The SFG experiments were carried out with a femtosecond broadband laser system. Briefly, a Ti:sapphire amplifier (Spitfire ACE, Spectra Physics) produces ~7 W of 800-nm, 35-fs pulses at a 2-kHz repetition rate. Forty percent of the 800-nm beam passed through a beam splitter to pump an optical parameter amplifier followed by a difference frequency generation stage (TOPAS-C, Spectra Physics). The rest was reflected from a Bragg filter (N013-14-A2, OptiGrate) to generate a narrow band beam of ~0.5-nm bandwidth. The broadband IR and narrowband 800-nm pulses overlapped at the sample surface with incident angles of 57° and 45°, respectively. The SF signal was collected along the reflected direction with a spectrograph (Acton SP2300) and a liquid nitrogen–cooled charge-coupled device camera (Princeton Instruments PyLoN 1340×100).
The UV illumination was carried out in vacuum at a base pressure of 10−7 torr to exclude ambient oxygen. The UV lamp illuminated the sample at a power density of about 0.9 mW/cm2 with a wavelength centered at 365 nm (see fig. S3). SF spectra reported in the main text were taken with the SSP polarization combination at 295 K after 30 min of UV illumination.
The UV illumination was carried out in vacuum at a base pressure of 10−7 torr to exclude ambient oxygen. The UV lamp illuminated the sample at a power density of about 0.9 mW/cm2 with a wavelength centered at 365 nm (see fig. S3). SF spectra reported in the main text were taken with the SSP polarization combination at 295 K after 30 min of UV illumination.