Doubly Resonant Sum-Frequency Generation Spectroscopy of Adsorbates at an Electrochemical Interface

In this paper we report the first spectroelectrochemical in situ doubly resonant sum frequency generation (DR-SFG) spectroscopy experiment carried out with a free-electron laser tunable IR source and IR and VIS Optical Parametric Oscillators. We studied the potential-dependent adsorption and reaction of the quinoline dye 4-{2-[1-(2-cyanoethyl)-1,2,3,4-tetrahydroquinolin-6-yl]diazenyl} benzonitrile (CTDB) onto a Cu(100) electrode. SFG band assignments were based on density functional theory calculations. Strong SFG enhancement was obtained in the IR range 1100-1300 cm-1, corresponding to Ar-C≡N stretching, with the VIS beam set at 441.6 nm and in the IR range 2000-2300 cm-1, corresponding to nitrile stretching, with the VIS beam set at 532 nm and the pH adjusted in order to match the bathocromic shift of the adsorbed CTDB chromophore with the input VIS laser frequency. Doubly resonant conditions were achieved via resonance of the VIS input with the electronic structure of the adsorbate orbitals and of the adsorption bond.