TRIR and 2DIR Spectroscopy

Time resolved infrared (IR) spectroscopy is highly informative about the structural modifications occurring upon photoexcitation in molecular systems. We have developed infrared based pump-probe spectroscopy, where the excitation pulse can be either a visible pulse, enabling electronic excitation, or a an infrared pulse, enabling transitions among the vibrational levels of the system under investigation in the electronic ground state. 

Our laser system is based on a Ti:sapphire regenerative amplifier (Coherent Legend)  pumped by a Ti:sapphire oscillator (Coherent Micra). The system produces 1KHz, 40 fs pulses at 800 nm, with output power of about 3W.

Infrared pump pulses are obtained by pumping a commercial Optical Parametric Amplifier (OPA) system (TOPAS, Light Conversion) with a portion of the fundamental laser radiation. The Near IR (NIR) signal and idler OPA output are frequency mixed through a non linear crystal to create pump pulses in the Mid-IR. By frequency doubling or by mixing the NIR signal or idler with residual 800 nm light, visible pulses can be obtained in the 450-700 nm spectral range. Pump pulses in the UV region can also be obtained by further second harmonic generation.

The IR probe pulses are generated by pumping a second, home-made, OPA. Before reaching the sample the probe pulses are delayed respect to the pump pulses by passing through a motorized stage. Pump and probe pulses are focussed and overlapped on the sample. Detection is achieved through a 32 elements MCT array. 

The same setup can be extended for use as a 2DIR spectrometer. In that case a third IR pulse is sent to the sample, allowing to introduce a further time delay which translates into a second frequency axis upon Fourier transformation. Our system is implemented in a pump-probe geometry where two delayed pump pulses are produced by sending the TOPAS output through a Mark-Zender  interferometer, while the probe pulse is produced through the second OPA.