Femtosecond time-resolved absorption spectrometer

Ultrafast spectroscopy plays an important role in understanding the fundamental photochemical and photophysical processes such as excited-state proton transfer, electron transfer and photoisomerization processes, occurring within femto-to-nanosecond time scales. Femtosecond time-resolved absorption spectroscopy stands out as a key technique for tracking ultrafast population dynamics. This method enables the study of electron population transitions between electronic states upon photoexcitation.

In the femtosecond time-resolved absorption measurement, the pump pulse photoexcites the sample and initiates the photo-chemical reaction. Subsequently, after a variable delay time (∆t), a probe pulse measures the absorption spectrum of the sample. By adjusting ∆t, a series of time-resolved absorption spectra can be recorded with femtosecond time resolution, revealing the dynamics of population changes.

Our femtosecond time-resolved absorption spectrometer employs a Ti:Sapphire femtosecond regenerative amplifier (Legend, Coherent) as the light source. The output from the Ti:Sapphire laser is divided into two portions, and sent to the optical parametric amplifiers (TOPAS-C, Light Conversion) to generated the pump and probe pulses, respectively. The wavelength of the pump pulse can be tuned from the UV to near-infrared (IR) region depending on the sample absorption. The probe pulse is the white-light continuum covering from the visible to near-IR region, which can record the absorption spectrum with broad spectral range. An optical delay mounted on a motorized translational stage can variate the delay time ∆t from femtosecond (fs) to 1.5 nanosecond, allowing us to measure the time-resolved absorption spectra with ~200 fs time resolution.