Volume 225, Issue 9-10 - Special Issue Prof. Hippler

Femtosecond mid-infrared pump-probe spectroscopy was carried out to obtain information about the dynamics of vibrational energy relaxation in liquid ethanolamine at room temperature and ambient pressure. Through partial deuteration it was possible to disentangle the dynamics resulting from the OH and the NH stretching modes that proceed independently and simultaneously in the hydrogen-bonded liquid following an ultrafast vibrational excitation by a resonant mid-infrared pulse. The OH-stretching vibrational lifetime was determined to be 450 fs while the NH-stretching lifetime was found to be 1.2 ps. This large difference in lifetimes highlights the importance of the hydrogen-donating and the hydrogen-accepting character of the vibrating groups that are engaged in hydrogen-bonding.

A semi-classical model for transient anisotropy in degenerate excited states is developed on the basis of a more general ansatz presented recently (O. Schalk and A.-N. Unterreiner, Phys. Chem. Chem. Phys. 12 , 655 (2010)). This is the first model that can treat both rotational dephasing and the dynamics in degenerate systems, which is a prerequisite for a comprehensive theory to describe gas phase anisotropy experiments in small, highly symmetric systems. In the present contribution, it is shown that this model covers most of the features of the full quantum dynamical treatment and helps to give insights into the physical processes that are underlying these dynamics.

The HO 2 radical is one of the most important intermediate species in atmospheric chemistry. We report on the development of a new photoreactor with first in-situ measurement of HO 2 radical photostationary concentrations using continuous wave cavity ring-down spectrometry (cw-CRDS). Characterization of the actinic photon flux was carried out by NO 2 actinometry. Photolysis of Cl 2 /methanol mixtures in air under UV light allowed the measurement of HO 2 photostationary concentrations of a few 10 10 molecules cm -3 with an HO 2 detection limit of 1.5 × 10 10 molecules cm -3 at 6638.207 cm -1 . The feasibility of HO 2 direct measurement in a reaction chamber is demonstrated through the measurement of the HO 2 overall loss at different pressures showing the importance of HO 2 diffusion and wall loss in such low pressure quartz reactor. The rate coefficient for the HO 2 +HO 2 reaction has been measured at 6.6, 24 and 118 mbar and found to be in good agreement with the recommended value.

The ultrafast response of singly negatively charged C 60 fullerene in solution has been investigated by femtosecond pump-probe absorption spectroscopy and transient anisotropy in the visible and near-infrared region. Pump excitation within the near-infrared band demonstrates that this spectral feature can be described as a vibrational progression associated with a single electronic transition. Relaxation of the first electronically excited state occurs primarily by internal conversion with a time constant of 3 ps, slightly depending on the solvents, tetrahydrofuran or o-dichlorobenzene, and also on the excitation wavelength. An excitation of the second electronically excited state around 530 nm leads to an ultrafast internal conversion to the first excited state with a pulse-limited time constant of less than 100 fs. As a minor channel, stimulated emission in the spectral regime of 1150–1300 nm was observed from the first electronically excited state both after near-infrared and visible excitation. After internal conversion to the electronic ground state, C 60 − dissipates its excess internal energy into the solvent on a longer timescale of 40–70 ps. The transient anisotropy associated with directly populating the first excited state reveals an ultrafast component decaying within 100 fs, which is attributed to ultrafast vibrational motions, conceivably arising from excited state pseudorotation.

The kinetics of and the yield of OH in the reaction 3 CH 2 + O 2 has been studied at room temperature using pulsed laser photolysis of ketene at 351 nm with detection of OH by LIF. The kinetics were determined from a biexponential analysis of the growth and decay of the OH signal. The rate coefficients obtained from the profiles for OH( v =0,1) are (2.39 ± 0.26) and (2.33 ± 0.28) × 10 12 cm 3 molecule −1 s −1 respectively. The rate coefficient for 3 CH 2 + NO was also measured from the growth of the OH( v =1) signal, giving k 3 = (4.52 ± 0.48) × 10 11 cm 3 molecule −1 s −1 . The ratio of the yields of OH in the two reactions 3 CH 2 +O 2 and 3 CH 2 +NO was determined from analysis of back to back OH profiles with O 2 and NO as co-reactant; the ratio obtained, at 300 K, was 2.96 ± 0.14. This result was combined with other experimental product yields on these two reactions to obtain a consistent set channel yields for 3 CH 2 +O 2 and 3 CH 2 +NO at ∼300 K.

A comparison of representative experimental data [296< T /K<1920; 10 3 < P /atm<10 1 ] and “theory” reveals some of the strengths and weaknesses of current understanding of the title reaction. The available data has been reconciled with a theoretically derived value of the high-pressure rate constant for methyl radical combination using a hindered-Gorin transition state using the Multiwell suite of codes.

