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Dissociative photoionization of quinoline induced by vacuum ultraviolet radiation is investigated using photoelectron–photoion coincidence spectroscopy. Branching ratios of all the detectable fragment ions are measured as a function of internal energy ranging from 2 to 30 eV. A specific generation hierarchy is observed in the breakdown curves of a set of dissociation channels. Moreover, a careful comparison of the breakdown curves of fragments among the successive generations allowed to establish a decay sequence in the fragmentation of quinoline cation. This enabled us to revisit and refine the understanding of the first generation decay and reassign the origin of a few of the higher generation decay products of quinoline cation. With the help of the accompanying computational work (reported concurrently), we have demonstrated the dominance of two different HCN elimination pathways over previously interpreted mechanisms. For the first time, a specific pathway for acetylene elimination is identified in quinoline+ and the role of isomerization in both acetylene as well as hydrogen cyanide loss is also demonstrated. The experiment also established that the acetylene elimination exclusively occurs from the non-nitrogen containing rings of quinoline cation. The formation of a few astronomically important species is also discussed. [ABSTRACT FROM AUTHOR]

The interaction of fast protons (velocity between 1.41 and 2.4 a.u.) with naphthalene and azulene is investigated as a model of an ion-polycyclic aromatic hydrocarbon interaction system. Production of various intact and fragment ions in coincidence with electron emission, electron transfer to projectile, or both is analyzed. The two targets being isomers, the rather obvious similarity in the fundamental ion-molecule collision energetics is quantitatively verified. The fast isomerization processes of cationic azulene are observed to be influencing its further dissociation channels such as C2H2 and H eliminations. A first ever attempt is presented here wherein single plasmon excitation in conjunction with isomerization dynamics is reported. Evidence from dication evaporation energetics is used to invoke the double plasmon excitation model. A model based on the multiplasmon resonance explains the observed proton velocity dependence of double to single ionization cross sections. Moreover an attempt is made to reinforce the proposition of double plasmon excitation by explaining the observed suppression of neutral H loss from dications as opposed to monocations. [ABSTRACT FROM AUTHOR]

The internal energy dynamics of three examples of PAHs are probed using a high resolution energy time of flight spectrometer. The measured H-loss rates are converted to internal energy values for a range of UV photon wavelengths. Multi step multi photon absorption scheme is proposed to estimate the possible internal energy values with reasonable accuracies. This was possible because of the ultrafast radiationless internal conversion process in PAHs and their structural rigidity. Thus, a method is proposed to produce PAH cations with narrow and known internal energy distribution using a single nanosecond pulsed laser, paving a way to study statistical unimolecular decay in PAHs very efficiently. [ABSTRACT FROM AUTHOR]

Testing and performance validation of a new multipurpose time-of-flight mass spectrometer followed by an energy analyzer is presented. The instrument with high mass and energy resolution is primarily designed to study cations of polycyclic aromatic hydrocarbons (PAHs) and their dehydrogenation process. The energy correlated time-of-flight measurement is supplemented by Monte Carlo simulation to probe the dehydrogenation process in a relatively small PAH cation. The experiment is carried out on fluorene+ on a timescale of several microseconds. Fluorene cations with high internal energies were produced using UV multiphoton ionization. Specific n-photon processes leading to ionization as well as H-loss reaction were identified. The average value of dehydrogenation rate is estimated by fitting the measured data to the outcome of simulations. The quantification of H loss decay rate is in agreement with previously reported decay rate measurement. This corresponds to the internal energy available by inner valence electron emission caused by three photon process. The effectiveness of the instrument to access a range of decay rates (103–107 s−1) in a single measurement is demonstrated. [ABSTRACT FROM AUTHOR]

In search of the cause behind the similarities often seen in the fragmentation of PANHs, vacuum ultraviolet (VUV) photodissociation of two pairs of isomers quinoline–isoquinoline and 2-naphthylamine-3-methyl-quinoline are studied using the velocity map imaging technique. The internal energy dependence of all primary fragmentation channels is obtained for all four target molecules. The decay dynamics of the four molecules is studied by comparing their various experimental signatures. The dominant channel for the first pair of isomers is found to be hydrogen cyanide (HCN) neutral loss, while the second pair of isomers lose HCNH neutral as its dominant channel. Despite this difference in their primary decay products and the differences in the structures of the four targets, various similarities in their experimental signatures are found, which could be explained by isomerization mechanisms to common structures. The fundamental role of these isomerization in controlling different dissociative channels is explored via a detailed analysis of the experimental photoelectron–photoion coincidences and the investigation of the theoretical potential energy surface. These results add to the notion of a universal PANH fragmentation mechanism and suggests the seven member isomerization as a key candidate for this universal mechanism. The balance between isomerization, dissociation, and other key mechanistic processes in the reaction pathways, such as hydrogen migrations, is also highlighted for the four molecules. [ABSTRACT FROM AUTHOR]

A detailed exploration of the potential energy surface of quinoline cation (C9H7N·+) is carried out to extend the present understanding of its fragmentation mechanisms. Density functional theory calculations have been performed to explore new fragmentation schemes, giving special attention to previously unexplored pathways, such as isomerization and elimination of HNC. The isomerization mechanisms producing five- to seven-membered ring intermediates are described and are found to be a dominant channel both energetically and kinetically. Energetically competing pathways are established for the astrochemically important HNC-loss channel, which has hitherto never been considered in the context of the loss of a 27 amu fragment from the parent ions. Elimination of acetylene was also studied in great detail. Overall, the computational results are found to complement the experimental observations from the concurrently conducted PEPICO investigation. These could potentially open the doors for rich and interesting vacuum ultraviolet radiation-driven chemistry on planetary atmospheres, meteorites, and comets. [ABSTRACT FROM AUTHOR]