Development of Measuring CO_2 Concentration by Laser Induced Multiphoton Fluorescence

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In order to develop strateges for global warming, it is needed to develop non-intrusive measurement technique of CO_2 concentration. It can be applied, for example, direct injection engine system which reduces CO_2 for 30 percent. In the present study, we developed a measurement technique using multiphoton absorption type laser induced fluorescence. A high-power pulse YAG laser light of 355nm (3.5eV) was focussed into a cell, where prescribed CO_2 concentration gas with Nitrogen flows through it. The laser induced fluorescence spectrum intensity was detected by an monocromator and CCD diode camera equipped with an image-intensifier. CO2 molecules was excited into a higher level of C,CO,CO_+ and CO_<2+> and characteristic laser induced fluorescence spectrum was detected. The mechanism of LIF was made clear. Effect of CO_2 concentration on the laser induced fluorescence intensity was clarified. In addition, laser intensity and pressure on the present measurement technique was elucidated. The present technique requires laser pulse enery larger than 200mJ/pulse. The pressure condition needs not to be low and the presetn method could be applied in atmospheric pressure condition. The nitrogen is also breaks into N_+ by the high power laser and the magnitude of its spectrum reduces when concentration of nitrogen becomes low. Its phenomena could be applied to an advanced measurement technique without considering laser power fluctutation and sensitivity of CCD diode array since the ratio of N_+ to CO_<2+> becomes only a function of CO_2 concentration.