Insertion of a nitrogen atom modifies the electronic structures and photochemistry of polycyclic aromatic hydrocarbons by introducing nπ* states into the molecules. To better understand the electronic structures of isolated polycyclic aromatic nitrogen-containing hydrocarbons (PANHs) and their dimers as well as the influence of the position of the nitrogen atom in the molecule, we investigate three different azaphenanthrenes, benzo[f]quinoline, benzo[h]quinoline, and phenanthridine, in a joint experimental and computational study. Experimentally, resonance-enhanced multiphoton ionization (REMPI) spectroscopy is applied to characterize the excited electronic states. The REMPI spectra of the azaphenanthrene monomers have a rather similar appearance, with origins between 3.645 and 3.670 eV for the 1ππ* ← S0 transition. In contrast to the phenanthrene parent, 2ππ* ← S0 is broad and unstructured even at the band origin. The experiments are accompanied by density functional theory computation, and vibrationally resolved spectra are simulated using a time-independent approach. The differences between phenanthrene and the azaphenanthrenes are assigned to perturbations due to the low-lying 1(nπ*) state, which accelerates nonradiative deactivation. For the dimers, it is found that two π-stacked isomers with two electronic transitions each contribute to the electronic spectrum, leading to overlapping bands that are difficult to assign.

Due to their unusual electronic structure, the biradical m-benzyne, C6H4, and its cation are of considerable interest in chemistry. Here, the photoion mass-selected threshold photoelectron spectrum of the m-benzyne biradical is presented. An adiabatic ionization energy of 8.65 ± 0.015 eV is derived, while a vibrational progression of 0.10 eV is assigned to the ν9+ ring breathing mode, in excellent agreement with computations. The experimental spectrum was reproduced well by Franck-Condon spectral modeling of the 2A1 ← X 1A1 transition, in which the cation retains a monocyclic C6 framework. The energetically close-lying bicyclic 2A2 cation state exhibits low Franck-Condon factors, due to the large change in geometry, and thus cannot be observed.

Anja Röder opened a general discussion of the papers by Jonathan Marangos: I have a question concerning the temporal resolution that can be achieved. The electrons produced via Auger decay in the detection step, are not produced instantaneously. Does this limit the time-resolution of such an ex

Mass selected slow photoelectron spectra (SPES) of three boron-containing reactive species, BH2, BH, and BF were recorded by double imaging photoion-photoelectron coincidence spectroscopy (i2PEPICO) using synchrotron radiation. All species were generated in a flow reactor from the H-abstraction of B2H6 by F atoms created in a F2 microwave discharge. The spectrum of BH2+ exhibits a long bending mode progression with a 970 cm-1 spacing due to the large geometry change from bent to linear upon ionization. Its ionization energy was determined as 8.12 ± 0.02 eV. For BH, photoionisation from both X1Σ+ singlet and a3Π triplet state was observed, permitting the experimental determination of the singlet/triplet gap (ΔEST) from the observed IE's of 9.82 eV and 8.48 eV. In addition, a threshold photoelectron spectrum of BF was recorded, which leads to an IE of 11.11 eV and an improved value for νBF+ of 1690 cm-1. All spectra were simulated by calculating Franck-Condon factors from optimised structures based on quantum chemical calculations.

A recent review on the photoionisation of the C6H4 isomer ortho-benzyne suggests that bands reported in earlier photoelectron spectra might be due to side products or contaminations, while computations raise doubts, whether the cation has a planar geometry. We therefore reinvestigate the photoionisation of ortho-benzyne, generated by pyrolysis from benzocyclobutenedione, by photoion mass-selected threshold photoelectron (ms-TPE) spectroscopy using synchrotron radiation. The experiments are accompanied by a theoretical study that investigates the structure of the ortho-benzyne cation systematically as a function of the computational method, up to CASPT2(11,14) ab initio computations. Our study leads to a re-evaluation of the ionisation energy of ortho-benzyne. It reveals that the orthobenzyne cation has indeed a twisted C-2 geometry rather than a C-2v structure. A vertical ionisation energy IEvert of 9.77 eV and an adiabatic ionisation energy of IEad = 9.56 eV are computed for orthobenzyne. A Franck-Condon simulation of the photoelectron spectrum based on the CASPT2 results and including three electronic states of the cation is in agreement with the experiment and yields IEad = 9.51 eV (+50 meV/-100 meV). Since this value is in contrast with previous work, the ionisation energy has to be revised based on our study. Computational methods based on density functional theory give a reasonable description of the cationic ground state, but fail for the corresponding excited electronic states that are indispensible for a proper assignment of the photoelectron spectrum.

