Transient absorption spectrometer (TAS) setup is an instrument suitable for pump and probe experiments in liquid and thin film samples. The molecules or particles are excited with a pump laser pulse and the transient absorption features are monitored with a spectrometer. The photophysical and photochemical processes can be followed in the ultrashort time scale (50 fs- 2 ns) and an electrochemical control is also available for the measurements.
Key areas of science include semiconductor research when the electron-hole generation and charge carrier relaxation processes can be monitored in nanoparticles or in thin films, for example in components of solar cell devices. The TAS setup is coupled with a potentiostat to perform in situ transient spectroelectrochemical experiments. Different research fields of chemistry could also apply the transient absorption technique, for example, the in situ synthesis of core-shell structured nanomaterials or doped nanoparticles can be studied during the synthesis or chemical modifications. The photocatalytic processes for the synthesis of organic molecules can be followed by applying gold nanoparticles under plasmonic excitation. Materials sensitive to light can be explored for nanolithography applications for the electronic industry. Photosensitive materials can be also characterised for biomedical applications with this technique.
The attenuated HR-1 laser beam (1030 nm, 100 kHz, 10-30 fs) enters the TAS setup and splits into two parts with a beam splitter. Typically, we use 10 W average power and 30 fs pulse duration. In this direction 80 % of the beam is used for the generation of the pump light. The beam is focused in a BBO crystal in which the second harmonic light is generated with 515 nm central wavelength. Before the BBO crystal, we also have an optical chopper that can be used for generating pump on and pump off periods at 2000 Hz. The pump light can be filtered if we want a shorter pump wavelength range. This green pump light is then collimated and it goes to the delay stage. The beam travels in this direction and is reflected back from the retroreflector parallel to the incoming beam. Therefore moving the delay stage we can change the distance of the pump beam path till the sample and thus the delay time can be changed from ~50 fs to 2 ns. We can also generate UVC light (257 nm) with a second BBO crystal. The green or UVC pump light is focused with a spherical mirror with the focal plane behind the sample. In the other direction, we have the probe light generation. We focus the near infrared beam on a sapphire plate and generate the white light continuum in the 500-800 nm wavelength range. Other substrates such as CaF2 or LiF could provide different white light spectra. We have a QPod unit with magnetic stirring and heating options as a sample holder for the cell containing the liquid sample. Changing the delay stage position, we can generate a well defined time difference between the arrival of the pump and probe pulses with about 10 fs time resolution in the ~50 fs-2 ns time range. We can also measure thin film samples in reflection mode. The probe beam is measured with an Ocean Insight FX spectrometer.
Figure 1: Transient absorption setup for liquid samples with temperature control at the HR1 beamline
Figure 2 Electrochemical cell for in situ transient spectrolectrochemical experiments
|Configuration of the TAS spectroscopy system
|Parameters of the TAS spectroscopy system
|1st config HR pump
|1030 nm/515 nm, 100 kHz, 30 fs, HR-1
|900-1160 nm/450-580 nm/225-290 nm; 100 kHz, 6-30 fs, HR-1 and HR-2
|1st config HR Probe - a
|white light (CaF2, Sapphire)
|white light (CaF2, Sapphire and LiF)
|1st config HR Time resolution
In the TAS setup we can measure liquid samples in static and flow cells with different optical path lengths (1-10 mm). Thin film samples can be also measured typically with an active area of a few cm2.
We have an Ocean Insight FX spectrometer in the TAS setup. The shortest integration time is 10 μs and can be triggered at 4000 Hz triggering frequency.