CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 430
- Based on the source of the photons used, spectra are classified asemission, when we
measure the characteristics of photons originating in the substance,absorption, when
we measure the amount of photons absorbed by the substance, andRaman, when we
measure the change of the photon wavelength after their collision with molecules. - Based on the type of the quantum states of molecules, spectra are classified aselectron,
vibrational,rotationalandmagnetic resonance. - Based on the appearance of the originating spectrum, spectra may be roughly distin-
guished aslinespectra, where interaction occurs in narrow wavelength regions (peaks)
which are usually clearly separated from each other, andcontinuousspectra, where ab-
sorption occurs in broader wavelength regions (bands) which, in addition, often overlap.
Rough connection of different spectra classifications based on the individual categories fol-
lows from the following table:
Type of Radiation characteristic Type of quantum
spectrum frequency [Hz] wavelength [m] transitions
X-ray 3 × 1016 to 3× 1020 1 × 10 −^12 to 1× 10 −^8 transition of
inner electrons
ultraviolet 7. 7 × 1014 to 3× 1016 1 × 10 −^8 to 4× 10 −^7 transition of
valence electrons
visible 3. 8 × 1014 to 7. 7 × 1014 4 × 10 −^7 to 7. 9 × 10 −^7 transition of
valence electrons
infrared 1 × 1011 to 3. 8 × 1014 7. 9 × 10 −^7 to 3× 10 −^3 vibrations and ro-
tations
microwave 3 × 108 to 1× 1011 3 × 10 −^3 to 1 ESR^1 , rotations of
heavy molecules
radiofrequency 3 × 106 to 3× 108 1 to 100 NMR^112.3.4 Electronic spectra
The simplest spectrum of electron transitions is given by the absorption or emission of a photon
in an isolated hydrogen atom. The energy of the hydrogen atom acquires the values given by
(^1) ESR—electron spin resonance and NMR—nuclear magnetic resonance, see(12.3.7)