Electron-triggered processes in halogenated carboxylates: dissociation pathways in CF3COCl and its clusters.

Trifluoroacetyl chloride, CF3COCl, is produced in the Earth's atmosphere by photooxidative degradation of hydrochlorofluorocarbons, and represents a potential source of highly reactive halogen radicals. Despite considerable insight into photochemistry of CF3COCl, its reactivity towards electrons has not been addressed so far. We investigate the electron ionization and attachment in isolated CF3COCl molecules and (CF3COCl)N, max. N ≥ 10, clusters using a molecular beam experiment in combination with quantum chemical calculations. The ionization of the molecule at 70 eV electron energy leads to strong fragmentation: weakening of the C-C bond yields the CF3+ and COCl+ ions, while the fission of the C-Cl bond produces the major CF3CO+ fragment ion. The cluster spectra are dominated by Mn·COCl+ and Mn·CF3CO+ ions (M = CF3COCl). The electron attachment at energies between 1.5 and 11 eV also leads to the dissociation of the molecule breaking either the C-Cl bond at low energies below 3 eV yielding mainly Cl- ions, or dissociating the C-C bond at higher energies above 4 eV leading mainly to CF3- ions. In the clusters, the intact Mn- ions are stabilized after electron attachment at low energies with contribution of Mn·Cl- fragment ions. At higher energies, the Mn·Cl- fragments dominate the spectra, and C-C bond dissociation occurs as well yielding Mn·CF3-. Interestingly, Mn·Cl2- ions appear in the spectra at higher energies. We briefly discuss possible atmospheric implications.

On the Wavelength-Dependent Photochemistry of the Atmospheric Molecule CF3COCl.

The wavelength control of photochemistry usually results from ultrafast dynamics following the excitation of different electronic states. Here, we investigate the CF3COCl molecule, exhibiting wavelength-dependent photochemistry both via (i) depositing increasing internal energy into a single state and (ii) populating different electronic states. We reveal the mechanism behind the photon-energy dependence by combining nonadiabatic ab initio molecular dynamics techniques with the velocity map imaging experiment. We describe a consecutive mechanism of photodissociation where an immediate release of Cl taking place in an excited electronic state is followed by a slower ground-state dissociation of the CO fragment. The CO release is subject to an activation barrier and is controlled by excess internal energy via the excitation wavelength. Therefore, a selective release of CO along with Cl can be achieved. The mechanism is fully supported by both the measured kinetic energy distributions and anisotropies of the angular distributions. Interestingly, the kinetic energy of the released Cl atom is sensitively modified by accounting for spin-orbit coupling. Given the atmospheric importance of CF3COCl, we discuss the consequences of our findings for atmospheric photochemistry.

Pattern recognition as a new strategy in high-resolution spectroscopy: application to methanol OH-stretch overtones.

We further develop a strategy for a line-by-line assignment of complex high-resolution overtone spectra. A search for specific line patterns in the spectrum allows to identify upper rotational states by extending the concept of ground state combination differences (GSCD). Simultaneous use of all GSCDs relating to a given upper state significantly reduces a probability of incorrect assignments. To test this approach, we have analysed a newly recorded spectrum of methanol in the first OH-stretch overtone region, 2νOH, between 7170 cm-1 and 7220 cm-1 at temperature of 19 K by combining a tunable-laser-diode absorption spectrometer with a slit-jet supersonic expansion. The spectrum consists of 1002 lines at this low temperature reflecting the fact that methanol is an asymmetric rotor with a hindered internal rotation. In total, 295 lines have been reliably assigned, representing 63% of the total intensity. Rotational energies and rotational quantum numbers for 52 upper states have been determined. Many of these states have the same quantum numbers, suggesting couplings to a manifold of 'dark' vibrational states in this overtone region.

Energy partitioning and spin-orbit effects in the photodissociation of higher chloroalkanes.

We investigate the photodissociation dynamics of the C-Cl bond in chloroalkanes CH3Cl, n-C3H7Cl, i-C3H7Cl, n-C5H11Cl, combining velocity map imaging (VMI) experiment and direct ab initio dynamical simulations. The Cl fragment kinetic energy distributions (KEDs) from the VMI experiment exhibit a single peak with maximum close to 0.8 eV, irrespective of the alkyl chain length and C-Cl bond position. In contrary to CH3Cl, where less than 10% of the available energy is deposited into the internal excitation of the CH3 fragment, for all higher chloroalkanes around 40% to 60% of the available energy goes into the alkyl fragment excitation. We apply the classical hard spheres and spectator model to explain the energy partitioning, and compare the classical approach with direct ab initio dynamics simulations. The alkyl chain appears to be a soft, energy absorbing unit. We further investigate the role of the spin-orbit effects on the excitation and dynamics. Combining our experimental data with theory allows us to derive the probability of the direct absorption into the triplet electronic state as well as the probabilities for intersystem crossing. The results indicate an increasing direct absorption into the triplet state with increasing alkyl chain length.

