Diatomic molecules, vibrational spectra

We previously found the selection rule A7 = 1 for a 2 diatomic-molecule vibration-rotation or pure-rotation transition. The rule (4.138) forbids A/ = 1 for homonuclear diatomics this gives us no new information as far as vibration-rotation spectra are concerned, since the absence of a dipole moment insures the absence of a vibration-rotation or pure-rotation spectrum, anyway. 

Figure 6.7 Rotational transitions accompanying a vibrational transition in (a) an infrared spectrum and (b) a Raman spectrum of a diatomic molecule...

The harmonic potential is a good starting place for a discussion of vibrating molecules, but analysis of the vibrational spectrum shows that real diatomic... 

For this case, the primary change that is observable in the IR spectrum is due to changes in the vibrahonal frequencies of the probe molecule due to modificahons in bond energies. This can lead to changes in bond force constants and the normal mode frequencies of the probe molecule. In some cases, where the symmetry of the molecule is perturbed, un-allowed vibrational modes in the unperturbed molecule can be come allowed and therefore observed. A good example of this effect is with the adsorption of homonuclear diatomic molecules, such as N2 and H2 (see Section 4.5.6.8). 

Absorption and Emission Spectra of Small Molecules. In diatomic molecules the number of vibrational and rotational levels is small, so that their energy spacing remains relatively large. Their absorption spectra are therefore line spectra which correspond to transitions to stable , associative excited states, but if a dissociative excited state is reached then the absorption spectrum becomes a continuum since such states have no vibrational levels. 

Diatomic molecules such as HC1 have one vibrational mode, but it is important to note that symmetrical diatomic molecules, such as 02, N2, Cl2, F2, and H2, do not absorb in the infrared region of the spectrum. This is... 

Exercise 28-4 Explain qualitatively how temperature could have an effect on the appearance of the absorption spectrum of a diatomic molecule A—B with energy levels such as are shown in Figure 28-1, knowing that most molecules usually are in their lowest vibrational state at room temperature. 

If one adopts the correct point of view that the complete wave function of any state of a diatomic molecule has contributions from all other states of that molecule, one can understand that all degrees of perturbation and hence probabilities of crossover may be met in practice. If the perturbation by the repulsive or dissociating state is very small, the mean life of the excited molecule before dissociation may be sufficiently long to permit the absorption spectrum to be truly discrete. Dissociation may nevertheless occur before the mean radiative lifetime has been reached so that fluorescence will not be observed. Predissociation spectra may therefore show all gradations from continua through those with remnants of vibrational transitions to discrete spectra difficult to distinguish from those with no predissociation. In a certain sense photochemical data may contribute markedly to the interpretation of spectra. 

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