Guiyin Zhang

Emission spectrum and relaxation kinetics of SO2 induced by 266 nm laser

Laser induced fluorescence (LIF) emission spectrum of SO(2) in the range of 270.0-470.0 nm has been obtained with the quadruple harmonic output (266 nm) of a pulsed Nd:YAG laser as excitation source. The spectrum is composed of a continuous envelope in the short wavelength side, while it shows the character of banded structure superimposed on a continuous one in the long wavelength region. Fluorescence emission from the hybrid states of A(1)A(2)+B(1)B(1) and X(1)A(1)+B(1)B(1) forms the continuous envelope and phosphorescence emission from the triplet state a(3)B(1) forms the banded progression. It is also found that direct emission from laser excited states is very weak. The primary portion of the emission is from the energy levels populated by collision relaxation or collision induced intersystem crossing process. The harmonic frequencies and inharmonic coefficients of the symmetric stretching vibration and the bending vibration of X(1)A(1) state are derived from the ascription of the phosphorescence progression.

Laser-induced dispersive fluorescence spectrum and the detection of NO2

Laser-induced dispersive fluorescence (LIDF) spectrum of NO2 molecule in the spectral region of 508.3-708.3nm is obtained with the 508.0nm excitation wavelength. It is found that at low sample pressure the spectrum is composed of a banded structure superimposed on a continuous one. While the spectrum show itself as a continuous envelope centered at 630.0nm when the pressure with a higher value. NO2 molecules are excited to the first excited state A2B2 by absorbing laser photons. Owing to the strong interaction between X2A1~A2B2 and A2B2 ~ B2B1states, some excited molecules redistribute to X2A1 and B2B1 states by the process of internal energy conversion or quenching. This induces the fluorescence come from different excited states. Based on the experimental data, the vibration frequencies of the ground electronic state of NO2 molecule are obtained. They are ω1=(1319±12)cm-1, ω2=(759.8±0.7)cm-1,and ω3=(1635±29)cm-1. The optimum-receiving wavelength for detecting NO2 gas with the technique of LIDF is proposed.

Influence of Doppler effect on the phenomenon of electromagnetically induced transparency

Electromagnetically induced transparency (EIT) is a significant nonlinear optical phenomenon. Based on the theory of density matrix equation, we presented the influence of Doppler effect on EIT. A cascade type three-level system and Na atomic vapor is adopted during the course. The results showed that EIT is determined by Rabi frequency of the couple and probing field. It is independent of temperature usually. But when we take Doppler effect into account, it is found that the full transparency appeared at the condition of ΩP=0.01GHz, ΩC=1GHz will vary with temperature. An obvious transparent window can be observed only when the temperature is less than 50K. With the increase of temperature, EIT phenomenon disappeared quickly. At room temperature, we can see that the double peaks of Aulter-Townes will instead of the EIT transparent window as Rabi frequency of the couple field is larger than 1.5GHz.

Temperature effect on ultraviolet differential absorption cross section of SO2

When the technique of differential optical absorption spectroscopy (DOAS) is applied to the pollutant monitoring, the differential absorption characteristics of pollution gases will change greatly owing to the flue gas is often with high temperature. This will bring the influence on the detection results. This article mainly aims at the temperature effects for SO2 differential absorption cross section by recordings the absorption spectra. The results show that the differential absorption property changes dramatically with temperature. The differential absorption peaks in the region of 280.0-320.0nm decrease with the increase of temperature while the valleys will increase. So the entire differential absorption cross section decreases with the increase of temperature, but no wavelength drift and differential absorption structure change appear with temperature. By measuring the differential absorption cross section of a few peaks at different temperature, it is found that the reduction regularity at different wavelength is varied. The variation at 286.7nm, 293.9nm and 304.0nm with temperature is in a manner of cubic polynomial, while the variation at 300.0nm presents a nearly linear decline. When the temperature rises from 300K to 450K, the relative change of the differential absorption cross section at 286.7nm is 77.1%, while it can reach 84.0% at 300.0nm.

Competition between spontaneous radiation and ionization in the process of resonance enhanced multi-photon ionization

Photo-ionization probability is an important factor in the practical use of resonance enhanced multiphoton ionization (REMPI). To a certain experimental condition, it depends on the competition between spontaneous radiation and ionization of the excited particles. In this work, we investigate the influence of laser resonance detuning, Rabi frequency and ionization rate on spontaneous radiation and ionization in the process of REMPI with the theory of density matrix equation. A model of three energy level system is adopted. It is found that the spontaneous radiation and ionization probability increase with the decrease of laser resonance detuning. They get to the maximum when resonance detuning equals zero. The line width of spontaneous emission will decrease with the increase of ionization rate due to the competition between spontaneous radiation and ionization. In addition, the spontaneous radiation and ionization probability increase with Rabi frequency until gets to saturation. Laser resonance detuning has no influence on the saturation value. It only influences the Rabi frequency for saturation. If Rabi frequency increases further after saturation, the spontaneous radiation will decrease because of the phenomena of energy level splitting in strong laser field. Now that resonant absorption and large laser intensity can increase the ionization probability greatly, so we must select suitable laser frequency and large laser intensity in the practical use of REMPI, in order to get optimum detection result.

Investigation of Rydberg states of nitrogen dioxide with high-power laser

A survey of the Rydberg states of NO2 accessed in optical-optical two-color double-resonant (OODR) manner by the technique of multi-photon ionization (MPI) spectroscopy is presented. The pump laser is the double-frequency output of a Nd:YAG laser. While the probe laser is an optical parameter generator and optical parameter amplifier (OPG/OPA) pumped by the triple-frequency output 355nm of the former. The OODR-MPI spectrum of NO2 is obtained by scanning the probe laser in the range of 465-535nm under the condition that the pump laser is unfocused and the probe laser is focused on the center of the pump laser beam. The ionization peaks could be attributed to E2∑u←A2B2←X2A1(1+2) resonant transitions. This means that NO2 molecule is excited to the appropriate level of the first excited A2B2 state by absorbing one pump laser (ω1) photon. Then from the first excited state it should take three probe photons (ω2) and via final resonant E2∑u state for the ionization. The bending vibration frequency of NO2 E2∑u state obtained from above ionization spectrum is (608.6±2.2)cm-1. It is consistent with the literature.

Two-photon laser-induced fluorescence spectrum of NO

Two-photon laser-induced fluorescence spectrum(TP-LIF) of NO is obtained by using high power laser as excitation source. A few stronger band can be attributed to A2??X2 ? transition. Some molecule constants about NO which in the ground state are deduced by the spectrum. The nature radiation lifetime of NO which is in the excited state A2? is about 180ns by fitting the curve oflifetime verse pressure.