Understanding the Gap in a HCl Absorption Spectrum

[v_pino]

Looking at a HCl spectrum (from hyerphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/vibrot.html), I don't understand why there is a gap in the middle of the spectrum that seems to act as a line of symmetry in the x-axis.

Can anyone help please?

 

[_PJ_]

http://www.cartage.org.lb/en/themes/Sciences/Physics/MolecularPhysics/MolecularSpectra/Rotation/Rotation.htm

 

[v_pino]

Hi PJ, thanks for the link. But can you be more specific as to which bit of the article you're referring to? Or do I have to go through the math for the explanation? Thanks.

 

[v_pino]

From the article, I gather that the "gap" is the Q branch, which is the forbidden region. But why?

 

[Claude Bile]

See reference (http://en.wikipedia.org/wiki/Rovibrational_coupling)

Vibrational spectra can be broken into 3 parts.

P branch; -1 change in rotational quantum number (J).
Q branch; no change in rotational quantum number.
R branch; +1 change in rotational quantum number.

The Q-branch is forbidden for linear (straight-line) molecules, hence the gap in the center of the spectrum. In the 1st approximation, the spacing between the energy levels in the P and R branch should be equal, assuming a perfect harmonic oscillator. Molecules are not, in general, perfect harmonic oscillators however, and so the energy spacing tends to decrease with increasing change in rotational quantum number.

Claude.