We attribute this lack of vibrational enhancement to be resulting from the fact that the overall vibrational energy of the incident beam is still much smaller compared to the dissociation barrier. On the other hand, numerous previous studies where significant vibrational enhancement has been reported, are systems where the vibrational energy is comparable to the dissociation barrier height. Another point that should be noted is that previous experimental work, such as HREELS studies of CO 2 adsorbed on low-index planes of Cu, have also shown no evidence of chemisorption or bent CO 2 state on Cu(110) surface. However, a chemisorption state was observed for high-index planes of copper surfaces. The absence of any role for vibrational energy in our measurements suggests that the bent configuration as a transition state is unlikely for low-index planes such as Cu(110).