Two Color Electrochemical Tip-Enhanced Raman Spectroscopy

This proposal requests funds to acquire a broadly tunable (350-950 nm) CW laser system for AFOSR MURI FA9550-14-1-0003 “Electrochemical Imaging and Mechanistic Studies on the Nanometer Scale.” This laser will allow us to simultaneously observe the oxidized and reduced species, which have electronic transitions that differ by hundreds of nanometers, in electrochemical tip-enhanced Raman spectroscopy (TERS) experiments. With the fixed frequency laser sources currently available, one is limited in the selection of target redox systems to those whose molecular electronic resonances serendipitously overlap with available fixed excitation wavelengths. This is particularly important for studying single molecule electrochemical processes because single molecule Raman detection has not yet been demonstrated without exploiting resonance enhancement. Picosecond pulsed excitation from an existing Ti:Sapphire pumped optical parametric oscillator (OPO) can, in principle, be used for this purpose; however, in practice, we find that these high peak power pulses photodecompose many adsorbates tested and/or degrade the TER(R)S enhancement factor. The equipment requested will greatly increase the generality of the electrochemical systems that can be studied under FA9550-14-1-0003. The ability to tune our excitation source to match the electronic resonances of any electroactive species will allow us to perform two color EC-TER(R)S experiments in which all reactants, products and intermediates of the electrochemical system can be monitored down to the single molecule level.