Why does diamond absorb infra-red radiation?

Diamond is being increasingly used as a window for transmitting infra-red radiation. We show here that absorption at the important (CO laser) wavelength of 10.6 ?m involves the simultaneous destruction and creation of phonons. Its strong temperature dependence in the range 300 K < T < 650 K is accurately described, without any adjustable parameters, in terms of three main components: the destruction of one phonon of 335 cm and the creation of a second of 1275 cm; the shift to lower energy of the phonons; and a three-phonon process involving the destruction of one and the creation of two phonons. The analysis demonstrates why diamond has to be effectively cooled when used for the windows of a high-power CO laser.