Rovibrational Transitions

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Ro-Vibrational Transitions

1 Order of Magnitude Energies

The Born-Oppenheimer approximation allows us to treat the electrons in a molecule as a cloud– they are much less massive and therefore have much higher velocities than the nuclei. If is the molecular size, typical electrons have momentum and the electronic energy spacings can be expressed as:

The nuclei feel an equivalent potential that only depends on the internuclear distance and the electronic state. The internuclear potential has a minimum, and vibrations about the minimum can be roughly modeled as a harmonic oscillator. This potential is about , where is the vibration frequency and is the displacement from equilibrium. If the displacement is the same order as the size of the molecule, the electronic energies should be about :

The nuclei can also rotate, and have rotational energies that depend on their angular momentum. If is the quantum angular momentum number,

For small ,

So the electronic, vibrational, and rotational energy states have contributions that scale with the electron-to-nucleus mass ration:

2 Rotation-Vibration Spectra

While it is possible to have a pure rotational spectrum, a pure vibrational spectrum is very unlikely: energies required to excite vibrations are much larger than those required to excite rotation. However, a combination of rotation and vibrational modes can be excited.

Rotational energies can be described using the angular momentum number :

where , is the moment of inertia, and is the reduced mass:

The vibrations can again be modeled as a harmonic oscillator:

where and where is the effective spring constant.

The total energy of rovibrational transitions, then, is:

The selection rules for rovibrational transitions tell us that and . is called the R branch, and is called the P branch. This notation matches the videos above, but is opposite the notation in Rybicki & Lightman. We can use our rovibe energy expression to find the frequency of emitted photons for the R branch:

and the P branch:

However, we should note that the potential is not perfectly harmonic; the slight asymmetries we can see in the potential cause the bond length to increase as increases. So if increases, increases, causing to increase. Our rovibrational energy expression depends on , so the energy decreases as increases. This effectively pulls all of our spectral lines to the left, which decreases the separation between lines on the R branch and increases the separation between lines on the P branch. Similar effects can also be found when comparing the expressions for the observed frequencies. Putting everything together, we can plot our rovibrational spectrum.