Towards Probing High Order Anharmonicities of a Strong Oscillator

Overtones and combination bands relate directly to the potential energy surface and grant insight into the couplings present within a molecule. Rhodium dicarbonyl (RDC) is a model compound for nonlinear spectroscopy due to the high oscillator strength of its carbonyl modes. Previous works have studied the anharmonicities of RDC, yet recent developments enable studies of higher order vibrational anharmonicities. We use a novel femtosecond laser system to perform coherent multidimensional spectroscopy, probing high order overtone and combination bands of the carbonyl stretches. This experiment determines the amount of anharmonicity present and maps out the potential energy surface of the molecule. Two independent infrared frequency beams scan across the resonances of the carbonyl stretches and their overtones and combination bands, forming multiple quantum coherences of the vibrational states. A visible excitation upconverts the coherent output, enabling better detection and isolation of the coherent, phase-matched output beam. Our laser system delivers pulses intense enough and broad enough in bandwidth for multiple interactions to occur for each pulse. The multiple laser interactions will coherently climb the vibrational ladder of the molecule to probe the higher order anharmonicities. The results of this experiment demonstrate the extent of the anharmonic oscillator that may be probed using coherent multidimensional spectroscopic techniques.