ABSTRACT
Isotope ratio measurements have been increasingly used in geochemistry, geochronology, cosmos chemistry and environmental science. Precise and accurate isotope ratio measurements are an important task in many applications such as the determination of isotope variations in geological and cosmic samples. Due to its high sensitivity, high ionization efficiency and high element selectivity, laser resonance ionization spectroscopy has nowadays become one of the key techniques, including isotope ratio measurements and trace amount analyses. Because of the isotope shifts and hyperfine structure, there is laser-induced isotopic discrimination effect in the process of laser resonance ionization. The different isotope ionization efficiency can affect precise and accurate measurement of isotope ratios. In the present paper, the dependences of the laser-induced isotopic discrimination effect on some of the laser parameters were studied by theoretical methods. Based on the numerical simulation of the population rate equations, laser-induced isotopic discrimination effect of lead isotopes was studied, by calculating laser resonance ionization transition "6s2 6p23 P0-6s2 6p7 s3 P1(0) --> ionization". The population rate equations was approximated considering some factors which affect the probability of laser resonance transition such as spectral lines width of laser and atom, isotope shifts and hyperfine structure. According to the approximated population rate equations, "1+1" laser resonant ionization process was employed to calculate the ionization probability of lead isotopes by means of computer simulation. The dependences of laser-induced isotopic discrimination effect on the laser parameters, such as laser central wavelength, bandwidth and intensity were investigated. The calculated results show that the laser-induced isotopic discrimination effect of lead isotopes could be almost eliminated by operating at optimized wavelength and could be lessened by using wide band laser. Saturated ionization of lead isotopes was achieved by using a high intensity laser, consequently the laser-induced isotopic discrimination effect of lead isotopes could be eliminated.
ABSTRACT
Resonance ionization spectroscopy (RIS) is an important technique for trace analyses. In the present paper, based on the population rate equations, "2+1", "1+1" and "1+1+1" multiphoton resonant ionization processes of Mn atom are studied by computer simulation. The phenomena and the threshold condition of saturated excitation and ionization processes are discussed. The threshold laser intensities for saturated ionization of Mn atom with different RIS schemes are calculated. With the typical Q-switched laser duration of 10 ns, the obtained saturation threshold laser intensity for the ionization steps in all the resonant ionization processes of Mn is about 10(8) W x cm(-2) with the exception of the "1+ 1" resonant ionization of Mn which is 3 orders of magnitude lower. The obtained threshold laser intensity for the saturated excitation steps is about 10(2)-10(3) W x cm(-2) in "1+1" and "1+1+1" ionization processes. It is also shown that the threshold laser intensity required for saturated excitation and ionization processes turns smaller with the laser duration prolonged.
ABSTRACT
Fluorescence techniques based on emission spectrum, synchronous fluorescence spectrum (SFS), or three-dimensional (3D) fluorescence spectrum, have been widely used in medical, biological and chemical analyses. In the present paper, 25 crude oil samples from 15 different borefields in the concentration range from 10(-4) g x L(-1) to 10 g x L(-1) have been investigated with different spectral approaches. It was shown that, compared with conventional emission spectrum based technique used in oil logging, the 3D fluorescence technique can provide much more information in the analysis of crude oil samples but at the price of time consuming. While the SFS spectrum of a crude oil sample, taken under the emission and excitation wavelength synchronous scan with deltalambda=40 nm, can represent the main characteristic of the 3D spectrum provided, with even better spectral qualities. All the results suggest that SFS technique is a more suitable approach in crude oil sample analysis than the other two, and has a great potential to be developed as a new quantitative analysis method in petroleum logging.
