Picosecond IR study of UV-induced peroxide decomposition: Formation and vibrational relaxation of CO2 in CH2Cl2 solution

Picosecond UV pump (248.5 nm) / IR probe spectroscopy has been applied to t he study of the decomposition of several aromatic diacyl peroxides, peroxyc arbonates, and of a peroxyester dissolved in CH2Cl2. Measuring the IR trans ient absorbance in the 2100 to 2450 cm(-1) range allows to monitor the form ation and vibrational relaxation of the photoproduct CO2 via the asymmetric stretching mode (vg) with a time resolution of 1.8 ps. With each of the si x peroxides a CO2 molecule is released at delay times below 10 ps. The ener gy relaxation of the initially formed vibrationally hot CO2 is followed ove r 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 monoexpon ential decay of internal energy. Irrespective of the type of peroxide a sin gle relaxation time of 67 +/- 5 ps is found to adequately represent the coo ling behavior of CO2 in liquid CH2Cl2. The "initial" temperatures of vibrat ionally hot CO2 at a delay time of 20 ps after applying the pump pulse diff er considerably, between 1400 and 2700 K for the decompositions of tert-but yl benzoyl peroxide and tert-butyl benzoyl carbonate, respectively. These r emarkably high temperatures are assumed to originate from energy release as sociated with structural relaxation of the bent OCO moiety to form the line ar CO2 molecule.