Thermochemistry and chemical activation kinetics for the reaction of the secondary radical of 2-butanone, 2-butanone-3yl, with 3 O 2 are reported. Thermochemical and kinetic parameters are determined for reactants, transition states structures and intermediates. Standard enthalpies and kinetic parameters are evaluated using ab initio (G3MP2B3 and G3), density functional (B3LYP/6-311g( d,p )) calculations and group additivity (GA). The C–H bond energies are determined for the three carbons of the 2-butanone, showing that the C–H bond energy (BE) on the secondary carbon is low at 90.5 kcal mol −1 . The CH 3 C(=O)CH • CH 3 radical + O 2 association results in chemically-activated peroxy radical with 26 kcal mol −1 excess of energy. The chemically activated adduct can dissociate to butanone-oxy radical + O, react back to butanone-yl + O 2 , form cyclic ethers or lactones, eliminate HO 2 to form an olefinic ketone, or undergo rearrangement via intramolecular abstraction of hydrogen to form hydroperoxide and/or OH radicals. The hydroperoxide-alkyl radical intermediates can undergo further reactions forming cyclic ethers (lactones) and OH radicals. Quantum RRK analysis is used to calculate k(E) and master equation analysis is used for evaluation of pressure fall-off in these chemical activated reaction systems.

The unimolecular thermal decomposition reaction CHF 3 (+M)→CF 2 + HF (+M) was studied in shock waves by monitoring the UV absorption of the forming CF 2 radicals. The results of the present and previous experiments on the temperature and pressure dependence of the rate constants are analyzed in terms of unimolecular rate theory. Falloff curves (for 1500–1900 K) are represented in terms of fitted limiting low pressure rate constants k 0 = [Ar] 1.1 × 10 16 exp(−53.0 kcal mol −1 / RT ) cm 3 mol −1 s −1 , limiting high pressure rate constants k ∞ = 1.25 × 10 15 exp(−75.8 kcal mol −1 / RT ) s −1 from quantum-chemical calculations, and center broadening factors F cent = 0.170(± 0.04) including strong and weak collision contributions from unimolecular rate theory. With these results, approximate analytical expressions of the falloff curves well represent the measured and calculated rate constants over wide ranges of pressure and temperature.

The combustion chemistry of the two butane isomers represents a subset in a comprehensive description of C1–C4 hydrocarbon and oxygenated fuels. A critical examination of combustion models and their capability to predict emissions from this class of fuels must rely on high-quality experimental data that address the respective chemical decomposition and oxidation pathways, including quantitative intermediate species mole fractions. Premixed flat low-pressure (40 mbar) flames of the two butane isomers were thus studied under identical, fuel-rich ( φ =1.71 ) conditions. Two independent molecular-beam mass spectrometer (MBMS) set-ups were used to provide quantitative species profiles. Both data sets, one from electron ionization (EI)-MBMS with high mass resolution and one from photoionization (PI)-MBMS with high energy resolution, are in overall good agreement. Simulations with a flame model were used to analyze the respective reaction pathways, and differences in the combustion behavior of the two isomers are discussed.

In the present work, the density distribution of rhodamine 6G ions (R6G) in the gas phase and the droplets of an electrospray plume was studied by spatial and spectral imaging. The intention is to contribute to the fundamental understanding of the release mechanism of gaseous R6G in the electrospray ionization (ESI) process. Furthermore, the influence of ESI-parameters on the release efficiency of R6G, e.g. solvent flow, R6G and salt concentration were examined via direct fluorescence imaging of R6G. A solvent-shift of the fluorescence maximum, λ max = 555 nm in methanolic solution and λ max = 505 nm in gas phase, allows the discrimination between solvated and gaseous R6G. Two experimental setups were used for our measurements. In the first experiment, the R6G fluorescence and the light scattered from the spray plume were imaged in two spatial dimensions using a tunable wavelength fil ter. The second experiment was designed for obtaining 1-dimensional spatially resolved emission spectra of the spray. Here, the intensity distribution of solvated and gaseous R6G as well as scattered light ( λ =355 nm) were measured simultaneously. The results show the distribution of gaseous R6G in the plane, orthogonal to the ESI capillary, decreasing slightly towards the spray center and showing maxima at the cone margins. The distribution of gaseous R6G confirms the preferred release of gaseous ions from nano-droplets, indicating the ion evaporation model (IEM) to be the dominating release mechanism. Up to now, only a few fluorescence spectra of ionic compounds in the gas phase were published because the measurement of emission spectra of mass-selected ions in an ion trap is experimentally challenging. The fluorescence spectrum of gaseous lucigenin at atmospheric pressure is reported for the first time. This spectrum of lucigenin in the gas phase exhibits a blue shift of about Δ λ =10 nm in comparison to the corresponding spectrum in methanol.

