Infrared polarization spectroscopy of CO2 at atmospheric pressure
The thesis work, aimed at the development of laser-spectroscopic detection schemes, consists of three parts, concerned with the development and application of different techniques. The development of UV filtered Rayleigh scattering (FRS) for thermometry and fuel/air ratio measurements, using a frequency tripled, singlelongitudinal-mode alexandrite laser at ∼254 nm together with an isotopically enhanced mercury filter, is examined first. Mercury has almost ideal features for serving as an atomic
... filter, having sharp cut-off slopes and a high extinction ratio, making it possible to effectively block unbroadened scattered light from particles, for example, and still let Doppler broadened light from the gas-phase pass through. The S6-model was utilized to model the Rayleigh-Brillouin scattering lineprofile. The FRS was employed in several applications, the thermometry measurements agreeing well with earlier measurements and calculations. Fuel/air ratio measurements were also obtained, various preliminary results being discussed. A second area considered was use of polarization spectroscopy (PS) to probe species relevant to combustion. The CH radical was studied in a low-pressure methane/oxygen flame and compared with laser-induced fluorescence (LIF) measurements acquired simultaneously. Atomic hydrogen was imaged at ambient pressure in a hydrogen/oxygen flame, utilizing a two-photon-pump polarization spectroscopy probe (TPP-PSP) approach in which a 243 nm laser beam was used to excite the atoms through a two-photon process to the 2s state. The 2s state was then probed by use of the 2s-4p transition through a PS scheme employing 486 nm. PS was also extended into the mid-infrared spectral regime and used to probe ro-vibrational transitions of species not accessible in the UV and visible spectral range. Use of infrared PS (IRPS) for detection of minor species such as OH and C 2 H 2 produced in flames, as well as of stable species such as CO 2 , CH 4 and H 2 O was demonstrated. The signal-to-noise ratio obtained was higher than for IRLIF. Finally, the development of planar LIF (PLIF) for CH involving improved detection sensitivity for a single-shot investigation of lean and turbulent premixed flames was reported. An alexandrite laser characterized by a long pulse duration (∼150 ns) and the possibility to operate both in a single-mode manner with a narrow bandwidth (∼100 MHz) and in a multimode manner with a broad bandwidth (∼8 cm -1 ), was utilized to excite the B-X (0,0) band. A sharp and thin CH layer was visible from Φ = 0.6 to Φ = 1.5 in methane/air flames. Finally, use of the technique in a lean and partially premixed methane/air flame in a co-axial jet flame burner was demonstrated. v List of papers I. Li, Z.S., Afzelius, M., Zetterberg, J., and Aldén, M., Applications of a single-longitudinal-mode alexandrite laser for diagnostics of parameters of combustion interest. Review of Scientific Instruments, 75:3208-3215, 2004 II. Zetterberg, J., Li, Z.S., Afzelius, M., and Aldén, M., Two-dimensional temperature measurements in flames using filtered Rayleigh scattering at 254 nm. Determination of surface normal temperature gradients using thermographic phosphors and filtered Rayleigh scattering. Applied Physics B-Lasers and Optics, 84:537-541, 2006 V. Kiefer, J., Li, Z.S., Zetterberg, J., Linvin, M., and Aldén, M., Simultaneous laser-induced fluorescence and sub-Doppler polarization spectroscopy of the CH radical. Optics Communications, 270:347-352, 2007 VI. Linvin, M., Li, Z.S., Zetterberg, J., and Aldén, M., Single-shot imaging of ground-state hydrogen atoms with a nonlinear laser spectroscopic technique. Optics Letters, 32:1569-1571, 2007 VII. Alwahabi, Z.T., Zetterberg, J., Li, Z.S., and Aldén, M., High resolution polarization spectroscopy and laser induced fluorescence of CO 2 around 2 μm. Detection of methane with mid-infrared polarization spectroscopy. Applied Physics B-Lasers and Optics, 79:135-138, 2004 IX. Li, Z.S., Rupinski, M., Zetterberg, J., and Aldén, M., Mid-infrared PS and LIF detection of CH 4 and C 2 H 6 in cold flows and flames at atmospheric pressure. Development of improved PLIF CH detection using an Alexandrite laser for single-shot investigation of turbulent and lean flames.