Ultraviolet absorption spectral region

Less restrictive selection rules are just one of several benefits that can arise when using multiphoton excitation methods. Experimental convenience is another. A multiphoton excitation using visible or near-ultraviolet (UV) photons can often prove the easiest route to populating an excited state lying at energies that, in one-photon absorption, would fall in the technically much more demanding vacuum ultraviolet (VUV) spectral region. Other benefits derive from the fact that multiphoton excitations normally require the use of a focused pulsed laser... 

UV-vis refers to absorption spectroscopy in the ultraviolet-visible spectral region. The absorption in the visible range directly affects the perceived color of the chemicals involved. The UV-vis spectra are measured using spin casted films, or dilute polymer solutions. Films are more representative for the active layer behavior in SCs, while solutions are more reliable when comparing the different polymers or blends. For example, the film deposited on ITO substrate may be dissolved in a suitable solvent (e.g., in chloroform at concentration of 25 pg/mL, in 10 mm quartz cell) and either directly used in a spectrometer, or spin casted on glass slides, vacuum dried and measured at the wavelength 280 to 900 nm at a rate of 300 nm/min.i ... 

The inverse of the energy separation, AE, between ground and excited electronic states of the molecule. This means that there will be a correlation between NMR spectra and absorption in the visible and ultraviolet spectral regions. 

The photodissociation products of the homonuclear halogens in the visible and ultraviolet are now comparatively well established in view of the detailed spectroscopic studies that have been made. The strongest absorption system observed in this spectral region is associated with a transition to the 3II0u+ state which correlates with X / ) + X(2Pyz). Thus photoexcitation to the continuum associated with this state leads directly to the formation of an excited atom, while excitation to the banded region followed by predissociation will lead only to ground state atoms. 

While inferring k from measurements of absorption by particulate samples may be valid for many kinds of sohds at visible wavelengths, it may not be valid in spectral regions where the optical constants rapidly vary, such as the infrared or far ultraviolet. 

The interaction of electromagnetic radiation with matter in the domain ranging from the close ultraviolet to the close infrared, between 180 and 1,100 nm, has been extensively studied. This portion of the electromagnetic spectrum, called UV/Visible because it contains radiation that can be seen by the human eye, provides little structural information except the presence of unsaturation sites in molecules. However, it has great importance in quantitative analysis. Absorbance calculations for compounds absorbing radiation in the UV/Visible using Beer-Lambert s Law is the basis of the method known as colorimetry. This method is the workhorse in any analytical laboratory. It applies not only to compounds that possess absorption spectra in that spectral region, but to all compounds that lead to absorption measurements. 

To study the excited state one may use transient absorption or time-resolved fluorescence techniques. In both cases, DNA poses many problems. Its steady-state spectra are situated in the near ultraviolet spectral region which is not easily accessible by standard spectroscopic methods. Moreover, DNA and its constituents are characterised by extremely low fluorescence quantum yields (<10 4) which renders fluorescence studies particularly difficult. Based on steady-state measurements, it was estimated that the excited state lifetimes of the monomeric constituents are very short, about a picosecond [1]. Indeed, such an ultrafast deactivation of their excited states may reduce their reactivity something which has been referred to as a "natural protection against photodamage. To what extent the situation is the same for the polymeric DNA molecule is not clear, but longer excited state lifetimes on the nanosecond time scale, possibly of excimer like origin, have been reported [2-4],... 

Many classifications of spectra exist those describing the spectral region involved (ultraviolet, infrared) the appearance of the spectra (line, band) the method of observation (absorption, emission) or the species producing the spectra (atoms, molecules). With respect to processes and properties of expls and proplnts, classification by species is most appropriate since information concerning reaction kinetics is frequently provided by spectroscopic techniques, From a spectroscopic viewpoint, it is convenient to divide the electromagnetic spectrum into a number of sections (see Fig 1). 

Table 6.2 lists the ultraviolet cutoff for a variety of solvents commonly used in UV-VIS spectroscopy. The solvent chosen must dissolve the sample, yet be relatively transparent in the spectral region of interest. Typically, very low concentrations of sample will be present in the solvent. It is therefore important to avoid solvents that have even weak absorptions near the solute s bands of interest. Methanol and ethanol are two of the most commonly used solvents. Care must be exercised when using the latter that no benzene (an azeotropic drying agent) is present as this will alter the solvent s transparency. Normally, this will not be a problem in spectral grade solvents. 

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