Ultrafast Vibrational Energy Transfer in the Layers of D₂O and CO on Pt(111) Studied with Time-Resolved Sum-Frequency-Generation Spectroscopy

Ultrafast dynamics of vibrational energy transfer in overlayers of D 2 O and CO on Pt(111) have been investigated by femtosecond time-resolved (TR) IR−visible sum-frequency-generation (SFG) spectroscopy under ultrahigh-vacuum conditions. About 10 layers of D 2 O ice were epitaxially grown on c(4 × 2)-CO/Pt(111). The surface was excited by subpicosecond laser pulses, and subsequent energy transfer through low-frequency modes of adsorbates was monitored in terms of peak shifts and broadenings of C−O and O−D stretching bands in SFG spectra as a function of the pump−probe delay. Because D 2 O ice forms islands, there are two types of CO: one interacting with D 2 O and the other free from D 2 O. Simulations of the TR-SFG spectra by using a phenomenological model for the energy-transfer dynamics indicate that the coupling rate of perturbed CO is larger than that of free CO by a factor of 1.7; this is probably because CO 2π* states shift toward the Fermi level due to interaction with D 2 O. Two isolated bands at 2668 and 2713 cm −1 were assignable to the OD stretching bands of D 2 O directly interacting with CO at the D 2 O/CO interface and D 2 O at the vacuum/ice interface, respectively. Analysis of the temporal spectral changes of free D 2 O by using a diffusive thermal transport model indicates that heat transfer through low-frequency phonons of the ice layers occurs within 3 ps; this is substantially faster than the pulsed laser-induced melting of thin ice films reported previously.