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.