This is a "pump-probe" experiment. A "pump" laser pulse generates excitations and/or ionisations in the sample and a "white" pulse, the "probe", measures the absorption spectrum of the sample at a given time relative to the "pump" pulse. By comparing the absorption of the sample after and before the "pump" laser pulse, the transient absorption spectrum for a given pump-probe time is obtained. The pump-probe delay Dt is controlled by a delay line that changes the optical path length of the "probe" relative to the "pump"; by varying this delay, it is possible to study the dynamics and kinetics of ultrafast evolution of the system.

Principle of the femtosecond transient absorption “pump-probe” experiment(SHG/THG : second/third harmonic generation, WLC: white light continuum)

• • • • • • • • Laser source: the laser used is a commercial amplified Titanium Sapphire laser (Amplitude) delivering pulses centred at 780 nm with an energy of ~2mJ and a duration of ~130 fs at a repetition rate of 1 kHz, but only 640 µJ of the energy is used for the transient absorption setup.
              •  Harmonic generator: 90% of the laser beam energy is used for the pump beam and sent to a harmonic generator (Spectra Physics) to produce either the second harmonic (SHG) at 390 nm or the third harmonic (THG) at 260 nm.
              •  White light continuum: 10% of the laser beam energy is focused in a fused silica or sapphire plate to produce a white light continuum (WLC) which will provide the "probe" pulse, but also a reference pulse allowing to take into account the laser fluctuations.
              •  Delay line: a translation stage (Micro-Control) 20 cm long with 1 µm steps enables studies till the nanosecond.
              •  Detection: a spectrograph with a 3 grating turret (ACTON) coupled to a 1300 x 400 pixel CCD camera (Princeton Instruments) enables simultaneous recording of the probe and reference beams over a spectral range between 300 nm and 850 nm.
              •  Samples: it is possible to work with liquid samples (in quartz cells with an optical path of 1 mm, with or without circulation, or as a laminar jet of 300 µm thickness), or with solid samples (thin slides or films, supported or not).

• • • • • • • • Solvation of the electron produced by photoionisation of the solvent
              • Dynamics of excited states of molecular compounds
              • Excited state proton transfer between pyranine and water (M2 students lab work)
              • Electron transfer in porphyrin-polyoxometallate complexes
              • Dynamics of photogenerated charge carriers in semiconductors

Examples of signals (A spectra et B kinetics) obtained during the study of excited-state proton transfer from pyranine to water [Changenet2020]