Time-resolved spectroscopy
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. Ultrafast transient absorption spectroscopy, an example of non-linear spectroscopy, measures changes in the absorbance/transmittance in the sample. 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.
Time resolved absorption spectroscopy relies on our ability to resolve two physical actions in real time. The shorter the detection time, the better the resolution. This leads to the idea that femto-second laser based spectroscopy offers better resolution than nano-second laser based spectroscopy. In a typical experimental set up, a pump pulse excites the sample and later, a delayed probe pulse strikes the sample. In order to maintain the maximum spectral distribution, two pulses are derived from the same source. The impact of the probe pulse on the sample is recorded and analyzed with wavelength/ time to study the dynamics of the excited state.
Absorbance (after probe)-Absorbance (after pump)= Δ Absorbance
Δ Absorbance records any change in the absorption spectrum as a function of time and wavelength. As a matter of fact, it reflects ground state bleaching (-ΔA), further excitation of the excited electrons to higher excited states(+ΔA), stimulated emission(-ΔA) or product absorption(+ΔA). Bleaching of ground state refers to depletion of the ground state carriers to excited states. Stimulated emission follows the fluorescence spectrum of the molecule and is Stokes shifted relative to bleach signal. Product absorption refers to any absorption changes caused due to formation of intermediate reaction products. TA measurements can also be used to predict non emissive states and dark states unlike time resolved photoluminescence.
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