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The recently constructed vacuum ultraviolet (VUV) free electron laser (FEL) at the Dalian Coherent Light Source (DCLS) is yielding a wealth of new and exquisitely detailed information about the photofragmentation dynamics of many small gas-phase molecules. This Review focuses particular attention on five triatomic molecules—H2O, H2S, CO2, OCS and CS2. Each shows excitation wavelength-dependent dissociation dynamics, yielding photofragments that populate a range of electronic and (in the case of diatomic fragments) vibrational and rotational quantum states, which can be characterized by different translational spectroscopy methods. The photodissociation of an isolated molecule from a well-defined initial quantum state provides a lens through which one can investigate how and why chemical reactions occur, and provides numerous opportunities for fruitful, synergistic collaborations with high-level ab initio quantum chemists. The chosen molecules, their photofragments and the subsequent chemical reaction networks to which they can contribute are all crucial in planetary atmospheres and in interstellar and circumstellar environments. The aims of this Review are 3-fold: to highlight new photochemical insights enabled by the VUV-FEL at the DCLS, notably the recently recognized central atom elimination process that is shown to contribute in all of these triatomic molecules; to highlight some of the potential implications of this rich photochemistry to our understanding of interstellar chemistry and molecular evolution within the universe; and to highlight other and future research directions in areas related to chemical reaction dynamics and astrochemistry that will be enabled by increased access to VUV-FEL sources. [ABSTRACT FROM AUTHOR]

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory‐based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH2OO, CH3CHOO, (CH3)2COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene‐derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH2OO and (CH3)2COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever‐growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing 1ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo‐induced fission of the terminal O–O bond, and speculates about possible alternate decay processes that could occur following non‐adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI. [ABSTRACT FROM AUTHOR]

This study of the photodissociation of dimethyl sulfide at λ = 227.5 nm demonstrates the opportunities (and some of the challenges) of product detection using vacuum ultraviolet photoionization combined with recently developed multi-mass imaging methods. The capability of imaging different charged products simultaneously allows determination of the primary fragmentation dynamics through, for example, product fragment momentum and angular distribution matching and reveals potential complications from dissociative ionization, product alignment-dependent photoionization probabilities, and the effects of space charging. [ABSTRACT FROM AUTHOR]

Non-adiabatic couplings between Born–Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating 'reactive' outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase H2O molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products. This article is part of the theme issue 'Chemistry without the Born–Oppenheimer approximation'. [ABSTRACT FROM AUTHOR]

Velocity map imaging methods, with a new and improved ion optics design, have been used to explore the near ultraviolet photodissociation dynamics of gas phase 2-bromo- and 2-iodothiophene molecules. In both cases, the ground (X) and spin-orbit excited (X*) (where X = Br, I) atom products formed at the longest excitation wavelengths are found to recoil with fast, anisotropic velocity distributions, consistent with prompt C-X bond fission following excitation via a transition whose dipole moment is aligned parallel to the breaking bond. Upon tuning to shorter wavelengths, this fast component fades and is progressively replaced by a slower, isotropic recoil distribution. Complementary electronic structure calculations provide a plausible explanation for this switch in fragmentation behaviour--namely, the opening of a rival C-S bond extension pathway to a region of conical intersection with the ground state potential energy surface. The resulting ground state molecules are formed with more than sufficient internal energy to sample the configuration space associated with several parent isomers and to dissociate to yield X atom products in tandem with both cyclic and ring-opened partner fragments. [ABSTRACT FROM AUTHOR]

The provenance of oxygen on the Earth and other planets in the Solar System is a fundamental issue. It has been widely accepted that the only prebiotic pathway to produce oxygen in the Earth's primitive atmosphere was via vacuum ultraviolet (VUV) photodissociation of CO2 and subsequent two O atom recombination. Here, we provide experimental evidence of three-body dissociation (TBD) of H2O to produce O atoms in both 1D and 3P states upon VUV excitation using a tunable VUV free electron laser. Experimental results show that the TBD is the dominant pathway in the VUV H2O photochemistry at wavelengths between 90 and 107.4 nm. The relative abundance of water in the interstellar space with its exposure to the intense VUV radiation suggests that the TBD of H2O and subsequent O atom recombination should be an important prebiotic O2-production, which may need to be incorporated into interstellar photochemical models. Three-body dissociation of water, producing one oxygen and two hydrogen atoms, has been difficult to investigate due to the lack of intense vacuum ultraviolet sources. Here, using a tunable free-electron laser, the authors obtain quantum yields for this channel showing that it is a possible route to prebiotic oxygen formation in interstellar environments. [ABSTRACT FROM AUTHOR]

