Measurements of the concentration of organic solutions by femtosecond time-resolved ERE-CARS

A variant of all-resonant CARS named electronic-resonant enhancement CARS (ERE-CARS) is applied to measure the methanolwater solution concentration at room temperature. The measurements are performed using the ERE-CARS signal of the Raman vibrations near the C-H stretching modes (at 2835 and 2942 cm ^-1) in methanol. By changing the timing (Δt>0) of the laser pulses of this non-degenerate four wave mixing technique, the concentration information based on the vibrational dynamics of the C-H bonds can be successfully detected as the frequency-spread dephasing rate during the first few hundred fs in the ERE-CARS signal with high sensitivity and accuracy. Femtosecond time-resolved ERE-CARS technique is applied to the concentration analysis of a mixture of the organic solution. This investigation indicates that femtosecond time-resolved ERE-CARS technique might be a powerful tool for real-time detection for solution concentration of different liquids.