Volume 216, Issue 3 - 3

The technique of femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (fs-CARS) was used to study the vibrational dephasing dynamics in hydrogen-bonded (pyridine + water) complexes as a function of pyridine mole fraction x (Py). By detecting the spectrally resolved CARS signal, a mapping of the vibrational coherence dynamics of ring modes at ∼990 and ∼1030 cm -1 was achieved. The quantum beatings among different modes of the hydrogen-bonded network are clearly exhibited in the transient signal. Its spectral analysis yields the involved frequencies by employing FT methods to the time domain signal. The oscillatory pattern in the CARS transients are adequately explained when contributions from several modes are accounted for. The individual vibrational dephasing times ( T 2 ) in the range ∼1.7-5.3 ps were obtained with high precision. The assumption of homogeneous broadening of the ring modes under investigation is made for those mole fractions, where the pyridine molecules have a surrounding consisting mainly of only one species. According to the concentration profile this is valid for x (Py)=1.00 (neat pyridine) and x (Py)=0.20, for which the hydrogen-bonded network is dominated by PyW n species i.e . water-water hydrogen bonding). For both mole fractions the condition (Δν˜ 1/2 ) hom =1/π c T 2 , which relates line width in wavenumber from Raman data and dephasing time from time-resolved data, is fulfilled. Inhomogeneous broadening is assumed for the intermediate concentration x (Py)=0.60 since several distinct species, such as: Py 2 W, PyW, and PyW 2 are known to co-exist leading to complex chemical equilibria.

Transient stimulated Raman scattering (SRS) is a convenient method to extend the tuning range of short-pulse lasers. In this paper we present results obtained by using 400 nm pump pulses of 2.5 ps duration. After briefly reviewing earlier work on transient SRS in pure hydrogen and in H 2 /CH 4 mixtures, we discuss new experiments on transient SRS in mixtures of HD with hydrogen and deuterium. In gas mixtures additional components, corresponding to the sum- and difference-frequencies of the components of the individual gases, are observed. In HD/H 2 mixtures, up to 6% of the pump pulse energy can be converted into the strongest combination line (-1/-1). From the experimentally determined optimum partial pressure ratios a differential Raman-cross section of 3.0×10 -30  cm -2 /sr was derived for the HD molecule.

The Karhunen-Loève (KL)-decomposition is a standard method to analyze spatiotemporal patterns which are characteristic for non-linear chemical dynamics on surfaces. We apply the KL-decomposition to quantum dynamical problems. Using a variational principle it is shown how to arrive at the KL-expansion of an arbitrary complex valued function. In the case of an one-dimensional bound state motion, the KL-decomposition yields the eigenstates of the system in a straightforward manner. The time-resolved spectroscopy of an atom-molecule collision serves as another application. We demonstrate that the orthonormal decomposition into KL-modes provides an efficient tool to calculate long-time pump-probe signals.

Quinolinium cations and quinolinium betaines were investigated in the representative solvents water and acetonitrile at room temperature using stationary and time-resolved fluorescence spectroscopy ( S ingle- P hoton- C ounting-method). Experimental results reveal that sulfoalkyl- and carboxyalkyl-quinolinium compounds display a strikingly different behavior in the two solvents. Furthermore, the fluorescence lifetime depends on the length of the spacer for the sulfoalkyl compounds in acetonitrile and the carboxyalkyl compounds in water, respectively. This suggests an intramolecular interaction of the anionic headgroups with the quinolinium system in the excited state. To support this idea, different positions at the chromophore are substituted by a methylgroup in order to perturb the proposed interaction. With the intention to understand the dynamics of the postulated photoinduced electron transfer from the anionic group onto the excited quinolinium chromophore, semiempirical quantum chemical calculations were performed on the species using the PM3 hamiltonian including solvent effects by a self consistent reaction field (SCRF). We show that the Marcus theory of electron transfer may serve as a theoretical basis for a natural interpretation of the dynamic fluorescence quenching behavior.

