Utrafast transient absorption (TA)

For time resolved pump-probe experiments two independently tunable and synchronized pulses are needed. An amplified pulse is divided into two parts with relative intensities chosen depending on the experiment. A relatively strong pump beam, approximately 95%, is used for sample excitation by sending it through a harmonic generator or an optical parametric amplifier (OPA). The remaining 5% is sent through a computer-controlled delay line and then focused close to 3 mm thick sapphire plate to generate a white light continuum. A weak probe pulse subsequently measures the transmission (the absorption of the excited sample) as a function of the time delay t_D between the pump and probe pulses, see Figure 1. 

Figure 1. Schematic diagram of pump-probe spectroscopy

The schematic layout of a typical femtosecond transient absorption apparatus is shown if Figure 2.
Figure 2. Layout schematic for a typical femtosecond transient absorption experiment.

Focusing a small fraction of the amplified laser beam carefully on a sapphire plate leads to self focusing of the beam in a transparent medium and generation of a while light continuum around the fundamental frequency. Most of the pulse energy is centered around the fundamental frequency but a portion of the intensity distribution from the generated light covers the wavelength region from 450-800 nm and an equally wide wavelength range in the IR side of the fundamental frequency, see Figure 3.

Due to its weak intensity the white light continuum is suitable for probing purposes. During propagation in the transparent medium the red wavelengths of the pulse travel faster than the blue wavelengths. This phenomenon is called group velocity dispersion. Pulses where the red wavelengths lead the blue wavelengths are said to be positively chirped. In Figure 4 we can see light in the white light continuum is typically strongly chirped due to strong dispersion of the sapphire crystal and the collimating optics.
Figure 4. White light continuum generation.

In pump-probe experiments the white light continuum is used as the probe source. The initial time t₀(λ)=0 varies from wavelength to wavelength and must be accounted for if the transient spectra is being measuring. In order to measure the transient absorption, every second pump pulse must be blocked. In most cases it is sufficient to use a mechanical chopper. The optical chopper is used to modulate the excitation beam in order to switch the sample between excited and ground states, while the probe pulse is measured twice, once when the pump pulse is on and once when the pump pulse is off,(see Figure 5).

Figure 5. Transient signal detection.

The difference between the two probe pulse measurements corresponds to the transient absorption changes accompanying the reaction initiated with the pump pulse. The transient absorption is then calculated using the following equation:

There are a variety of detectors for collecting transient data, though a CCD spectrometer is typically better suited for transient spectroscopy. A CCD spectrometer allows the acquisition of the whole spectrum while being compact and easy to align.