Transcript Document

32nd International Symposium on Free Radicals, 21-26 July, Potsdam, Germany
Oxygen Atom Recombination
in the Presence of Singlet
Molecular Oxygen
Valeriy Azyazov
P.N. Lebedev Physical Institute
of RAS, Samara Branch, Russia
A.A. Chukalovsky, K.S. Klopovskiy,
D.V. Lopaev, T.V. Rakhimova
Skobeltsyn Institute of Nuclear Physics,
Moscow State University, Russia
Michael Heaven
Department of Chemistry
Emory University, USA
The Pure Oxygen Kinetics (POK)
O atom formation O2 + h (<242 nm)  O + O
Ozone formation
O + O2 + M  O3+ M
O3 photolysis
O3 + h (320 nm) O2(a) + O(1D)
 O2(X) + O(3P)
Odd oxygen removal
O + O 3  O2 + O2
O + O + M  O2 + M
O2(a1∆) deactivation
O2(a1∆) O2(X) +h (1268 nm)
O2(a1∆) +O2(X)  O2(X) + O2(X)
G.P. Brasseur, S. Solomon, Aeronomy of the Middle Atmosphere. Chemistry and Physics of the
Stratosphere and Mesosphere Series: Atmospheric and Oceanographic Sciences Library, Vol. 32,
2005, Published by Springer, P.O. Box 17, 3300 AA Dordrecht, The Netherlands
What’s missing in the POK?
1) Ozone molecule formed in recombination process
O + O2 + M  O3(v) + M
is vibrationally excited!
W.T Rawlins et al. J. Geophys. Res., 86, 5247 (1981) observed infrared
emission originated from high vibrational levels of ozone (up to 3=6)
formed during recombination.
2) O3(v) has a high reactivity!
M.J. Kurylo, et al., J. Photochem. 3, 71 (1974) found that the rate constant
for O2(a1Δ) quenching by O3() that has one quantum of vibrational energy
is faster by a factor of 3820.
W.T. Rawlins et al. J. Chem. Phys., 87, 5209 (1987) estimated that the rate
constant for quenching of O2(a1) by ozone with two or more quanta of the
stretching modes excited to be in the range 10-11-10-10 cm3s-1.
V.N. Azyazov et al. Chem. Rhys. Lett., 482, 56 (2009) observed fast
quenching of O2(a1Δ) in the O/O3/O2 system.
G.A. West et al. , Chem. Phys. Lett., 56, 429 (1978) observed that
vibrationally excited ozone reacts effectively with oxygen atom.
The fate of O3(v)
O3(υ) formation
1. O(3P) + O2 + M  O3(υ) + M
O3(υ) destruction
2. O3(υ) + O2(1)  O(3P) +2O2
4a. O3(υ) + O(3P)  2 O2
5. O3(υ) + X  products
O3(υ) stabilization
3. O3(υ) + M  O3 + M (O2, N2)
4b. O3(υ) + O(3P)  O3 + O(3P)
6. O3(υ)  O3 + h
Present work
(1)
The rates of O2(a1∆) removal, O atom recombination and O3 recovery were measured in the
O/O2(a1∆)/O2/O3 system using laser-pulse
technique, time-resolved emission/absorption
spectroscopy and O+NO chemiluminescent
reaction.
(2)
New experimental data showing that vibrationally
excited ozone is effectively quenched by O2(a1∆)
molecule and O atom are reported.
The contribution of these quenching channel on
the O2(a1∆) and O3 budgets in the middle
atmosphere and oxygen-containing plasma is
discussed.
Experimental setup
O2/O3/buffer
Power
meter
1268 nm
filter
Ge photo
detector
To pump
O3 + h (248 nm)  O(1D) + O2(a1), h,O3 =0.9
 O(3P) + O2(3)
O(1D) + O2  O(3P) + O2(b1)
O2(a1)  O2(3)+ h (1268 nm)
Details of the flow cell
7
Schematic view of time-resolved absorption
spectroscopy for O3 concentration measurements
О2/О3/М
Supply
fiber
Withdraw
fiber
Monochromator
LED
PMT
258 nm
Laser beam
8
Temporal profiles of O2(a1Δ) emission after laser
photolysis of O3 with different buffer gases
PO3=1 Torr
E =87 mJ cm-2
T=300 K.
Temporal profiles of O2(a1Δ) emission
after laser photolysis of O2/O3/He
mixture + model predictions
PO2=460 Torr
PO3=1 Torr,
E=87 mJ cm-2,
T=300 K.
PHe varied:
0 – 244 Torr
Temporal profiles of O2(a1Δ) emission
after laser photolysis of O2/O3/CO2
mixture + model predictions
PO2=460 Torr
PO3=1 Torr,
E=87 mJ cm-2,
T=300 K.
PCO2 varied:
0 – 97 Torr.
O Atom removal in O3/O2
photochemistry
O+NO+MNO2*+M, Trace [NO] used for detection
Model without O atom
regeneration from
secondary reactions of
O3 does not fit the O
atom decay rate.
Without O atom
regeneration the
accepted rate constant
must be reduced by a
factor of two.
O3 recovery in O3/O2/Ar/CO2
photochemistry
a)
O3 density temporal profiles at E=90 mJ/cm2, total
gas pressure Ptot =712 Torr, PO2 =235 Torr, gas
temperature T=300 K for several CO2 pressure.
O3 density temporal profiles at E=90 mJ/cm2, total
gas pressure Ptot =706 Torr, gas temperature T=300 K
for several O2 pressure.
The degree of O3 recovery depends on gas composition while the POK model
predicts a full recovery of the ozone at our experimental conditions
Observations
(1) The degree of O3 recovery depends on gas composition
and for O3/O2/Ar mixtures (the lower curves it
amounts to about 70 %). The standard pure oxygen
kinetics (POK) predicts that it must be restored to its
initial value (100 %) at our experimental conditions. Odd
oxygen is removed in the process
O + O3(v) – O2 + O2
(2) The O3 recovery time depends also on gas composition
and for O3/O2/Ar mixtures and for the lower curves it
is about 50 msec against 13 msec predicted by POK.
Oxygen atoms regenerate in the process
O2(1) + O3(v) – O + O2 + O2
(3) Ar quenches O3(v) worse than CO2 or O2. Replacement
of Ar by CO2 or O2 results in increasing both the degree
and the rate of O3 recovery.
Atmospheric applications
The ratio of the rate of O2(1) removal in the
process (2) to the rate of the process (13)
2) O3(υ2) + O2(1)  O(3P) +2O2
13) O2(1∆) +O2(X)  O2(X) + O2(X)
R =
RΔ =
k2=5.2×10-11 cm3/s
k13=3.0×10-18 cm3/s
k2 [O3 (  2)][O 2 (a )]
k13[O 2 ][O 2 (a )]
k 2k 1M [O][M]
k 13  k 2 [O 2 (a)]+k M
3 [M]+k 4 [O]+A υ 
Atmospheric applications
The fraction of O3(v) that dissociates in the
processes (1) and (4a)
2) O3(υ2) + O2(1)  O(3P) +2O2
4) O3(υ) + O(3P)  O3 + O(3P)
4a) O3(υ) + O(3P)  2 O2
Rloss
R loss =
k1=5.2×10-11 cm3/s
k4=1.5×10-11 cm3/s
k4a=4.5×10-12 cm3/s
O3 (v) dissociation rate
=
O3 (v) stabilization rate
k 4a [O]+k 2 [O2 (a)]
 k [O (a)]+k
2
2
M
3
[M]+k 4 [O]+A υ 
Measurement errors of the rate constant
of process O+O2+M O3+M
A systematic error caused by reaction
O3(v) + O2(1)  O(3P) +2O2


