UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 193

Increasing energy

Increasing wavelength

X-Ray Ultraviolet Visible Infrared Microwave Radio wave

Inner
electron
excitation

Valence
electron
excitation

Molecular
vibration

Molecular
rotation

Nuclear
spin

UV-Visible
Spectroscopy

Infrared (IR)
spectroscopy

Sample
extraction

XRF NMR

Figure 8.3 Molecular responses to radiations of various wavelength ranges (not to scale)

ground state by emitting a new and longer wavelength radiation termed as
the fluorescence radiation (Kebbekus and Mitra, 1998) (Fig. 8.4).

8.1.2 Origins of Absorption in Relation to Molecular
Orbital Theories

The above discussion as to why UV radiation causes electron excitation and why
infrared radiation causes molecular vibration may not be intuitive without the

Radiation
emitted

Radiationless
decay

Electronic

translational
levels E 2

UV

Vibrational
levels

E 1

VIS

Fluorescence
emission

IR

Rotational
levels

E 0

Absorption Emission Fluorescence

E 0 = Ground level; E 1 , E 2 = Excited states

Energy spacing: vibration > rotation >> translation

Figure 8.4 Three types of spectroscopy—absorption, emission, and fluorescence