The kinetics of the combustion-relevant reaction of the allyl radical, a-C3H5, with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring, using the CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence Spectroscopy) setup. The ability to measure threshold photoelectron spectra enables a background-free detection of reactive species as well as an isomer-specific analysis of reaction products. Allyl was generated by direct photodissociation of allyl iodide at 266 nm and 213 nm and indirectly by the reaction of propene with Cl atoms, which were generated by photolysis from oxalyl chloride at 266 nm. Experiments were conducted at room temperature at low pressures between 0.8 and 3 mbar using Ar as the buffer gas and with excess O2 to maintain nearly pseudo-first-order reaction conditions. Whereas allyl was detected by photoionisation using synchrotron radiation, the main reaction product allyl peroxy was not observed due to dissociative ionisation of this weakly bound species. From the concentration-time profiles of the allyl signal, second-order rate constants between 1.35 × 1011 cm3 mol-1 s-1 at 0.8 mbar and 1.75 × 1011 cm3 mol-1 s-1 at 3 mbar were determined. The rates obtained for the different allyl radical generation schemes agree well with each other, but are about a factor of 2 higher than the ones reported previously using He as a buffer gas. The discrepancy is partly attributed to the higher collision efficiency of Ar causing a varying fall-off behavior. When allyl is produced by the reaction of propene with Cl atom, an unexpected product is observed at m/z = 68, which was identified as 1,3-butadienal in the threshold photoelectron spectrum. It is formed in a secondary reaction of allyl with the OCCl radical, which is generated in the 266 nm photolysis of oxalyl chloride.

In this paper, we investigate HNCO by resonant and nonresonant Auger electron spectroscopy at the K-edges of carbon, nitrogen, and oxygen, employing soft X-ray synchrotron radiation. In comparison with the isosteric but linear CO2 molecule, spectra of the bent HNCO molecule are similar but more complex due to its reduced symmetry, wherein the degeneracy of the pi-orbitals is lifted. Resonant Auger electron spectra are presented at different photon energies over the first core-excited 1s -> 10a' resonance. All Auger electron spectra are assigned based on ab initio configuration interaction computations combined with the one-center approximation for Auger intensities and moment theory to consider vibrational motion. The calculated spectra were scaled by a newly introduced energy scaling factor, and generally, good agreement is found between experiment and theory for normal as well as resonant Auger electron spectra. A comparison of resonant Auger spectra with nonresonant Auger structures shows a slight broadening as well as a shift of the former spectra between -8 and -9 eV due to the spectating electron. Since HNCO is a small molecule and contains the four most abundant atoms of organic molecules, the reported Auger electron decay spectra will provide a benchmark for further theoretical approaches in the computation of core electron spectra. Published by AIP Publishing.

We present a joint experimental and computational study of the nonradiative deactivation of the benzyl radical, C7H7, after UV excitation. Femtosecond time-resolved photoelectron imaging was applied to investigate the photodynamics of the radical. The experiments were accompanied by excited state dynamics simulations using surface hopping. Benzyl has been excited at 265 nm into the D-band (ππ*) and the dynamics was probed using probe wavelengths of 398 nm or 798 nm. At a probe wavelength of 398 nm a single time constant of around 70-80 fs was observed. When the dynamics was probed at 798 nm, a second time constant τ2 = 1.5 ps was visible, which can be attributed to further non-radiative deactivation to the lower-lying D1/D2 states.

The propargyl radical is considered to be of key importance in the formation of the first aromatic ring in combustion processes. Here we study the bimolecular (self-) reactions of propargyl in a high-temperature pyrolysis flow reactor. The aromatic reaction products are identified by IR/UV ion dip spectroscopy, using the free electron laser FELIX as mid-infrared source. This technique combines mass selectivity with structural sensitivity. We identified several aromatic reaction products based on their infrared spectra, among them benzene, naphthalene, phenanthrene, indene, biphenyl, and surprisingly a number of aromatic compounds with acetylenic (ethynyl) side chains. The observation of benzene confirms that propargyl is involved in the formation of the first aromatic ring. The observation of compounds with acetylenic side chains shows that, in addition to a propargyl- and phenyl-based mechanism, the HACA (hydrogen abstraction C2H2 addition) mechanism of polycyclic aromatic hydrocarbons formation is present, although no acetylene was used as a reactant. On the basis of the experimental results we suggest a mechanism that connects the two pathways.

