Water vapor rotational spectrum

Figure 1.32 Fourier transforms of the two time domain waveforms shown in Figure 1.31. The signal is plotted on a logarithmic scale to show the dynamic range of the instrument. Pure rotation lines due to water vapor are seen on the background spectrum (above). The sample spectrum (below) is that of crystalline silicon. Reproduced with permission from Ref [79].

Kauppinen, J., Kyro, E. High resolution pure rotational spectrum of water vapor enriched by H2l O and H2l 0, J. Mol. Spectry. 1980, 84,405-23. 

WF Murphy. The Rayleigh depolarization ratio and rotational Raman spectrum of water vapor and the polarizability components of the water molecule. J Chem Phys 67 5877-5882, 1977. 

W. F. Murphy, /. Chem. Phys., 67, 5877 (1977). The Rayleigh Depolarization Ratio and Rotational Raman Spectrum of Water Vapor and the Polarizability Components for the Water Molecule. 

As the absorption bands caused by water vapor (its rotational spectrum) are intense throughout the far-infrared region, it is important to purge efficiently the inside of the spectrometer with dried air or nitrogen. Some commercial spectrometers can be evacuated, but usually their sample compartment needs to be purged with dried air or nitrogen. This is commonly needed also in terahertz time-domain spectrometry, which is described in the following section. 

In Figure 19.5, a high-resolution absorption spectrum (rotational spectrum) of water vapor in the atmosphere at a reduced pressure of 60 Pa is shown, together with the measured THz-TDS signal shown in (a). The abscissa range shown in (b) and (c) covers 0-150 cm , although the spectrum actually measured covers a region from about 1.3 to 230 cm The phase-delay spectrum is also shown in (c), but it does not have any particular analytically useful information. The instrument used for this spectral measurement was manufactured... 

Figure 19.5 TFIz-TDS measurement of the rotational spectrum of water vapor, (a) Measured Interference signal Intensity In arbitrary unit, (b) transmission spectra of 0 vacuum and (g) water vapor in the atmosphere at a reduced pressure of 60 Pa, and (c) absorbance and phase-delay spectra of water vapor In the atmosphere at a reduced pressure of 60 Pa.

Fig. 1.6 Absorption spectrum for water (gaseous, solution, and liquid). Above the vapor band is Mecke s rotational analysis [11,12].

This is a crude assumption. However, it appears that a quantum picture of discrete rotational lines, placed in the submillimeter wavelength range (ca. from few to 150 cm-1), is essentially determined by a form of a molecule only for a gas. In the case of a liquid, discrete spectrum is not revealed, since separate rotational lines overlap due to strong intermolecular interactions, which become of primary importance. So, due to these interactions and the effect of a tight local-order cavity, in which molecules reorient, the maximum of the absorption band, situated in the case of vapor at 100 cm-1, shifts in liquid water to... 

Quantum mechanical and electromagnetic theory provide an additional extremely important restriction upon the occurrence of rotational transitions, namely, to a first and generally adequate approximation they can occur only for molecules having non-zero electric dipole moments. Thus, microwave spectra occur for the polar molecules of water, carbon monoxide and acetone but not for the non-polar moleculess of methane, carbon dioxide and benzene. It is worth stressing also that rotational spectra are produced only by gaseous molecules, not by liquids or solids. While this seems at first a serious limitation it should be noted that it is possible to vaporize even very refractory materials at elevated temperatures. Thus, the microwave spectrum of gaseous sodium chloride (Na-Cl) molecules is perfectly well known. 

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