Definitions

The following are a few notes from my reading of Higher Excited States of Polyatomic Molecules, vol. 1, by M. .B. Robin [1].

Energy regions can be associated with certain types of transitions. Robin divides the spectrum as follows:

0 - 10 000 cm^-1:

Mostly vibrational and rotational transitions of polyatomic molecules.

10 000 - 50 000 cm^-1:

Electronic transitions in molecules which are or unsaturated or contain lone-pairs. The presence of air presents no experimental difficulties and quartz optics may be used. The electronic transitions in this region (visible and quartz ultraviolet) can be described as valence shell transitions and hence easier for theory. In the absence of theoretical predictions, comparison to similar systems is often helpful.

50 000 - 100 000 cm^-1:

This is the vacuum ultraviolet, a much more treacherous region. Quartz is opaque in this region and must be replaced by high-quality sapphire or salt optics or other spectroscopic techniques must be used. The solvents and the presence of air may also hamper experiments. Furthermore, the density of valence transitions with repsect to wavenumber increases dramatically after 50 000 cm^-1 (it tails off again eventually, at a few hundred thousand wavenumbers), making theoretical modelling and assignment difficult. Even worse, Rydberg transitions typically begin around 50 000 cm^-1. This energy region is the focus of Robin's work.

100 000 + cm^-1:

The extreme vacuum-ultraviolet region; there are no longer transparent, strong window materials, and transitions are often ionizing or autoionizing.

A few other definitions regarding the transitions are in order:

Vertical:

The most intense band in the vibrational Franck-Condon envelope.

Adiabatic:

The frequency for a transition where both upper and lower states have no vibrational quanta (i.e., a transition between zero-point levels).

Advertical:

A transition which is both vertical and adiabatic.