Experimental Setup and Procedure for the Laser Optical Measurements
Experimental setup
Fig. 1 shows a scheme of the experimental setup. A pulsed laser system is used (Infinity-XPO laser, Coherent). The laser diode pumped Nd:YAG laser generates pulses with the fundamental wavelength of 1064 nm. The repetition rate is variable in the range of 0 to 100 Hz. The pulse is amplified by two Nd:YAG rods, the pulse energy accounts up to 500 mJ. After the passage of the pulse through the amplifying system, it is sent through the SHG (Second Harmonic Generation), an angle tuned BBO crystal (Beta Barium Borate). In the crystal, the infrared radiation undergoes doubling. The two beams of 1064 nm and 532 nm are sent through a second angle tuned BBO crystal (THG, Third Harmonic Generation) where they undergo a frequency mixing. The radiation produced at 354.7 nm is thereafter separated from the infrared and doubled radiation. The pulse energy rates up to 160 mJ.
Fig. 1: Experimental setup for spectrally resolved or time-resolved laser-induced fluorescence (LIF) measurements in different flames.
In a tunable Optical Parametric Oscillator (OPO, Type II), light at 354.7 nm is injected in a crystal, which provides light of two other wavelengths, the "signal" and the "idler". In the experiments, only the signal wavelength is used. The signal wavelength is tunable from 709.4 nm to 420 nm, dependent on the angle position of the crystal. Behind of the OPO, the SHG arrangement allows doubling of the OPO signal wavelength. Therefore, in the UV range the pulsed laser system is tunable from 325 nm down to 210 nm. At wavelengths from 325 nm to 210 nm the maximum UV laser energy of a single pulse is about 5 mJ. The pulse duration is 3 ns in average and the laser bandwidth of a single shot measures about 8 cm-1. The laser light is linearly polarized with its E-vector lying in the vertical plane, i.e. perpendicular with the detection axis.
The laser beam is focused by a convex cylindrical lens and crosses the first flame (methane/air Bunsen burner, hydrogen/oxygen welding torch or acetylene/oxygen welding torch). The second flame and ICCD camera is just for two-dimensional spatial-resolved images and not for measurements of excitation emission spectra (EES).
Two intensified CCD cameras (ICCD) were used for detection (FlameStar, LaVision). The cameras include an UV-sensitive gateable microchannel plate image intensifier (I/I) and a slow scan CCD sensor. The ICCD camera 1 delivers information about wavelength and position of excited molecules using an UV-light permeable objective, focal length 105 mm and f/4.5 aperture (UV Nikkor, Nikon) and an imaging spectrograph (250IS, CHROMEX). The spectrograph is a 250-mm-focal-length, f/4.0 aperture ratio model equipped with three different gratings. In these tests the gratings with 1200 grooves/mm, blaze wavelength 200 nm and 100 grooves/mm, blaze wavelength 450 nm were used. The spectrograph is mounted with the entrance slit (50 mm to 100 mm) oriented parallel to the laser beam's propagation direction . The spectral system response was calibrated using a mercury argon lamp, a xenon lamp, a krypton lamp and a neon lamp.
The second ICCD camera permits spatial-resolved and time-resolved 2D-images of the flames. Rayleigh scattering light was separated from LIF signal with UV transmitting black glass filters (UG11 and UG5, Melles Griot).
The two CCD control units and two I/I control units are controlled by a personal computer (PC camera). With a second computer (PC laser), the laser system is managed and the crystals in the OPO and SHG are turned via a stepping motor control unit in order to vary laser output wavelength. The laser system provides a trigger signal for the camera system and the image intensifier (I/I units).
Experimental procedure
To record the excitation-emission spectra (EES), the laser beam is focused about 2 to 4 centimetres outside the flame front/reaction zone in order to avoid two-photon photo dissociation of H2O (Frodermann, 1996).
For each line of the EES 10 to 100 laser pulses were summed up and signal intensities of the reaction zone / flame front were averaged. The laser wavelength steps varry with the measuremets (0,01 nm to 0,04 nm steps). For more details see the measurement reports of each EES.
