Infrared spectroscopy provides a comprehensive view of chemical and conformational structures of molecules. Spin cast amorphous and semicrystalline PET thin films on silicon substrates were investigated with transmission IR to reveal the effect of deep UV (172 nm) irradiation on molecular architecture. Difference spectra by subtracting original samples from the UV treated ones show features consistent with a Norrish type I based decarbonylation and a Norrish type II process producing terminal carboxylic acid groups. Conformation selectivity of the photochemistry is discussed on the basis of three major structure factors related to the formation of a cyclic transition state of the Norrish type II process. Quantitative analysis was carried out by simulating carbonyl stretching band in the difference spectra with ester and acid type carbonyl model spectra. The effects of UV fluence and molecular mobility on quantum yields were elucidated. Quantum yields of 172 nm UV photochemistry in bulk PET were calculated to be no different from those of longer wavelength (200-400 nm). UV modified depth were also estimated by means of plasma etching of the samples. (c) 2005 Elsevier Ltd. All rights reserved.