Determination of the heat of formation of O_3 using vacuum ultraviolet laser-induced fluorescence spectroscopy and two-dimensional product imaging techniques

摘要

Two different techniques, vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy and two-dimensional (2D) ion counting product imaging, have been used to determine the bond energy for the dissociation of jet-cooled O_3 into O(~1D) + O_2(a ~1#DELTA#_g). The photofragment excitation (PHOFEX) spectrum for O(~1D) products is recorded by detecting the VUV-LIF signal associated with the 3s ~1D~0 - 2p ~1D transition at 115.22 nm while scanning the photolysis laser wavelength between 305 and 313 nm. A clear cut-off corresponding to the appearance threshold into O(~1D) + O_2(a ~1#DELTA#_g) is observed in this PHOFEX spectrum. The 2D image of the O(~1D) products from the O_3 photolysis near 305 nm is measured using an ion-counting method, with the detection of O(~1D) atoms by [2 + 1] resonance enhanced multiphoton ionization (REMPI) at 205.47 nm. The kinetic-energy distribution obtained from the 2D image shows rotational structure due to the O_2(a ~1#DELTA#_g, v" = 0) fragment. The bond energy into O(~1D) + O_2(a ~1#DELTA#_g) has been obtained from the rotational assignments in the kinetic-energy distribution. The two different experimental approaches give consistent results and an accurate value of the bond dissociation energy into O(~1D) + 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#_fH~0(O_3) = -144.31 +- 0.14 kJ/mol, has also been calculated from the bond energy obtained, in conjunction with thermochemical data for O_2 molecule and O atom. The uncertainty for the #DELTA#_fH~0(O_3) value obtained in the present study is smaller than the previous value which has been used widely.