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.

Temperature-Dependent Dynamic Response to Flash Heating of Molecular Monolayers on Metal Surfaces: Vibrational Energy Exchange

“…The vibrational dynamics of molecules adsorbed on metal surfaces play an important role in many chemical and physical processes, such as heterogeneous catalysis , and molecular electronics . Our previous studies ,,− revealed features that were relevant to those processes. Because the T -jumps persist for just 10 μs with flash heating, we can superheat molecular adsorbates well above the usual decomposition temperature without substantial decomposition because the molecules do not have enough time to decompose .…”

“…In this study, we examine the time-dependent vibrational response of molecular adsorbates on flash-heated Au surfaces and how the response depends on which molecular vibration was probed and the size of the temperature jump ( T -jump) Δ T , where Δ T ranged from 35 to 250 K. In several previous studies, our group had developed techniques to controllably flash-heat Au surfaces with self-assembled monolayer (SAM) adsorbates, where the specific vibrational transitions could be probed by ultrafast vibrational spectroscopy. − The specific type of spectroscopy used is termed broad-band multiplex vibrational sum-frequency generation with nonresonant suppression (SFG), , and this method provided high-quality time-resolved vibrational spectra of monolayers, despite the short time intervals (picoseconds) and the small number of molecules (∼10 11 ) being probed . The specific SAM studied here was 4-nitrobenzenethiolate (NBT), and we used SFG to probe both nitro and phenyl groups.…”

“…In addition, we can probe vibrational energy transport by monitoring vibrational reporting groups on specific parts of the absorbates . In this way, we have measured the flow of heat in several types of molecular wires consisting of long-chain alkanes , or linear polyphenyls . In the alkanes, , for instance, where SFG was used to probe the terminal methyl groups, energy flowed from the hot Au surface, through the linker groups, and along the length of the alkane chains.…”

Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy with Picosecond Excitation

“…Ultrafast dynamics of molecules on surfaces are of fundamental importance to numerous fields, including solar energy, molecular electronics, and heterogeneous catalysis. , Techniques such as surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS), , surface-enhanced coherent anti-Stokes Raman spectroscopy (SE-CARS), , and time-resolved sum frequency generation (TR-SFG) − can probe the ultrafast dynamics of molecules on surfaces, but they do not intrinsically possess spatial resolution beyond the optical diffraction limit. In contrast, tip-enhanced Raman spectroscopy (TERS) has become an established tool capable of obtaining the vibrational spectra of adsorbates with nanoscale spatial resolution. − Coupled with scanning tunneling microscopy (STM) or atomic force microscopy (AFM), TERS can be utilized to gather correlated chemical and topographic information.…”

Structure and Nonequilibrium Heat‐Transfer of a Physisorbed Molecular Layer on Graphene

“…[46] Still, some results on the thermal barrier between a gold film and SAMs have been reported using ultrafast flash-heating molecular thermal conductance. [22,48] With this technique, values of about 0.45•10 -8 K m 2 W -1 for the TBR between gold and a covalently bound thiol SAM have been reported. The 2±1•10 -8 K m 2 W -1 we report here falls on the low end of the range of values typically reported for physisorbed molecular layers.…”