Excitation vibrational and rotational

The energy of infrared radiation can excite vibrational and rotational transitions, but it is insufficient to excite electronic transitions. As shown in Figure 26-20, infrared spectra exhibit narrow, closely spaced absorption bands resulting from transitions among the various vibrational quantum levels. Variations in rotational levels may also give rise to a series of peaks for each vibrational state with liquid or solid samples, however, rotation is often hindered or prevented, and the effects of these small energy differences ai e not detected. Thus, a typical infrared spectrum for a liquid, such as that in Figure 26-20, consists of a series of vibrational bands. 

Lee joined the University of Chicago faculty in 1968, moving to Berkeley in 1974. His analyses of reactants, products, and complexes, including excited vibrational and rotational states, provided new mechanistic insights. For example, concerted reaction of F2 and I2 to form IF molecules is symmetry-forbidden, but occurs easily. Lee discovered that I2F, a radical species, is an intermediate and explains the rapid rate of this reaction. 

Absorption the light excites vibrational and rotational vibrating modes in the dielectric material since it is associated with an oscillating electric field. 

Excited vibrational and rotational levels of Ti electronic state... 

Can monochromatic electromagnetic radiation with wavelength A = 3 pm excite vibrational and rotational energies of the HE molecules if it is in a ground state ... 

Much of the previous section dealt with two-level systems. Real molecules, however, are not two-level systems for many purposes there are only two electronic states that participate, but each of these electronic states has many states corresponding to different quantum levels for vibration and rotation. A coherent femtosecond pulse has a bandwidth which may span many vibrational levels when the pulse impinges on the molecule it excites a coherent superposition of all tliese vibrational states—a vibrational wavepacket. In this section we deal with excitation by one or two femtosecond optical pulses, as well as continuous wave excitation in section A 1.6.4 we will use the concepts developed here to understand nonlinear molecular electronic spectroscopy. 

Faubel M 1983 Vibrational and rotational excitation in molecular collisions Adv. Atom. Mol. Phys. 19 345... 

Conventional spontaneous Raman scattering is the oldest and most widely used of the Raman based spectroscopic methods. It has served as a standard teclmique for the study of molecular vibrational and rotational levels in gases, and for both intra- and inter-molecular excitations in liquids and solids. (For example, a high resolution study of the vibrons and phonons at low temperatures in crystalline benzene has just appeared [38].)... 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Just as in the ground electronic state a molecule may vibrate and rotate in excited electronic states. The total term value S for a molecule with an electronic term value T,... 

The use of molecular and atomic beams is especially useful in studying chemiluminescence because the results of single molecular interactions can be observed without the complications that arise from preceding or subsequent energy-transfer coUisions. Such techniques permit determination of active vibrational states in reactants, the population distributions of electronic, vibrational, and rotational excited products, energy thresholds, reaction probabihties, and scattering angles of the products (181). 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

Color from Vibrations and Rotations. Vibrational excitation states occur in H2O molecules in water. The three fundamental frequencies occur in the infrared at more than 2500 nm, but combinations and overtones of these extend with very weak intensities just into the red end of the visible and cause the blue color of water and of ice when viewed in bulk (any green component present derives from algae, etc). This phenomenon is normally seen only in H2O, where the lightest atom H and very strong hydrogen bonding combine to move the fundamental vibrations closer to the visible than in any other material. 

Figure 5-20. Characteristic of forced vibration or resonance in rotating machinery. (Ehrich, F.F., Identification and Avoidance of Instabiiities and Seif-Excited Vibrations in Rotating Machinery, Adopted from ASME Paper 72-DE-21, Generai Eiectric Co., Aircraft Engine Group, Group Engineering Division, May 11, 1972.)...

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