Karl-Heinz Gericke

Dynamics of H2O2 photodissociation: OH product state and momentum distribution characterized by sub‐Doppler and polarization spectroscopy

Hydrogen peroxide has been optically excited at a wavelength of 266 nm and the OH photofragment completely characterized by Doppler and polarization spectroscopy using the laser‐induced fluorescence technique. The entire internal state distribution (vibration, rotation, spin, and Λ components), translational energy, angular distribution, rotational alignment, and vector correlations between rotational and translation motions of OH products is measured. The hydroxyl radicals are formed in the X 2Π3/2,1/2 ground state with 90% of the available energy (248 kJ/mol) being released as OH recoil translation. The angular distribution is nearly a sin2 θ distribution about the electric vector of the photolysis laser. The internal motion of OH is vibrationally cold (no vibrationally excited OH was found) while the rotational excitation in v‘=0 can be described by a Boltzmann distribution with a temperature parameter of Trot=(1530±150) K. The two spin states are found to be populated nearly statistically, in contrast...

Photo induced three body decay

The photo induced three body decay : ABC hnu A B C, where a molecule ABC decays into three fragments A, B and C upon irradiation, is reviewed. Various experimental and theoretical techniques for the investigation of this reaction and their application to a wide range of molecular species are discussed. Emphasis is laid on the distinction between concerted and stepwise processes, consisting of one single or two consecutive kinetic events, respectively. The concerted fragmentation scheme is further classified as being of either synchronous or asynchronous character, depending on whether or not the bond breaking processes take place in unison. The three body decays of acetone, azomethane and s-tetrazine are discussed in detail as prototypes for these mechanisms. A novel kinematic analysis approach, based on the evaluation of fragment kinetic energy distributions, is presented and applied to the ultraviolet photodissociation of phosgene. Competing pathways are found to be operative, dominated by the asynchron...

Doppler spectroscopy of OH in the photodissociation of hydrogen peroxide

The photofragmentation of hydrogen peroxide has been investigated at an excitation wavelength of 266 nm using polarized laser beams. Doppler spectroscopy in connection with the laser induced fluorescence technique was applied to determine the nascent translational distribution of OH products. The hydroxyl radicals are formed in their electronic ground state, X 2Π3/2,1/2. The released energy is transformed almost exclusively in translation of these photoproducts. The Q1(4) transition in OH(X 2Π3/2→A 2Σ+) was used to study the anisotropic angular distribution of recoiling OH photofragments and analyzed to obtain information about the lifetime and symmetry of the excited dissociative state. The Doppler profiles for different directions of the electric field vectors of dissociation and probe laser relative to the detector axis have been determined for two different geometries. The angular distribution measured by Doppler spectroscopy, peaks in the direction perpendicular to the electric vector of the dissocia...

The influence of vibrational and translational motion on the reaction dynamics of O(1D)+H2(1Σ+g,v)

The OH product state distribution from the reaction O(1D)+H2(v)→OH(v‘,J‘,Ω,Λ)+H was determined by laser‐induced fluorescence (LIF) in the Δv=−3 band for v‘=3 and 4 with resolution of the J‘, Ω, and Λ sublevels. The rotational state population distribution is inverted strongly in v‘=3, weaker in v‘=4. There is a higher propensity for production of OH in the Π(A’) Λ‐sublevels. Vibrationally excited H2 was used for a part of the experiments. Excitation was achieved by stimulated Raman pumping (SRP). The population ratio of the vibrational states was determined to be P(v=3)/P(v=4)=3.5 for the reaction with H2(v=0) and 3.0 when there is H2(v=1) in the reaction chamber. Higher OH product states are populated than it would be expected from the mean available energy of the reaction. The translational energy of the reactants is transferred into OH rotation.

State‐to‐state reaction dynamics of R+HCN (ν1νl22ν3)→RH+CN(v, J) with R=Cl, H

Vibrational overtone excitation of HCN in the wavelength region 6 500 cm−1–18 000 cm−1 is used to initiate the endothermic reaction of chlorine and hydrogen atoms with HCN. HCN is excited to the overtone levels (002), (004), (302), (105), and (1115). The labeling of the vibrational levels (ν1ν2l2ν3) corresponds to the normal modes ν1=CN, ν2=bend, ν3=CH, and l2=vibrational angular momentum. The product state distribution of CN(X 2Σ+) is completely analyzed by laser induced fluorescence (LIF). Excitation of the first overtone of CH‐stretch leads to vibrationally excited CN in the reaction of Cl+HCN(002), implying the existence of a long living complex. The CN vibrational excitation increases with increasing H–CN stretch excitation. However, a slightly higher CN vibrational excitation is found when at the same internal energy of HCN three quanta of CN‐stretch and two quanta of CH‐stretch are excited. Therefore, the energy is not completely redistributed in the collision complex. The ratio of rate constants b...

