In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10−16 seconds.
Transient-absorption spectroscopy, also known as flash spectroscopy, is an extension of absorption spectroscopy. Here, the absorbance at a particular wavelength or range of wavelengths of a sample is measured as a function of time after excitation by a flash of light. In a typical experiment, both the light for excitation ('pump') and the light for measuring the absorbance ('probe') are generated by a pulsed laser. If the process under study is slow, then the time resolution can be obtained with a continuous (i.e., not pulsed) probe beam and repeated conventional spectrophotometric techniques.
Examples of processes that can be studied:
- Optical gain spectroscopy of semiconductor laser materials.
- Chemical reactions that are initiated by light (or 'photoinduced chemical reactions');
- The transfer of excitation energy between molecules, parts of molecules, or molecules and their environment;
- The behaviour of electrons that are freed from a molecule or crystalline material.
Other multiple-pulse techniques
Transient spectroscopy as discussed above is a technique that involves two pulses. There are many more techniques that employ two or more pulses, such as:
- Photon echoes.
- Four-wave mixing (involves three laser pulses)
The interpretation of experimental data from these techniques is usually much more complicated than in transient-absorption spectroscopy.
Time-resolved infrared spectroscopy
Time-resolved infrared (TRIR) spectroscopy also employs a two-pulse, "pump-probe" methodology. The pump pulse is typically in the UV region and is often generated by a high-powered Nd:YAG laser, whereas the probe beam is in the infrared region. This technique currently operates down to the picosecond time regime and surpasses transient absorption and emission spectroscopy by providing structural information on the excited-state kinetics of both dark and emissive states.
Time-resolved fluorescence spectroscopy
Time-resolved fluorescence spectroscopy is an extension of fluorescence spectroscopy. Here, the fluorescence of a sample is monitored as a function of time after excitation by a flash of light. The time resolution can be obtained in a number of ways, depending on the required sensitivity and time resolution:
- With fast-detection electronics (nanoseconds and slower)
- With Time Correlated Single Photon Counting, TCSPC (picoseconds and slower)
- With a streak camera (picoseconds and slower)
- With intensified CCD (ICCD) cameras (down to 200 picoseconds and slower)
- With optical gating (femtoseconds-nanoseconds) - a short laser pulse acts as a gate for the detection of fluorescence light; only fluorescence light that arrives at the detector at the same time as the gate pulse is detected. This technique has the best time resolution, but the efficiency is rather low. An extension of this optical gating technique is to use a "Kerr gate", which allows the scattered Raman signal to be collected before the (slower) fluorescence signal overwhelms it. This technique can greatly improve the signal:noise ratio of Raman spectra.
This technique uses convolution integral to calculate a lifetime from a fluorescence decay.
Time-resolved Photoemission Spectroscopy and two-photon photoelectron spectroscopy (2PPE) are important extensions to Photoemission spectroscopy. These methods employ a pump-probe setup. In most cases the pump and probe are both generated by a pulsed laser and in the UV region. The pump excites the atom or molecule of interest, and the probe ionizes it. The electrons or positive ions resulting from this event are then detected. As the time delay between the pump and the probe are changed, the change in the energy (and sometimes emission direction) of the photo-products is observed. In some cases multiple photons of a lower energy are used as the ionizing probe.
- A. Stolow, A. E. Bragg, and D. M. Neumark, Femtosecond time-resolved photoelectron spectroscopy, Chem Rev, 104 (2004) 1719