The unexpected higher intensity at low momentum region for HOMO of W(CO) 6 was investigated using electron momentum spectroscopy at various impact energies, as well as non-relativistic, scalar relativistic and spin-orbit relativistic DFT-B3LYP calculations. There was no evident difference between the experimental momentum profiles at various impact energies. It was found that the low frequency vibration of W(CO) 6 can evidently change the momentum profile. The semi-quantitative vibrational averaged calculations agreed well with the experimental results. Therefore, it was suggested that the unexpected higher intensity at low momentum regions was possibly due to the vibration effects. (c) 2010 Elsevier B.V. All rights reserved.