Resolved sets of photoproducts arising from the photodissociation of axial and equatorial conformers of 3-pyrroline have been observed using H(Rydberg) atom photofragment translational spectroscopy following excitation in the wavelength range of 250-213 nm. 3-pyrroline (alternatively 2,5-dihydropyrrole) is a five membered partially saturated heterocycle in which the bonding around the N atom is pyramidal (sp3 hybridized) and the N-H bond can lie either axial or equatorial to the ring. Careful analysis of total kinetic energy release data derived from H atom time-of-flight measurements reveals excitation of the 3-pyrrolinyl cofragment consistent with N-H bond fission in both the axial and equatorial conformers. This allows determination of the energy difference between the ground state conformers to be 340±50 cm-1 and the N-H bond strength for axial and equatorial conformers as 31 610±50 and 31 270±50 cm-1, respectively. [ABSTRACT FROM AUTHOR]

This article reports a comprehensive study of the mechanisms of H atom loss in aniline (C6H5NH2) following ultraviolet excitation, using H (Rydberg) atom photofragment translational spectroscopy. N–H bond fission via the low lying 1πσ* electronic state of aniline is experimentally demonstrated. The 1πσ* potential energy surface (PES) of this prototypical aromatic amine is essentially repulsive along the N–H stretch coordinate, but possesses a shallow potential well in the vertical Franck–Condon region, supporting quasibound vibrational levels. Photoexcitation at wavelengths (λphot) in the range 293.859 nm≥λphot≥193.3 nm yields H atom loss via a range of mechanisms. With λphot resonant with the 11ππ*←S0 origin (293.859 nm), H atom loss proceeds via, predominantly, multiphoton excitation processes, resonantly enhanced at the one photon energy by the first 1ππ* excited state (the 11ππ* state). Direct excitation to the first few quasibound vibrational levels of the 1πσ* state (at wavelengths in the range 269.513 nm≥λphot≥260 nm) induces N–H bond fission via H atom tunneling through an exit barrier into the repulsive region of the 1πσ* PES, forming anilino (C6H5NH) radical products in their ground electronic state, and with very limited vibrational excitation; the photo-prepared vibrational mode in the 1πσ* state generally evolves adiabatically into the corresponding mode of the anilino radical upon dissociation. However, as the excitation wavelength is reduced (λphot<260 nm), N–H bond fission yields fragments with substantially greater vibrational excitation, rationalized in terms of direct excitation to 11ππ* levels, followed by coupling to the 1πσ* PES via a 11ππ*/1πσ* conical intersection. Changes in product kinetic energy disposal once λphot approaches ∼230 nm likely indicate that the photodissociation pathways of aniline proceed via direct excitation to the (higher) 21ππ* state. Analysis of the anilino fragment vibrational energy disposal—and thus the concomitant dynamics of 1πσ* state mediated photodissociation—provides a particularly interesting study of competing σ*←π and π*←π absorption processes and develops our appreciation of the photochemistry of aromatic amines. It also allows revealing comparisons with simple amines (such as ammonia and methylamine) as well as the isoelectronic species, phenol. This study yields a value for the N–H bond strength in aniline, D0(H-anilino)=31630±40 cm-1. [ABSTRACT FROM AUTHOR]

The photophysics of gas phase pyrazole (C3N2H4) and 2H-1,2,3-triazole (C2N3H3) molecules following excitation at wavelengths in the range 230 nm≥λphot≥193.3 nm has been investigated using the experimental technique of H (Rydberg) atom photofragment translational spectroscopy. The findings are compared with previous studies of pyrrole (C4N1H5) and imidazole (C3N2H4), providing a guide to H atom loss dynamics in simple N-containing heterocycles. CASPT2 theoretical methods have been employed to validate these findings. Photoexcitation of pyrazole at the longest wavelengths studied is deduced to involve π*←π excitation, but photolysis at λphot≤214 nm is characterized by rapid N–H bond fission on a 1πσ* potential energy surface. The eventual pyrazolyl radical products are formed in a range of vibrational levels associated with both the ground (2A2) and first excited (2B1) electronic states as a result of nonadiabatic coupling at large N–H bond lengths. The excitation energy of the lowest 1πσ* state of pyrazole is found to be significantly higher in energy than that of pyrrole and imidazole. Similar studies of 2H-1,2,3-triazole reveal that the lowest 1πσ* state is yet higher in energy and not accessible following excitation at λphot≥193.3 nm. The N–H bond strength of pyrazole is determined as 37 680±40 cm-1, significantly greater than that of the N–H bonds in pyrrole and imidazole. The correlation between the photochemistry of azoles and the number and position of nitrogen atoms within the ring framework is discussed in terms of molecular symmetry and orbital electron density. A photodissociation channel yielding H atoms with low kinetic energies is also clearly evident in both pyrazole and 2H-1,2,3-triazole. Companion studies of pyrazole-d1 suggest that these slow H atoms arise primarily from the N–H site, following π*←π excitation, and subsequent internal conversion and/or unintended multiphoton absorption processes. [ABSTRACT FROM AUTHOR]

