  We report vibrational sum frequency generation spectra from supported lipid
bilayers in which the OH and the CH stretching signals are probed at different
salt concentrations. Atomistic simulations show a negligible impact of salt on
the OH stretching spectra, indicating the observed SFG intensity changes are
due to chi(3) and potential dependent contributions. These are further analyzed
in the contact of exact-zero reference states. Further experiments and
simulations identify specific hydrogen bonding interactions between interfacial
water molecules at the PC head group of the zwitterionic DMPC lipids at 3200
wavenumbers.
