2.2. Raman experiment
The samples investigated were contained in 1800 ml quartz cuvettes
(Starna) and illuminated by the 266 nm line of a DPSS laser (FQCW 266-50) provided by CryLaS GmbH. The maximum excitation
power was approximately 55 mW. The laser was focused into
the sample using a set of quartz lenses and the scattered signal was
then recorded at a 90 geometry and dispersed using a single
monochromator (TRIAX 550, Jobin Yvon) using the 2400 grooves/
mm diffraction grating and an entrance slit width of 200 mm. For
signal detection an UV-enhanced back-illuminated CCD detector
(Symphony 3500, Jobin Yvon, France) was employed with liquid
nitrogen cooling and a chip size of 2048 1024 pixels. This system
yields a spectral resolution of about 3 cm1. The Raman spectra
were recorded in the range [400e3060 cm1]. Cyclohexane was
used to calibrate the Raman setup in both Ramanwavenumber and
spectral intensity as shown in Fig. 1. The baseline of each spectrum
was approximated by a fourth-order polynomial fit in order to
subtract the weak remaining fluorescence background. Computer
control of spectral recording and preprocessing was achieved using
commercial software (NGS LabSpec, Jobin Yvon). The Raman
spectrum using a near infrared excitation source at 785 nm has
been taken using a compact spectrometer (i-Raman, BWTek)
equipped with fiber optics for the excitation and the scattered light.
Raman spectra with excitations in the visible spectral range (488
and 514.5 nm) were obtained using the same single monochromator
as for UV Raman, but with a grating having 1200
grooves/mm and an argon-ion laser as excitation source.