Near-Infrared, Visible, and Ultraviolet Spectra

4 Near-Infrared, Visible, and Ultraviolet Spectra 2.1.1.3.4.1 General 

Eight absorption and/or emission systems have been found in the range from near IR at 1.2 pm to vacuum UV at 160 nm due to transitions between the known six lowest valence states of NH and ND, X a M, b A rij, c TT, and d An energy level diagram with the spectroscopically observed transitions is shown in Fig. 2, which was taken from the reference and completed by the a X and b a systems observed later. 

The most prominent and most intense transition is the triplet system A ITj t- X Z with its maximum around 336 nm. It has been known for a long time and extensively analyzed over a period of almost 60 years. Since it is the chief characteristic of the NH radical, the spectrally or temporally resolved A X system has been repeatedly used to prove the existence of the radical and to monitor its formation and decay processes in various environments, such as N- and H-containing gas molecules in electric discharges or during photolysis or radiolysis, molecular beams, flames, or noble-gas matrices. The A X spectrum even proved the existence of NH in the sun, in stellar atmospheres, and in comets. 

The prominent singlet system c appears at 324 nm just at the short-wavelength 

Recently, the transition between the two lowest metastable states, b Z - a A, was detected at 1.17 pm in the emission spectrum of matrix-isolated NH. 

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Although the interpretation of rotational spectra of diatomic molecules is relatively simple, such spectra lie in the far infrared, a region that at present is not as easily accessible to study as are the near infrared, visible, cr ultraviolet. Consequently, most information about rotational energy levels has actually been obtained, not from pure rotation spectra, but from rotation-vibration spectra. Molecules without dipole moments have no rotation spectra, and nonpolar diatomic molecules lack rotation-vibration spectra as well, Thus, II2, N2, 02, and the molecular halogens have no characteristic infrared spectra. Information about the vibrational and rotational energy levels of these molecules must be obtained from the fine structure of their electronic spectra or from Raman spectra. 

In the absorption spectra of Ln3+ ions, the 4f-4f transitions are located in the near infrared, visible and near ultraviolet regions. Only in the case of Eu3+ and Sm3+ is it possible to observe the transition from the first and second excited levels of the ground multiplet, having low energy values and populated at room temperature. 

Tunable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical infonnation on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B <—A band, and were able to produce excited-state complexes containing 5-state I2 in a wide range of vibrational states. From line w idths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed infonnation on potential energy surfaces [231. 

Representative emission spectra are shown schematically in Fig. 2.2 for hydrogen, potassium, and mercury on a common wavelength scale from the near infrared to the ultraviolet. Under the coarse wavelength resolution of this figure, the emitted light intensities are concentrated at single, well-defined emission lines. In H, the displayed emission consists of four convergent series of lines, the so-called Ritz-Paschen and Pfund series in the near infrared, the Lyman series in the vacuum ultraviolet, and the Balmer series in the visible. Johann Balmer, a schoolteacher in Basel in the late nineteenth century. 

The full-rotational group compatibility tables show how a free-ion J level is broken up into crystal-field levels when the ion is placed in a crystalline environment with a distinct point symmetry. The irreducible representations (irreps) are labelled according to the notations of Koster et al. (1963). The tables are given up to J = 8 for even-electron systems and up to J = 17/2 for odd-electron systems. The double groups are marked by an asterisk. Although higher J values may occur for divalent lanthanide ions, they are of less importance for the study of the energy levels in the ultraviolet, visible and near-infrared parts of the spectra. 

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 spectmm (13,14,17,24). Much evidence indicates that these bands arise from electronic transitions within the and bf shells in which the Af and hf configurations are preserved in the upper and lower states for a particular ion. 

For our purpose, it is convenient to classify the measurements according to the format of the data produced. Sensors provide scalar valued quantities of the bulk fluid i. e. density p(t), refractive index n(t), viscosity dielectric constant e(t) and speed of sound Vj(t). Spectrometers provide vector valued quantities of the bulk fluid. Good examples include absorption spectra A t) associated with (1) far-, mid- and near-infrared FIR, MIR, NIR, (2) ultraviolet and visible UV-VIS, (3) nuclear magnetic resonance NMR, (4) electron paramagnetic resonance EPR, (5) vibrational circular dichroism VCD and (6) electronic circular dichroism ECD. Vector valued quantities are also obtained from fluorescence I t) and the Raman effect /(t). Some spectrometers produce matrix valued quantities M(t) of the bulk fluid. Here 2D-NMR spectra, 2D-EPR and 2D-flourescence spectra are noteworthy. A schematic representation of a very general experimental configuration is shown in Figure 4.1 where r is the recycle time for the system. 

Fig. 9. Ultraviolet, visible (0.1 M HC1), near-infrared (0.1 M DCI/D2O), and infrared (KBr) absorption spectra of the electronic transitions of I (NH3)50spz0s(NH3)5]5+ (solid line) and l (NH3)5RupzRu(NHa)5l5+ (dashed line). The numbers refer to analogous transitions in Ru and Os complexes. The value of tmax for transition 1 is approximate because the absorption intensity has been measured in a KBr disk. Reprinted with permission from Inorganic Chemistry, Ref. 80. Copyright 1988, American Chemical Society.

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. 

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

Fluorescent molecules span the entire region from the ultraviolet, through the visible, to the near infrared. Many dyes exhibit fluorescence, but to be of practical use, fluorescent dyes must satisfy certain requirements they must produce a pure color dictated by their absorption and emission spectra, they must have a high molar extinction, and most important, they must have a high quantum yield. These requirements are met by very few dyes. A disadvantage shared by many fluorescent dyes is their poor lightfastness, but there are some exceptions. 

Figure 10.2 Absorption spectra of Fe(III) oxides in the ultraviolet-visible region (left) and visible-near-infrared region (right) (from Sherman Waite, 1985). (a) Goethite (b) lepidocrocite (c) maghemite and (d) hematite. Measured reflectance spectra were converted into absorption spectra by applications of the Kebulka-Munk function. The vertical bars indicate band positions (listed in table 10.2).

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