Spectroscopy

Spectroscopy refers to the measurement of radiation intensity as a function of wavelength/ frequency/ energy.

• Transitions involve changing energy.
• We will look at molecular transitions changing energy by absorption or emission of a photon.
  • Electronic - Visible to UV
  • Vibrational - Visible to Infrared
  • Rotational - Infrared
• From one quantum state to another.
• Notated by a change the the quantum numbers.

Transitions for Diatomics:

Beginners

• When a specific energy photon of light is absorbed by the molecule, the molecule can go from an initial quantum state to a final quantum state; this is called a transition. The energy of the photon is called the transition energy.

• Because the photon must cause the molecule to change from the initial state to the final state, there are selection rules that govern which transitions are allowed and which are forbidden. This is usually expressed in allowed changes in quantum numbers

Intermediate

• For example, J = +/- 1 or J=0 is a common selection rule

• v=0 or v=+/- 1 is another common selection rule

• The selection rules associated with electronic state are more complicated. The spin not changing is common. Lambda changing by +/- 1 or 0 is common.

• Rotational transitions involve change in J, no change in v or electronic state. These occur in microwave. There is usually a P and R branch (for -1 and +1).

• Vibrational transitions involve change in v. Rovibrational transitions have resolved J quantum number and can also involve changes in J. These occur in infrared.

• Electronic transitions involve change in electronic state. Rovibronic transitions have resolved v and J quantum number and can involve also changes in J and v. These occur in visible or UV usually.

Advanced

• Selection rules can be broken, but these normally lead to much weaker transitions.