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Picosecond UV pump (248.5 nm)/IR probe spectroscopy has been applied to the study of the decomposition of several aromatic diacyl peroxides, peroxycarbonates, and of a peroxyester dissolved in CH 2 Cl 2 . Measuring the IR transient absorbance in the 2100 to 2450 cm -1  range allows to monitor the formation and vibrational relaxation of the photoproduct CO 2  via the asymmetric stretching mode (ν 3 ) with a time resolution of 1.8 ps. With each of the six peroxides a CO 2  molecule is released at delay times below 10 ps. The energy relaxation of the initially formed vibrationally hot CO 2  is followed over the time range up to 500 ps. Analysis of the transient IR spectra, via an anharmonic oscillator model proposed by Hamm  et al.  [1], shows a monoexponential decay of internal energy. Irrespective of the type of peroxide a single relaxation time of 67±5 ps is found to adequately represent the cooling behavior of CO 2  in liquid CH 2 Cl 2 . The "initial" temperatures of vibrationally hot CO 2  at a delay time of 20 ps after applying the pump pulse differ considerably, between 1400 and 2700 K for the decompositions of  tert -butyl benzoyl peroxide and  tert -butyl benzoyl carbonate, respectively. These remarkably high temperatures are assumed to originate from energy release associated with structural relaxation of the bent OCO moiety to form the linear CO 2  molecule.

Picosecond IR Study of UV-Induced Peroxide Decomposition: Formation and Vibrational Relaxation of CO 2 in CH 2 Cl 2 Solution