State‐to‐state dissociation of OClO(Ã 2A2ν1,0,0)→ClO(X 2ΠΩ,v, J)+O(3P)

Applying the two‐photon laser‐induced fluorescence technique for nascent state resolved ClO(X 2ΠΩ,v,J) detection, the photofragmentation dynamics of OClO in the (A 2A211,0,0) and the (A 2A218,0,0) state is investigated at fixed photolysis wavelengths of 351 nm and 308 nm. In both experiments the product fragments are formed in their electronic ground states, namely ClO(2ΠΩ) and O(3P). A complete product state analysis proves the ClO radicals originating from the OClO(A 2A211,0,0) dissociation at 351 nm to be formed in v=0–4 vibrational states with a spin–orbit ratio of P(2Π3/2):P(2Π1/2)=3.8±0.5. The ClO fragment shows moderate rotational excitation. The obtained ClO product state distributions and the relatively high translational energy of the fragments can be explained by predissociation of the (A 2A2ν1,0,0)‐excited parent molecule in the course of which the initial symmetric stretch motion (ν1) of OClO is transferred into the dissociative asymmetric stretching mode (ν3). ClO line profile measuremen...

SPIN SELECTIVITY IN THE ULTRAVIOLET PHOTODISSOCIATION OF PHOSGENE

The ultraviolet photodissociation of phosgene in its first absorption band 1A2←1A1 was investigated by resonance enhanced multiphoton ionization and time of flight techniques. Nascent atomic chlorine fragments were observed and their state specific kinetic energy distributions were determined. Of the chlorine atoms 15% are produced in the excited 2P1/2 spin–orbit state with a mean kinetic energy of 3200 cm−1 compared to a value of 1500 cm−1 for the mean kinetic energy of the ground state chlorine atoms. The analysis of the kinetic energy spectra yielded evidence for a concerted three‐body decay. The formation of intermediate COCl is of minor importance in the dissociation process, the formation of a stable final COCl product can be excluded. Competing pathways on the upper potential energy surface are discussed. A significant excitation of the carbon monoxide CO fragments is predicted.

Reaction dynamics of C(1D)+H2(v)→CH(X 2Π)+H

The CH(X 2Π,v,J,Ω,Λ) product state distribution from the reaction C(1D)+H2(v)→CH+H was determined by laser‐induced fluorescence (LIF) where the B 2Σ–X 2Π transitions were probed. Most of the available energy is released as translation. A nearly thermal rotational distribution is obtained for CH(v=0,1). Only a small fraction, 4.1×10−4, of the CH products is formed in the vibrationally excited state. A higher propensity for the production of CH in the symmetric Π(A’) Λ sublevels is evident. For studying the influence of vibrational excitation on the reaction dynamics, H2 was excited to its first vibrational state via stimulated Raman pumping (SRP). H2(v=1) increases the reaction rate and enhances the population of higher rotational states, but diminishes the Λ selectivity. The vibrational population ratio P(v=0)/P(v=1) of the CH product remains unaltered. Insertion of the C(1D) atom into the H2 bond is the major reaction mechanism, but the probability for an abstractive process seems to increase when H2(v=1...

Effect of molecular bending on the photodissociation of OCS

At 230 nm, the photodissociation of OCS via a hot band and a triplet state was investigated by selective probing of high rotational levels of product CO (J=45–67) with photofragment imaging spectroscopy: OCS(v=0)+hν(UV)→CO(X 1Σ+,J⩾65)+S(3P), OCS(v=1)+hν(UV) →CO(X 1Σ+,J∼65)+S(1D). Additional two-photon IR excitation of the UV photoprepared OCS with intense 1.06 μm laser pulses bleaches the UV processes listed above and induces a new excitation channel of OCS: OCS(v=0)+hν(UV)→OCS*, OCS*+2hν(IR)→CO(X 1Σ+,J∼74)+S(1S). The bending mode of OCS in the excited states plays a central role in the excitation and dissociation dynamics. Additionally, the alignment effect of OCS by nonresonant infrared laser pulse, which appears on the angular distribution of the photofragment, is discussed.

3D-Resolved Investigation of the pH Gradient in Artificial Skin Constructs by Means of Fluorescence Lifetime Imaging

PurposeThe development of substitutes for the human skin, e.g., artificial skin constructs (ASCs), is of particular importance for pharmaceutical and dermatologic research because they represent economical test samples for the validation of new drugs. In this regard, it is essential for the skin substitutes to be reliable models of the genuine skin, i.e., to have similar morphology and functionality. Particularly important is the barrier function, i.e., the selective permeability of the skin, which is strongly related to the epidermal pH gradient. Because the pH significantly influences the permeation profile of ionizable drugs such as nonsteroidal anti-inflammatory drugs, it is of major importance to quantitatively measure the epidermal pH gradient of the ASC and compare it to that of genuine skin.MethodsUsing three-dimensional fluorescence lifetime imaging combined with two-photon scanning microscopy, we measured with submicron resolution the three-dimensional pH gradient in the epidermis of ASCs stained with 2′,7′-bis-(2-carboxyethyl)-5/6-carboxyfluorescein.ResultsSimilar to genuine skin, the surface of the artificial epidermis has an acidic character (pH 5.9), whereas in the deeper layers the pH increases up to 7.0. Moreover, the pH gradient differs in the cell interior (maximally 7.2) and in the intercellular matrix (maximally 6.6). Apart from the similitude of the pH distribution, the genuine and the artificial skin prove to have similar morphologies and to be characterized by similar distributions of the refractive index.ConclusionsArtificial skin is a reliable model of genuine human skin, e.g., in permeability studies, because it is characterized by a similar pH gradient, a similar morphology, and a similar distribution of the refractive index to that of genuine skin.