Atomic fluorine emission cross sections observed in the 600-900 nm region following 0-500 eV electron impact on fluoromethanes (CF4, CHF3, CH2F2 and CH3F)

Absolute emission cross sections (sigma(em)) of F I atom electronic transit ions in the 600-900 nm region, following the electron impact on CHxF4-x (x = 0-3) molecules and determined by the optical method, are reported. The ra nge of electron energies studied varies from threshold to 500 eV. In the el ectron impact on CF4 at pressures higher than 2 mTorr, a continuum backgrou nd band in the 500-700 nm region is observed and has been assigned to the C F3* ((2)A(2)" --> (1)A(1)' and/or E-1' --> (1)A(1)') transitions. Continuum bands of the same features were also found in collisional processes on CHF 3 and CH2F2, although their maxima are shifted to the blue with respect to that of CF4 and have been tentatively assigned to the F-2* ion pairs. For l ow F content fluoromethanes high electronic excited states (3d) of atomic F are observed and may be due to redistribution of the excess electron energ y over that for C-F bond dissociation.