Femtosecond coherent transient infrared spectroscopy of CO on Cu(111)

Femtosecond infrared coherent transients have been measured for the stretch vibration of CO on Cu(111). The free induction decay exhibits a dephasing time (T2) of 2±0.3 ps (and 2±0.1 ps assuming a single exponential decay between 2 and 3 ps). The decay was best fit by exponential relaxation, thereby suggesting that the CO vibrational band is almost entirely homogeneously broadened. The surface sum frequency spectrum was also measured at two coverages (0.10 and 0.45 L) using spectrally narrowed pulses. Interferences were observed leading to a determination of the relative phase and amplitude of the resonant and nonresonant second-order susceptibility in this system. The magnitude of the nonresonant susceptibility was only weakly dependent on coverage, suggesting that the nonresonant polarizability originates in the bulk Cu. Time and frequency domain results were in good agreement.