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

将在中红外区域发展超灵敏和定量吸收光谱技术，用于测定微量大气成分的混合比率和同位素丰度。基于布里斯托尔小组最近工作的技术发展将在新一代分析光谱仪中得到利用，该光谱仪基于最先进的红外激光技术，波长超过3微米。在这个区域，各种分子的强基本振动跃迁可以用光谱方法获得，与目前围绕电信二极管激光器设计的近红外仪器相比，在提高选择性和检测限制方面具有相当大的潜在优势。该项目建立在：(a)我们最近在3.25微米的腔增强光谱方面取得的进展，使用基于紧凑、低成本二极管激光器的高效差频产生（DFG）激光源；(b)一项正在进行的计划，利用光向激光的反馈，将7.8微米量子级联激光器与光学腔耦合，以提高耦合效率。研究生将专注于分析和环境应用。他/她将：(i)使用7.8微米波长连续波QCL激光器（目前PDRA正在设计和建造的设备），通过近距离研究13CH4和12CH4吸收谱线，测试光学反馈腔增强吸收光谱（of - ceas）光谱仪的性能，并优化碳同位素δ - 13c值测定的精度和准确性；（ii）将3.2微米波长DFG中红外光谱仪与先前开发的自动化预浓缩装置（通过使用适当孔径的分子筛被证明对乙烯具有高度特异性）相结合，以探索测定C2H4（在环境空气中小于几ppbv） δ - 13c的可行性，作为原理验证研究；（iii）利用从(i)和（ii）中获得的专业知识，评估CEAS方法的潜力，如果需要，再加上预浓缩阶段，用于测量空气中各种其他小化合物（如N2O， C2H2）的同位素比率；（iv）与R.P. Evershed教授（布里斯托尔大学化学系）和E. Hornibrook博士（布里斯托尔大学地球科学系）合作，在研究土壤类型的影响和甲烷营养细菌对大气甲烷的作用时，确定CH4 δ - 13c值；（五）探索部署基于量子计算机的光谱仪，就地测量甲烷通量及其同位素组成的可行性。这样的δ - 13c值测定是建立大气源和汇的关键手段，如果能够证明具有竞争力的准确性和精密度，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法具有相当大的优势。除了通过布里斯托尔化学研究生院提供的常规学术，研究和可转移技能培训外，PG学生还将接受以下方面的专家培训：激光和分子光谱；光学及光学腔；大气与环境化学；分析光谱;数据集分析；准备出版资料等。在布里斯托尔大学，研究生将通过与大气化学研究小组（由D.E. sharcross教授和S. O'Doherty博士领导）、有机地球化学小组（由R.P. Evershed教授和R.D. Pancost博士领导）和跨学院生物地球化学研究中心（包括化学、生物学、地球科学和地理学小组）的互动（例如联合小组会议和研讨会），获得研究领域的广阔视野。该学生将参加每年在格勒诺布尔举行的欧洲大气研究课程（ERCA）和计划于2011年在加拿大举行的腔增强光谱学研讨会。

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).