In situ measurements of the dioxins (PXDD) are critical issue for environmental conservation. We have been developing an in situ measurement system using non-resonant multi-ionization processes of femto-second laser radiation.

The femto-second laser (800-nm wavelength, 150- fs pulse width, 10-Hz repetition rate, and 10¹³-10¹⁵ W/cm² laser intensity), ionization chamber, time-of -flight mass-spectrometer, MCP ion detector and so on were used. Samples are injected into the chamber with a pulse valve system.  The samples are ionized by multi - ionization process with high- intense femto-second laser pulses. The ionized samples are accelerated by electrodes and detected by a micro channel plate giving TOF data after passing through a flight tube. 

In the experiments, the aromatic　hydrocarbons (chlorobenzene, pyridine, pyrazine etc.) were used as the samples to investigate the laser intensity and laser wavelength dependence of detection efficiency. The optimum laser intensities for ionization are 0.5-10×10¹⁴ W/cm². It has been found that the degree of fragmentation differs between samples.  To investigate the fragmentation process, we measured the light absorption spectrum of the ionized samples. Samples that have resonant -absorption-coefficients more than 200 M^-1cm^-1 at 800-nm wavelength are fragmented strongly while samples up to 30 M^-1cm^-1 are fragmented weakly. Below 30 M^-1cm^-1, the sample was ionized (with non-fragmentation). The key-factors for ionization of the samples are the laser intensities and the laser wavelengths.

From these results, we have the key-factors for controlling ionization of the samples. It would be possible to compose an efficient dioxins in situ measurement system with optimized laser intensity and laser wavelength.