  We present a 100 kHz broadband vibrational sum-frequency generation (VSFG)
spectrometer operating in the 5.5-10.5 $\mu$m (950-1750 cm$^{-1}$) wavelength
range. The mid-infrared beam of the system is obtained from a collinear, type-I
LiGaS$_{2}$ crystal based optical parametric amplifier seeded by a
supercontinuum and pumped directly by 180 fs, ~32 $\mu$J, 1.03 $\mu$m pulses
from an Yb:KGd(WO$_{4}$)$_{2}$ laser system. Up to 0.5 $\mu$J mid-infrared
pulses with durations below 100 fs were obtained after dispersion compensation
utilizing bulk materials. We demonstrate the utility of the spectrometer by
recording high-resolution, low-noise vibrational spectra of Langmuir-Blodgett
supported lipid monolayers on CaF$_{2}$. The presented VSFG spectrometer scheme
offers superior signal-to-noise ratios and constitutes a high-efficiency,
low-cost, easy-to-use alternative to traditional schemes relying on optical
parametric amplification followed by difference frequency generation.
