Time-resolved spectroscopy

In physics and physical chemistry, time-resolved spectroscopy is the study of dynamical processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied that occur after illumination of a material, but in principle, the technique can be applied to any process which leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes which occur on time scales as short as 10⁻¹⁴ seconds. The rest of the article discusses different types of time-resolved spectroscopy.

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Transient-absorption spectroscopy

Transient-absorption 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:

• 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.

Nuclear magnetic resonance and electron spin resonance are often implemented with multiple-pulse techniques, though with radio waves and micro waves instead of visible light.

Time-resolved infrared spectroscopy

The king of time-resolved spectroscopic techniques, 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 whilst 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 a streak camera (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.