Understanding the Gap in a HCl Absorption Spectrum

Click For Summary

Discussion Overview

The discussion revolves around the gap observed in the HCl absorption spectrum, specifically questioning its nature and significance within the context of molecular spectroscopy. Participants explore the theoretical underpinnings of vibrational spectra and the implications of rotational quantum numbers.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant expresses confusion about the gap in the HCl spectrum, seeking clarification on its meaning and implications.
  • Another participant references an article but requests more specific guidance on which section is relevant to the discussion.
  • A participant identifies the gap as the Q branch, describing it as a forbidden region, but questions the reason behind this classification.
  • Further elaboration is provided on the vibrational spectra, detailing the P, Q, and R branches and their associated changes in rotational quantum numbers, with an emphasis on the Q branch being forbidden for linear molecules.
  • It is noted that the assumption of perfect harmonic oscillators does not hold for real molecules, leading to variations in energy level spacing.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the reasons for the gap in the spectrum, with ongoing questions regarding the Q branch and its implications. Multiple viewpoints and clarifications are presented without resolution.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about molecular behavior and the definitions of the branches in vibrational spectra. The complexity of real molecular behavior versus idealized models is acknowledged but not fully resolved.

v_pino
Messages
156
Reaction score
0
Physics news on Phys.org
http://www.cartage.org.lb/en/themes/Sciences/Physics/MolecularPhysics/MolecularSpectra/Rotation/Rotation.htm
 
Last edited by a moderator:
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.
 
From the article, I gather that the "gap" is the Q branch, which is the forbidden region. But why?
 
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.
 

Similar threads

  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 17 ·
Replies
17
Views
8K
  • · Replies 9 ·
Replies
9
Views
8K
  • · Replies 23 ·
Replies
23
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K