Time-resolved spectroscopy
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 impact of the probe pulse on the sample is recorded and analyzed with wavelength/ time to study the dynamics of the excited state.
Stimulated emission follows the fluorescence spectrum of the molecule and is Stokes shifted relative to and often still overlaps with the bleach signal.
Transient absorption is a highly sensitive technique that can provide insightful information regarding chemical and material processes when achieving sufficient spectral resolution.
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.
ΔIprobe = γIpumpIprobeL[4] The above equation describes the change in intensity relative to the number of photons absorbed (γ) and the thickness of the sample (L).
Sapprox = 0.43∙IprobeIref[4] 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.
[5] TA measurements are highly sensitive to laser repetition rate, pulse duration, emission wavelength, polarization, intensity, sample chemistry, solvents, concentration and temperature.
Transient absorption spectroscopy helps study the mechanistic and kinetic details of chemical processes occurring on the time scales of few picoseconds to femto-seconds.
Transient absorption spectroscopy can be used to trace the intermediate states in a photo-chemical reaction; energy, charge or electron transfer process; conformational changes, thermal relaxation, fluorescence or phosphorescence processes, optical gain spectroscopy of semiconductor laser materials.
