Ultraviolet-Visible-Near Infrared spectra

Figure 2a shows the ultraviolet-visible-near infrared spectrum of an —2% solution of as-synthesized emeraldine base in A -methyl-2-pyrrolidone (NMP) [16]. The two absorption peaks at 328 and 635 nm have been assigned as the tt-tt and exciton transitions, respectively. The ratio of the intensity of the exciton transition to that of the TT-TT transition is 0.886. This ratio and the position of the exciton peak are very sensitive to the oxidation state of the polyaniline sample. Thin films of EB spin coated on quartz plates from the NMP solution give similar spectra with the positions of the two absorption peaks at 329 and 623 nm, respectively. The ratio of these two peaks also changes to 0.68, an indication of conformational change during the evaporation of the solvent. 

Figure 18-6 (a) Absorption spectrum of KMnO at four different concentrations, (b) Peak absorbance at 555 nm is proportional to concentration from 0.6 xM to 3 mM. The Cary 5000 ultraviolet-visible-near infrared spectrophotometer used for this work has a wider operating range than many instruments. It is difficult to measure absorbance accurately above 2 or below 0.01. [From A. R. Hind, Am. Lab., December 2002, p. 32. Courtesy Varian, Inc., Palo Alto, CA.]... 

Figure 2.4 Hydrogen atom energy levels and transitions. The Lyman, Balmer, Ritz-Paschen, and Brackett series occur in the vacuum ultraviolet, visible, near-infrared, and infrared regions of the electromagnetic spectrum, respectively.

Although this book is concerned only with the infrared region of the electromagnetic spectrum, it will be useful to look at the entire spectrum normally used in analysis. The spectrum is divided into several regions, each of which is used for a different purpose, and each of which furnishes a different type of information. These are the X-ray, ultraviolet, visible, near-infrared, infrared, far-infrared, and microwave regions. This division of the spectrum is arbitrary, and the boundaries... 

Absorption and Fluorescence Spectra. The absorption spectra of actinide and lanthanide ions in aqueous solution and in crystalline form contain narrow bands in the visible, near-ultraviolet, and near-infrared regions of the spectrum. 

Absorption and Fluorescence Spectra. The absorption spectra of actinide and lanthanide ions in aqueous solution and in crystalline form contain narrow bands in the visible, near-ultraviolet, and near-infrared regions of the spectrum (13,14,17,24). Much evidence indicates that these bands arise from electronic transitions within the 4f and 5/shells in which the 4f and bf configurations are preserved in the upper and lower states for a particular ion. 

Rapid-scanning spectroscopy (RSS) is a method in which a selected portion of the ultraviolet, visible, or near-infrared spectrum is scanned on a time scale ranging from several sec to a few /isec. The applications of this technique to systems in which short-lived transient species exist or large reaction rates are encountered are numerous... 

The light-emitting diode (LED) is a semiconductor device that emits incoherent, narrow-spectrum light. The color of the emitted light depends on the semiconducting material used, and can be near ultraviolet, visible, or infrared. 

Figure 45.4 The transmittance of a siiica aerogei in the ultraviolet, visible, and near infrared spectrum (a) and the infrared spectrum (b) showing IR bands of alcohol at 3600-3200cm , of carbonyl at 1760-1690cm" and of carboxyl at 1300-1080cm". (Redrawn from Ref. (67).)...

Spectrophotometry proper is mainly concerned with the following regions of the spectrum ultraviolet, 185-400 nm visible 400-760 nm and infrared, 0.76-15 /tm. Colorimetry is concerned with the visible region of the spectrum. In this chapter attention will be confined largely to the visible and near ultraviolet region of the spectrum. 

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible and, for many transition metal complexes, the near infrared region of the electromagnetic spectrum. Spectroscopic methods that probe these electronic transitions can, in favourable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

The reaction should be capable of operating over a wide bandwidth of the visible and ultraviolet portions of the solar spectrum with a threshold wavelength well into the red or near infrared. 

Techniques employing the ultraviolet (UV), visible, and near-infrared parts of the spectrum have the advantage of high sensitivity (single photon), high time resolution (femtoseconds), and moderate spatial resolution (on the order of 100 nm). Structural information is obtainable by infrared to radio-frequency techniques (e.g., magnetic resonance). Together, these techniques have enabled the visualization of individual molecules and the measurement of excited state dynamics from such molecules on the picosecond time scale. It is also possible to follow the time course of chemical reactions on the femtosecond time scale when... 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain spectrum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the spectrum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectrum the instrument used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

The term light , discussed in this book, includes the following spectral ranges of electromagnetic spectrum near infrared (NIR), visible, and ultraviolet (UV) A, B, and C bands (Figure 15.1). 

The visible spectrum extends approximately from 4,000 to 8,000 A. The human eye is not sensitive to light of wave lengths much shorter or longer than these. The near infrared region extends from 8,000 up to perhaps 200,000 A, and the far infared region extends still further toward the long waves from meters to thousands of meters, which are familiar to anyone who turns a radio dial. On the shorter side of the visible spectrum, we have ultraviolet light... 

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