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

4.4 Flash photolysis (FP) 83
— decay of intensity for pass through cavity (length , mirrorreflectivity ,
absorption path length ):
µ
1 −
= −
−
(4.10)
| {z } | {z }
due to reflection losses at mirrors due to absorption
— with
y
— empty cavity ( =0):
— with absorber ( 6= 0):
= (4.11)
= − [(1 − )+] (4.12)
−1 (1 − )
0 =
(4.13)
−1 =
[(1 − )+]
(4.14)
— The difference of the ringdown times measures the absorption coefficient
and is related to absorber concentration [A] by
• Performance of a CRD spectrometer:
— mirrors:
— measuredemptycavitydecaytime:
— effective absorption pathlength (1 m cell):
• applications to chemical kinetics:
−1 − −1
0 = = [A] (4.15)
=99999% (4.16)
0 ≈ 300 s (4.17)
eff =90km (4.18)
— slow reactions: pump-probe scheme with variable time delay.
first-order reaction with rate constant 1 :
[A] = [A] 0
− 1
(4.19)
y
ln ¡ ¢
−1 − −1
0 = − 1 (4.20)
— fast reactions: kinetics and ringdown occur on the same time scale. y
(extended) Kinetics and Ringdown model ((e)SKaR); see Brown2000 and
Guo2003 for details.