Photofragmentaion mechanisms of H2O studied by dispersed fluorescence spectroscopy

Full text: The photofragmentation of H2O has been studied by florescence spectroscopy at photon energies between hv = 16.9 - 54.5 eV. Monochromatized undulator radiation was used for the primary photoexcitation source. The florescence in the wavelength range of 280 - 720 nm was dispersed with an imaging spectrograph. At hv ∼ 30 eV many vibrational bands of the OH+(A-bar3ΠΩ → X-bar3Σ-) and OH(A-bar2Σ+ → X-bar2ΠΩ) transitions can be identified. The appearance energies of the OH (A-tilde → X-tilde) and OH(A-tilde → X-tilde) transitions are found to be 25.5 ± 0.3 and 29 - 30 eV, respectively. These values are much higher than the dissociation limits for the OH+(A-tilde3ΠΩ) + H(n=1) and OH(A-tilde2Σ+) + H*(n ≥2) channels, respectively. The values are, however, consistent with the vertical transition energies from H2O( X-tilde1A1) to H2O+[(1b1)-2 (4a1) 12A1] and H2O*[(2a1)1 (nl)1 ≥ 2], respectively, that have been assumed to correlate with the above dissociation limits in the literature. Thus, the potential energy surfaces of these excited states are considered to be highly repulsive along the dissociation coordinate as shown. On the other hand, the Balmer emission observed between hv = 22 and 30 eV is ascribable to the three-body dissociation channel of H*(n ≥3) + H(n=1) + O(3Pg), which has the thermochemical threshold of 21.7 eV with n = 3