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lecture notes6

If iaibic eg nh34 the energy levels are 2 2 2

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Unformatted text preview: r environments can be strong enough for maser action.2 Hyperfine structure splits this line into 4 components at 1612, 1665, 1667, and 1720 MHz. 4. Polyatomic Molecules We will only briefly discuss the physics of polyatomic molecules due to their complexity. We may consider two cases: linear and nonlinear molecules. A. LINEAR MOLECULES A linear molecule (e.g. HCN, CO2) is in many ways similar to a diatomic molecule: the key difference is that it has many more vibrational degrees of freedom, and from a symmetry perspective it is important that some of these can bend the molecule (i.e. the vibrational excitations can carry angular momentum!). However, the rotation spectrum, which is all that we will discuss here, is similar to that of a diatomic; we need only consider the appropriate moment of inertia. We note that some linear molecules have zero dipole moment by symmetry and hence no rotation spectrum (e.g. CO2). Such molecules do develop dipole moments when deformed so they do show vibrational spectra. The rotational constants for some linear molecules found in the ISM are:3 Molecule 2B 1H12C14N 88.6 GHz 1H12C16O+ 89.2 GHz Of particular interest is that due to their small number of rotational states, one can search efficiently for the microwave rotational transitions of quite large linear molecules. This is how the interstellar cyanopolyynes (HC3N, HC5N, HC7N, HC9N, HC11N) were discovered. 2 See discussion in Dopita & Sutherland, §4.4. 3 From the NIST database: http://physics.nist.gov/PhysRefData/MolSpec/Diatomic/index.html B. NONLINEAR MOLECULES The vast majority of molecules are nonlinear (H2O, NH3, CH4, CH3OH, etc.). In some cases, they h...
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