Different Dynamics of CH3 and Cl Fragments from Photodissociation of CH3Cl in Clusters.

We investigate the photodissociation of CH3Cl at 193.3 nm using the velocity map imaging technique in (CH3Cl)n clusters in comparison with isolated molecules. Our results for the isolated molecules are in excellent agreement with the previous study of Cl fragments, and we extend it by detecting also the CH3(ν = 0) fragments. For the clusters, the Cl (and Cl*) and CH3 fragment images are dominated by intense central isotropic features. The corresponding kinetic energy distributions (KEDs) reveal significant differences in the CH3 and Cl fragment dynamics. While the CH3 fragments exhibit a very narrow near-zero kinetic energy peak, pointing to almost complete caging of CH3 fragments, the Cl (and Cl*) fragments show more structured KEDs extending all the way to the maximum available kinetic energy. The Cl KED spectra have a bimodal character with two broad peaks close to zero and around 0.6 eV. We observe a higher ICH3(ν=0)/ICl signal ratio from the clusters compared to the monomers. This is attributed to an efficient quenching of the higher vibrationally excited ν2 states of the CH3 fragments generated in the photodissociation. Collisional quenching of these excited states in clusters enhances the detected CH3(ν = 0) state. Finally, we determine the [Cl*]/[Cl] branching ratio for the photodissociation pathways in the clusters as ≈0.55 ± 0.15 compared to 0.86 for the isolated molecules, which is also attributed to the collisional quenching of the excited state in the clusters. The clusters and photofragment dynamics are discussed.

Ion and radical chemistry in (H2O2)N clusters.

We investigate the ionization induced chemistry of hydrogen peroxide in (H2O2)N clusters generated after the pickup of individual H2O2 molecules on large free ArM, M[combining macron]≈ 160, nanoparticles in molecular beams. Positive and negative ion mass spectra are recorded after an electron ionization of the clusters at energies 5-70 eV and after a slow electron attachment (below 4 eV), respectively. The spectra demonstrate that (H2O2)N clusters with N≥ 20 are formed on argon nanoparticles. This is the first experimental report on hydrogen peroxide clusters in molecular beams. The major negative cluster ion series (H2O2)nO2- indicates O2- ion formation. The dissociative electron attachment to H2O2 molecules in the gas phase yielded only OH- and O- (Nandi et al., Chem. Phys. Lett., 2003, 373, 454). These ions and the series containing them are much less abundant in the clusters. We propose a sequence of ion-molecule and radical reactions to explain the formation of O2-, HO2- and other ions observed in the negatively charged cluster ion series. Since hydrogen peroxide plays an important role in many areas of chemistry from the Earth's atmosphere to biological tissues, our study opens new horizons for experimental investigations of hydrogen peroxide chemistry in complex environments.

OH-stretch overtone of methanol: empirical assignment using a two temperature technique in a supersonic jet.

This paper describes a novel approach for empirical lower state assignments in complex high resolution ro-vibrational overtone spectra of molecules with low rotational constants and complex intramolecular dynamics. Methanol, CH3OH, was chosen as a representative of such molecules - it is an asymmetric top with two non-hydrogen nuclei and hindered internal rotation leading to dense and disordered rotational structure of vibrational overtone bands. We report the first rotationally resolved methanol spectra of the OH-stretch overtone 2ν1 band using sub-Doppler diode laser spectroscopy in a supersonic jet, and describe how the combination of two temperature analysis (TTA) and analysis by ground state combination differences (GSCDs) is used to reliably identify spectral lines that originate from lowest rotational states. In the first step of the analysis, the TTA was utilized to obtain a set of possible rotational assignments for each spectral line using the line intensity variation between two different temperatures in the supersonic jet (13, and 56 K respectively). Thereafter, the GSCDs were used to confirm specific lower state assignment for those spectral lines that have been identified to have low rotational ground states by the TTA. We show that the TTA pre-selection leads to fast and reliable confirmation by GSCDs and avoids false assignments due to accidental GSCD matches. The procedure yields an important subset of reliably assigned spectral lines in the complex ro-vibrational structure that provides a convenient starting point for subsequent application of traditional spectral analysis techniques.