Molecule spectrum, diatomic larger

The second promising method is the use of the spectrum of a diatomic or larger molecule. As discussed in Section II-C, if one can describe accurately the population distribution for the molecule under excitation conditions, then the temperature can be extracted from the measured spectrum. The difficulty lies in capturing the spectrum in a sufficiently short time period. This can be accomplished through the use of a multiple detector array, or Optical Multichannel Analyzer such as is manufactured by Princeton Applied Research Co. (30). 

In principle, it is not possible to equate the frequency of a diatomic A-B molecule with the frequency of an A-B group in a larger molecule. This is demonstrated by some triatomic molecules. A well-known example which illustrates this point is the difference between the CO frequency in the IR spectrum of the CO molecule (2136 cm ) and... 

Molecular species are also formed in the plasma or in the ion beam formed by ions produced in the plasma. These molecular species are predominantly diatomic in structure, although triatomic and larger molecular species are possible. For most triatomic and higher molecular species, the energy required to break them up into simpler molecules is sufficiently small that the probability of them remaining stable in the high-temperature plasma is very low. Therefore, the absence of the majority of these species in the spectrum is expected and observed. A notable exception occurs when the individual components of the polyatomic molecule are present at high... 

This is, first of all, due to the manifold of rotational and vibrational levels within each electronic state and furthermore to a larger variety of angular momentum coupling, such as spin-rotation interaction, A-type doubling, fine and hyperfine structure. In addition different kinds of perturbations may further increase the line density and the complexity of the spectrum. Even for small molecules, such as diatomic or triatomic molecules, the spacings between rotational lines of an electronic transition may become much smaller than the Doppler-width. This implies that single rotational lines often cannot be resolved with "classical" Doppler-limited techniques. 

A diatomic molecule approximates the model just discussed such that the mass of one atom is much larger than the other atom such as hydrogen bromide. In infrared spectroscopy, the absorbed infrared radiation results in transitions in both the vibrational and rotational states of a molecule. Considering only the vibrational transitions, what would an infrared spectrum of hydrogen bromide look like based on the classical result According to classical mechanics, would infrared spectroscopy be a useful tool in chemistry ... 

The RRHO approximation predicts that the infrared spectrum of a diatomic molecule will have a number of peaks all of equal separation (2Bo) except one larger gap of 4Bo between the set of peaks where J is increasing by one (R-branch) and the set where J is decreasing by one (P-branch). As can be seen by Figure 6-2, the RRHO approximation predicts the infrared spectrum of a diatomic molecule remarkably well in spite of the harmonic oscillator approximation for vibrational motion and the sevare truncation of the power series in Equation 6-18. The distinct gap between the P and R-branches can be clearly seen. However, note that the distance of separation between the peaks in the infrared spectrum has some variation whereas the... 

---