pi 3/2).

We have extended our investigation of the v'-dependent predissociation dynamics of the ClO A(2)Pi(3/2) state using velocity-map ion imaging. Correlated fine-structure branching ratios are reported for v' = 0-5. The measured branching ratios are non-statistical and are qualitatively inconsistent with adiabatic dissociation dynamics. The coupling constants between the A(2)Pi(3/2) state and several dissociative excited state potentials have been optimized, as have the locations of the crossing points, based on comparison to previously reported v'-dependent predissociation rates. Using these optimized potentials we have modeled the branching ratios in the diabatic limit but the lack of agreement with experiments suggests the importance of exit channel coupling. Coupled channel calculations including 9 coupled potentials provide modest improvement with experiment but differences remain.