Raman spectroscopy is a widely used analytical tool capable of providing valuable information about the chemical structure and composition of molecules. In order to detect substances also at a very low concentration levels, Surface Enhanced Raman Scattering (SERS) was introduced. The different amplification mechanisms result in extreme sensitivity, however, a quantitative use of SERS appears to be problematic. Especially, when deploying silver sols as SERS substrates, the reproducibility of the signal intensities for a given substance concentration is questionable. Experimental results of an investigation of this problem for low concentrations of adenine are presented. Comparison with results obtained for different SERS substrates by other authors reveals clearly different dependencies. Only in a very limited concentration range reproducible and therefore quantitatively utilizable data could be obtained.

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In this study, we have employed femtosecond time-resolved CARS (coherent anti-Stokes Raman scattering) spectroscopy in order to gain information about the molecular dynamics evolving on the electronic excited as well as ground state potential energy surfaces (PES) of iodine and bromine in the gas phase. The coherences of the wave packet in the temporal transients of the molecules excited by the ultrashort laser pulses are analyzed by means of fast Fourier transform calculations as well as by a simple model describing the purely rotational contribution. The latter is able to describe the rotational coherence seen for the excited but not for the ground-state dynamics. Information about vibrational dynamics is extracted from the transients reflecting the wave packet motion on both the ground and excited-state PES.

Raman spectroscopy is a common tool for the qualitative and quantitative chemical analysis of molecules. Although the unique identification of molecules is possible via their vibrational lines, high concentrations (mmol/l) are needed for their nonresonant excitation owing to their low scattering cross section. The intensity of the Raman spectra is amplified by the use of the surface-enhanced Raman scattering (SERS) technique. While the use of silver sols results only in a limited reproducibility of the Raman line intensities, lithographically designed, nanostructured gold surfaces used as SERS-active substrates should, in principle, combine the high sensitivity with better reproducibility. For this purpose, we have produced gratings of gold dots on Si(001) surfaces by means of electron beam lithography. Qualitative and quantitative investigations of crystal violet (CV) performed using nanostructured surfaces give high reproducibility and enhancement of the Raman lines. The substrates are reusable after cleaning; all results presented could be obtained from a single SERS substrate. For the experiments very low laser powers were used.

The population flow from the first excited singlet state (S 1 ) to the electronic ground state (S 0 ), facilitated through the S 1 -S 0 internal conversion, and the subsequent internal vibrational energy redistribution and vibrational cooling processes are monitored selectively with respect to the different forms of nuclear motion in all-trans-b-carotene. This is realized by exciting the molecule into the second excited singlet state (S 2 ) with an auxiliary pump laser pulse and interrogating the population recovery into the electronic ground state by means of a time-delayed, coherent anti-Stokes Raman (CARS) process. This spectroscopic scheme is referred to as a pump-CARS scheme in analogy to the classical pump-probe scheme in timeresolved spectroscopy. Here, the profound enhancement of the CARS signal intensity in the case of a Raman resonance to a specific vibrational mode is utilized as an intensity filter that amplifies the signal from the vibrational modes of interest, making the contributions from other vibrational modes negligible. This filter allows for the population flow in a specific vibrational mode to be monitored as the radiationless electronic transition between the S 1 and S 0 state takes place. This spectroscopic scheme opens up the possibility of identifying the vibrational motion with a large-amplitude motion in the C C double bond symmetric stretch as the primary acceptor of population from the S 1 state. A mechanism with which the other normal modes are populated is postulated.