Heteronuclear diatomic rotational spectra

Most values of heteronuclear diatomic rotational constants B fall between the extremes of HF (20.9 cm ) and ICl (0.114cm ). For these species, the frequencies of the J = 0 -+ 1 rotational transitions are 41.8 and 0.228 cm both are in the far-infrared to microwave region of the electromagnetic spectrum. Some representative rotational constants are listed in Table 3.1. 

When interpreting time-resolved mid-IR spectra, it is beneficial to consider the influence of rotational dynamics on the vibrational spectrum of a heteronuclear diatomic. It was shown more than 30 years ago that the vibrational absorption spectrum of a diatomic is related to its transition dipole correlation function (/z(0) /r(t)> through a Fourier transform (10) ... 

Let s now consider how rotational spectroscopy can give information about the structure of a molecule. For example, if the energy of the photon necessary to promote a heteronuclear diatomic molecule from E0 (J = 0) to Ej (/ = 1) is determined, the value of / for the molecule can be calculated, which in turn allows the calculation of RL.. Thus the rotational spectrum of a diatomic molecule provides an accurate method for measuring its average bond length. 

Radiation in the far (long-wavelength) infrared and microwave regions of the electromagnetic spectrum excites rotational states of molecules. In order to absorb radiation, the molecule must have a permanent dipole moment. Moreover, for a heteronuclear diatomic molecule, absorption of light is possible only between... 

---