Chemisorbed films of ethyltrichlorosilane (ETS) at the glass/air interface
are studied by IR-excitation sum-frequency (SF) spectroscopy. Using two ind
ependently tunable infrared pulses in combination with a visible upconversi
on pulse for the SF-probing process, a time-resolved vibrational spectrosco
py is accomplished for the molecular monolayer at the dielectric surface. R
apid energy transfer in the CH-stretching region is observed with a time co
nstant of 1.5 +/- 1 ps. The population relaxation of the CH3-stretching mod
es is found to be bimodal with a decay time of T-1 = 4.5 +/- 1 ps, while th
e ground state is re-populated rather slowly with 130 +/- 50 ps. (C) 1999 E
lsevier Science B.V. All rights reserved.