tailieunhanh - Optoelectronics Devices and Applications Part 5

Tham khảo tài liệu 'optoelectronics devices and applications part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Detection of Optical Radiation in NOx Optoelectronic Sensors Employing Cavity Enhanced Absorption Spectroscopy 149 Absorption spectra can be defined as the set of all electron crossings from lower energy levels to higher ones. They cause an increase in molecules energy. In case of the emission spectra there is inverse situation. The spectra correspond to the reduction of molecules energy as a result of electrons transitions from higher energy levels to lower ones. Scattering spectra rely on a change in the frequency spectra diffuse radiation in relation to the frequency of incident radiation due to the partial change of the photon energy as a result of impact with the molecules. However in this case there is no effect of radiation absorption or emission Saleh Teich 2007 Sigrist 1994 . 2. Principles of absorption spectroscopy Each gas molecule has a very characteristic arrangement of electron energy levels vibrational and rotational . As a result of light absorption particles go to one of the excited states and then in various ways lose energy. Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation as a function of wavelength due to its interaction with a sample. The sample absorbs energy . photons from the radiating field. The intensity of the absorption varies as a function of wavelength and this variation is the absorption spectrum Sigrist 1994 . Absorption spectroscopy is performed across the electromagnetic spectrum. A source of radiation and very sensitive photoreceiver is used which records radiation passing through the absorber sample. During the last several years absorptions methods for gas detection were significantly developed. The simple setup which shows the idea of absorption method is presented in Fig. 2. Fig. 2. The absorption method idea. An arc lamp LED Light Emitting Diode or laser emitting a wavelength matched to the absorption lines of the test gas could be applied as the source of radiation. If .