<P> These figures do not represent the "recoil" of the C atoms, which, though necessarily present to balance the overall movements of the molecule, are much smaller than the movements of the lighter H atoms . </P> <P> The simplest and most important or fundamental IR bands arise from the excitations of normal modes, the simplest distortions of the molecule, from the ground state with vibrational quantum number v = 0 to the first excited state with vibrational quantum number v = 1 . In some cases, overtone bands are observed . An overtone band arises from the absorption of a photon leading to a direct transition from the ground state to the second excited vibrational state (v = 2). Such a band appears at approximately twice the energy of the fundamental band for the same normal mode . Some excitations, so - called combination modes, involve simultaneous excitation of more than one normal mode . The phenomenon of Fermi resonance can arise when two modes are similar in energy; Fermi resonance results in an unexpected shift in energy and intensity of the bands etc . </P> <P> The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample . When the frequency of the IR is the same as the vibrational frequency of a bond or collection of bonds, absorption occurs . Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This measurement can be achieved by scanning the wavelength range using a monochromator . Alternatively, the entire wavelength range is measured using a Fourier transform instrument and then a transmittance or absorbance spectrum is generated using a dedicated procedure . </P> <P> This technique is commonly used for analyzing samples with covalent bonds . Simple spectra are obtained from samples with few IR active bonds and high levels of purity . More complex molecular structures lead to more absorption bands and more complex spectra . </P>

2. describe the conditions for ir absorption to occur