[Laser induced breakdown spectra of coal sample and self-absorption of the spectral line].

The LIBS of one kind of household fuel coal was obtained with the first harmonic output 532 nm of an Nd·YAG laser as radiation source. With the assignment of the spectral lines, it was found that besides the elements C, Si, Mg, Fe, Al, Ca, Ti, Na and K, which are reported to be contained in coal, the presented sample also contains trace elements, such as Cd, Co, Hf, Ir, Li, Mn, Ni, Rb, Sr, V, W, Zn, Zr etc, but the spectral lines corresponding to O and H elements did not appear in the spectra. This is owing to the facts that the transition probability of H and O atoms is small and the energy of the upper level for transition is higher. The results of measurement also show that the intensity of spectral line increases with the laser pulse energy and self-absorption of the spectral lines K766.493 nm and K769.921 nm will appear to some extent. Increasing laser energy further will make self-absorption more obvious. The presence of self-absorption can be attributed to two factors. One is the higher transition rate of K atoms, and the other is that the increase in laser intensity induces the enhancement of the particle number density in the plasma.

[Investigation on internal energy transfer and relaxation kinetics of NO2 by photoacoustic and fluorescence emission spectra].

With 532 nm laser as excitation source, the excitation and relaxation process of NO2 molecule was investigated by the technique of photoacoustic and fluorescence emission spectra. The results show that NO2 molecules will be pumped to the first excited electronic state by laser photon. When the sample pressure is lower, some of the excited molecules relax to the ground state by radiation process directly; the other parts are redistributed to a few of the excited rovibronic energy levels by the process of fast internal energy transfer. With the increase in the sample pressure, continual collisions dominate the relaxation process gradually. This makes the excited molecules to be redistributed to many excited rovibronic energy levels. Emission from these excited levels forms a continuous spectrum. Just then, the efficiency of fluorescence emission from laser excited level decreases and the fluorescence intensity on the long wavelength side increases. The intensity of PA signals increases also. These phenomena indicate that besides the relaxation process of radiation, there is a strong relaxation process of continual collision under the condition of higher sample pressure. It converts vibration energy of the excited molecules into translation one. This induces the increase in gas temperature and a sound wave is produced.

[Theoretical analysis of photoacoustic spectrum of NO molecule].

The technique of photoacoustic (PA) spectrum is based on the conversion of photon to acoustic energy by collision quenching of the excited molecules. It holds the characteristic of higher detection sensitivity, wide detection spectral region, no damage to the sample etc. It is used in many scientific observation areas such as gas composition analysis, research on chemistry and biology, environmental monitor and so on In the present paper, the analytic formula of the PA signal produced from the in teraction of intense laser with gas system was deduced by solving the dynamic rate equation about the interaction of photon and material. The results show that the magnitude of the PA signal depends on the factors of molecular absorption cross-section, laser intensity, photon number absorbed by the molecule and collision relaxation rate. With the aid of the relation of the PA signal versus laser intensity, the PA spectrum of NO molecule in the wavelength region of 420.0-470.0 nm is ascribed to the transition of X 2 pi (v" = 0) --> A 2 sigma (v' = 0, 1) and X 2 pi (v" = 0) --> E 2 sigma (v' = 2, 3, 4), F 2 sigma (v' = 1, 2, 3) and R 2 sigma (v' = 0, 1). These transitions are realized via two or three-photon process. The vibration constants of NO A 2 sigma, E 2 sigma, F 2 sigma and R 2 sigma electronic states were calculated from the wavelength of the spectral peaks. They are 2 346, 2 342, 2 397 and 2 381 cm(-1) respectively. The results are consistent with the one of other method. The phenomenon of saturation appears when the buffer gas pressure is high enough. This is owing to the finite excited molecules.

[Time decay spectrum of NO2 fluorescence radiation].

Investigation of the optical absorption and fluorescence of NO2 molecule has long been of interest because it is not only one of the key substances of air pollution, but also a stable molecule of nonzero spin and has many special properties such as that the vibronic levels of the first excited state are coupled strongly to the high vibration levels of the ground state, so that once NO2 molecules are excited, they must undergo complicated quenching process. The quenching mechanism influences the lifetime of the excited molecule severely. In the present paper, the fluorescence lifetime of NO2 excited electronic states are observed experimentally by the technique of LIF time decay spectroscopy and with an optical parameter generator and amplifier pumped by a Nd:YAG laser as excitation source. The results show that the fluorescence lifetime of excited NO2 molecules depends on the excitation wavelength and sample pressure. The time decay curves present a property of bi-exponential when the excitation wavelength is selected as 429.0, 452.0, 509.0 and 532.0 nm, respectively. This indicates that the fluorescence is composed of two components. One has a long lifetime, while the other has a short one. The short-lived component comes from the radiation of the molecules excited by A2B2, B2B1<--X2 A1 transition And the long one is owing to the radiation of the molecules excited to the high rovibronic levels of the ground electronic state. These levels are correlated with A2B2 state. The de-excitation mechanism of the excited molecules is investigated by measuring the variation in fluorescence lifetime versus the sample pressure. The conclusion is that the excited molecules corresponding to the short lifetime quench mainly through the process of radiation and fast inner conversion. As to the excited molecules with long lifetime, the de-excitation process is not only radiation, but also the non-radiation process of collision.