Editing Time-resolved spectroscopy (section)

=== 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>