Instant Notes: Analytical Chemistry

Section E – Spectrometric techniques

E2 ATOMIC AND MOLECULAR

SPECTROMETRY

Transitions For a given set of energy levels, defined by their quantum numbers, there are
several possible types of transition (Figs 1 and 2). Consider just two energy
levels, the upper with energy EUand the lower, EL, separated by an energy
difference DE, where

DE =EU- EL

There is a definite statistical probability of transitions between these levels,

which depends on the structure and population of the energy levels and also on

DE. These are summarized by selection ruleswhich govern allowed transitions.

If energy is supplied to the sample, for example, by passing electromagnetic

radiation through it, the sample atoms or molecules may absorb energyand be

promoted into the higher energy level. The radiation emerging from the sample

Key Notes

Transitions between energy levels may occur in several ways, providing

different types of spectra for analysis.

The intensity of the spectral emission or the reduction of intensity by

absorption is related to the concentration of the species producing the

spectrum.

An equation relating the concentration of an absorbing species and the

path length through the sample to the absorbance at a particular

wavelength was derived by Beer and Lambert and is used extensively in

quantitative absorption spectrometry.

Certain transitions are favored by quantum theory rules. Others are much

less favored or are forbidden. The allowed transitions are summarized by

selection rules.

Related topics Other topics in Section E.

Transitions

Quantitative

spectrometry

Beer–Lambert

absorption law

Selection rules

EU

E

EL

Absorption

EU

EL

Emission

Fig. 1. Absorption and emission of radiation.