Design And Optimization Of Highly Sensitive Resonance Raman System

Resonance Raman spectroscopy(RRS)can improve Raman intensity effectively.When excitation wavelength is chosen to match the electron absorption energy level of target molecules,the Raman intensity of some vibration modes will be enhanced exponentially,so more and more molecular information can be obtained from Raman spectra.Due to significant differences on the highly sensitive detection of trace substances between RRS in visible region and in ultraviolet(UV)region,the thesis will introduce the instrument construction and detection study in these two different wavelength regions respectively.As for RRS in visible region,the enhancement ratio is relatively low which accompanied with strong fluorescence interference.It usually needs surface-enhanced technology to improve the ratio and solve the fluorescence problem.The ideal enhancement way is achieving the resonance of electron energy levels for target molecules and surface plasmons resonance(SPR)of substrates under the same excitation wavelength.However,the traditional methods to obtain the best resonance wavelength,such as absorption spectra,extinction spectra and scattering spectra,have limitations or misleading results.Therefore,highly sensitive detection can be achieved by the continuously wavelength-tunable(CWT)Raman spectrometer.On account of above requirements,a CWT micro-resonance Raman system will be introduced in the first part,which equipped with unique reflecting lens and filter linkage optical path,so Raman excitation optical path remains unchanged when turning filters to adjust wavelength,which can input laser lights with different wavelengths and output different Raman signal lights.The tunable band length is 45 nm;The CWT output laser is originated from a supercontinuum white light laser and a self-constructed monochromator.The self-constructed CWT micro-resonance Raman system has the following characteristics:highly adjustable precision;good stability of wavelength and power;good pointing control in tunable wavelength range.Except for that,we test and optimize the performance of this system by solid sample Si wafer,liquid sample CCl4.Several parameters including CCD width(rows number),slit size and slit position are reset and optimized.The malachite green(MG)dye molecules are detected by wavelength scanning process,618nm is MG molecules’ best resonance wavelength.The results show that the best resonance wavelength and high detection sensitivity of samples can be obtained by wavelength scanning detection using this CWT microresonance Raman system.As for the ultraviolet resonance Raman spectroscopy(UVRRS),especially in deep UV region,it is a highly sensitive characterization method which can not only selectively obtain the strongest Raman signal of target molecules,but also perfectly avoid fluorescence interference by exciting in UV region,so UVRRS is widely applied in analysis and detection of environmental pollution,food safety,biological macromolecules and other fields.However,under a long-term exposure of UV laser,the molecules will be photolyzed obviously,which is hard to gather the chemical"fingerprint" information and obtain the semi-quantitative analysis results.If the photolysis problem generated by UV light can be optimized under the conditions of laboratory,which will strongly promote the application and promotion of UVRRS technology in detection and analysis.Therefore,in the second part of this thesis,we study the UVRRS of nitrobenzene aqueous solution by a deep UV microscopic resonance Raman system with 266 nm laser,which was independently constructed by our group.After optimizing sample cell,the spectral analyzing results prove that the improved sample cell can avoid photolysis problem effectively.After comparing the Raman spectra of 0.1 μM/L nitrobenzene exciting by different lasers,the results proves the system can realize in-situ monitoring of nitrobenzene in drinking water.Moreover,for further improving the efficiency of collecting Raman scattered light in more detection and analysis occasions by the spectrometer mentioned above.Firstly,we analyze the relationship between NA of objective lens,the rear aperture length D and Raman scattering light collection efficiency,we find that the improvement space of collection efficiency is not very high.Meanwhile,we also find that Raman intensity ratio is equal to ratio square of focal length f,so we replace the 100 mm long-focus lens with a 45 mm short-focus lens.Through spectral analysis,it is found that the signal-to-noise(S/N)ratio of the improved Raman spectrum is not consistent with theoretical calculated value,the reason might be that the space angle of signal beam is close to the limitation of the spectrometer’s F number,and the aberration is caused by some light beam passing through the edge region of lens due to the incident angle.Finally,the Raman signal intensity can be improved by about 3.12 times through the optimization of optical path devices.