Threshold and cage effect for photodissociation of H2O in solid Ne and Ar

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Abstract
We determined the threshold for photodissociation of H2O to form OH in matrices by dissociating a matrix sample in situ on scanning the energy of synchrotron radiation and by detecting the increase in intensity of laser-induced fluorescence of OH. The derived threshold, 6.89±0.04 eV for H2O in solid Ar, is almost identical to the previously reported value of 6.9±0.04 eV. The photodissociation threshold in solid Ne, 6.87±0.02 eV, is within experimental uncertainty of that determined in solid Ar. They are identical to recently determined threshold of absorption of H2O in solid Ne and Ar. Hence, photodissociation of H2O in matrices near the threshold is determined by absorption from the ground X̃ state to the repulsive à state. The thresholds of absorption of H2O correspond to ∼1.7 eV excess energies of dissociation; at these energies sudden exit should dominate, consistent with the results from H2S photolysis in matrices.