We present an experimental investigation of the UV photochemistry of diacetylene under collisionless conditions. The H loss channel is studied using DC slice ion imaging with two-color reduced-Doppler detection at 243 nm and 212 nm. The photochemistry is further studied deep in the vacuum UV, that is, at Lymanalpha (121.6 nm). Translational energy distributions for the H ؉ C 4H product arising from dissociation of C 4H2 after excitation at 243, 212, and 121.6 nm show an isotropic angular distribution and characteristic translational energy profile suggesting statistical dissociation from the ground state or possibly from a low-lying triplet state. From these distributions, a two-photon dissociation process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a one-photon dissociation process prevails. The results are interpreted with the aid of ab initio calculations on the reaction pathways and statistical calculations of the dissociation rates and product branching. In a second series of experiments, nanosecond time-resolved phototionization measurements yield a direct determination of the lifetime of metastable triplet diacetylene under collisionless conditions, as well as its dependence on excitation energy. The observed submicrosecond lifetimes suggest that reactions of metastable diacetylene are likely to be less important in Titan's atmosphere than previously believed.ion imaging ͉ photochemistry ͉ Titan S aturn's moon, Titan, is the only solar system body besides Earth and Venus with a dense atmosphere (1, 2). It is widely considered as a natural laboratory on the planetary scale in understanding the prebiotic chemistry on proto-Earth. Diacetylene is believed to play a key role in the formation of polyynes and polycyclic aromatic hydrocarbons (PAHs) that partially comprise the haze layer in Titan's upper atmosphere (2-4). It is well established that the formation of diacetylene is initiated by photodissociation of acetylene below 217 nm (2, 5-8) according to the following reaction mechanism:The importance ascribed to diacetylene arises in part because it absorbs light at longer wavelengths, where the solar flux is higher, than any other major constituents of Titan's atmosphere; moreover, experimental results suggest it is still photochemically reactive even well below the threshold for dissociation (9-12). Understanding the dynamics of diacetylene photoexcitation is thus key to revealing the factors driving the chemistry of Titan's atmosphere.To date, no experiments on the photochemistry of diacetylene have been performed under collisionless conditions. In a pioneering study, Glicker and Okabe (9) determined a quantum yield of 2.0 Ϯ 0.5 for diacetylene photodissociation in the wavelength region of 147-254 nm. Between 184 and 254 nm, no free-radical products were detected and polymeric material was found to coat the inside of the reaction cell. The upper limit for the quantum yield of C 4 H formation was then determined to be only 0.06 at 228 nm based on experimental uncertainty. However, at the time, t...

The anion photoelectron spectra of ortho-, meta-, and para-methylphenoxide, as well as methyl deprotonated meta-methylphenol, were measured. Using the Slow Electron Velocity Map Imaging (SEVI) technique, the Electron Affinities (EAs) of the o-, m-, and p-methylphenoxyl radicals were measured: 2.1991 ± 0.0014, 2.2177 ± 0.0014, and 2.1199 ± 0.0014 eV, respectively. The EA of mmethylenephenol was also obtained, 1.024 ± 0.008 eV. In all four cases, the dominant vibrational progressions observed are due to several ring distortion vibrational normal modes that were activated upon photodetachment, leading to vibrational progressions spaced by ~500 cm -1 . Using the

The photodissociation of cyanoacetylene, one of the key minor constituents in Titan's atmosphere, was studied in a molecular beam under collisionless conditions using direct current slice ion imaging at 121.6, 193.3, and 243.2 nm. The experimental results were augmented by high-level theoretical calculations of stationary points on the ground-state and second excited singlet potential surfaces, and by statistical calculations of the dissociation rates and product branching on the ground-state surface. Results at 121.6 and 243.2 nm are nearly identical, suggesting that the 243.2 nm photodissociation is the result of a two-photon process. The translational energy distributions show only a modest fraction of the available energy in translation and are consistent with barrierless dissociation from the ground state. The results at 193.3 nm are quite distinct, showing up to half of the available energy in translation, implying dissociation with an exit barrier. The 193 nm result is ascribed to dissociation on the S(1) potential energy surface. The theoretical calculations show significant rates for H loss on the ground state at 193 nm and significant branching to CN + CCH at 157 nm and higher.

We report an ion imaging and time-of-flight mass spectroscopy study of the photodissociation of a variety of heptane isomers using 157 nm dissociation and ionization. Time-of-flight mass spectra show that C(3)H(7) + C(4)H(9) is the dominant detected product channel following one-color 157 nm dissociation/ionization of heptanes. The results further allow determination of the relative ionization efficiencies of 1- and 2-butyl and propyl radicals at 157 nm. Momentum matching for the two radical products indicates that, for the C3-C4 products, neutral dissociation followed by ionization is the main source of the detected signals. The images show isotropic angular distributions and the translational energy distributions peak at very low energy, with only approximately 0.3 eV or 8% of the available energy appearing in translation. This is consistent with dissociation from the ground state or low-lying triplet states following non-radiative electronic relaxation.

