The use of ultrafast infrared pulses to characterize the transient res
ponse of visible and ultraviolet pulse-excited CO/Pt(111) is described
. A 15 200 cm-1 (1.9 eV) pump pulse of 1.0 ps duration and incident en
ergy density of ca. 16 mJ cm-2 excites electron-hole pairs in the plat
inum. The accompanying transient response leads to an overall heating
of the system, as revealed by a decrease in infrared reflectivity of t
he platinum and a shift to lower frequency of the 2106 cm-1 CO-stretch
ing mode. Unlike the transient response observed for resonant infrared
pumping of the CO stretch, the shift associated with visible pumping
persists for over 50 ps and is believed to reflect activation of the 6
0 cm-1 frustrated CO-translation mode which is anharmonically coupled
to the CO stretch. By using a density matrix formalism for the IR resp
onse to the pump-induced CO(v = 0 --> 1) frequency shift, and incorpor
ating a three-temperature model for the CO/Pt(111) surface (T(e), T(la
t), and T(ads)), fitting of the measured spectral response as a functi
on of time-delay places limits on the timescale for energy transfer fr
om the substrate to the adsorbed CO. Similar results were obtained wit
h 34 612 cm-1 (4.3 eV) pump pulses of < 1.2 ps duration.