Vibration spectra

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Segall J, Zare R N, Dubai H R, Lewerenz M and Quack M 1987 Tridiagonal Fermi resonance structure in the vibrational spectrum of the CM chromophore in CHFg. II. Visible spectra J. Chem. Phys. 86 634-46... 

Dubai H-R, Ha T-K, Lewerenz M and Quack M 1989 Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore In CHXg molecules J. Chem. Phys. 91 6698-713... 

We now present one of the many examples of interfacial vibrational spectroscopy using SFG. Figure Bl.5.15 shows the surface vibrational spectrum of the water/air interface at a temperature of 40 °C [83]. Notice that... 

Once the vibration calculation completes, you can analy/eand display Ihe results by using Vibrational Spectrum menu item. 

Figure 5-10 Partial MM3 Output as Related to the Vibrational Spectrum of H2O. The experimental values of the two sti etching and one bending frequencies of water are 3756, 3657, and 1595 cm. The IR intensities are all very strong (vs).

Polyatomic molecules vibrate in a very complicated way, but, expressed in temis of their normal coordinates, atoms or groups of atoms vibrate sinusoidally in phase, with the same frequency. Each mode of motion functions as an independent hamionic oscillator and, provided certain selection rules are satisfied, contributes a band to the vibrational spectr um. There will be at least as many bands as there are degrees of freedom, but the frequencies of the normal coordinates will dominate the vibrational spectrum for simple molecules. An example is water, which has a pair of infrared absorption maxima centered at about 3780 cm and a single peak at about 1580 cm (nist webbook). 

Experimental. The vibrational spectrum of an ideal harmonic oscillator would consist of one line at frequency v corresponding to A = hv, where A is the distance between levels on the vertical energy axis in Fig. 10-la. In the harmonic oscillator, AE is the same for a transition from one energy level to an adjacent level. A selection rule An = 1, where n is the vibrational quantum number, requires that the transition be to an adjacent level. 

In order to analyze the vibrations of a single molecule, many molecular dynamics steps must be performed. The data are then Fourier-transformed into the frequency domain to yield a vibrational spectrum. A given peak can be selected and transformed back to the time domain. This results in computing the vibra-... 

To perform a vibrational analysis, choose Vibrationson the Compute menu to invoke a vibrational analysis calculation, and then choose Vibrational Dectrum to visualize the results. The Vibrational Spectrum dialog box displays all vibrational frequencies and a simulated infrared spectrum. You can zoom and pan in the spectrum and pick normal modes for display, using vectors (using the Rendering dialog box from Display/Rendering menu item) and/or an im ation. 

Figure 6.15 The infrared vibrational spectrum of crotonaldehyde. The parts marked (a), (b) and (c) refer to a 10 per cent (by volume) solution in CCI4, a 1 per cent solution in CCI4, and a thin liquid film, respectively. [Reproduced, with permission, from Bowles, A. J., George, W. O. and Maddams, W. F J. Chem. Soc. (B), 810, 1969]...

Collection and analysis of vibration signatures is a complex procedure. By looking at a vibration spectrum, one can identify which components of the pump system are responsible for a particular frequency component. Comparison of vibration signatures at periodic intervals reveals if a particular component is deteriorating. The following example illustrates evaluation of the frequency composition of an electric motor gear pump system. 

Another conventional simplification is replacing the whole vibration spectrum by a single harmonic vibration with an effective frequency co. In doing so one has to leave the reversibility problem out of consideration. It is again the model of an active oscillator mentioned in section 2.2 and, in fact, it is friction in the active mode that renders the transition irreversible. Such an approach leads to the well known Kubo-Toyozawa problem [Kubo and Toyozava 1955], in which the Franck-Condon factor FC depends on two parameters, the order of multiphonon process N and the coupling parameter S... 

Figure 16-7 shows the vibration spectrum with the speed and suction pressure kept constant but with a small 20psig increase in discharge pressure. Notice the large increase in the 9000 rpm component from 0.2 to 1.5 mil (0.0127-0.0381 mm). A further small increase in discharge pressure would have increased the subsynchronous vibrations to more than 1.0 mil (0.0254 mm) and wrecked the unit. 

The monitoring of turbomachinery mechanical characteristics, such as vibrations, has been applied extensively over the past decade. The advent of the accelerometer and the real-time vibration spectrum analyzer has required a computer to match and utilize the extensive analysis and diagnostic capability of these instruments. 

State or vibrational energies. If the current across the metal-insulator-metal sandwich is recorded as a function of applied voltage, the current increases as the threshold for each state or vibrational mode is crossed. The increases in current are in fact very small, and for improved detectability the current is double-differentiated with respect to voltage, thereby providing, in effect, a vibrational spectrum that can be compared directly with free-molecule IR and Raman spectra. 

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