The production of H2 in highly excited vibrational and rotational states (v=0–5, J=0–17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H2(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H2(v=0 and 1) populations in this case were below the detection limit. The H2 products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields H2 molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations. [ABSTRACT FROM AUTHOR]

The laser-induced photodissociation of formaldehyde in the wavelength range 309≤λ<330 nm has been investigated using H (Rydberg) atom photofragment translational spectroscopy. Photolysis wavelengths corresponding to specific rovibronic transitions in the à 1A2←X 1A1 201403, 202401, 202403, 203401, and 201501 bands of H2CO were studied. The total kinetic energy release spectra so derived can be used to determine partial rotational state population distributions of the HCO cofragment. HCO product state distributions have been derived following the population of various different NKa levels in the à 1A2 2243 and 2341 states. Two distinct spectral signatures are identified, suggesting competition between dissociation pathways involving the X 1A1 and the ã 3A2 potential energy surfaces. Most rovibrational states of H2CO(à 1A2) investigated in this work produceH+HCO(X 2A′) photofragments with a broad kinetic energy distribution and significant population in high energy rotational states of HCO. Photodissociation via the à 1A2 2243 11,1 (and 11,0) rovibronic states yields predominantly HCO fragments with low internal energy, a signature that these rovibronic levels are perturbed by the ã 3A2 state. The results also suggest the need for further careful measurements of the H+HCO quantum yield from H2CO photolysis at energies approaching, and above, the barrier to C–H bond fission on the ã 3A2 potential energy surface. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of amorphous solid water (ASW) films and polycrystalline ice (PCI) films at a substrate temperature of 100 K have been investigated by analyzing the time-of-flight (TOF) mass spectra of photofragment hydrogen atoms at 157 and 193 nm. For PCI films, the TOF spectrum recorded at 157 nm could be characterized by a combination of three different (fast, medium, and slow) Maxwell-Boltzmann energy distributions, while that measured at 193 nm can be fitted in terms of solely a fast component. For ASW films, the TOF spectra measured at 157 and 193 nm were both dominated by the slow component, indicating that the photofragment H atoms are accommodated to the substrate temperature by collisions. H atom formation at 193 nm is attributed to the photodissociation of water species on the ice surface, while at 157 nm it is ascribable to a mixture of surface and bulk photodissociations. Atmospheric implications in the high latitude mesopause region of the Earth are discussed. [ABSTRACT FROM AUTHOR]

The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145>=λphot>=206.00 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Many of the total kinetic energy release (TKER) spectra so derived show structure, the analysis of which confirms the importance of O–H bond fission and reveals that the resulting phenoxyl cofragments are formed in a very limited subset of their available vibrational state density. Spectra recorded at λphot>=248 nm show a feature centered at TKER ∼6500 cm-1. These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1←S0 (π*←π) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S0) state levels carrying sufficient O–H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S0 and S2(1πσ*) potential energy surfaces (PESs) at larger RO–H, en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes ν16a and ν11 can both promote nonadiabatic coupling in the vicinity of the S0/S2 CI. Spectra recorded at λphot≤=248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER ∼12 000 cm-1. These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1(1ππ*) and repulsive S2(1πσ*) PESs. Parent mode ν16b is identified as the dominant coupling mode at the S1/S2 CI, and the resulting phenoxyl radical cofragments display a long progression in ν18b, the C–O in-plane wagging mode. Analysis of all structured TKER spectra yields D0(H–OC6H5)=30 015±40 cm-1. The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of 1πσ* states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the 1πσ* state is optically dark. (ii) The probability of observing strikingly mode-specific product formation, even in “indirect” predissociations, if the fragmentation is driven by ultrafast nonadiabatic couplings via CIs between excited (and ground) state PESs. [ABSTRACT FROM AUTHOR]

