Electronic spectroscopy space selection

We have in this review presented a number of applications of the CASSCF/CASPT2 method in electronic spectroscopy. The general conclusion to be drawn from these and a large body of other applications is that if the method can be applied, it yields accurate results. The limitations of the approach is set by the possibility to select a small number of active orbitals, which describe the nondynamical correlation. This is almost always possible, but we have seen cases where the smallest acceptable active space is already outside the limits of the program. The CrFg molecule is one example, Cgg is another. The application to... 

As was the case for XANES, here electronic structure calculations including solvent effects are used to draw conclusions on the most probable coordination isomer and from that structure the interatomic bond distances can be directly compared to the computed EXAFS spectra [150-152]. As EXAFS spectroscopy probes the structure of a statistically averaged system, the most appropriate way of comparing theoretical EXAFS data to experimental ones is to use molecular dynamics trajectories to sample the configuration space, select snapshots and finally compute a statistically average spectrum [89,153] with a direct estimate of the mean square relative disorder (MSRD, also called or the EXAFS Debye-Waller term). However, as discussed in Section 11.2.3, the relevance of MD simulations hinges on how accurate intermolecular interactions are, given that these are usually obtained at DFT level (in Car-Parinello MD simulations) or with force-fields (in classical MD simulations). 

Resonance ionization spectroscopy is a photophysical process in which one electron can be removed from each of the atoms of a selected type. Since the saturated RIS process can be carried out with a pulsed laser beam, the method has both time and space resolution along with excellent (spectroscopic) selectivity. In a recent article [2] we showed, for example, that all of the elements except helium, neon, argon, and fluorine can be detected with the RIS technique. However, with commercial lasers, improved in the last year, argon and fluorine can be added to the RIS periodic table (see figure 2). 

The basal spacing (d 001) (DRX-Kristalloflex-805 Siemens) and the surface area (Micromeritics ASAP 2400) was obtained on the solids calcined at different temperatures. X-Ray diffraction patterns have also been obtained after ethylenglycol saturation of selected samples. High resolution transmission electron microscopy (HREM) was performed (Jeol 100 CX Temscan) on ultrathin preparations (LKB Ultratome type 8802A). TPD (NH3) and infrared spectroscopy (pyridine) allowed to evaluate the acid properties of the solid calcined at 4(X) and 600°C. 

The chemisorption bond between CO or NO and Pt is mainly attributed to the interaction of a electron of the adsorbate with the d hole of Pt, whereas the site selection of the adsorbate is assisted by the interaction between the adsorbate 2tt and the Pt 5d orbital. The shape of the d orbital is important for this site selection. The five d orbitals in the Oh space group are generally classified into eg (dx2 y2 and d3,2 r2) and r2g (dxy, dyz, and dM) orbitals with respect to the cubic coordinate. Of the two states of Pt, t2g has more d hole character [90], and furthermore, only the f2g state is upward-shifted and the eg is preferentially filled as a result of s-d hybridization [89]. The d band filling in the eg state due to the s-d hybridization for the alloy is supported by the band structure of this alloy in the Y-L direction observed by angle-resolved photoemission spectroscopy using synchrotron radiation [88]. 

Femtosecond photoelectron spectroscopy was employed to study the excitation of trons-stilbene above the isomerization reaction barrier [82]. Apart from the contribution, evidence of a second electronic state was found on the basis of two different transients measured across the photoelectron spectrum. Time-dependent density functional theory calculations on So, Si, S2, and Do, tt ether with simulations of the electron energy distribution, supported the experimental findings for selective photoelectron energies of the So, Si,... electronic states. The photoelectron spectra of trans-stilbene following the excitation with 266 nm laser pulses consisting of a pronounced three-peak structure were subjected to a substantia] broadening, due to the large number of closely spaced vibrational states involved in the excitation scheme. 

In the atomic spectroscopy experiment in Figure 20-1, a liquid sample is aspirated (sucked) through a plastic tube into a flame that is hot enough to break molecules apart into atoms. The concentration of an element in the flame is measured by absorption or emission of radiation. For atomic absorption spectroscopy, radiation of the correct frequency is passed through the flame (Figure 20-2) and the intensity of transmitted radiation is measured. For atomic emission spectroscopy, no lamp is required. Radiation is emitted by hot atoms whose electrons have been promoted to excited states in the flame. For both experiments in Figure 20-2, a monochromator selects the wavelength that will reach the detector. Analyte concentrations at the parts per million level are measured with a precision of 2%. To analyze major constituents, a sample must be diluted to reduce concentrations to the ppm level. Box 20-1 describes an application of atomic emission for space exploration. 

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