Photoionization computation

Until recently, photoionization cross sections and recombination (radiative and di-electronic) coefficient sets used in photoionization computations were not obtained self-consistently. Photoionization and recombination calculations are presently being carried out using the same set of eigenfunctions as in the IRON project (Nahar Pradhan 1997, Nahar et al. 2000). The expected overall uncertainty is 10 - 20%. Experimental checks on a few species (see e.g. Savin 1999) can provide benchmarks for confrontation with numerical computations. 

The near-ir spectmm of ethylene oxide shows two peaks between 1600—1700 nm, which are characteristic of an epoxide. Near-ir analyzers have been used for verification of ethylene oxide ia railcars. Photoionization detectors are used for the deterrnination of ethylene oxide ia air (229—232). These analyzers are extremely sensitive (lower limits of detection are - 0.1 ppm) and can compute 8-h time-weighted averages (TWAg). 

The 5950A ESCA spectrometer is interfaced to a desktop computer for data collection and analysis. Six hundred watt monochromatic A1 Ka X-rays are used to excite the photoelectrons and an electron gun set at 2 eV and 0.3 mAmp is used to reduce sample charging. Peak areas are numerically integrated and then divided by the theoretical photoionization cross-sections (11) to obtain relative atomic compositions. For the supported catalyst samples, all binding energies (BE) are referenced to the A1 2p peak at 75.0 eV, the Si 2p peak at 103.0 eV, or the Ti 2p3/2 peak at 458.5 eV. 

Figure 4 outlines a portion of ES 2 for choice of the specific instrumental configuration and conditions which are indicated by the decisions and factors provided in ES 1. This is a critical step, since the databases generated in ES 3 must be directly correlated to the specific instrumental configuration and conditions in ES 2 for the concerted analysis of samples, references, etc. e.g., pattern comparisons between analyses with specialty GC detectors (FID-flame ionization, TCD-thermal conductivity, NPD-nitrogen/phosphorus, PID-photoionization). This stage focuses on the attributes of modern analytical instrumentation flexible, modular, microprocessor/computer-... 

The development of lasers has opened up several new techniques for monitoring pollutants in the atmosphere. Sensitivities down to the parts-per-billion range are claimed, and continuous monitoring is possible. The photoionization mass spectrometer has been developed as a sensitive detector for fi radicals in the gas phase. A high-resolution mass spectrometer coupled to a computer is capable of detecting up to 300 compounds in air, both in particulate form and in the gas phase. 

At this point photoionization cross sections have been computed mostly for diatomic molecules, rr-electron systems, and other relatively small molecules [see Rabalais (242) for a summary of this work up to 1976]. Very few photoionization cross section calculations have been performed (108) on transition metal systems and the agreement with experimental intensities is rather poor. For the most part, therefore, one must rely on empirical trends when dealing with the photoionization of metal-containing molecules. A number of such trends have now emerged and are useful for spectral assignment. 

Taking advantage of advances in computational atomic and plasma physics and of the availability of powerful supercomputers, a collaborative effort - the international Opacity Project - has been made to compute accurate atomic data required for opacity calculations. The work includes computation of energy levels, oscillator strengths, photoionization cross-sections and parameters for pressure broadening of spectral lines. Several... 

Two general mechanisms are usually advanced to explain ionization of molecules in flames direct ionization by thermoionization, photoionization, or chemiionization and indirect ionization by charge transfer with other ions. The assessment of both mechanisms requires knowledge of the ionization potential of molecules. In the following discussion, computations developed in the Appendix are used to estimate approximate ionization potentials of polynuclear aromatic hydrocarbons. 

The deep inner shell orbitals such as Is, 2s and 2p are not very sensitive both to the scaling parameter a and to the oxidation state. On the other hand, the shallow inner shells like 3s and 3p and outer-shell orbitals 3d and 4s strongly depend upon a and the effective charge. Accordingly, the theoretical photoionization cross section computed by equation (12) is affected by the change of spatial extent of the atomic orbital. The theoretical photoionization cross sections for Fe orbitals shown in Table 2 are calculated for the photon energy of hv = 1487eV (A1 Ka) and indicated in Table 3. In the case of a=l. 0, the atomic orbitals are somewhat contracted compared with... 

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