Femtosecond Cars Applications In Molecular Ultrafast Dynamics And Flame Temperature Measurements

With the development of ultrashort pulse laser technology, femtosecond science(including femtosecond chemistry, femtosecond physics, femtosecond biology, etc.) is widely studied. Femtosecond coherent anti-Stokes Raman spectroscopy(CARS) is an important nonlinear spectroscopy technology in femtosecond science, in which the Raman vibrational modes are excited by femtosecond laser pump and Stokes pulses, and then the time evolution of the Raman vibrational modes is detected by probe pulses. Femtosecond CARS not only can reflect the material microscopic molecular ultrafast dynamics process, but also can reflect the macroscopically molecular temperature information. Therefore the femtosecond CARS is an important tool for molecular dynamics research and gas combustion temperature measurement. In this paper, using the femtosecond time-resolved CARS, by optimizing the experimental parameters such as delay time bet ween pump and Stokes pulses, the polarization characteristics and the laser wavelength, the theoretical and experimental research about molecular ultrafast dynamics of the BBO crystals(β phase barium metaborate crystals), ethanol solution, distilled water, PMMA(Poly methyl methacrylate) sheets and rhodamine B water solutions and gas combustion temperature measurement was carried out.The properties of the quantum beats were analyzed with a function for quantum beats. The effects of nonresonant background on the performance of the quantum beats are discussed. If the quantum beats only are Fourier transformed in time domain with a little or little nonresonant background, the signal to noise ratio in the Fourier transform power spectra of the quantum beats signal will be enhanced. The best delay time is on the first peak of the time-domain signal. The time-domain CARS signals of BBO crystals and PMMA sheets were Fourier transformed after the best delay time. The difference frequency of the Raman modes in them was obtained.The vibrational dynamics in PMMA sheets and BBO crystals were investigated by femtosecond time-resolved CARS. The C-H stretch modes in PMMA sheets(at 2870 cm-1 and 3008 cm-1) are excited and detected. The Raman modes of B-O stretching vibrations at 1214 cm-1 and 1437 cm-1 in BBO crystals were obtained at room temperature. The dephasing times for the four vibrational modes are obtained by fitting function of quantum beats.The Raman vibrational modes can be changed by wavelengths of the laser pulses. The ultrafast dynamics process of ethanol solution was studied by using femtosecond time-resolved CARS. The three Raman modes of C-H stretching vibrations from 2800 cm-1 to 3000 cm-1 in ethanol were excited and successfully investigated at room temperature. The three Raman vibrational modes of ethanol are around 2973 cm-1, 2927 cm-1 and 2878 cm-1. The coherence relaxation times for the three Raman vibrational modes of C-H stretching vibrations in ethanol were measured.The femtosecond time-resolved CARS was performed to investigate the vibrational dynamics in distilled water. The ultrafast dynamics process of the OH-stretching modes between 3100 cm-1 and 3700 cm-1 in water was obtained and analyzed. The dephasing times of four Raman vibrational modes in water between 3100 cm-1 and 3700 cm-1 were detected and compared.Femtosecond time-resolved CARS has emerged as an attractive method for studying the vibrational dynamics of Raman vibrational modes in time-domain. Multiple Raman vibrational modes can be coherently excited simultaneously due to the spectrally broad femtosecond laser pulses. The restriction of the femtosecond CARS resolutions is especially severe in the case of biological macromolecule, the separation and recognition of individual Raman modes become rather challenging. As three laser pulses are used for the generation of the CARS signal, many degrees of freedom can be varied, such as the delay time between pump and Stokes pulses and the polarization characteristics.The ultrafast vibrational dynamics in ethanol and rhodamine B water solutions were investigated by spectrally dispersed femtosecond time-resolved CARS where we combined both wavenumber and time resolution. Many Raman vibrational modes of rhodamine B dye molecules which are from 300 cm-1 to 1800 cm-1 was selectively excited by changing the delay time between pump and Stokes pulses. The Raman modes of C-H stretching vibrations from 2700 cm-1 to 3500 cm-1 in ethanol were obtained at room temperature. The Raman vibrational modes of C-O and C-C stretching vibrations from 800 cm-1 to 1100 cm-1 in ethanol were also obtained. This technique allows one to track and determine the wavenumber of the excited Raman transitions. The ratio of intensity between Raman vibrational modes in BBO crystals can be changed by the polarization of pump pulses or crystal geometries.Femtosecond CARS can be obtained in the femtosecond time domain information about the ultrafast process, capture the transient change information. Especially in recent years, the developed femtosecond time-resolved CARS can be used to measure the atmospheric pressure nearly adiabatic flame temperature, which has greatly improved in the temperature measurement accuracy and range than nanosecond CARS measurement.Femtosecond CARS was utilized to overcome most of the problems associated with nanosecond CARS recently. Femtosecond CARS has many of advantages such as high spatial resolution, high temporal resolution and high sensitivity, etc, which is widely utilized to measure the methane/air flame temperat ure at atmospheric-pressure. The femtosecond time-resolved and single-shot CARS signals of the nitrogen molecule were measured and then simulated the theoretical results with a simple model.The effects of laser parameters on temperature measurements are discussed. Timing jitter is added to the pump/probe pulses and Stokes pulses. In 2000 K, the results indicate that timing jitter of 10% lead to less than 2% error for temperature measurements. In the higher temperature measurement, the impact of the error in laser parameters is greater.The methane/oxygen/nitrogen flame temperatures 300 K and 1325 K are extracted from the comparison of theoretical and experimental data(femtosecond time-resolved CARS) by least-square fit. The procedure for fitting theoretical spectra to experimental spectra is explained. The experimental results show good agreements with theoretical ones and present a good repeatability.The influence of the broadening of the probe beam on nitrogen femtosecond single-shot CARS was theoretically calculated. Femtosecond single-shot CARS was utilized to measure the flame temperatures 300 K and 1000 K at atmospheric-pressure. Femtosecond single-shot CARS temperature measurement speed reached a millisecond level.