The spectral analysis of the inhomogeneous signal obtained from femtosecond (fs) time-resolved degenerate four-wave mixing spectroscopy on iodine vapor with the help of CCD broadband detection is reported. It is shown with this spectral analysis that the broad band detection technique makes the simultaneous observation of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS), coherent Stokes Raman scattering (CSRS) and degenerate four-wave mixing (DFWM) processes possible. The fs lasers employed in our studies possess spectral widths which allow not only for a DFWM process but also other forms of four-wave mixing (FWM) processes such as CARS and CSRS to take place simultaneously in the molecular sample. These processes show themselves at different spectral positions within the FWM signal. The separation of the FWM signal with help of a monochromator and subsequent broadband detection with a CCD camera allow all the spectral components of the signal to be detected simultaneously under identical experimental conditions. As a result, the different FWM processes can be compared with one another directly and the continuous transition between these processes can be observed. Furthermore, the high quantum efficiency innate to a CCD camera allows for an evident improvement in the quality of the recorded transient FWM signal in comparison with detection with a fast photomultiplier.

We report on an experimental setup which has been developed to perform efficient nanosecond time-resolved coherent anti-Stokes Raman scattering (CARS) experiments using either the scanning CARS or the broadband CARS technique. A special electronic device has been constructed that allows the CARS probing laser system to be triggered with a variable delay after the excitation pulse of a nitrogen laser and furthermore prevents any temporal drifts of the nitrogen laser emission. The acquisition of the broadband CARS spectra is based on the recently developed scanning multichannel technique, which is applied in the present paper to CARS spectroscopy. Several examples of ground- and excited-state CARS spectra are shown to demonstrate the performance of the setup and to point out the benefits and limitations of the various measurement techniques. Moreover, we present CARS spectroscopic investigations of triplet relaxation processes in order to illustrate the feasibility of the designed spectrometer for nanosecond time-resolved CARS experiments.

There is, of course, a disadvantage because of the slightly increasing measurement time since the number of active elements on the multichannel detector is reduced. However, a combination of measurements with small middle slits in the low-frequency Raman region and middle or even wide slit width in the higher-frequency Raman region combined with the use of the full detector size can yield complete Raman spectra in much shorter times than is possible by means of single-channel detection. Appropriate slit setting could be made computer-controlled during a scanning multichannel measurement.

An experimental CARS set-up for resonant solid-state coherent spectroscopy is described. Resonance CARS spectra of one unsymmetrically (TSFBS) and two symmetrically (TS6, FBS) substituted diacetylene monomer single crystals, with a low content of polymer, were obtained with this apparatus. Particularly, spectra of the C-C and the C=C stretching vibrations of the polymer chains, recorded for different pump laser frequencies, were investigated. The occurrence of broad spectral CARS features which are non-resonant and Stokes shifted relative to other observed CARS lines is discussed.

This article shows that, by means of a conventional Raman spectrometer which has been carefully designed for near-IR Raman excitation, results can be obtained that are similar to those achieved with Fourier transform Raman instrumentation. We demonstrate the performance of such an apparatus on a variety of samples including examples of gaseous samples. As far as we know, up to now, no Raman spectra of gases have been published for excitation at 1064 nm. From a comparison of gas spectra excited at 1064 nm and 514 nm, we estimate the sensitivity of our apparatus to be at least three orders of magnitude lower than that for excitation in the green spectral region. Nevertheless, the main features in Raman spectra of gases can be recorded. High-performance solid-state spectra are compared with spectra recorded by means of FT techniques. They show superior sensitivity in the low-frequency range.