Lymanhyphen;agr; fluorescence has been observed from the atomic hydrogen photodissociation fragments of CH4and H2O, and the fluorescence cross sections have been measured throughout the synchrotron radiation continuum from 175 to 780 Aring;. The maximum fluorescence cross sections are 4.6times;10minus;20cm2at 568 Aring; and 1.72times;10minus;18cm2at 703 Aring; for CH4and H2O, respectively. The photodissociation is mainly a direct process following absorption of a photon into the continuum state(s). Structure in the excitation functions is attributed to predissociation of Rydberg series. It is noted that in H2O, but not in CH4, there is a strong competition between the predissociation and photoionization channels. It is shown in CH4, in particular, that the use of fluorescence detection of the dissociation fragments provides very useful information for interpreting photoabsorption processes in the high energy region. The Lymanhyphen;agr; emission resulting from the dissociation process CH4+hngr;rarr;CH3+H(n= 2) has been observed in the present work although the threshold for this process has not been determined. However, the threshold corresponding to the process CH4+hngr;rarr;CH2+H+H(n= 2) and/or CH+H2+H(n= 2) has been measured and found to agree with that determined from electron impact studies.
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