Development and application of infrared laser excitation spectroscopy with an array detector

在本研究中，我们的目标是构建一种能够检测高灵敏度红外发射的光谱系统。到目前为止，红外发射主要是用傅里叶变换光谱方法观测到的。然而，该方法主要局限于双原子分子等物种。由于红外区域的发射概率不高，我们考虑用激光激发光谱来观测诱导发射，即用红外激光饱和分子的振动-转动跃迁。本研究的优点之一是建立了一个红外阵列探测器来观测微弱的发射光谱。将InSb X56×256红外阵列探测器安装在35K的杜瓦瓶中，用COGITO-3系统读出数据，传输到工作站，用IRAF软件进行分析。该探测系统与光栅式光谱仪耦合，用于观测发射光谱；对于激发源，利用商用激光元件在12~80K的低温下工作，建立了红外半导体激光系统。该系统应用于吸收光谱分析，利用多光程池测量大气中N_2O分子的同位素丰度比。由于半导体激光器的输出功率不足以饱和振动-旋转跃迁，因此量子级联激光器_(QC)激光器被认为是激发源的候选。我们与Hamamatsu HOTONIKUSU开展了一项合作研究，以考察QC激光器的性能。结果表明，输出功率足够大(约100 mW)，并已应用于激光激发光谱分析。

In the present study we had an aim to construct a spectroscopic system to detect infrared emission with high sensitivity. So far infrared emission has been observed mainly by Fourier transform spectroscopic method. However, the method is limited to species such as diatomic molecules mainly. Since the probability of emission in the infrared region is not high, we considered laser excitation spectroscopy to observe induced emission, where an infrared laser is used to saturate a vibration-rotational transitions of molecules.One of advantages of the present study is to rise an infrared array detector to observation of weak emission spectra. An infrared array detector with InSb X56 x 256 elements was installed in a dewar of 35 K. Data from the detector were read out by using COGITO-3 system and transferred to a work station and analyzed by an IRAF software. The detection system was coupled to a grating spectrometer to observe emission spectrum.For excitation source, an infrared diode laser system was set up by using commercially available laser elements which operate under low temperature of 12-80 K. The system was applied to absorption spectroscopy by using multi-path cell to determine isotopic abundance ratios of the atmospheric N_2O molecule.The output power of the diode laser was not enough for saturating vibration-rotation transitions, so a quantum cascade laser_(QC laser) is considered as the candidate of excitation source. We carried out a co-operative study with HAMAMATSU HOTONIKUSU to investigate the performance of the QC lasers. It was found that the output power was enough large (about 100 mW) for the present purpose and application to laser excitation spectroscopy has been carried out.