RESUMO
Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular vibrational dynamics. A quantitative study on nonlinear media for upconversion is still essential for wide applications, particularly at the frequencies below 2000 cm-1. We evaluate wide-bandgap nonlinear crystals of Li-containing ternary chalcogenides based on their performance as the upconversion medium for femtosecond MIR spectroscopy. The upconversion efficiency is measured as a function of the MIR pulse frequency and the chirped pulse energy. LiGaS2 is found to be an efficient crystal for the upconversion of MIR pulses in a wide frequency range of 1100-2700 cm-1, especially below 2000 cm-1. By using LiGaS2 as an efficient upconversion crystal, we develop a MIR pump-probe spectroscopy system with a spectral resolution of 2.5 cm-1, a time resolution of 0.2 ps, and a probe window of 120 cm-1. Vibrational relaxation dynamics of CO stretching modes of Mn2(CO)10 in cyclohexane and bovine serum albumin in D2O are demonstrated with a high signal-to-noise ratio.