The emission behaviour of the two polarity sensitive probes Prodan and Patman in phospholipid vesicles was studied as a function of the concentration of ethanol. Comparing the spectral shifts in both the symmetric lipid 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) showing a phase transition from a normal to a fully interdigitated gel phase and the strongly asymmetric lipid 1-stearoyl-2-lauroyl-sn-glycero-3-phosphatidylcholine (C(18):C(12)-PC) favouring a mixed interdigitated gel phase we show that the huge red shifts of Prodan in presence of higher ethanol concentrations cannot be easily attributed to a specific lipid phase transition. Rather, probe relocation and a pronounced increase in solvent relaxation (SR) as monitored by time-resolved emission spectra (TRES) in presence of ethanol contribute to the large shifts observable in both lipid systems in case of Prodan. While Patman exhibits a red shift caused by increased SR due to the ethanol induced formation of a fully interdigitated phase in DPPC, hardly any shift occurs in C(18):C(12)-PC, which is supposed not to undergo an ethanol-induced phase transition.

We present numerical simulations of the ozone concentration in the troposphere based on a six-variable model of tropospheric chemistry. Although the model is not meant to describe the detailed mechanism of tropospheric chemistry it contains nonlinear steps leading to oscillatory behaviour. Statistical noise imposed upon the emission intensity of nitric oxide may lead to the interesting phenomenon of "internal signal stochastic resonance". Here the imposed noise amplifies oscillatory dynamics of the system under conditions which lead to a stable stationary state in the absence of noise. A combination of statistical perturbations with a slow modulation of the rates of photosensitive steps allows to qualitatively reproduce observed time series of the ozone concentration.

A combination of density functional theory and multi-reference configuration interaction method (DFT/MRCI) is used to investigate the excited state intramolecular charge transfer (ICT) reaction for the ortho - and meta -cyano-isomers of {N,N}-dimethyl-4-aminobenzonitril (DMABN), 2-CDMA and 3-CDMA, and two di-cyano derivatives of DMABN, 3,4-DCDMA and 3,5-DCDMA. Experimentally, a red-shifted emission was only observed for DMABN and attributed to an ICT state. The other systems emit fluorescence solely from a locally excited state. The results of our computations show that the emission properties of all systems are best explained by the TICT (twisted intramolecular charge transfer) model: The first excited state of the investigated systems is of ICT character and its energy increases with increasing the twist angle about the donor-acceptor single bond. This finding is contrary to the stabilization in energy observed for DMABN. Also in contrast to DMABN, the dipole moment of both the first and second excited states increases for increasingly decoupled geometries, resulting in two highly polar twisted states. Consequently, only the TICT state of DMABN becomes the global minimum on the first excited state energy surface resulting in an ICT emission. The PICT model (planar ICT) is also discussed for explanation of the emission properties of this class of molecules. This model requires a small energy gap between the first and second excited state and the formation of a planar ICT state as the energy minimum of the first excited state. A significantly larger energy gap Δ E ( S 2 - S 1 ) between the first and second excited state found in the calculations for the investigated derivatives in respect to DMABN is in favor of the PICT hypothesis. However, the planarization energies are comparable for all systems including DMABN and this rules out the PICT model as a general explanation for the experimentally observed emission properties.

Oscillatory behaviour in the pH value was observed during the oxidation of sulfite by hydrogen peroxide mediated by hemin in a continuous-flow stirred tank reactor. The dynamics of this reaction was studied for a variety of flow rates of the reactants. As the flow rates increase, the oscillations evolve from relaxation oscillations to more complex shapes, displaying, among others, bursting behaviour. A reaction mechanism is proposed that involves the autocatalytic oxidation of HSO 3 - by H 2 O 2 , while equilibria between pH-dependent forms of hemin account for the consumption of protons. These equilibria provide the feedback steps which give rise to oscillatory dynamics. The contributions of the individual H + -consuming reactions to the dynamic behaviour are evaluated. Numerical simulations show that two pH-dependent reactions suffice to obtain bursting oscillations which resemble those observed in the experiments.