Determination of the heat of formation of O-3 using vacuum ultraviolet laser-induced fluorescence spectroscopy and two-dimensional product imaging techniques

Two different techniques, vacuum ultraviolet laser-induced fluorescence (VU V-LIF) spectroscopy and two-dimensional (2D) ion counting product imaging, have been used to determine the bond energy for the dissociation of jet-coo led O-3 into O(D-1)+O-2(a (1)Delta(g)). The photofragment excitation (PHOFE X) spectrum for O(D-1) products is recorded by detecting the VUV-LIF signal associated with the 3s D-1(0)-2p D-1 transition at 115.22 nm while scannin g the photolysis laser wavelength between 305 and 313 nm. A clear cut-off c orresponding to the appearance threshold into O(D-1)+O-2(a (1)Delta(g)) is observed in this PHOFEX spectrum. The 2D image of the O(D-1) products from the O-3 photolysis near 305 nm is measured using an ion-counting method, wi th the detection of O(D-1) atoms by [2+1] resonance enhanced multiphoton io nization (REMPI) at 205.47 nm. The kinetic-energy distribution obtained fro m the 2D image shows rotational structure due to the O-2(a (1)Delta(g),v(') =0) fragment. The bond energy into O(D-1)+O-2(a (1)Delta(g)) has been obtai ned from the rotational assignments in the kinetic-energy distribution. The two different experimental approaches give consistent results and an accur ate value of the bond dissociation energy into O(D-1)+O-2(a (1)Delta(g)) is found to be 386.59 +/- 0.04 kJ/mol. The standard heat of formation of O-3, Delta(f)H(0)(O-3)=-144.31 +/- 0.14 kJ/mol, has also been calculated from t he bond energy obtained, in conjunction with thermochemical data for O-2 mo lecule and O atom. The uncertainty for the Delta(f)H(0)(O-3) value obtained in the present study is smaller than the previous value which has been use d widely. (C) 1999 American Institute of Physics. [S0021-9606(99)02338-7].