Product branching ratios (BRs) are reported for ion-molecule reactions of state-prepared nitrogen cation (N(2)(+)) with methane (CH(4)), acetylene (C(2)H(2)). and ethylene (C(2)H(4)) at low temperature using a modified ion imaging apparatus. These reactions are performed in a supersonic nozzle expansion characterized by a rotational temperature of 40 ± 5K. For the N(2)(+) + CH(4) reaction, a BR of 0.83:0.17 is obtained for the dissociative charge-transfer (CT) reaction that gives rise to the formation of CH(3)(+) and CH(2)(+) product ions, respectively. The N(2)(+) + C(2)H(2) ion-molecule reaction proceeds through a nondissociative CT process that results in the sole formation of C(2)H(2)(+) product ions. The reaction of N(2)(+) with C(2)H(4) leads to the formation of C(2)H(3)(+) and C(2)H(2)(+) product ions with a BR of 0.74:0.26, respectively. The reported BR for the N(2)(+) + C(2)H(4) reaction is supportive of a nonresonant dissociative CT mechanism similar to the one that accompanies the N(2)(+) + CH(4) reaction. No dependence of the branching ratios on N(2)(+) rotational level was observed. In addition to providing direct insight into the dynamics of the state-prepared N(2)(+) ion-molecule reactions with the target neutral hydrocarbon molecules, the reported low-temperature BRs are also important for accurate modeling of the nitrogen-dominated upper atmosphere of Saturn's moon, Titan.

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Direct current (DC) slice imaging of state-selected ions is combined with high-level ab initio calculations to give insight into reaction pathways, dynamics, and energetics for ethylamine cation photodissociation at 233 nm. These reaction pathways are of interest for understanding the rich chemistry of Titan's ionosphere recently revealed by the Cassini mission. The result for the H-loss product has a bimodal translational energy distribution, indicating two distinct H-loss pathways: these are assigned to triplet CH(3)CH(2)NH(+) product ions and the singlet CH(3)CHNH(2)(+) species. The distribution shows a modest fraction of energy available in translation and is consistent with barrierless dissociation from the ground state. HCNH(+) formation is observed as the dominant channel and exhibits a bimodal translational energy distribution with the faster component depicting a significant angular anisotropy. This suggests a direct excited-state decay pathway for this portion of the distribution. We have also observed the H + H(2) loss product as a minor secondary dissociation channel, which correlates well with the formation of CH(2)CNH(2)(+) ion with an exit barrier.

Negative ion photoelectron spectra of ortho (o-), meta (m-), and para (p-) deprotonated benzonitrile (o-, m-, p-C 6 H 4 (CN) − ) isomers as well as the associated thermochemical values corresponding to deprotonation at o-, m-, and p-positions in C 6 H 5 (CN) are presented. Quantum mechanical results based on the density functional theory (DFT) utilizing the aug-cc-pVQZ basis set indicate that the o-, m-, p-C 6 H 4 (CN) • radicals have electron affinity values (EAs) of 1.901, 1.778, and 1.789 eV, respectively. The computed Franck−Condon (FC) factors give rise to o-, m-, and p-C 6 H 4 (CN) − negative ion spectra with FC active ring distortion vibrational modes with harmonic vibrational frequencies of ∼450, 760, and 1000 cm −1 as the dominant vibrational progressions. Deprotonation at the o-, m-, and ppositions in C 6 H 5 (CN) results in calculated gas-phase acidity values (Δ acid H 298K o ) of 383.9, 385.7, and 385.3 kcal mol −1 , respectively. The calculated Δ acid H 298K o is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 383.4.0 ± 4.4 kcal mol −1 . The computed Δ acid H 298K o and EAs are utilized to estimate the bond dissociation energy (DH 298 (H-C 6 H 4 CN)) associated with the formation o-, m-, and p-C 6 H 4 (CN) • using the negative ion thermochemical cycle: DH 298 (C 6 H 5 CN) = Δ acid H 298K o (H-C 6 H 4 (CN) + EA (C 6 H 5 CN) • − IP(H). The respective values of DH 298 (H-C 6 H 4 CN) corresponding to the formation of ortho, meta, and para C 6 H 4 (CN) radicals are 114.15, 113.11, and 113.51 kcal mol −1 .