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. This is done to overcome the hampering background interference that often disrupts and challenges Raman measurements to improve spectra quality. All time-resolved spectra are suitable to be analyzed using the two-dimensional correlation method for a correlation map between the peaks.
Time-gated Raman spectroscopy The most common issue in conventional (CW) Raman spectroscopy (RS) is sample-induced fluorescence emission making the identification or quantification of materials challenging or impossible. An effective solution to this problem is time-gating (TG), which is a general technique used in signal processing. An integral part of Time-gated (TG) Raman spectroscopy (RS) is the temporally precise synchronization (picosecond range) between the pulsed laser excitation source and the sensitive and fast detector. The detector is able to collect the Raman signal during the short laser pulses, while fluorescence emission, which has a longer delay, is rejected during the detector dead-time. TG-Raman is also resistant against ambient light as well as thermal emissions, due to its short measurement duty cycle. Time-gating is based on Timegated® Raman Technology.
Transient-absorption spectroscopy Transient-absorption spectroscopy (TAS), also known as flash photolysis, 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 the ability to resolve two physical actions in real time. The shorter the detection time, the better the resolution. As a result, femtosecond laser spectroscopy offers better resolution than nanosecond laser 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 pump) – Absorbance (before pump) = ΔAbsorbance ΔAbsorbance 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 and often still overlaps with the bleach signal. This is a lasing effect (coherent emission) of the excited dye molecules under the strong probe light. This emission signal cannot be distinguished from the absorption signal and often gives false negative Δ absorbance peaks in the final spectra that can be decoupled via approximations. 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. Transient absorption can be measured as a function of wavelength or time. The TA curve along wavelength provides information regarding evolution/decay of various intermediate species involved in chemical reaction at different wavelengths. The transient absorption decay curve against time contains information regarding the number of decay processes involved at a given wavelength, how fast or slow the decay processes are. It can provide evidences with respect to inter-system crossing, intermediate unstable electronic states, trap states, surface states etc.
Spectral Resolution of Transient Absorption Transient absorption is a highly sensitive technique that can provide insightful information regarding chemical and material processes when achieving sufficient spectral resolution. Beyond the obvious consideration of a sufficiently short pulse width, the dependence of the frequency bandwidth must be accounted for. The equation
ΔνΔt ≥ K demonstrates that, for any beam shape (K), the beam bandwidth (Δν) is inversely proportional to its pulse width. Therefore, a compromise must be made to achieve maximum resolution in both the time and frequency domains. The use of high-power lasers with ultrashort pulse widths can create phenomena that obscure weak spectral data, commonly referred to as artifacts. Examples of artifacts include the signal resulting from two-photon absorption and stimulated Raman amplification. Two-photon absorption occurs in samples that are generally transparent to UV-Vis wavelengths of light. These media are able to absorb light efficiently when simultaneously interacting with multiple photons. This causes a change in intensity of the probe pulse. ΔIprobe = γIpumpIprobeL The above equation describes the change in intensity relative to the number of photons absorbed (γ) and the thickness of the sample (L). The change in absorption signal resulting from this event has been approximated to the below equation. Sapprox = 0.43∙IprobeIref A common baseline correction technique used in spectroscopy is the penalized root mean square error. A variant of this technique, the asymmetric penalized root mean square, has been used to correct signals affected by artifacts in transient absorption.
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