Polarization-sensitive femtosecond mid-infrared spectrometer with a tunable Fabry–Perot filter and chirped-pulse upconversion

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular structure and vibrational dynamics. By using a polarized narrowband pump pulse in which center frequency is rapidly scanned with an electrically tunable Fabry–Perot filter, we have developed a femtosecond MIR spectrometer that is more sensitive to molecular structure. The pump energy and polarization dependences of transient MIR absorption spectra measured with the spectroscopic system revealed the appearance of a dark (infrared-inactive) mode due to a slight decrease in the molecular symmetry of a metal carbonyl complex, Mn2(CO)10, in solution. In addition, the rotational relaxation time of Mn2(CO)10 in solution was determined as approximately 20 ps.