Resonance-enhanced multiphoton ionization and velocity map imaging of the Cl(2P3/20) fragments of BrCl photolysis at 467.16 nm have been used to obtain a complete set of orientation parameters (with ranks K=1 and 3) describing the polarization of the electronic angular momentum. The experiments employ two geometries distinguished only by the circular or linear polarization of the photolysis laser beam. Normalized difference images constructed from the data accumulated using a right or left circularly polarized probe-laser beam, counterpropagating with the photolysis laser, were fitted to basis images corresponding to contributions from various odd-rank anisotropy parameters. Expressions are given for the difference images in terms of the K=1 and 3 anisotropy parameters, which describe coherent and incoherent parallel and perpendicular excitation and dissociation mechanisms. The nonzero values of the anisotropy parameters are indicative of nonadiabatic dissociation dynamics, with likely contributions from flux on the A 3Π(1),B 3Π(0+),C 1Π(1), and X 1Σ+(0+) states as well as one further Ω=1 state, all of which correlate adiabatically to Cl(2P3/20)+Br(2P3/20) photofragments. The magnitudes of the parameters depend both on the amplitudes of dissociative flux in these states, and also on the phases accumulated by the nuclear wave functions for different dissociation pathways. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425-485 nm by high-resolution velocity map ion imaging. Four images of the Cl(²P3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(² P[sup o][sub 3/2]) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s[sub 2], α[sub 2], η2, and γ2, and determination of the "alignment-free" recoil anisotropy parameter, β[sub 0], as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B ³Π(0[sup +]) and C ¹Π (1) states and excitations to the Ω = ± 1 components of the C state contributing to the latter. The deduced values of the alignment-free β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorption spectrum that are currently available, and with determinations of the orientation parameter γ[sub 1]' obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is... [ABSTRACT FROM AUTHOR]

The photodissociations of jet-cooled IBr and Br[sub 2] molecules have been investigated using high resolution ion imaging methods, at excitation energies just above the thresholds for forming, respectively, I(²P[sup o,sub 3/2])+Br(²P[sup o,sub 3/2]) and Br(²P[sup o,sub 3/2])+Br[sup *](²P[sup o,sub ½]) products from parent molecules in their ν' = 0 levels. For such molecules, we observe in both cases, that fragments with larger recoil velocities have markedly reduced angular anisotropy, whereas those from photolysis of IBr molecules with ν' = l show an essentially constant, limiting anisotropy. Given the monochromaticity of the photolysis radiation, increased recoil velocity of fragments resulting from photolysis of ν' = 0 molecules can only be derived from increased parent internal (rotational) energy. The measurements thus provide a particularly clear and direct observation of the breakdown of the axial recoil approximation as applied to the photodissociation of a diatomic molecule, and have been modeled, quantitatively, using both quantum and semiclassical methods together with the best available potential energy curves for the relevant excited states of IBr and Br[sub 2]. [ABSTRACT FROM AUTHOR]

Exciting a molecule with an ultraviolet photon often leads to bond fission, but the final outcome of the bond cleavage is typically both molecule tnd phase dependent. The photodissociation of an isolated gas-phase molecule can be viewed as a closed system: Energy and momentum are conserved, and the fragmentation is irreversible. The same is not true in a solutionphase photodissociation process. Solvent interactions may dissipate some of the photoexcitation energy prior to bond fission and will dissipate any excess energy partitioned into the dissociation products. Products that have no analog in the corresponding gas-phase study may arise by, for example, geminate recombination. Here, we illustrate the extent to which dynamical insights from gas-phase studies can inform our understanding of the corresponding solution-phase photochemistry and how. in the specific case of photoinduced ring-opening reactions, solution-phase studies can in some cases reveal dynamical insights more clearly than die corresponding gas-phase study. [ABSTRACT FROM AUTHOR]

The near ultraviolet photolysis of jet-cooled acetylene molecules has been further investigated using the technique of H atom photofragment translational spectroscopy. Analysis of the rotational structure evident in the total fragment kinetic energy spectrum yields a precise value for the C–H bond dissociation energy: D0(HCC–H)=46 074±8 cm-1 (551.2±0.1kJ mol-1). [ABSTRACT FROM AUTHOR]

