2.2 SFG-VS measurement.
A detailed description of the spectrometer can also be found in our perious work.39,40 Briefly, the visible beam and infrared beam came from the same side of the surface normal (a co-propagating geometry) of the sample. The incident angle of infrared beam (IR) was 40°, and 63° for the visible beam against the surface normal. Each spectrum was monitored with SSP, SPS, PPP combinations (for example, SSP refers to s-polarized sum-frequency, s-polarized visible and p-polarized infrared beam). Under these polarizations, The spectrum was collected and normalized to the spectrum of Z-cut crystalline quartz. To study the temperature dependence of the SFG intensity, an electric heater device is applied. The sample is load in a cylindrical Teflon cell placed right at the middle of the heating module. The temperature of the bulk was detected before and after a single measurement by a thermocouple whose standard deviation was ±0.3K. The detector was stretched into the middle of the bulk phase for about 5 seconds in order to read a stabilized value. The composition of liquid sample was confirmed as a constant before and after the evaporation by the gas chromatograph ((PE Auto System XL, PE, U.S.A.) after the thermal experiments.