In this manuscript we present threshold photoelectron spectra (TPES) of the amidogen radical, NH2, recorded at two vacuum ultraviolet synchrotron radiation beamlines, the DESIRS beamline of Synchrotron SOLEIL and the VUV beamline of the Swiss Light Source (SLS). Amidogen radicals were generated by two different methods, (a) H-atom abstraction of ammonia in a fluorine microwave discharge flow tube and (b) flash pyrolysis of methylhydrazine and diphenylmethylamine. Due to the large geometry change upon photoionization from the bent neutral molecule NH2 (X[combining tilde] (2)B1) to the quasi-linear cation NH2(+) (X[combining tilde] (3)B1), significant activity in the bending vibration υ2(+) of NH2(+) (X[combining tilde] (3)B1) is observed in the TPES. Transitions into a large number of υ2(+), Ka(+) levels of the cation are resolved.

The self-reaction of the phenyl radical is one of the key reactions in combustion chemistry. Here we study this reaction in a high-temperature flow reactor by IR/UV ion dip spectroscopy, using free electron laser radiation as mid-infrared source. We identified several major reaction products based on their infrared spectra, among them indene, 1,2-dihydronaphthalene, naphthalene, biphenyl and para-terphenyl. Due to the structural sensitivity of the method, the reaction products were identified isomer-selectively. The work shows that the formation of indene and naphthalene, which was previously considered to be evidence for the HACA (hydrogen abstraction C2H2 addition) mechanism in the formation of polycyclic aromatic hydrocarbons and soot can also be understood in a phenyl addition model.

An attempt to control the three structural features linkage, structural dimensionality and coordination mode is presented for the combination of Fe(II) and Fe(III) chlorides together with the ditopic ligand piperazine (pipz). Thermal conversion of small units like complexes into highly aggregated structures such as coordination polymers is demonstrated for the framework formation of ∞³[Fe(II)Cl2(pipz)] starting from both FeCl2 and FeCl3. Depending on the oxidation state of iron, different reaction paths and metastable products are observed. Iron(II)chloride reacts with piperazine to form the previously unknown 2-dimensional network, ∞²[Fe(II)2Cl4(pipz)3]·pipz, which can be thermally converted into the 3-dimensional framework at elevated temperatures by release of piperazine. The use of iron(III)chloride starts with an internal redox reaction giving the divalent complex [Fe(II)Cl3(Hpipz)(pipz)] in the first step, followed by thermal conversion yielding the framework ∞³[Fe(II)Cl2(pipz)] and the salt [Fe(II)Cl3(Hpipz)(pipz)](Hpipz)Cl2. Both thermal conversion mechanisms were further investigated by in situ IR-spectroscopy, differential thermal analysis and thermogravimetry.

AbstractWe discuss the role of isolated radicals in various chemical environments, such as combustion engines, low‐temperature plasmas, planetary atmospheres, and interstellar space. Several methods to generate radicals are presented, for example, pyrolysis, photolysis, discharges, chemical reactions, and neutralization–reionization mass spectrometry. The breadth of each method is illustrated and species generated by these methods are introduced. It is shown that photoionization is an important tool to detect radicals and unravel their reactions. However, the often small Franck–Condon‐factors have to be considered when interpreting photoionization data. Selected results on the unimolecular and bimolecular reactions of radicals are discussed with the intention to show the scope of present methods to study radical chemistry in the gas phase. Among these methods are time‐resolved laser spectroscopy, mass‐spectrometric techniques such as charge‐tagging and schemes based on detecting radicals with synchrotron radiation.

The VUV photochemistry of the radicals propargyl, C3H3, and ethyl, C2H5, is investigated using synchrotron radiation. The radicals are produced by flash pyrolysis from propargyl bromide and ethyl iodide, respectively. It is shown that dissociative photoionisation of propargyl is associated with loss of a H-atom, leading to formation of c-C3H2+. The process sets in between 12.5 and 13 eV, in agreement with computations. The alternative pathway, leading to C3H(+) + H2 is only tentatively observed at energies above 15.5 eV. The onset for dissociative photoionisation of ethyl according to C2H5 --> C2H3(+) + H2 is observed at 11.6 eV, roughly 1 eV higher than expected from ab initio calculations. In addition we monitor the hitherto unobserved three-body dissociative ionisation of the precursor molecules. The process C2HsI --> C2H3 + H2 + I sets in above 12.7 eV, the dissociative ionisation of C3H3Br --> C3H+ + H2 + Br above 15 eV.