Rolf Bombach

Intermolecular bonding and vibrations of the carbazole⋅B complexes (B=H2O, D2O, NH3)

Carbazole⋅B complexes (B=H2O, D2O, NH3) were synthesized and cooled in pulsed supersonic nozzle beams. The intermolecular hydrogen‐bond vibrations and dissociation energies were studied by several laser‐spectroscopic techniques (fluorescence excitation and emission, resonance‐two‐photonionization with mass‐specific detection). The following results were obtained for both S0 and S1 electronic states: (1) determination of the structural symmetry of the complexes, (2) measurement of the intermolecular stretching (νσ) and bending (νβ) frequencies, (3) determination of stretching force constants, (4) hydrogen‐bond dissociation energies D0 for carbazole⋅H2O/D2O, and (5) electronic spectral shifts δν relative to the bare carbazole molecule. The latter are large (500–710 cm−1) and reflect an increase of the hydrogen‐bond energy by ≈40% upon electronic excitation. Fermi resonance couplings between the intermolecular νσ and an intramolecular b1 vibration of carbazole are observed and partially analyzed. To complem...

Order–disorder transitions in quasi‐two‐dimensional argon solvent clusters

Structures and order–disorder transitions of the rare‐gas solvent clusters carbazole⋅Arn were studied for n=1–7 by molecular dynamics and Monte Carlo simulation methods. This study was motivated by the investigation of these clusters by size‐specific electronic spectroscopy. Cluster structures corresponding to absolute and local energy minima were obtained by molecular‐dynamics slow cooling and annealing. For n=1–3, single minimum‐energy structures of Cs symmetry were found, with all n atoms on the same side of the molecular substrate. For n=4–7, the number of energetically low‐lying monolayer isomers rises very rapidly with n. Orientational ordering of the adcluster by the substrate is unimportant for n≥4. The isomers for n≥4 differ by (i) rotational/translational displacements of the cluster relative to the substrate, (ii) promotion of atoms to the second solvent layer, and (iii) adsorption of atoms on the second side of the substrate. Order–disorder transitions of the monolayer solvent clusters were st...

Unimolecular dissociations of excited C3H6O+: A comparative photoelectron—photoion coincidence study of 1,2-epoxypropane and acetone

Abstract The unimolecular fragmentation of internal energy selected 1,2-epoxypropane cations has been studied by fixed-wavelength photoelectron—photoion coincidence spectroscopy. Branching ratios for the prominent fragment ions are reported up to an ionization energy of I = 14 eV. It is shown that 1,2-epoxypropane cations initially formed with none or only little vibrational excitation in the electronic ground state do not dissociate, though their excess energy with respect to the lowest energetic fragmentation pathway is 1.25 eV. As the internal energy is increased, slow fragmentation into several dissociation channels is observed. This is used to explain a comparably slow dissociation process observed in the case of acetone molecular ions initially excited to their electronic A state. CH 2 C(OH)CH 3 + and/or CH 3 CHCHOH + are proposed as precursors for these low-rate unimolecular reactions.

The rate/energy functions for the competitive fragmentation processes of ethylene and ethane cations

Abstract A generally applicable strategy is presented in order to derive the rate/energy functions of competing unimolecular fragmentations of excited organic cations directly from their breakdown diagrams obtained from photoelectron/photoion coincidence spectroscopy. As an illustration, the energy dependence of the unimolecular rate constants for the loss of an H2 molecule or an H atom from ethylene and ethane cations is determined. The main difficulties connected with the analysis of larger systems are discussed.

Branching ratios and partition of the excess energy for the predissociation of CO+2 C̃ 2Σ+g molecular cations

He–Iα photoelectron–photoion coincidence spectroscopy is used to study the decay of CO+2 molecular cations, initially prepared in various vibrational levels of the C 2Σ+g state. The branching ratios for the energetically accessible fragmentation pathways are determined. Owing to the precisely known energetics and to an accurate measurement of the kinetic energies released, the partition of the respective excess energy among internal and external degrees of freedom of the separating fragments could be established. The formation of highly rotationally excited diatomic fragments must be proposed in order to explain the experimental results. The disappearance of O+ ions in favor of CO+ fragments as soon as the energy exceeds the dissociation limit for CO+ formation is consistent with a reinterpretation of previous ab initio calculations of potential‐energy surfaces. The observed branching ratio for the population of the v=0 and v=1 levels of the CO fragment can be accounted for by an application of the stati...

The rate—energy functions for the formation of tropylium and benzylium ions from toluene molecular cations

Abstract The rate—energy functions for the decay of toluene molecular cations into tropylium and bezylium cations have been established by analyzing photoelectron—photoion coincidence data with a straightforward formulation of the RRKM theory. The threshold energies for the two corresponding fragmentation reactions could be determined, although both product ions have identical mass. The 0 K enthalpies of formation derived agree closely with appropriately corrected 298 K literature data.

Fragmentation of formaldehyde molecular cations

Abstract The fragmentation behaviour of internal-energy-selected formaldehyde CX2O+ (X = H, D) molecular cations has been explored using the fixed-wavelength photoelectron—photo-ion coincidence technique. The breakdown curves for the CX2O+ parent ion and the CXO+, CO+ and X2+ fragment ions are reported up to an ionisation energy of 18 eV. Values for the mean kinetic energy released during the formation of the above-mentioned fragment ions are given.

The fragmentation of formaldehyde molecular cations: the lifetime of CD2O+(Ã 2B1)

Abstract He Iα photoelectron-photoion coincidence spectroscopy was employed to identify CD2O+(A 2B1) as precursor of the unusual metastable signal in the mass spectrum of dideuteroformaldehyde. The lifetimes of this electronically excited state of the molecular cation are reported.

Solute-solvent interactions in microhydrate clusters: Carbazole - (H2O)n

Abstract We report the laser-induced fluorescence (LIF) and the resonant two-photon ionization spectra of the carbazole-(H2O)n (n = 1–3) hydrogen-bonded complexes. Use of carbazole as a symmetric “probe” molecule allows detailed insight into the symmetries, structures and intermolecular vibrations of these microhydrate complexes.

Unimolecular dissociations of excited C3H6O+: A photoelectron—photoion coincidence study of cyclopropanol and allyl alcohol

Abstract The photoelectron spectrum of cyclopropanol has been determined and the first few bands have been assigned to the respective ionization processes. The dissociative photoionization of cyclopropanol and allyl alcohol has been studied by means of HeIα photoelectron—photoion coincidence spectroscopy. The breakdown diagrams of the two corresponding radical cations are practically indistinguishable within the entire energy range investigated (= 5 eV). This implies that extensive isomerizations to a single or to a mixture of common precursor structure(s) precede the dissociative processes. The currently available information on the structure of this reactant and, in particular, the role of other C 3 H 6 O + isomers is discussed. The kinetic and thermodynamic aspects of the present results are outlined. Although three generations of daughter ions are involved, the relative importance of the different fragmentation pathways could be determined. The RRKM analysis of the coincidence data suggests that the formation of the ethylene cation by loss of a CO molecule from the parent ion hardly competes with the other four primary fragmentation reactions in the sense of the statistical theory of unimolecular reactions.