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The infrared absorption in microwave plasma-deposited, polycrystalline diamond films has been measured for the C-H stretch-mode and the diamond two-phonon mode, giving absorption coefficients of -26 and 7.5 cm' respectively for a 2Oun thin film, assuming that the whole thickness of the film is responsible. The C-H absorption band envelope has been deconvoluted into its various components, allowing some estimation ofthe relative importance ofCH, CH2, and CH3 groupings in the films. A significant absorption at 3 j.m due to N-H has been measured in films exposed to atomic nitrogen. 1JNFRARED ABSORPTION IN DIAMONDThe principal infrared absorptions in diamonds are due to one-phonon absorption (near lOjim), two-phonon absorption (near 5jun), and three-phonon absorption (near 3ji.m)'; in addition there are impurity-related bands in doped material (eg a 7.8un band for nitrogen)2 and C-H stretch-mode absorptions near 3.5un fordiamond grown in hydrogen 3. With the advent of "synthetic" diamond prepared by chemical vapour deposition techniques, the relative importance of these bands has changed from the domination by impurities in natural diamond to the significant C-H absorptions observed in microwave plasma deposited films .Since plasma CVD processes can produce polycrystalline diamond having no detectable incorporation of metals, nitrogen or oxygen, it is now possible to grow material in which the two-phonon and C-H absorptions are the absorption limitations for mid-JR transparency. Since the two-phonon absorption appears to be a fundamental property of the diamond lattice, the only possible improvement may be to reduce the hydrogen content of the polycrystalline diamond.2JNFRARED SPECTROMETRY 0.0200-' 0.0150-H w C-)