Editing Time-resolved spectroscopy (section)

==Transient-absorption spectroscopy==
{{main|Flash photolysis}} {{main|Ultrafast laser spectroscopy}}
Transient-absorption spectroscopy (TAS), also known as [[flash photolysis]], is an extension of [[Spectroscopy#Absorption|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 [[Excited state|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 [[spectrophotometry|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.<ref>{{Cite journal |pmid = 30410276|pmc = 6217845|year = 2018|last1 = Wang|first1 = L.|title = Ultrafast Transient Absorption Spectra of Photoexcited YOYO-1 molecules call for additional investigations of their fluorescence quenching mechanism|journal = Journal of Photochemistry and Photobiology A: Chemistry|volume = 367|pages = 411–419|last2 = Pyle|first2 = J. R.|last3 = Cimatu|first3 = K. A.|last4 = Chen|first4 = J.|doi = 10.1016/j.jphotochem.2018.09.012}}</ref> 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 
[[File:Spectral_domain_resolution_as_pulse_widths_broaden.gif|thumb|Change in wavelength distribution as pulse widths broaden.]]
ΔνΔt ≥ K<ref>{{Cite journal |last=Freek |first=Ariese |last2=Khokan |first2=Roy |last3=Venkatraman |first3=Kumar |last4=Hanehalli |first4=Sudeeksha |last5=Surajit |first5=Kayal |last6=Siva |first6=Umapathy |date=2017 |title=Time-resolved Spectroscopy: Instrumentation and Applications |journal=Encyclopedia of Analytical Chemistry}}</ref>

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.

ΔI<sub>probe</sub> = γI<sub>pump</sub>I<sub>probe</sub>L<ref name=":0">{{Cite journal |last=Lorenc |first=M. |last2=Ziolek |first2=M. |last3=Naskrecki |first3=R. |last4=Karolczak |first4=J. |last5=Kubicki |first5=J. |last6=Maciejewski |first6=A. |date=2002 |title=Artifacts in femtosecond transient absorption spectroscopy |journal=Applied Physics B |volume=74 |pages=19-27}}</ref>

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.

S<sub>approx</sub> = 0.43∙I<sub>probe</sub>I<sub>ref</sub><ref name=":0" />

A common baseline correction technique used in spectroscopy is the penalized [[Root-mean-square deviation|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.<ref>{{Cite journal |last=Olivier |first=Devos |last2=Nicolas |first2=Mouton |last3=Michel |first3=Sliwa |last4=Cyril |first4=Ruckebusch |date=2011 |title=Baseline correction methods to deal with artifacts in femtosecond transient absorption spectroscopy |journal=Analytica Chimica Acta |volume=705 |pages=64-71}}</ref>

===Conditions===
TA measurements are highly sensitive to laser repetition rate, pulse duration, emission wavelength, [[Polarization (waves)|polarization]], intensity, sample [[chemistry]], solvents, [[concentration]] and [[temperature]]. The excitation density (no. of photons per unit area per second) must be kept low; otherwise, sample annihilation, saturation and orientational saturation may come into play.

===Application===
Transient absorption spectroscopy helps study the mechanistic and kinetic details of chemical processes occurring on the time scales of a few picoseconds to femtoseconds. These chemical events are initiated by an ultrafast laser pulse and are further probed by a probe pulse. With the help of TA measurements, one can look into non-radiative relaxation of higher electronic states (~femtoseconds), vibrational relaxations (~picoseconds), and radiative relaxation of excited singlet state (occurs typically on a nanoseconds time scale).

Transient absorption spectroscopy can be used to trace the intermediate states in a photochemical reaction; energy, charge, or electron transfer process; conformational changes, thermal relaxation, fluorescence or phosphorescence processes, [[semiconductor optical gain|optical gain]] spectroscopy of semiconductor laser materials, etc. With the availability of UV-Vis-NIR ultrafast lasers, one can selectively excite a portion of any large molecule to desired excited states to study the specific molecular dynamics, such as photo-protective functions of carotenoids in photosynthesis.<ref>https://link.springer.com/article/10.1007/s11120-009-9454-y#Abs1</ref>

Transient absorption spectroscopy has become an important tool for characterizing various electronic states and energy transfer processes in nanoparticles, to locate trap states, and further helps in characterizing the efficient passivation strategies.<ref>C. Burda and M. A. El-Sayed, Pure Appl. Chem., 2000, Vol. 72, No. 1-2, pp. 165-17.</ref>