Ultraviolet and visible dispersed spectroscopy for the photofragments produced from H2O in the extreme ultraviolet

摘要

The photofragmentation of H2O has been studied by fluorescence spectroscopy at photon energies between E-hnu = 16.9-54.5 eV. The primary photon beam was monochromatized undulator radiation supplied from the UVSOR synchrotron radiation facility. The fluorescence in the wavelength range of 280-720 nm was dispersed with an imaging spectrograph. The dispersed spectra exhibit the hydrogen Balmer lines of H*[n(2)L'(J') --> 2 L-2"(J") (n = 3 - 9)] and the emission band systems of H2O+ [(A) over tilde (2)A(1) (0, v'(2), 0) -->(X) over tilde B-2(1) (0,0,0)], OH+ ((A) over tilde (3)Pi(Omega), v' --> (X) over tilde (3)Sigma(-), v"), and OH((A) over tilde (2)Sigma(+), v' --> (X) over tilde2Pi(Omega), v"). The fluorescence cross sections for these transitions have characteristic dependences on E h n and vibrational quantum numbers. The cross section summed over the Balmer lines takes a minimum value at E-hnu = 21.7 eV and is very small even at 24.9 eV beyond which it steadily increases with increasing E-hnu. This behavior is understood as that the superexcited states correlating with H*(ngreater than or equal to3) + OH((A) over tilde (2)Sigma(+)) are too repulsive to be accessible below E-hnu similar to30 eV by the Franck-Condon transitions from H2O((X) over tilde (1)A(1)) and as that the Balmer emission below 30 eV is mainly due to the H*(ngreater than or equal to3) +H(n=1) +O(P-3(g)) channel. The appearance energy of the OH+ ((A) over tilde (3)Pi(Omega), v' --> (X) over tilde (3)Sigma(-), v") transitions is found to be ca. 25.5+/-0.3 eV. This value is much higher than the dissociation limit of 21.5 eV for the OH+ ((A) over tilde (3)Pi(Omega))+H(n=1) channel, but is consistent with the vertical ionization energy to H2O+ [(1b(1))(-2) (4a(1)) (1 2) A(1)] that has been assumed to correlate with the above dissociation limit in the literature. The vibrational distribution of OH+ ((A) over tilde (3)Pi(Omega)) evaluated from the OH+ ((A) over tilde (3)Pi(Omega), v' --> (X) over tilde (3)Sigma(-), v") band intensities is similar to the prior distribution in the rigid-rotor harmonic-oscillator approximation. (C) 2002 American Institute of Physics. [References: 34]