Infrared rotational spectra

It passes through the sample absorption cell made of Pyrex with polyethylene windows and is detected with a liquid helium cooled bolometer. One of the lasers is frequency-modulated at 1 kHz and the detector output is processed with a lock-in amplifier, as shown. Far-infrared rotational spectra of CO, HC1 and HF have been recorded [67], and as an example of the excellent sensitivity achieved, we refer the reader to the spectrum of the OH radical [68] shown later in this chapter. Evenson s spectrometer operates over a wide range of the far-infrared region up to 9 THz, with excellent frequency stability. 

Linear molecules have / , the moment of inertia about the molecular axis, equal to zero and equal to f, which are the moments of inertia about two axes perpendicular to the molecular axis and to each other. Polyatomic linear molecules and diatomic molecules have identical rotational energy equations. However, pure infrared rotational spectra can only be observed for those molecules which possess a permanent dipole moment. In carbon dioxide and acetylene, for example, the permanent dipole moment is zero because of symmetry. 

Birnbaum G. Quantized rotational motion in liquids Far infrared rotational spectrum of HF and NH3 in liquid SF6, Mol. Phys. 25, 241-5 (1973). 

We conclude this section on modulation spectrometers by describing a particularly novel and important method, known as velocity modulation, which was originally developed by Gudeman, Begemann, Pfaff and Saykally [10]. It applies specifically to ionic species, and has been used primarily to study the infrared vibration-rotation spectra of molecular ions. If we need an excuse to include it in this book, however, it is provided by Matsushima, Oka and Takagi [11] who used the method to study the far-infrared rotational spectrum of the HeH+ ion. 

Spherical top molecules which have three equal moments of inertia have a rotational energy equation identical to that for linear molecules. However, since spherical top molecules have no permanent dipole moment (it is equal to zero), no pure infrared rotational spectrum is observed. 

The fluctuating cage model presented in Chapter 7 is an alternative. The idea came from comparison of the different kinds of absorption spectra of HC1 found in liquid solutions (Fig. 0.5). In SFg as a solvent the rotational structure of the infrared absorption spectrum of HC1 is well resolved [15, 16], while in liquid He it is not resolved but has... 

The chemistry of all of these molecules is fascinating but, concentrating on the origins of life, a detailed look at the organic species is appropriate to see what molecules are present and how they might have been formed. The only alkane detected directly in the ISM is methane but this is due to the problem of detection. All alkanes are non-polar and so do not have a pure rotation spectrum. However, there is one allowed vibration of methane that is infrared active and with the low moment of inertia of methane the vibration-rotation spectrum can be observed and a rotational progression identifies the molecule with confidence. 

Infrared radiation of frequencies less than about 100 cm-1 is absorbed and converted by an organic molecule into energy of molecular rotation. This absorption is quantized thus a molecular rotation spectrum consists of discrete lines. 

The a3 n state of CO was first identified through its ultraviolet emission spectrum to the ground state, producing what are now known as the Cameron bands [160, 161, 162], Its radioffequency spectrum was then described by Klemperer and his colleagues in a classic series of molecular beam electric resonance experiments. Its microwave rotational spectrum was measured by Saykally, Dixon, Anderson, Szanto and Woods [163], and the far-infrared laser magnetic resonance spectrum was recorded by Saykally, Evenson, Comben and Brown [164], In the infrared region both electronic... 

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