kM

3 [M]
k 2 [O 2 (a)][O 3 (   2)] 
k 4 [O]

M
2 =
=
1



k 1M [O][O 2 ][M]
k 2 [O 2 (a)] k 2 [O 2 (a)] 




M
1
A systematic error caused by reaction
O3(v) + O (3P)  2 O2

  k 3 [M] 
k 3 [M] 
k 4a [O][O 3 (υ  2)]  k 4 
   3+ M

 4a =
=
+ M

k 1[O][O 2 ][M]  k 4a
k 4a [O] 
k 4a [O] 






M
-1
-1
At [O2(a)]≈0.9[O]3×1016 cm-3 [O2]=2.1×1019 cm-3 – 2=0.58, 4a=0.14.
Klais et al. (Int. J. Chem. Kinet. 12, 469-490 (1980)) experiments T=219 K,
[O2]=4.41017 cm-3, [O]≈1015 cm-3 4a = 0.22.
Conclusions
1. O3(v) is a significant quenching agent of O2(a1)
in the O/O2/O3 systems.
2. Odd oxygen is effectively removed
in the process O + O3(v)  O2 + O2.
3. Processes involving active oxygen species effect
significantly on the balance of O2(a1) and O3
at the atmospheric altitudes 80 - 105 km.
4. Processes involving excited oxygen species
may make large systematic errors in the
measurements of rate constants
in the O/O2/O3 systems.