The photochemistries of HCN and DCN at the H(D) Lyman-α wavelength have been reinvestigated using the technique of H(D) Rydberg atom time-of-flight spectroscopy, with angular resolution of the H/D atom signal about the polarization vector of the photolysis radiation. In the case of HCN photodissociation, the previous assignment of substantial branching to H+CN(A [sup 2]Π)[sub v=0] products is confirmed. Analysis of the profile taken under parallel polarization of the Lyman-α radiation relative to the time of flight axis reveals additional structure attributable both to a progression in CN(A) products with high rovibrational excitation (v=4-9, with N∼26-41, for all v), and to various rotationally excited levels associated with CN(B [sup 2]Σ[sup +])[sub v=0,1]. From these various assignments an improved value for the dissociation energy, D[sub 0](H-CN)=43 710±70 cm[sup -1], is obtained. The determined β parameter, which is a measure of the angular part of the photofragment velocity distribution about the polarization vector of the photolysis radiation, shows an increasingly parallel distribution of H atom velocities with increasing CN internal energy. DCN photolysis at the D Lyman-α wavelength yields both CN(A)[sub v=0] and a range of rovibrationally excited CN(A) products but, in contrast to HCN, no significant branching to CN(B) products is observed. The corresponding β parameter, which is found to be relatively invariant with CN internal energy, indicates a near limiting perpendicular distribution of D atom velocities about the photolysis radiation. These results are interpreted with reference to the available ab initio calculated potential energy surfaces of A[sup ′] and A[sup ″] symmetry, and the relative propensities for excitation to, and the likely dynamics on, these various excited states is discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The isomerization dynamics of α-pyrone dissolved in CH3CN have been probed by femtosecond 267 nm pump/broadband infrared (IR) probe spectroscopy. A novel experimental setup allowed the populations of the parent molecule and ring-opened photoproducts to be monitored over pump/probe time delays ranging between 2 ps and 100 μs within a single experiment, and at 5 different temperatures between 0 and 40 °C. The photochemically prepared α-pyrone(S1) molecules decay rapidly (<10 ps) through internal conversion to the S0 potential energy surface, with an initial quantum yield for parent molecule re-formation of ∼60%. Probing the antisymmetric ketene stretch region (2100-2150 cm-1) confirms the presence of at least two ring-opened photoproducts, which are assumed to have an E-configuration with respect to the central CC double bond. These ketenes are observed to undergo two distinct, thermally driven, isomerization processes which occur on the nanosecond and microsecond time scales, respectively. The former reaction is ascribed to thermalization of the initially prepared E-isomer populations, while the slower (microsecond) process involves rotation around the central CC double bond leading to formation of Z-isomers. Subsequent rapid Z → Z isomerizations (occurring on a nanosecond time scale) result in ring-closure and a second, longer time recovery of parent molecule population. By determining rates as a function of the sample temperature, barrier heights of 0.23(3) eV and 0.43(2) eV are obtained for the E → E and E → Z transformations, respectively. [ABSTRACT FROM AUTHOR]

A combinationof ultrafast transient electronic absorption spectroscopy(TEAS) and transient vibrational absorption spectroscopy (TVAS) isused to investigate whether photoinduced N–H bond fission,mediated by a dissociative 1πσ* state, is activein aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visibleand IR spectral signatures of the adeninyl radical (Ade[-H]), formedas a result of N–H bond fission, TEAS and TVAS are performedon Ade in D2O under basic conditions (pD = 12.5), whichforms Ade[-H]−anions via deprotonation at the N7or N9 sites of Ade’s 7H and 9H tautomers. At 220 nm we observeone-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 wavelengthdependent TEAS measurements between 240–260 nm identify a thresholdof 4.9 ± 0.1 eV (∼250 nm) for this photodetachment processin D2O. Analogous TEAS experiments on aqueous Ade at pD= 7.4 generate a similar photoproduct signal together with eaq–afterexcitation at 266 and 220 nm. These eaq–are born from ionization ofAde, together with Ade+cations, which are indistinguishablefrom Ade[-H] radicals in TEAS. Ade+and Ade[-H] are foundto have different signatures in TVAS and we verify that the pD = 7.4photoproduct signal observed in TEAS following 220 nm excitation issolely due to Ade+cations. Based on these observations,we conclude that: (i) N–H bond fission in aqueous Ade is inactiveat wavelengths ≥220 nm; and (ii) if such a channel exists inaqueous solution, its threshold is strongly blue-shifted relativeto the onset of the same process in gas phase 9H-Ade (≤233nm). In addition, we extract excited state lifetimes and vibrationalcooling dynamics for 9H-Ade and Ade[-H]−. In both cases, excited state lifetimes of <500fs are identified, while vibrational cooling occurs within a timeframe of 4–5 ps. In contrast, 7H-Ade is confirmed to have alonger excited state lifetime of ∼5–10 ps through bothTEAS and TVAS. [ABSTRACT FROM AUTHOR]