Vibrational energy transfer of CO/Cu(100): Nonadiabatic vibration/electron coupling

Vibrational energy relaxation of the internal C–O stretching mode of carbon monoxide in the c(2×2) overlayer on the Cu(100) surface at 120 K is measured by picosecond pump–probe spectroscopy. A resonant 1.5 ps infrared pulse at ν=2085 cm−1 pumps the C–O stretching mode. The energy relaxation is monitored by sum frequency generation from a delayed pair of 1.5 ps infrared and visible pulses. A single component decay, with a decay time of 2.0 ±0.5 ps, is reported. Uncertainties in the actual excited state lifetime are discussed, and the actual lifetime is estimated to be 2.0 ±1.0 ps. This lifetime is close to the lower limit of 1.2 ps set by the observed vibrational linewidth of 4.5 cm−1. The energy relaxation process is interpreted to occur by nonadiabatic energy transfer to the electrons (electron-hole pair excitations) of the copper substrate, and the measurement supports previous assertions that the nonadiabatic energy transfer rate for this system is very rapid. The nonadiabatic energy transfer lifetime of this mode has previously been estimated by density-functional calculations T. T. Rantala and A. Rosen, Phys. Rev. B 34, 837 (1986), and has recently been calculated by extrapolation of ab initio Hartree–Fock electronic structure calculations for CO on copper clusters M. Head-Gordon and J. Tully, preceding paper, J. Chem. Phys. 96, 3939 (1992). The calculated lifetimes in both cases are in the 1–3 ps range, in good agreement with the experimentally measured value.