Rotational spectra instrumentation

Fig. 28 Pure rotational spectrum of C>2. Trace (a) is the S3 transition recorded at a pressure of 1.0 atm. Trace (b) is the result of deconvolving the S3 profile with a Voigt profile to remove most of the pressure broadening, Doppler broadening, and instrument effects. Trace (c) was calculated using a 0.035-cm-1 Gaussian profile and calculated spin splittings. The traces are scaled to the same height.

The infrared spectrum, also known as molecule vibration and rotation spectrum, is used in fundamental research of molecular structure and analysis of chemical components, and the latter is the widest application of the infrared spectrum. The structure of an unknown sample can be deduced according to the position and shape of absorption peaks in the spectrum and contents of the components of the mixture can be detected by the strength of the characteristic peaks. Infrared spectrometry has become the most widely used analysis and test instrument because of its analytical characteristics of high efficiency, high sensitivity, lower sample quantity, and good sample applicability. 

The two molecules whose vibration-rotation spectrum is shown in Figures 1.2 and 1.3, CO2 and H2O, are often encountered as interferences when mid-infrared spectra are measured (although the rotational lines in the spectrum of CO2 are often unresolved when the spectrometer resolution is 4 cm or poorer). In fact, it is good practice to eliminate all traces of these molecules in the beam path of an infirared spectrometer by purging the instrument with dry C02-free air or pure nitrogen gas, as the bands shown in Figures 1.2 and 1.3 will often be seen in the spectra. As noted above, because collisions occur- at a greater rate than the rotational frequency of molecules in the liquid state, no rotational fine structure is seen. 

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... 

A microwave pulse from a tunable oscillator is injected into the cavity by an anteima, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-mduction decay signal, which is detected with an anteima-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transfomied to yield the spectrum whose bandwidth is detemimed by the quality factor of the cavity. Hence, such instruments are called Fourier transfomi microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

If the resolving capacity of the instruments is ideal then vibrational-rotational absorption and Raman spectra make it possible in principle to divide and study separately vibrational and orientational relaxation of molecules in gases and liquids. First one transforms the observed spectrum of infrared absorption FIR and that of Raman scattering FR into spectral functions... 

If we consider FTIR instrumentation then the situation is trickier, since the equivalent resolution in nm varies across the spectrum. But even keeping the spectrum in its natural wavenumber units, we again find that, except for rotational fine structure of gases, the natural bandwidth of many (most) absorbance bands is greater than 10 wavenumbers. So again, using that figure shows the typical user how he can expect his own measured spectra to behave. 

Convolve the synthetic spectrum with the surface velocity field (rotation, granulation) and with the instrumental profile. 

The distinction between a truly continuous absorption spectrum and a banded absorption spectrum for diatomic molecules may be made by instruments of relatively low resolving power. Even though individual rotational lines are not resolved, a discrete spectrum will have sharp band heads and the appearance will in no way resemble the appearance of a continuum. 

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