The surface photochemistry of submonolayer to multilayer amounts of CF
3I, adsorbed on Ag(111) at 95 K, has been studied using 248 and 193 nm
pulsed laser excitation. For low doses, there is some thermally activ
ated dissociation, limited to 30% of the first monolayer, to form adso
rbed CF3 and I. The remaining CF3I adsorbs molecularly. Neither CF3 no
r I is photoactive, but adsorbed CF3I is photodissociated, by C-I bond
cleavage, at both 248 and 193 nm. A fraction of the resulting CF3 and
I desorbs during photolysis; the remainder is retained as chemisorbed
CF3 and I. The former processes were probed using time-of-flight and
Fourier transform mass spectrometry. The retained products were detect
ed by post-irradiation temperature programmed desorption and Auger ele
ctron spectroscopy. The photochemistry varied with wavelength and cove
rage. Regarding the mechanism, for both 193 and 248 nm, there is good
evidence that both submonolayer and multilayer CF3I molecules absorb p
hotons and dissociate into CF3 and I, i.e., direct photodissociation.
There is evidence, based on time-of-flight distributions of CF3 photof
ragments, that I((2)p(1/2)), electronically excited I, is produced at
both wavelengths, while ground state atomic iodine is produced only at
248 nm. At both 193 and 248 nm, and for coverages up to three monolay
ers, there is also evidence for a charge transfer process involving ho
t electrons produced by photon absorption in Ag(111), i.e., substrate
mediated photodissociation. These hot carriers attach to CF3I, and the
resulting anion dissociates into CF3 and I-. The latter is detected b
y Fourier transform mass spectrometry and the former as a low-velocity
component in time-of-flight mass spectrometry. At 193, but not 248 nm
, there is evidence for a second, substrate independent, charge transf
er process also leading to CF3 and I-. For coverages exceeding ten mon
olayers, approximately 80% of the reaction was through this channel, t
he remaining 20% occurring through the direct photodissociation channe
l. To account for this second charge transfer channel, photoinduced in
termolecular charge transfer is proposed. (C) 1995 American Institute
of Physics.