Browsing by Author "Assmann, J."

The operation of a liquid polymer jet laser-plasma target and the characterization of the absolute x-ray emission in the extreme ultraviolet wavelength window from 9-19 nm is reported. The target is a liquid polymer (perfluoro-polyether) that is exposed to pulsed and focused laser light at 532 nm in the form of a thin, liquid microjet (d=40 to 160 mum) in vacuum. The spectral brightness of the source in the 13 nm range is relatively high because a large fraction of radiative energy is emitted in one single line only, which is assigned to be the 2p-3d F-VII doublet at 12.8 nm, with a laser energy conversion efficiency of 0.45% (2pi sr, 2% bandwidth) in our initial experiment. A further increase of the relative emission has been found in the wavelength range between 7 and 17 nm when the jet diameter was increased from 40 to 160 mum. The two-dimensional spatial profile of the source plasma (d=40 to 50 mum) has been analyzed with a pinhole camera. (C) 2004 American Institute of Physics.

A time resolved study of intramolecular vibrational energy redistribution (IVR) in competition with intermolecular vibrational energy transfer (VET) of vibrationally excited methylene iodide (CH2I2) and allyl iodide (C3H5I) in solution is reported. Near IR-laser pulses between 1.7 and 2.4 mum selectively excited the molecules in the C-H-stretch overtone or combination bands and transient absorption at 400 nm monitored IVR on different ps timescales as well as VET to the solvent on a longer timescale. The transient absorption was calibrated against thermal absorption spectra at high temperatures measured in shock waves. With a simple model we have been able to determine global rate coefficients tau (IVR) for the intramolecular equilibration of vibrational energy in the range 12-22 ps for CH2I2 and 2-3 ps for C3H5I. The picture that emerges from these studies is that tau (IVR) for methylene iodide is only weakly dependent upon the excitation energy and the excited (zeroth order) mode and even less dependent on the solvent. The fact that it depends on the nature of the solvent at all supports our conclusion that IVR for CH2I2 is weakly solvent assisted, as opposed to C3H5I. In the case of allyl iodide we find considerably smaller values for tau (IVR) and a much faster decay due to VET with almost no dependence on the excitation energy and the solvent. Since IVR rates for both molecules do not scale at all with calculated densities of states, we conclude that the rate of intramolecular energy transfer in solution for both molecules in all solvents investigated in this study is still dominated by specific intramolecular interactions causing non-statistical IVR.

The light-induced (266 nm) ultrafast decarboxylation of two peroxides R-1-C(O)O-OR2, with R-1 = phenyl and R-2 = benzoyl or tert-butyl, in solution has been studied on the picosecond time scale by absorption spectroscopy with a time resolution typically of 100 to 200 fs. The reaction was investigated in various solvents of different polarity and viscosity to elucidate the influence of the solvent environment on the decarboxylation rate. Transient intermediate benzoyloxy radicals, R-1-CO2, were monitored at wavelengths between 300 and 1000 nm. While the primary dissociation of the peroxide is too fast to be resolved, the dissociation of intermediate benzoyloxy radicals is clearly detected on the picosecond time scale. The mechanism of light-induced two-step dissociation is discussed, as is the dependence of reaction dynamics on the type of substituent R-2 as well as the branching ratio between prompt and delayed CO2 formation. A model for the decarboxylation process is presented that is based on molecular structure parameters and energies. The latter quantities, which are obtained from density functional theory calculations, serve as input data for calculations of the specific decomposition rate coefficients of benzoyloxy intermediates via statistical unimolecular rate theory. The predicted benzoyloxy radical decay data are compared with corresponding experimental concentration versus time traces.

Femtosecond IR-pump-UV-probe spectroscopy allows us to directly observe the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of selectively excited aromatic molecules in solution (CF2ClCFCl2). In this article, we report global IVR and VET rate coefficients for benzonitrile, p-difluorobenzene, and pyrazine in a weakly interacting (nonpolar) solvent which are excited in overtones or combination bands of C-H stretch vibrations. The experimental findings are compared with recent results for benzene, toluene, and alpha,alpha,alpha-trifluorotoluene. While we have found a characteristic variation of the relaxation dynamics of the aromatic systems upon chemical substitution, the remarkable similarity of the time scales of IVR for a large group of molecules investigated is striking. Furthermore, the intermolecular vibrational relaxation is characteristically different for different chemical substitutions; however, the correlation of VET rates with the presence of low-frequency modes in the molecules is poor.

Intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of two alkyl iodides (CF3CH2I and CH3CH2-I) selectively excited in the two quanta overtone region of the CH stretch vibration (upsilon(CH) = 2) were measured in real time in solution. In this study, we have focused on the effect of chemical substitution on the mechanisms and time scales of IVR and VET of this family of molecules. With a simple model. we have obtained global IVR and VET rate coefficients for both molecules. The magnitude and the variation of the relaxation rates upon chemical substitution provide evidence for a survival of hierarchical IVR in these solvated molecules. which is governed by specific low-order intramolecular interactions and which can be rationalized with a simple low-order coupling model. At the same time, the general assumption that VET is simply dominated by the lowest-frequency modes in a molecule is not supported.

Competition between intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of excited methylene iodide (CH2I2) in solution has been measured in real time. After excitation of the C-H- stretch overtone and C-H- stretch containing combination bands of CH2I2 between 1.7 and 2.4 mum an increase followed by a decrease in the transient electronic absorption at 400 nm has been monitored. The transient absorption has been attributed to vibrational energy flow from the initially excited degrees of freedom to vibrational states with larger Franck-Condon (FC) factors for the electronic transition (long wavelength wing) and energy loss due to energy transfer to the solvent. A model based upon the dependence of the electronic absorption on the internal energy of CH2I2 has been used to determine the times for intramolecular vibrational energy redistribution and intermolecular energy transfer to the solvent. in the simplest version of our model the internal energy of the molecule probed by the population of the FC-active modes rises and decays exponentially on a picosecond (ps) time scale, which reflects the initial intramolecular vibrational energy redistribution and the subsequent energy transfer to the solvent. This simple approach was able to accurately describe the measured transient absorption for all solvents and excitation wavelengths. Overall time constants for IVR have been found to be on the order of 9-10 ps, almost independent of the excitation wavelength, the excited modes, and the solvent. In contrast, energy transfer to the solvent takes significantly longer. Overall time constants for VET have been determined in the range between 60 and 120 ps depending on the solvent, the excitation energy, but not on the mode which was initially excited.

Experimental and theoretical investigations on the ultrafast photoinduced decomposition of three tert-butyl peroxides of general structure R-C(O)O-O-tert-butyl with R = phenyloxy, benzyl, or naphthyloxy in solution are presented. Photoinduced O-O bond scission occurs within the time resolution (200 fs) of the pump-probe experiment. The subsequent dissociation of photochemically excited carbonyloxy radicals, R-CO2, has been monitored on a picosecond time scale by transient absorption at wavelengths between 290 and 1000 nm. The measured decay of R-CO2 is simulated via statistical unimolecular rate theory using molecular energies, geometries, and vibrational frequencies obtained from density functional theory (DFT) calculations. The results are compared with recent data for tert-butyl peroxybenzoate (R = phenyl). While benzoyloxy radicals exhibit nanosecond to microsecond lifetimes at ambient temperature, insertion of an oxygen atom or a methylene group between the phenyl or naphthyl chromophore and the CO2 moiety significantly decreases the stability and thus lowers the lifetime of the carbonyloxy radicals in solution to picoseconds. The reasons behind this structural effect on decomposition rate are discussed in terms of barrier heights for decarboxylation on the ground-state potential energy surface and of a fast reaction channel via electronically excited states of carbonyloxy radicals. Arrhenius parameters are reported for thermal rate constants, k(T), of R-CO2 decarboxylation as deduced from modeling of the time-resolved experimental data in conjunction with the DFT calculations.

Transient ferritosecond IR-pump-UV-probe spectroscopy is employed to investigate the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of benzene, toluene (CH3-C6H5), and alpha,alpha,alpha,-trifluorotoluene (CF3-C6H5) selectively excited in overtones or combination bands of C-H stretch vibrations in solution. Global IVR and VET rate coefficients are derived from the measured transient absorption profiles using a simple kinetic model. The study reveals the effect of a methyl rotor and the effect of methyl rotor fluorination on the mechanisms and time scales of IVR and VET in aromatic model systems. For the present case, it turned out that the methyl rotor in toluene is not simply an enhancer for IVR; however, its fluorination accelerates IVR significantly. These results suggest that the methyl rotor effect on an aromatic ring in solution is more subtle than expected from previous gas-phase studies. In particular, the corresponding relaxation rates in this case are not simply governed by the number of lowest order resonances, such as found for aliphatic molecules. Instead, in aromatic molecules also, the very large number of higher order anharmonic resonances may play a pronounced role. Because the IVR rates are not at all correlated with the total density of states, we conclude that intramolecular vibrational energy relaxation of a zeroth order C-H stretch overtone or combination vibration in these molecules is not in its statistical limit and that hierarchical IVR, such as known for isolated molecules, still survives to some extent in solution. Our results further suggest that VET rates are not always simply correlated with the lowest frequency modes of the molecules.