Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

Appendix B 265
I
Figure B.1: Exponential decay curve of a particular vibration-rotation state of the
CH 3 O radical resulting from the unimolecular dissociation reaction of the radical according
to CH 3 O → H+H 2 CO (Dertinger 1995). The small box is the output box
from a fit usingtheORIGINprogram.
I Example 2: Multiexponential decay. In femtosecond spectroscopy, we often observe
ultrafast multiexponential decays of laser-excited molecules. The laser prepares
an excited wavepacket, which usually does not decay single-exponentially. Further, we
need to take into account the final duration of the pump laser pulse (by deconvolution,
or by forward convolution).
• Model function to be fitted to measured data:
() = X exp (− )
(B.11)
with adjustable parameters and .
• Instrument response function (IRF): Often represented by a Gaussian centered at
time 0 Ã !
1
() = √ exp − ( − 0) 2
(B.12)
IRF 2 2 2 IRF
with width parameter IRF related to the full width at half maximum (FWHM)
of the IRF by
FWHM = √ 8ln2≈ 2355 IRF
(B.13)