Ultrafast Excitation of Molecular Adsorbates on Flash-Heated Gold Surfaces

Abstract

An ultrafast flash-thermal conductance technique is used to study energy transfer from a flash-heated polycrystalline Au(111) surface to adsorbed thiolate self-assembled monolayers (SAMs). The focus is on understanding energy transfer processes to parts of SAM molecules situated within a few carbon atoms of the Au surface, by probing specific SAM functional groups with vibrational sum-frequency generation (SFG) spectroscopy. The SFG intensity drop after flash-heating for benzenethiol (BT) CH-stretch transitions shows a substantial overshoot lasting several tens of picoseconds before BT and Au equilibrate at a higher temperature estimated at 600 degrees C. The thermal redshift of BT CH-stretch transitions also shows an overshoot. Other aromatic molecules and aliphatic molecules such as cyclohexanethiol (CHT) and hexanethiol (C6) have an overshoot as well. A model is proposed where the overshoot is primarily the result of hot surface electrons existing only during the flash-heating pulses. The intensity overshoot is caused by electron excitation of the probed vibrations and the redshift overshoot is caused by electron excitation of lower-energy vibrations anharmonically coupled to the probed vibration. Although electron excitation causes a substantial perturbation, up to 50% in some cases, of the SFG signal, the total amount of energy deposited into SAMs by electrons is much smaller than the heat transferred by Au surface phonons. Studies of a variety of molecular structures including substituted benzenes, biphenyl and terphenyl, and benzene rings connected to the Au surface by alkane linkers show that the likelihood of electron excitation becomes small for distances of 4-5 carbon atoms above the surface.

Spin inelastic currents in molecular ring junctions

“…Advances in experimental techniques at nanoscale shift focus of research in molecular electronics [1][2][3][4][5] from ballistic transport 6,7,99 to inelastic effects 9,10,12,93 (and closely related energy transfer processes [13][14][15][16][17][18] ), to noise characteristics [19][20][21] and optical response [22][23][24][25][26] in current carrying junctions. Recent spin-transport experiments 27,28 demostrated potential possibility of using organic molecules to construct molecular spin devices, indicating the emergence of molecular spintronics [30][31][32][33][34][35] (see Ref.…”

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

“…5 We did not try to compensate for spectral chirp, which was less than 1 ps. The white light was split into signal and reference beams which were focused to 100 lm and were incident at 30 .…”

“…5 We showed it was possible to use SFG to probe hot electron-and phonon-excited vibrations at multiple locations on the adsorbate molecules, 2 and it was shown that the hot electrons excited atomic groups most effectively if they were no farther than 4-5 carbon atoms away from the surface. 5 Flash-heating of Au surfaces by short pulses of light has been studied extensively over the past 25 years using transient reflectance probes. 6,7 Reflectance transients are sensitive to changes in the conduction electron densities caused by optical excitation and by heat.…”

“…The actual values of DT with 800 nm flashheating turned out to be closer to 35 K. However during the earliest stages of the flash-heating process, the atoms nearest the Au surface were pumped by both phonons and hot electrons, causing it to appear that the chains were reacting to much larger T-jumps. 5 Motivated by those previous studies, we set out to combine our nonlinear vibrational spectroscopy methods that probe the absorbate response with optical reflectivity measurements that probe the Au surface. This combination allows us to produce calibrated ultrafast T-jumps and to separate the effects of electron and lattice heating on the adsorbates.…”

“…These terminal methyl groups become excited after a delay time t 0 that resulted from vibrational excitations that had to travel along the chains to the methyl groups after being generated by surface electrons and phonons heating a base region 4-5 carbon atoms in length. 1,5 Some of the measurements here were made using 800 nm flash-heating pulses to produce DT ¼ 35 K as used in previous works. [1][2][3][4][5] In addition, we used 400 nm (blue) pulses, which are more strongly absorbed by Au, to make the same DT ¼ 35 K and also much larger-amplitude T-jumps where DT ¼ 175 K.…”