Probing of molecular adsorbates on Au surfaces with large-amplitude temperature jumps

Abstract

Methods are described to probe vibrational transitions of molecules adsorbed on Au films subjected to calibrated ultrafast large-amplitude temperature jumps (T-jumps). The probe technique, vibrational sum-frequency generation (SFG), can monitor vibrations localized on specific parts of adsorbate molecules in the form of self-assembled monolayers (SAMs). Substrates had a thin Cr adhesion layer and an Au film that could withstand millions of T-jumps without laser damage of film or adsorbate. The substrate flash-heating process was characterized using ultrafast reflectance measurements. Reflectance transients induced by both 800 nm or 400 nm femtosecond pulses had overshoot-decay-plateau structures. The overshoots and decays represented optically generated hot electrons, and the plateaus gave the equilibrium temperature increase ΔT, which was in the 30–175 K range. The combination of SFG adsorbate and Au surface reflectance measurements was used to assess the effects of adsorbate vibrational heating by both hot electrons and the hot Au lattice. Two types of SAMs were investigated, nitrobenzenethiolate (NBT), where SFG probed nitro groups located 4 carbon atoms from the surface, and octadecylthiolate (ODT), where SFG probed terminal methyl groups 17 carbon atoms from the surface. With ΔT = 175 K, the NBT nitro transition νs(NO2) showed time-dependent intensity loss, redshifting, and broadening. These three kinds of transients also had overshoot-decay-plateau structures, which resulted from the interplay of hot electron excitation of higher-frequency vibrations including the probed vibration, and Au lattice heating of lower-energy vibrations and the conformational modes that cause reversible disordering of the SAM structure. The relative importance of these effects was different for the overshoot and plateau regions, and for the intensity, redshifting, and broadening effects. With ODT, T-jumps caused the terminal methyl groups to become disordered, and the disordering process was nonexponential in time. From the ratio of symmetric to antisymmetric CH-stretching intensities, the ensemble-averaged methyl tilt angle could be determined. With smaller T-jumps, the methyl groups gradually increased their tilt by a small amount during ∼200 ps, while with larger T-jumps where ΔT = 175 K, the methyl groups abruptly reoriented toward the surface normal and then tilted gradually away from the normal in the next 20 ps.