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

4.3 The discharge flow (DF) technique 78
4.3 The discharge flow (DF) technique
DF technique is used for gas phase reactions on ms timescale:
• laminar flow along tubular reactor:
∆ = ∆
with =meanflow speed, ∆ = distance along the flow tube.
(4.1)
• y first-order reaction gives exponential decay of concentration along ∆.
• in reality ⇒ Hagen-Poisseuille flow (parabolic flow profile),
• but fast radial diffusion of reactants (at pressures of 1 to 10 mbar) gives “plug
shaped” radial concentration profiles despite parabolic flow profile.
• more complicated, when “back diffusion” and radial diffusionhavetobetaken
into account (at higher pressures).
• problem: heterogeneous reactions lead to first-order decay of reactive radicals
even in the absence of a reactant.
Combinations with numerous highly sensitive detection techniques, for instance:
• mass spectrometry (MS): universal, but often low sensitivity; problems with fragmentationofmoleculesintheionsource,
• laser induced fluorescence (LIF): excitation to electronically excited state followed
by relaxation by spontaneous emission. very sensitive (e.g., for OH ( 2 Σ ← 2 Π)
detection limit below 10 6 molecules cm 3 ),
• electron spin resonance (ESR): for paramagnetic atoms by exploiting the Zeeman
effect depending on magnetic field
= 0 + (4.2)
∆ =1y ∆ = (4.3)
ESR transitions are in the microwave region (X band: 9GHz).
• laser magnetic resonance (LMR): very sensitive; based on observation of rotational
transitions between Zeeman-shifted rotational levels of paramagnetic radicals
( rot = rotational constant).
00 = rot ( +1)+ 00
00 (4.4)
0 = rot ( +1)+ 0 0 (4.5)
∆ = ±1 ∆ =0 ±1 : (4.6)
y 0 = rot ( 00 +1)( 00 +2)+ 0 0 (4.7)
y ∆ = =2 rot ( 00 +1)+ ( 0 0 − 00
00 ) (4.8)
LMR transitions are in the FIR region (100 m ≤ ≤ 2000 m).