Environmental applications of cavity enhanced spectroscopy in the mid infra-red region

将在中红外区开发超灵敏和定量吸收光谱技术，以确定微量大气组分的混合比和同位素丰度。基于布里斯托尔集团最近工作的技术发展将被用于新一代分析光谱仪，该光谱仪基于最先进的红外线激光技术，波长超过3微米。在这一区域，各种分子的强基本振动跃迁可以通过光谱学获得，与目前围绕在近红外下工作的电信二极管激光器而设计的仪器相比，具有相当大的潜在优势，即提高了选择性和检测极限。该项目的基础是：(A)我们在3.25微米的腔增强光谱方面的最新进展，该光谱学使用基于紧凑、低成本的紧凑型低成本二极管激光器的高效差频产生(DFG)激光源；以及(B)正在进行的将7.8微米量子级联激光器(QCL)与光学腔耦合的项目，该项目使用光到激光器的反馈来提高耦合效率。PG专业的学生将专注于分析和环境应用。他/她将：(I)利用7.8微米波长的CW QCL激光器(由现有的PDRA利用现有设备设计和建造的装置)测试光反馈腔增强吸收光谱(OF-CEAS)光谱仪的性能，研究近光谱范围内的13CH4和12CH4吸收线，并优化碳同位素-13C值测定的精密度和准确度；(2)将3.2微米波长的DFG中红外光谱仪与先前开发的自动预浓缩装置(通过使用适当孔径的分子筛证明对乙烯高度专一)相结合，以探索作为一项原则验证研究对C2H4(在环境空气中的含量低于几ppbv)进行Delta-13C测定的可行性；(3)利用从(I)和(Ii)获得的专门知识，评估CEAS方法的可能性，如有必要，再加上预浓缩阶段，测量空气中各种更小的化合物(如N2O、C2H2)的同位素比率；(Iv)与R.P.埃弗雷德教授(布里斯托尔化学)和E.霍尼布鲁克博士(布里斯托尔地球科学)合作，在土壤类型的影响和甲烷营养细菌对大气甲烷的作用的研究中确定CH4 Delta-13C值；以及(V)探讨部署基于QCL的光谱仪以现场测量甲烷通量及其同位素组成的可行性。这种增量-13C值测定是确定大气源和汇的关键手段，如果能够证明具有竞争力的准确度和精密度，便携式分析光谱仪在尺寸、成本和易于部署方面比使用同位素比质谱仪具有相当大的优势。除了通过布里斯托尔化学研究生院提供的定期学术、研究和可转让技能培训外，PG学生还将接受以下方面的专家培训：激光和分子光谱学；光学和光学腔；大气和环境化学；分析光谱学；数据集分析；在布里斯托尔，研究生研究生将通过与由D.E.Shallcross教授和S.O‘Doherty博士领导的大气化学研究小组、有机地球化学组(由R.P.埃弗雷德教授和R.D.Pancost博士领导)以及跨系生物地球化学研究中心(包括来自化学、生物、地球科学和地理的小组)的互动(例如联合小组会议和研讨会)，获得对研究领域的广泛看法。学生将参加欧洲大气研究课程(ERCA，每年在格勒诺布尔举行)和计划中的腔增强光谱学研讨会(计划于2011年在加拿大举行)。

Ultrasensitive and quantitative absorption spectroscopy techniques will be developed in the mid infra-red region for determination of mixing ratios and isotopic abundances of trace atmospheric constituents. Technological developments based on recent work in the Bristol group will be exploited in a new generation of analytical spectrometers based on state-of-the-art laser technology in the infra-red at wavelengths beyond 3 microns. In this region, the strong fundamental vibrational transitions of a variety of molecules can be accessed spectroscopically, presenting considerable potential advantages of improved selectivity and detection limits over current instruments designed around telecoms diode lasers operating in the near infra-red. The project builds on: (a) our recent advances in cavity enhanced spectroscopy at 3.25 microns using efficient difference frequency generation (DFG) laser sources based on compact, low-cost diode lasers; and (b) an on-going project to couple a 7.8 micron quantum cascade laser (QCL) with an optical cavity using feedback of light to the laser to enhance the coupling efficiency. The PG student will focus on analytical and environmental applications. He/she will: (i) test the performance of an optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) spectrometer with the 7.8 micron wavelength cw QCL laser (an apparatus being designed and built by a current PDRA, using existing equipment) by studying 13CH4 and 12CH4 absorption lines in close spectral proximity, and optimizing the precision and accuracy of determination of carbon isotope delta-13C values; (ii) couple the 3.2 micron wavelength DFG mid-IR spectrometer with a previously developed and automated pre-concentration apparatus (proven to be highly specific to ethene through use of a molecular sieve of appropriate pore size) to explore the feasibility of delta-13C determinations on C2H4 (present at less than a few ppbv in ambient air) as a proof-of-principle study; (iii) with the expertise gained from (i) and (ii), evaluate the potential for CEAS methods, coupled to pre-concentration stages if required, for isotopologue ratio measurements for a variety of further small compounds in air (e.g. N2O, C2H2); (iv) establish a collaboration with Prof R.P. Evershed (Chemistry, Bristol) and Dr E. Hornibrook (Earth Sciences, Bristol) to determine CH4 delta-13C values in studies of the effects of soil type and the role of methanotrophic bacteria on atmospheric methane; and (v) to explore the feasibility of deployment of a QCL-based spectrometer for in situ measurements of methane fluxes and their isotopic composition. Such delta-13C value determinations are a key means of establishing atmospheric sources and sinks, and a portable analytical spectrometer has considerable advantages of size, cost and ease of deployment over use of isotope ratio mass spectrometry if competitive accuracy and precision can be demonstrated. In addition to the regular academic, research and transferable skills training delivered through the Bristol Graduate School of Chemistry, the PG student will receive expert training in: lasers and molecular spectroscopy; optics and optical cavities; atmospheric and environmental chemistry; analytical spectroscopy; analysis of data sets; preparation of data for publication, etc. Within Bristol, the PG student will gain a broad perspective of the research area through interaction (e.g. joint group meetings and seminars) with the Atmospheric Chemistry Research Group headed by Prof D.E. Shallcross and Dr S. O'Doherty, the Organic Geochemistry Unit (led by Prof R.P. Evershed and Dr R.D. Pancost) and the cross-faculty Biogeochemistry Research Centre (encompassing groups from Chemistry, Biology, Earth Sciences, and Geography). The student will attend the European Research Course on Atmospheres (ERCA, held annually in Grenoble) and a planned workshop on Cavity Enhanced Spectroscopy (scheduled for 2011 in Canada).