Rotational absorption, pure

IR emission spectroscopy makes use of the reciprocal effect of IR absorption spectroscopy. At temperatures above 0 °K, molecules undergo a number of vibrational, vibrational-rotational or purely rotational movements. The relaxation of these excited states leads to the emission of thermal radiation, primarily in the IR region. 

At room temperature, many rotational levels of a molecule are well-populated because of the small energy differences between rotational levels. Thus, in contrast to electronic and vibrational absorption spectra, most of the lines in a pure-rotational absorption spectrum do not involve the ground state. 

Which of the following molecules will exhibit a pure rotational absorption spectrum ... 

Second, due to the nonzero matrix elements of the operator of the dipole moment, transitions between the rotational sublevels of the same vibronic level with the change of the projection K by unity (AA = 1) are possible. In the work of Child and Longuet-Higgins (1961) it is noted that the possible vibronic pure rotational absorption spectrum within the same vibronic level is somewhat similar to the pure rotational spectrum in the excited degenerate dipolar-type state of harmonic oscillators predicted by Mizushima and Venkatesvarlu (1953). 

E13.12(b) Polar molecules show a pure rotational absorption spectrum. Therefore, select the polar molecules based on their well-known structures. Alternatively, determine the point groups of the molecules and use the rule that only molecules belonging to C , C v, and Cs may be polar, and in the case of C and C v, that dipole must lie along the rotation axis. Hence all are polar molecules. 

EXAMPLE The lowest-frequency pure-rotational absorption line of occurs at... 

For a certain diatomic molecnle, two of the pure-rotational absorption lines are at 806.65 GHz and 921.84 GHz, where 1 GHz = 10 Hz, and there are no pure-rotational lines between these two lines. Find the initial 7 value for each of these transitions and find the molecular rotational constant B. 

Pure rotational absorption by gases is observed in the far-IR region, provided the molecules have permanent dipole moments. Examples include H,0.0,. HCl, and AsH). Absorption by water is troublesome elimination of its interference requires evacuation or at least purging of the spectrometer. 

By definition, molecules that are spherical tops do not have a permanent dipole moment, so they do not have a pure rotational spectrum. However, imder some conditions they may have rotational absorptions superimposed in their vibrational spectrum. 

The same information may be obtained from purely rotational far infrared spectroscopy (FIR) and depolarized Rayleigh spectra. Dielectric relaxation measurements are also used for the same goal, most successfully in combination with far-infrared data. The absorption coefficient of a periodic electric field... 

Despite the fact that relaxation of rotational energy in nitrogen has already been experimentally studied for nearly 30 years, a reliable value of the cross-section is still not well established. Experiments on absorption of ultrasonic sound give different values in the interval 7.7-12.2 A2 [242], As we have seen already, data obtained in supersonic jets are smaller by a factor two but should be rather carefully compared with bulk data as the velocity distribution in a jet differs from the Maxwellian one. In the contrast, the NMR estimation of a3 = 30 A2 in [81] brought the authors to the conclusion that o E = 40 A in the frame of classical /-diffusion. As the latter is purely nonadiabatic it is natural that the authors of [237] obtained a somewhat lower value by taking into account adiabaticity of collisions by non-zero parameter b in the fitting law. 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

---