Sphingomyelin(SM)is specifically enriched in the plasma membrane of mammalian cells.The functions of the SM molecules play crucial roles in the cellular activities by the interaction between the sphingomyelin and Ca2+ions.In this report,we investigate the conformational differences between the egg sphingomyelin(ESM)and milk Sphingomyelin(MSM)monolayer caused by Ca2+ions using sub-wavenumber high-resolution broadband sum frequency generation vibrational spectroscopy(HR-BB-SFG-VS).