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.
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.
Analytic expression of the ionization probability about 3+1 resonance enhanced multi-photon ionization (REMPI) process is deduced with the theory of rate equation, which implies the interaction of photon and material. Based on the expressions, the influence of laser intensity, laser pulse duration and spontaneous radiation lifetime on the ionization probability is analyzed theoretically. It is found that the ionization probability increases with laser intensity and laser pulse duration until gets to saturation. After that, the ionization probability will oscillate around the saturation value if laser intensity increases further. The amplitude of oscillation increases with laser intensity at first, and then it will decrease even get to zero after a maximum peak comes out. We attribute the appearance of the oscillation to the phenomena of quantum coherence caused by the splitting of energy level in strong laser field. As to the fact that the ionization probability becomes to zero with the increase of laser intensity, it indicates that laser intensity is strong enough so as to make the neutral particles getting to the region of ionization suppression. It is also found that the variation of ionization probability with spontaneous radiation lifetime is far smaller than the one with ionization rate. So the influence of the spontaneous radiation lifetime on ionization probability could be ignored.
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.
The analytic formula of the ionization efficiency in the process of resonance enhanced multiphoton ionization is derived from the population rate equation. Based on this formula, the ionization efficiency of NO molecule, which is ionized via A2Σ, E2Σ intermediate resonant states and by (2+2) or (3+1) multiphoton process, versus laser intensity and pulse duration is analyzed by computer simulation. It is shown that the ionization efficiency of NO molecule increases with the laser intensity and pulse duration. When the photon flux is 2×1029photon•cm-2•s-1, all of the two steps in both processes are not get saturation as the pulse duration of the laser is 35ps. While the second excitation step is already saturated when the pulse duration is 6ns. And both of the two steps get saturation when the pulse duration is 50ns.Owing to the higher absorption transition cross section in the (2+2) process, the ionization efficiency via A2Σresonant state is with a much larger value than that of via E2Σstate. The ionization efficiency of NO molecule reaches saturation under lower laser intensity when it is ionized via A2Σresonant state. The optimum ionization pathway is decided when one detect NO by the technique of REMPI and with visible light as excitation source. It is the (2+2) multiphoton ionization process and via A2Σ intermediate resonant state. We wish the results can provide useful information for the detection of NO molecule.
The fluorescence lifetime of the excited NO2 molecules are observed experimentally by the technique of LIF time decay
spectrum. The results show that the time decay spectrum presents a property of bi-exponential. 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 B2B1←X2A1 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 deactivation mechanism of the excited molecules is investigated by measuring the
variation of fluorescence lifetime versus the sample pressure. The conclusion is that the excited molecules that
corresponding to the short lifetime quench mainly by the process of radiation and fast inner conversion. As to the excited
molecules with long lifetime, the deactivation process is not only radiation, but also the non-radiation process of
collision. At the same time, the optimum-receiving wavelength of 630nm for detection NO2 gas with the technique of
LIF is proposed by measuring the dispersive spectrum. Under the condition of standard atmosphere, a detection limit of
6ppb is obtained with the experimental apparatus.
The PA absorption property of NO2 is tentatively surveyed under the condition of room temperature. Laser radiation of
438.0nm is used as excitation source. It is shown that owing to the enhancement of V-T transfer energy with buffer gas
pressure, the intensity of the PA signal increases when the pressure of NO2 maintains at 665Pa and the pressure of the
buffer gas is increased. But the PA signal is almost invariable when the buffer gas pressure is more than 3×104Pa. The
quantity of NO2 is changeless are responsible for this phenomena. The PA signal shows itself as linearity variation with
NO2 concentration. Trace concentrations of NO2 are detected under ambient conditions. The detection limit of 6.4ppm is
obtained on the basis of SNR=1 with the homemade apparatus. But one can expect much lower value of the detection
limit of this method by improving the detection setup. The velocity of the sound in NO2 gas is measured from the PA
signal. It is about 270m/s. It also finds that the sound velocity varies slightly with the buffer gas pressure.
The absorption property of NO in the wavelength region of 420-470nm is surveyed by the technique of photo-acoustic
(PA) spectroscopy. It is found that NO molecule is excited though multi-photon process. The optimum excitation
wavelength in this region is decided when one detect NO with PA technique. It is 452.4nm or 429.6nm which
corresponding to the transition of NO A2Σ(v'=0,1)←X2Π (v=0). The variation of PA signal versus buffer gas pressure and
NO concentration is measured. It is shown that owing to the enhancement of V-T transfer energy with buffer gas pressure,
the intensity of the PA signal increases when the pressure of the buffer gas is increased. But the PA signal is almost
invariable when the buffer gas pressure is more than 5.32×104 Pa. The PA signal shows itself as linearity variation with
NO concentration. Under the condition of standard atmosphere, a detection limit of 5ppm is obtained on the basis of
SNR=1 with a homemade apparatus.
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.
Multiphoton ionization spectrum of NO in the wavelength region of 495.0~575.0 nm with a Nd:YAG laser pumped an
Optical Parameter Generator and Amplifier as excitation source is presented. The spectral bands are assigned by
measuring the variation of the ionization signal versus laser intensity together with calculation. The results show that NO
molecules are ionized via C2Π(v'=2,3), D2Σ(v'=2,3) intermediate resonant states and by (3+1) process in 495.0~535.0 nm
wavelength region. While the spectral bands come from the transition of NO molecule from the ground electronic state
to C2Π(v'=0,1), D2Σ(v'=0,1) and N2Δ(v'=1,2) intermediate resonant ones in 535.0~575.0 nm wavelength region. In this
region, NO molecule is ionized by (3+2) and (4+1) multiphoton process. The molecule constants about NO(C2Π, D2Σ
and N2Δ) states are calculated from the center wavelength of the spectral bands. It is also found that owing to the special electron configuration of NO, this molecule doesn't follow the normal transition selection rule of the diatomic molecule
during the multi-photon process.
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.
The resonance enhanced multiphoton ionization (REMPI) spectrum of SO2 in the region of 420~540nm is obtained with a picosecond Nd:YAG laser pumped an Optical Parameter Generator and Optical Parameter Amplifier as radiation source. The ionization pathway is analyzed. SO2 molecule is ionized though (4+1) or (4+2) process and via 4p, 5p and 6p Rydberg resonant states. The near quintic variation of the ionization signal versus laser intensity verified this conclusion further. So the spectral lines can be assigned to np Rydberg series. The adiabatic ionization potential and the quantum defect of SO2 are obtained based on the experimental datum, which is 99586 cm-1 and 1.85 respectively.
The resonance-enhanced multiphoton ionization (REMPI) spectrum of NO has been obtained in the range of 420-480nm with a Nd.YAG pumped optical parametric generator and amplifier. The spectral lines can be attributed to NO X2?(v'' = 0,1) ? A2?(v ' = 0,1) and X2?(v'"= 0) ?* E2?(v ' = 0~4)transitions. In this wavelength range, NO molecules are ionized via the resonant intermediate states A2?+ or E2? and by (2+2) or (3+1) REMPI process. The dependence of ion signals on laser intensity is in good agreement with these results.
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.