Polarization-resolved broad-bandwidth sum-frequency generation spectroscopy of monolayer relaxation

A novel implementation of broad-bandwidth sum-frequency generation (SFG) spectroscopy is presented, which allows for the simultaneous recording of SFG spectra with different polarization combinations of the SFG, visible, and infrared beams. This method is particularly advantageous for studies in which surface properties are time-dependent, such as kinetic studies. The technique is illustrated by a study that mimics lung surfactant relaxation during the breathing cycle. The time-dependent molecular order of lung surfactant lipids is quantified through the vibrational response of the terminal CH₃ group.