Excitation spectrum, ultraviolet-visible

Yon can use a sin gle poin t calculation that determines energies for ground and excited states, using configuration interaction, to predict frequencies and intensities of an electron ic ultraviolet-visible spectrum. 

The ultraviolet/visible absorption spectrum of a polyene shows an intense absorption band and an extremely weak absorption band which is located below the strong absorption band, as described in the following section. This spectral pattern is a general property of linear polyenes of all chain lengths independent of local symmetry and/or the presence of cis bonds. This is the reason why in the literature on polyenes the labels 1 kg for So, 2 kg for Si and 1 feu for Si are used even in cases where Cih symmetry is not realized. The ordering that the 2 kg excited state is located below the 1 feu excited state is peculiar to linear polyenes. 

In LIBS analysis, a pulsed laser is focused on the gem surface. The laser energy ablates a small amount of gem material which burns in a short-lived plasma. As the plasma cools, excited electrons decay into lower-energy orbitals, releasing energy in the form of photons in the ultraviolet-visible-infrared range. This light is collected by optic fiber, diffracted, and recorded as a spectrum, generally between 200 and 1000 nm. 

The fluorescence of TPHA 3 is not a mirror image of its absorption spectrum and the emission intensity is sensitive to concentrations greater than 10 M. The excitation profile of 3 also varies with concentration, believed to be due to aggregation of TPHA in solution and only emulates the ultraviolet-visible (UV-Vis) spectrum at concentrations less than 10 M. The Aem decreased from 633 nm in toluene to 619 nm in dimethyl sulfoxide (DMSO), and this is thought to be indicative of a polar ground state and nonpolar excited state <1998JA2989>. 

As the time scale of the Raman scattering event ( 3.3 x 10 14 s for a vibration with a Stokes wave number shift of 1000 cm 1 excited in the visible) is much shorter than that of the fastest conformational fluctuations, an ROA spectrum is a superposition of snapshot spectra from all the distinct conformations present in a sample at equilibrium. Since ROA observables depend on absolute chirality, there is a cancellation of contributions from enantiomeric structures arising as a mobile structure explores the range of accessible conformations. Therefore, ROA exhibits an enhanced sensitivity to the dynamic aspects of biomolecular structure. In contrast, conventional Raman band intensities are blind to chirality and so are generally additive and therefore less sensitive to conformational mobility. Ultraviolet circular dichroism (UVCD) also demonstrates an enhanced sensitivity to the dynamics of chiral structures ... 

This is the simplest type of transfer. The efficiency depends only on the extent of overlap of the donor emission spectrum with the acceptor absorption spectrum. Ultraviolet or visible light is emitted by the excited donor molecule and absorbed by the acceptor molecule in a two-step process. Increasing the acceptor concentration results in a decrease of fluorescence yield of the donor but the decay time of the donor fluorescence remains unaffected. The range of energy transfer by this mechanism depends on the undisturbed path of the emitted light and falls off with distance as R. ... 

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. 

In this chapter we will concentrate on the optical excitation and its theoretical treatment. This process consists of the absorption of a photon by the system of interest and the consecutive existence of the system in the excited state. We will therefore skip the reverse process, i.e. the emitting of energy reaching a lower or the ground state. We will also discard any vibrational excitations or even ionizations, which leaves us with the treatment of electronic excitations within the visible and ultraviolet region of the electromagnetic spectrum. 

The units of wavenumber are almost always chosen as reciprocal centimeters (cm ), so we can picture the wavenumber of radiation as the number of complete wavelengths per centimeter. The frequencies, wavelengths, and wavenumbers of the various regions of the electromagnetic spectrum were summarized in Fig. F.7. In this chapter we concentrate on vibrational and electronic transitions, which can be excited by the absorption of infrared and ultraviolet-visible radiation, respectively. 

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