Dynamical implications of the dissociation of a molecular ion under the influence of interferometrically generated light fields have been investigated by numerical solution of time dependent Schrödinger equations. As an example the dissociation of DCl + ions by means of two interfering 7 fs laser pulses at 800 nm has been chosen. We demonstrate that product branching ratios D + :Cl + can be manipulated from 10:1 to 1:10 not only by adjusting the appropriate delay time in the time-shifted two-pulse approach, but also by choosing the proper carrier envelope phase (CEP) of the two partial light fields. The effects of the phase shift related to the time shift and that of the CEP can be clearly distinguished.

A comparison of combined IR and UV/Vis pump-probe measurements of diphenylcyclopropenone (DPCP) and diphenylacetylene (DPA) provides clear evidence that the photodecarbonylation of DPCP following excitation to its S 2 -state proceeds non-adiabatically to the electronic ground state of DPA. It is shown that transient absorption of electronically excited DPA is caused exclusively by photoexcitation of ground state DPA generated by preceding DPCP photodecarbonylation.

The HO 2 radical yield in the OH initiated oxidation of SO 2 has been determined by direct observation of HO 2 concentration time profiles following the 248 nm photolysis of H 2 O 2 /SO 2 /O 2 mixtures. Initial OH radical concentrations have been deduced from a fit of the absolute HO 2 concentration time profiles after 248 nm photolysis of H 2 O 2 in the absence of SO 2 , an increase in the HO 2 concentration upon addition of SO 2 to this reaction mixture is observed and can be explained by a decrease of HO x -radical losses due to a faster decay of OH radicals in the presence of SO 2 . Simulations of theses profiles using recommended rate constants in a simple model are in agreement with an HO 2 -yield of 1.0 ± 0.1 from the OH initiated oxidation of SO 2 .

Ethyl iodide is a well known H atom precursor in shock tube experiments. In the present work, we study peculiarities, when C 2 H 5 I is used under conditions, where its decomposition is not longer fast compared to consecutive bimolecular reactions. On the basis of shock tube experiments with detection of H and I atoms by resonance absorption spectrometry, accompanied by quantum chemical (CCSD(T)/6-311G//CCSD/6-311G) and statistical rate theory calculations, we propose a small mechanism (5 reactions, 7 species) and kinetic data, which allow an adequate description of C 2 H 5 I pyrolysis as a H atom source down to temperatures between 950 and 1200 K at pressures ranging from 1 to 4 bar: C 2 H 5 I→C 2 H 5 + I (1), k 1 = 9.9 × 10 12 exp(−23200 K/ T ) s −1 ; C 2 H 5 + M→C 2 H 4 + H + M (2), k 2 = 1.7 × 10 −6 exp(−16800 K T ) cm 3 s −1 [D. L. Baulch et al ., J. Phys. Chem. Ref. Data 34 (2005) 757]; C 2 H 5 I→C 2 H 4 + HI (3), k 3 = 1.7 × 10 13 exp(−26680 K T ) s −1 ; H + HI→H 2 + I (4), k 4 = 7.9 × 10 −11 exp(−330 K T ) cm 3 s −1 [D. L. Baulch et al. , J. Phys. Chem. Ref. Data 10 (Suppl. 1) (1981) 1]; C 2 H 5 I + H→C 2 H 5 + HI (5), k 5 = 7.0 × 10 −9 exp(−3940 K T ) cm 3 s −1 . The latter bimolecular abstraction step turned out crucial for an adaquate d escription of the hydrogen atom concentration-time profiles in the above mentioned temperature and pressure range for initial concentrations [C 2 H 5 I] 0 > 2 × 10 13 cm −3 corresponding to mole fractions > 1 ppm.

There has been a lot of speculation about the role of water in gas phase reactions involving neutrals, radicals and ions. The reaction of NO and OH has attracted a lot of attention in the past due to its relevance for ozone chemistry in the atmosphere. In the present contribution we report low temperature measurements of the recombination of OH and NO at low temperatures in Laval nozzle expansions between 300 K and 60 K. We find an increase of the bimolecular rate constant in the presence of water of up to 40%. This effect has been attributed to water molecules acting either as an efficient collider releasing energy from the intermediate (in collisions) or – which is more likely for the present experimental conditions – as a cluster partner of the reaction intermediate HONO that also dissipates energy via cluster dissociation, which can in turn both stabilize the reaction intermediate, decrease back reaction to OH and NO, and enh ance finally the overall reaction to the products. The supersaturation of water vapor in the cold Laval nozzle expansion strongly favors the formation of clusters in the nozzle throat; their exact concentration is, however, difficult to estimate due to non-equilibrium conditions. The possible role of clusters in the recombination of OH and NO is investigated using ab initio molecular dynamics calculations. Beyond the reaction intermediate HONO and intramolecular proton transfer events also transient HOON was observed in the theoretical study.