Implementation of Deep Ultraviolet Raman Spectroscopy

The present thesis, “Implementation of Deep Ultraviolet Raman Spectroscopy”, mainly contains two sections. Deep Ultraviolet here is named DUV standing for the wavelength range from 200 to 300 nm. The first section, Chapters 1, 2, 3 and 4, is about the instrumental technology of the DUV Raman system; the second section, Chapters 5 and 6, is devoted to some application examples of the DUV Raman spectroscopy.

Chapter 1 is a brief introduction of Raman spectroscopy in general and its relation to DUV. The DUV Raman spectrometry has many remarkable advantages. The major limitations are the difficulty of the DUV Raman instrumentation, the DUV excitation laser source, DUV optical components, eventual instability of the sample and the non-transparency of ordinary glass containers.

With respect to the first limitation, most lasers do not directly emit the DUV lines. The DUV lines can typically be obtained by means of frequency conversions that are implemented by optical nonlinear processes as well as Second Harmonic Generation (SHG) or higher order harmonic generations. Chapter 2 aims at investigating three main type lasers, semiconductor, solid-state and gas lasers, to recognize the different ways to implement the DUV emission.

The important optical properties as well as the laser beam quality, line width, emission wavelength, output power and pulsed / CW operation of each kind of laser were discussed to help to choose the best suitable light source. With respect to semiconductor lasers, the most compact DUV laser has been reported to be the use of the waveguide SHG technique with a 266 nm emission wavelength.

This deeper wavelength has become possible to be obtained due to recent developments of blue semiconductor lasers. However, at present the probably best commercial laser - at the price – is a DUV laser in which the line is achieved by a highly complex frequency quadrupling device. Such a laser is obtainable as a product from TOPTICA Photonics.

With respect to the solid-state laser possibility, we devoted ourselves to distinguish between wavelength tuneable and un-tuneable lasers. With respect to gas lasers, we have focused on introducing two types of lasers: hollow cathode metal ion lasers and Argon ion lasers. The hollow cathode metal ion lasers emit DUV lines directly, however one should consider the poor beam quality and the relatively weak output powers possible and only for particular applications.

A frequency-doubled Argon ion laser (95-SHGQS) manufactured in USA by LEXEL-lasers, Inc. was finally adopted for our project. The basic structure and technological principles of the laser are discussed in detail as these are important requisites to safely understand the physics needed to change among the several lines (257.3, 244.0 and 229.0 nm) possible for a frequency-doubled Argon laser.

Chapter 3 is devoted to establishing a DUV Raman system - made by Renishaw PLC - associated with the adopted DUV laser. The chosen Renishaw InVia Reflex spectrometer was the best spectrometer that could be obtained for the available amount of money. Compared with a traditional Raman system working in the range of visible or near Infrared, the chosen DUV Raman system requires more stringent conditions for the optical components.

The various DUV optical components have been described, with particular focus on obtaining high quality of the final measurements. This naturally involves themes such as spectral resolution, sensitivity, elimination of background noise, and so on. Compared to Raman spectra excited with visible light, the DUV excited Raman spectra tend to have a markedly lower spectral resolution.

The spectral resolution is an important factor to consider when using the DUV excited Raman spectroscopy. In line with this insight is the fact that we found a way to improve the knowledge on the spectral resolution of the DUV excited spectrum. A new method was invented during the project to determine the spectral resolution of Raman spectrometers by means of principal experimental factors and one equation.

This equation is described in detail in chapter 4, and the associated Mathematica program is detailed in the appendixes. A manuscript on the same subject has been accepted for Applied Spectroscopy. The DUV Raman system was established so that it is possible to apply several different excitation lines, and even some visible lines can be included.

The operating convenience was considered carefully to realize the most user-friendly Raman system. The laser safety was also discussed, because DUV light is a potentially extremely dangerous kind of invisible radiation, and several personnel protection methods have necessarily to be presented. The fluorescence interference is a persistent problem in Raman spectroscopy.

It was met before many times when the wavelengths of excitation lasers are located in the visible range, e.g. for petroleum product analysis. Deep Ultraviolet Raman spectroscopy applied to this research field was claimed to be able to solve the problem. Chapter 5 is devoted to gasoline analysis by the use of the DUV Raman spectroscopy.

Firstly, some sampling difficulties (absorption, condensation) are described. We have found a way to solve the problems, and our solution, using a special designed gas gap cell to obtain measurements of extraordinary high quality, are presented. The DUV Raman spectra of gasoline were excited by three different wavelengths, 257.3, 244.0 and 229.0 nm were measured.

The results showed that the spectra obtained by use of the 257.3 nm excitation line are useless because of strongly fluorescent interferences; the spectra of the 244.0 nm excitation contained less fluorescent backgrounds and obtainment of clear Raman bands were possible; finally the fluorescent interference was completely eliminated by use of the 229.0 nm excitation line.

Obviously, the spectra were different when using the 244.0 and 229 nm excitation lines. By study of the Raman spectra of Toluene / Pentane mixtures using the 229 nm excitation, it is clear that Toluene dominated spectra of the mixtures because of resonance Raman scattering. By study of the Raman spectra of Toluene / Naphthalene mixtures using the 229 nm excitation, the results showed that Naphthalene has an even stronger resonance.

It was concluded that a small content (~0.5%) of Naphthalene was able to mainly dominate the Raman spectra of the gasoline samples. It is virtually unimportant what the rest of the sample consisted of. The most intense characteristic band is located at 1381 cm-1. The Raman spectra of home-made artificial gasoline mixtures - with gradually increasing Naphthalene contents - can be used to determine the concentration of Naphthalene by use of the 229.0 nm excitation.

For the Raman spectra of gasoline excited by the 244 nm line, Toluene and Naphthalene have comparable contributions. The intensity of the respective characteristic bands at 1004 cm-1 and 1381 cm-1 indicate the relative concentrations of the aromatics, Toluene and Naphthalene, in the gasoline. Chapter 6 shows examples of other applications of DUV Raman spectroscopy, for instance for the illegal red food additive: Sudan I.

For this dye Raman spectra - useful to indicate an unwanted presence - could not be obtained with green or blue laser line excitation because of fluorescence interferences. Raman spectra of Sudan I - without fluorescence interferences - could be achieved by use of the 244.0 nm excitation. The DUV Raman spectrometry could thus be a potential detection method for Sudan I, as an illegal food colorant.

The fluorescence-free DUV Raman spectrometry was further applied to detect another illegal food additive, Melamine, in milk sample. It was shown that the DUV constitutes a more sensitive measurement method than traditional Raman spectrometry and realizes a direct detection in liquid milk. In another research field regarding solid state analysis of Germanium and Silicon Carbide nanocrystals, the application of the DUV excited Raman spectroscopy as a research tool has also been demonstrated and the results are only rather briefly discussed because the results have already been published.

Obviously many more uses can be predicted.