Springer Series in Surface Sciences

The 2-year impact factor for year X equals the citations received in year X to items published in years X-1 and X-2, divided by the number of citable items published in those two years.

Impact factors are journal-level averages, not measures of individual articles. They vary strongly by field and can be influenced by publication mix (e.g., reviews) and database coverage, so compare within similar disciplines and time windows.