分步激发荧光检测大气氢氧自由基基础研究

OH radical is one of the most important object in forefront of scientific research on climate changes and atmospheric environment. Atmospheric OH radicals are so reactive and thin that it is very difficult to determine them in situ, only a few countries can do it. Fluorescence assay with gas expansion (FAGE) is of reliability and maturity. It consists of a pump laser and a dye laser to produce exciting light pulse scanning nearby at 308nm.The two lasers are big and heavy,which makes in-situ or aerial survey difficult. No available diode laser can perform the function in current OH detection device because there is no tunable diode laser in UV band or 616nm for frequency-doubled output.This proposal will employ series excition to excite the OH to first exciting electronic state and the resulting fluorescence to the ground electronic state will be recorded.Because of series excitation,the wavelengthes of exciting lasers lie in visible and infrared bands,which is a practicable way of replacing the two lasers in current device by diode lasers to reduce the volume and weight greatly.Moreover，it could eliminate artificial OH radicals in situ and avoid interference of exciting light, which exist in current devices using resonant fluorescence detection. Wavelength scanning for rotational state detection will be performed by a scanning Fabry-Perot interferometer following after the second exciting laser.The proposal is going to research exciting efficiency,conditions by series excitation and sensitivity for OH detection, as well as the detection ability for atmospheric OH radicals and replacing possibility of the two lasers by diode lasers.Exciting OH to higher vibration levels, precision scanning for rotational spectrum of the OH using a scanning Fabry-Perot interferometer and replacing the two lasers by diode lasers for OH detection are all original ideas proposed by the proposer.

气候变化和大气环境研究前沿几乎都与大气 OH 有关。大气中OH 的浓度低，活性高，现场检测十分困难，仅有几个国家能够在野外检测。其中采用激光诱导荧光原理的检测数据可靠。仪器包含泵浦和染料两台激光器，因笨重难以野外和航测。本项目提出分步激发荧光检测OH的方法，激发光源波长落在可见和近红外波段，有可能用半导体激光替代两台大型激光器，缩减体积和重量，便于野外和航测，也消除共振荧光装置中的人为产生OH和激发光干扰的影响。采用Fabry-Perot扫描干涉仪执行波长扫描，得到转动光谱支项，做为定性和定量检测依据。研究内容包括：分步激发荧光检测OH的激发效率、操作条件、检测灵敏度以及检测大气OH的可行性、半导体激光器替代的可能性。在大气OH自由基检测领域，本项目提出的高振动态激发、Fabry-Perot扫描干涉仪替代染料激光器做微区间扫描以及半导体激光替代两台大型激光器的思想，没有文献报道都是原创。

本项目针对气候变化和大气环境研究前沿急需的大气OH自由基检测仪器小型、便携、低造价的要求,进行基础性研究，提出分步激发荧光检测 OH 的方案，激发光源波长落在可见和红外波段，寻找半导体激光器替代现行共振荧光检测装置中两台大型激光器，缩减体积和重量，便于野外和航测，同时 消除共振荧光装置中的人为产生OH和激发光干扰的影响。. 项目实施中分期建造了两套OH自由基发生器，包括高纯氮干燥过滤及超声喷雾定量引入水蒸气的OH自由基标准发生源、一套Fabry-Perot扫描干涉仪和两套检测池及荧光检测器用于实验研究。实验中先后采用1430nm、979nm、351nm、392nm、449nm半导体激光光源以及LED光源，研究OH自由基浓度为10^9/立方厘米标准气体的分步激发的方案，得到OH自由基荧光信号。 研究表明：使用替代光源分步激发的方法是可行的，但是需要线宽窄的光源。目前缺乏窄线宽半导体激光器的条件下，还不能检测浓度低于10^9/立方厘米的OH自由基。.使用中科院化学所王玮罡课题组的可调谐激光器产生351nm激光，配合1430nm半导体激光器研究两步激发方案。由于使用的化学所激光器重复频率只有10Hz，308nm的荧光信号信噪比低、不稳定，数据重现性不好。又用北大环境学院朱彤、左澎实验室的飞秒激光器产生351nm紫外激光，研究新生振动激发态OH自由基的荧光信号，得到振动激发态的荧光计数信号。在低压汞灯电流为20mA，样品气中相对湿度大于80%时，新生振动态激发到电子激发态的荧光计数值在1-53/5min之间波动。计数值低、不稳定。原因是飞秒激光器谱线线宽约200cm^-1，尽管激光脉冲的功率很大，以至于空气几乎发生光致电离放电，由于所需的谱线线宽小于0.2^cm-1，其能量还是太低，致使荧光信号很弱。 .本研究得到以下结果：.1.在OH自由基标准发生器中采用超声喷雾方法定量向体系内加入水蒸气，从而达到控制生成的OH自由基浓度；.2.验证了分步激发荧光检测OH自由基使用替代光源的可行性；.3.依据本项目的研究和参考文献，得出结论：如果有了窄线宽的351和357nm的半导体激光器或者固体激光器，可以实现大气OH自由基检测仪器小型、便携、低价。