All polyatomic molecules and hetero-nuclear diatomic molecules absorb infrared radiation. The absorption changes the molecular rotation and vibration. The pattern of absorption therefore depends on the physical properties of the molecule--such as the number and type of atoms, the bond angles and bond strengths. This means that each spectrum differs from all others and may be considered the molecular "signature". Monatomic gases such as radon and homonuclear diatomic molecules such as oxygen and nitrogen do not have infrared bands and therefore must be measured by non-infrared means. Diatomic molecules such as NO, CO, HCl, and HF have a single major band that is an array of individual lines, each with a width of about 0.2 cm-1. Linear polyatomic molecules like CO2 , N2O and C2H2 also show arrays of individual lines. Non-linear polyatomic molecules like O3,

SO2, NH3, H2CO, CH4 and H2O have many apparent "lines" that are in fact small bundles of lines, with the widths of the bundles varying from 0.2 cm-1 to many cm-1. For larger polyatomic molecules at atmospheric pressure there are so many lines overlapping each other that the spectral features are broad and smooth, except for occasional "spikes". See the figure here.


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