Gas Phase Spectra

On the contrary, according to Eq. (1.70) gM becomes zero for o) = 0, the width of a gap in the centre of the spectrum being about l/xj. Note that the total width of a gas phase spectrum is much larger, namely 1/tc. This narrow gap in the centre of gM(oj) points to the existence of intercollisional correlation. The same is valid for the spectrum of random... 

There is of course attenuation of the signal, as shown in Fig. 5, taken from Joyner and Roberts (28) The gas phase spectrum will also be obtained, but this usually can be separated easily from the signal of the solid. This sample cell arrangement thus permits the study of the stationary-state surface during catalysis and also its evolution in response to pulses and step functions in the gas composition. The temperature of the sample should be controlled so that the surface can be studied during temperature-programmed desorption and reaction. 

The infrared spectrum of GeF2 has also been reported 10 3 It was necessary to study the matrix-isolated spectrum for two reasons. First, the examination of the ultraviolet absorption spectrum of GeF2 indicated that at least ten of the bending states were populated, and second, germanium has five abundant isotopes. These suggested that the gas phase spectrum would be broad and ill de-... 

Here a third selection rule applies for linear molecules, transitions corresponding to vibrations along the main axis are allowed if Aj = 1. The A/=0 transition is only allowed for vibrations perpendicular to the main axis. Note that because of this selection rule the purely vibrational transition (called Q branch) appears in the gas phase spectrum of C(X but is absent in that of CO. In both cases, two branches of rotational side bands appear (called P and R branch) (see Fig. 8.3 for gas phase CO). 

Experimentally, the spectrum of Ni(CO)4 has been measured in solution [76], in matrix [61] and, more recently, in gas-phase [65]. The solution spectrum exhibits a main peak at 6.0 eV, with two shoulders at 5.5 and 5.2 eV. Only the 5.2 eV band could be detected in the matrix isolation spectrum, with a new shoulder appearing at 4.5 eV. Finally, in the gas-phase spectrum a main peak at 6.0 eV and two shoulders at 5.4 and 4.6 eV could be discerned. Being a d10 system, d—>d ligand field (LF) transitions are absent in the spectrum. Since Ni(CO)4 has Ta symmetry, only excitations from the Ai... 

An alternative method was also studied. This involved ratioing the intensity of the 698 cm 1 styrene band to the intensity of the 2921 cm-1 C-H stretching vibration. Since oils and other additives would interfere with this approach they were extracted with acetone. Vacuum oven drying was then necessary to remove all traces of acetone prior to PA analysis. Otherwise the PA spectrum would be that of acetone rather than that of the rubber since the gas phase spectrum of the acetone would overwhelm that of the solid phase rubber. This technique allowed both solution and emulsion SBR to be analysed by a common method. The results can be expressed by a least squares linear regression equation over the range of 5%-40% styrene in SBR. 

Gas cells can be used for gaseous and volatile liquid samples. The use of the gas cell is simple, but care should be taken in emptying the cell when the sample is toxic. A gas sample can be sucked in to an evacuated cell from the atmosphere or from a container containing gas, if suitable connection is available. Volatile liquid samples can be placed as droplets into the cell, which is closed, and the chemical is allowed to vaporize. The gas-phase spectrum can then be measured. The droplets can also be placed in a separate bottle for the vaporization, and the vapor can then be transferred into the evacuated gas cell. 

The IR spectra of the chemicals are measured in the vapor phase, as the GC effluent is hot. One of the problems associated with the spectrum measurement in the gas phase is that a gas-phase spectrum of a chemical differs considerably from those measured in other phases. See Section 3.3 for further discussion on this subject. 

Figure 3-13. Gas phase spectrum (solid line) and spectrum of a single solute—solvent configuration (dashed line). Vertical transitions are convoluted by Gaussians with a width of 0.14 eV. Data from Ref. [160]...

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