Positive wavenumber shift

SAMs, with the bands associated with longer chain SAMs appearing at lower wavenumbers [50], Therefore, a positive wavenumber shift of these peaks is usually interpreted as evidence of increas conformational disorder of the chains in the SAM, where the word "disorder is intended to indicate that situation where departure from the all-trans conformation has taken place. 

Figure 3.17 presents ps-TR spectra of the olehnic C=C Raman band region (a) and the low wavenumber anti-Stokes and Stokes region (b) of Si-rra i-stilbene in chloroform solution obtained at selected time delays upto 100 ps. Inspection of Figure 3.17 (a) shows that the Raman bandwidths narrow and the band positions up-shift for the olehnic C=C stretch Raman band over the hrst 20-30 ps. Similarly, the ratios of the Raman intensity in the anti-Stokes and Stokes Raman bands in the low frequency region also vary noticeably in the hrst 20-30 ps. In order to better understand the time-dependent changes in the Raman band positions and anti-Stokes/Stokes intensity ratios, a least squares htting of Lorentzian band shapes to the spectral bands of interest was performed to determine the Raman band positions for the olehnic... 

When the charge on the electrode is made negative, the bond is weakened due to donation of charge from the metal into adsorbate x orbitals and the band frequency shifts to lower wavenumber. When the charge on the metal is made positive a shift to higher frequency occurs. At a mercury electrode, however, there are no p- or d-electrons available to participate in a back-bonding interaction. 

When Raman spectra are chosen, the x-axis is shifted and switched so that the Raman laser signal is at zero and the Stokes region of the spectrum is in positive wavenumbers. The Raman laser frequency is taken from the parameter RLW of the experimentally determined spectrum or can be set manually. 

The integrated intensity of the Raman signature peak Jp has to be considered in the data treatment instead of the intensity at the maximum. In principle, this last quantity could be used but the maximum position is very sensitive to any wavenumber shift which could be caused by the spectrometer drift or a change in the experimental conditions. Considering the intensity Jp integrated over a relatively wide range - at about of 30 cm -allows one to remove these difficulties. Moreover accuracy and precision in concentration, as derived from the calculation of J are better than if the intensity of peak maximum is used (see 5). 

The crystal size of the sample is too large, which leads to a scattering of radiation that becomes worse at the high-wavenumber end of the spectrum. Additionally, the bands are distorted and consequently their positions are shifted, thus leading to the possibility of an incorrect assignment. In this case, the sample needs to be ground further with a mortar and pestle. 

Since the ju s are known, the wavenumber shift can be calculated. It is of interest that if the atoms H and D are the two isotopes, then the factor relating the position of the two bands is approximately 2. 

Other precautions must be taken when spectra are collected if high photometric precision (repeatability) is required. Photometric precision is not only restricted to quantitative analysis, but is also required for spectral subtraction (Section 9.3), and spectral search systems (Section 10.8). Repeatable sample preparation and placement is critical. If the aperture of the sample cell is small enough to become the limiting aperture in the spectrometer, any shift in the position of the sample from the center of the beam will cause a shift in wavenumber and possibly a change in the ILS function (Section 2.6). A small wavenumber shift will cause a peak maximum to be displaced, and the bandshape and peak absorbance may change due to the change in the ILS function [6] and the discrete sampling of the spectrum (Section 3.1). 

Under carefully controlled conditions, wavenumber measurements may be precise to 0.01 cm (discussed below), but usually only when a sample is left undisturbed in the sample compartment. Even if the sample is simply removed and reinserted between measurements, the repeatability is often worse than 0.01 cm . Several reasons can be advanced to explain why band shifts occur. First, the temperature of the sample may change between measurements, which leads to small spectral shifts. Second, it was noted in Section 2.6 that changes in the effective solid angle of the beam through the interferometer can lead to small wavenumber shifts. Because the cell may represent a field (Jacquinot) stop, if a cell is not placed in exactly the same position for successive measurements, bands will appear to shift from one measurement to the next. Furthermore, if the cell is slightly tilted and the angle changes appreciably from one measurement to the next, the beam may be refracted to a different position on the detector, which also shifts the wavenumber scale. Loose or insecure sample mounts should be avoided if users require the wavenumbers of absorption band maxima to be repeatable to better than 0.1 cm . ... 

In three dimensions and for central potentials, the Schrodinger equation separates in spherical coordinates analogously to corresponding classical equations of motion. The incoming particle is described by a wavefunction with a local wavenumber ic(r). Repulsive potentials decrease the velocity of the wave packet and the resulting scattered wavefunction has fewer nodes, resulting in a negative phase shift. Conversely, attractive potentials result in an increase in velocity the wavefunction acquires more nodes and this results in a positive phase shift. The overall phase shift for scattered particles is... 

Q are the absorbance and wavenumber, respectively, at the peak (center) of the band, p is the wavenumber, and y is the half width of the band at half height. Liquid band positions ate usually shifted slightly downward from vapor positions. Both band positions and widths of solute spectra are affected by solute—solvent interactions. Spectra of soHd-phase samples are similar to those of Hquids, but intermolecular interactions in soHds can be nonisotropic. In spectra of crystalline samples, vibrational bands tend to be sharper and may spHt in two, and new bands may also appear. If polarized infrared radiation is used, both crystalline samples and stressed amorphous samples (such as a stretched polymer film) show directional effects (28,29). 

The spectra from strong oscillators have special features which are different from those from metallic and dielectric substrates. Different structures in tanf and A are observed on a metallic substrate, dependent on the thickness of the film (Fig. 4.65). For very thin films up to approximately 100 nm the Berreman effect is found near the position of n = k and n < 1 with a shift to higher wavenumbers in relation to the oscillator frequency. This effect decreases with increasing thickness (d > approx. 100 nm) and is replaced by excitation of a surface wave at the boundary of the dielectric film and metal. The oscillator frequency (TO mode) can now also be observed. On metallic substrates for thin films (d < approx. 2 pm) only the 2-component of the electric field is relevant. With thin films on a dielectric substrate the oscillator frequency and the Berreman effect are always observed simultaneously, because in these circumstances all three components of the electric field are possible (Fig. 4.66). 

Fig. 28 Raman spectra of polymeric sulfur (S ) prepared by various methods [109,173], of large disordered rings (S ) [182], and of photo-induced amorphous sulfur (a-S) [119], respectively. The spectrum of a-S has been smoothed for clarity. The position of the stretching vibration of a-S is pressure-shifted to higher wavenumbers. The very weak signals in the spectra of Sj, at ca. 150 and 220 cm are probably caused by the presence of Sg. In addition, the weak shoulder at ca. 470 cm observed in spectra of Sj, may originate from Sg, too...

Fig. 15. Thermal denaturation of triosephosphate isomerase with FTIR (upper left), second-derivative FTIR (upper right), and VCD (bottom) showing irreversible aggregation effects. The IR shift from a simple maximum at 1650-1640 cm-1 to a lower frequency distorted to low wavenumber is seen to be irreversible when the original spectrum is not recovered. The second-derivative result makes the changes more dramatic and shows the original native state spectrum to be more complex (negative second derivatives correspond to peak positions). Loss of structure is even more evident in the VCD, which loses most of its intensity at 60°C.

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