Ultraviolet spectroscopy electronic excitations

Visible, ultraviolet—valence electron excitations Experimental Methods in Molecular Spectroscopy... 

C. L. Wilson, Comprehensive Analytical Chemisty Ultraviolet Photoelectron and Photoion Spectroscopy Auger Electron Spectroscopy Plasma Excitation in SpectrochemicalAnalysis, Vol. 9, Elsevier Science, Inc., New York, 1979. 

Ultraviolet-visible spectroscopy (UV = 200 - 400 nm, visible = 400 - 800 nm) corresponds to electronic excitations between the energy levels that correspond to the molecular orbital of the systems. In particular, transitions involving n orbital and ion pairs (n = non-bonding) are important and so UV/VIS spectroscopy is of most use for identifying conjugated systems which tend to have stronger absorptions... 

Because of electronic excitation ultraviolet spectroscopy is also known as electronic spectroscopy. 

Spectroscopy provides one of the few tools available for probing the inner workings of molecules. Infrared and Raman spectroscopies provide information from which force constants and information about charge distributions can be obtained. Ultraviolet spectroscopy gives information on the nature of the electronically excited states of molecules, and is directly connected with their photochemical transformations. Photoelectron spectroscopy gives information on the nature of the radical cations that may be formed by ionization of a molecule, and NMR spectroscopy can give information on the hybridization associated with a given bond. As a result of the level of information that may be obtained, there have been a number of spectroscopic studies. 

The electron, upon excitation, is ejected from an inner shell into vacuum and the energy of the free electron is then measured. This technique is called X-ray photoelectron spectroscopy. If the electron is ejected from the valence band by ultraviolet radiation, the technique is called ultraviolet photoelectron spectroscopy. Excitation energies not greater than those provided by ultraviolet radiation are necessary for electron excitation from the valence band or for electrons from the valence shell of adsorbed molecules. 

Ultraviolet spectroscopy has great utility in the characterization of expls and related materials, and serves as a primary analytical tool for the quantitative determination of reactant composition and purity. Additionally, it can provide the principal method of monitoring expl kinetics and reaction mechanisms, since the high temps characteristic of expins are effective in creating electronic excitations... 

Rare-gas samples exist only at cryogenic temperatures and most of the optical spectroscopy of electronic processes should be done in the vacuum ultraviolet. Making experiments requires an indispensable combination of liquid-helium equipment with windowless VUV-spectroscopic devices and synchrotron radiation as a photon source. To study the electronic excitation energy pathways and a variety of subthreshold inelastic processes, we used the complimentary advantages of cathodoluminescence (possibility to vary the excitation depth beneath the sample surface), photoluminescence (selective-state excitation by synchrotron radiation at high-flux SUPERLUMI-station at HASYLAB, DESY, Hamburg) and... 

As instrumental advances continue and computational methods become more powerful, the frontiers of chiroptical spectroscopy are being continually pushed back. One important area has been in the characterization of biopolymers. Mark Manning and John Towell have contributed a chapter covering the use of ultraviolet-CD in the analysis of protein structure, and Max Diem has described the application of infrared-CD to the study of similar systems. Finally, the range of chiroptical investigations which can be performed on molecules in electronically excited states has been summarized by James Riehl. 

The ultraviolet spectroscopy of formaldehyde has been studied almost exhaustively, and there is an excellent review on this subject (171). A majority of the bands in the electric-dipole-forbidden vibronically allowed A 2 +X Aj transition have been assigned mostly due to the work of Brand (37), Robinson and DiGiorgio (196), Callomon and Innes (44), and Job, et al. (124). As briefly mentioned earlier, the ground electronic state (X) is planar and the first excited singlet state (A) is pyramidal. It is valid to use the C2V point group symmetry for both electronic states, rather than the C2 point group symmetry (see ref. 171), although the emission could certainly be treated as a -A" - 1A transition. 

The pump-probe pulses are obtained by splitting a femtosecond pulse into two equal pulses for one-color experiments, or by frequency converting a part of the output to the ultraviolet region for bichromatic measurements. The relative time delay of the two pulses is adjusted by a computer-controlled stepping motor. Petek and coworkers have developed interferometric time-resolved 2PPE spectroscopy in which the delay time of the pulses is controlled by a piezo stage with a resolution of 50 attoseconds [14]. This set-up made it possible to probe decoherence times of electronic excitations at solid surfaces. 

Luminescence spectroscopy is an analytical method derived from the emission of light by molecules which have become electronically excited subsequent to the absorption of visible or ultraviolet radiation. Due to its high analytical sensitivity (concentrations of luminescing analytes 1 X 10 9 moles/L are routinely determined), this technique is widely employed in the analysis of drugs and metabolites. These applications are derived from the relationships between analyte concentrations and luminescence intensities and are therefore similar in concept to most other physicochemical methods of analysis. Other features of luminescence spectral bands, such as position in the electromagnetic spectrum (wavelength or frequency), band form, emission lifetime, and excitation spectrum, are related to molecular structure and environment and therefore also have analytical value. 

Electron Spectroscopy. Electron spectroscopy is a rapidly developing field involving measurement of electrons ejected from a bombarded sample. It may be divided into categories according to the bombarding source, ultraviolet excitation (UPS), X-ray photoelectron spectroscopy (XPS), electron spectroscopy for chemical... 

---