1. QUANTITATIVE ABSORPTION AND KINETIC STUDIES OF TRANSIENT
SPECIES USING GAS PHASE OPTICAL CALORIMETRY
DMITRY G. MELNIK
The Ohio State University, Dept. of Chemistry, Laser Spectroscopy Facility,
120 W. 18th Avenue, Columbus, Ohio 43210

2. Realistic chemical processes (e.g., hydrocarbon oxidation):
Problem overview
Realistic chemical processes (e.g., hydrocarbon oxidation):
Complex mechanism with thousands of elementary steps
Most reactions involve radicals
Mostly second or pseudo-second order – require independent concentration
determination
Accurate quantitative data required for better understanding and management
Experimental method sought:
Based on simple and well understood principles
Broadly applicable (i.e. diverse chemical systems)
Robust
Cheap

3. Signatures and reporters
Chemical signature:
2l-CRDS:
used HCl absorption as
reference :
Precursors
Products
Chemical
mechanism
Thermal signature:
nph
Ni,j(t)
Thermo
chemical
data
Closing the loop,
dQ(t)nph; Nj(t)
dQ(t)

4. How do we measure heat? dT Q l dT Q n0 L df df Q Calorimetry:
Heat from the target is transferred to a body
whose thermal properties are well known, and
whose change of state is used to characterize
the target.
Gas phase implementation:
When a region within a gas sample is heated, it expands
and its density (and the refractive index) is lowered.
Variation of refractive index causes phase shift of the
transmitted light.
In Fabry-Perot interferometer, such a phase shift results
in shift, df, of resonance frequencies. n0 l L df
df Q

5. … … … Heat balance and timing of gas-phase reaction. 10 ns 1-10 ms
10 ns
1-10 ms
100 ms 1s …
Evacuation of
heated sample … …
Absorption
of photolysis
radiation
Heat Losses through
thermal conductivity
Sample
expansion
Heat generated by chemical reactions
laser
power dissipation

6. Thermodynamic processes in CRDS reaction cell
The setup is configured for kinetic measurements in slow flow regime
From flow mass
controllers operating
at ~ 5% capacity
Buffer
contracts
Needle
valve
To pump
Sample
expands
This part of the
system is essentially
sealed on 100ms
time scale
Sample cycle V V1 V2 P
Heat deposition from
photolysis
(isochore) V P
V0-V2
V0-V1
Buffer cycle
adiabatic
limit

7. Frequency shift and concentrations
probe beam V0 V1 l L
Properties of the gas
“Instrumental” parameter
(independent of gas composition)
Heat deposited by photolysis laser and reactions
Heat transfer effects ,
calculated numerically
df(t)
Q(t)
Thermodynamic
and optical
analysis
Thermochemical
data and
photochemical
mechanism

8. PD PD PD ECDL Experimental setup Kinetic measurements Calorimetric
AOM OI
1.3 mm,
P=3.5 mW
M3,
F*=102 PD
Additional cavity
serves dual purpose:
Provides an independent
frequency scale
2. ECDL can be locked to it PD PD
M2,
F*=102
Reactive sample
M1, 1.3 mm,
F*=105

9. Measurement of frequency shift with reference cavity
Frequency sweep
turning point
TEM01
TEM10
TEM00
TEM02
TEM11
TEM20
Fine frequency scale,
Dfmode=58.7(5)MHz
df(t)

10. Optical calorimetry data: ethyl peroxy radical
adiabatic
calculations
Precursor: 3-pentanone, f=7522 cm-1
Traces averaged over 100 shots (4 traces
taken ca. 1 hr apart are shown).
The actual concentration of ethyl
peroxy radicals calculated from
experimental trace:
Uncertainty correspond to variations
between traces.
calculations including heat
transfer
“Acoustic”
oscillations
Noise ~ 70 kHz RMS

11. Time resolved CW-CRDS measurements of C2H5O2 self-reaction
Raw data at 7596 cm-1
(R-branch head of the
AX origin band)
Absorption decay curve
due to ethyl peroxy
self-reaction

12. Rate constant from OC/CW-CRDS measurements

13. METHYL PEROXY MEASUREMENTS
Spectral range accessible for diode laser
M. Pushkarsky, S. Zalyubovsky, T. A. Miller, J. Chem. Phys., 112, 10695, (2001)
Note: for these measurements, the absolute frequency of the diode laser is not
calibrated. Instead, the laser frequency is step-scanned to optimize signal
and possibly, minimize (k/s). The A0(k/s) data obtained in the vicinity of the peak
absorption are combined to derive N0kL0.

14. Summary of the results on the two radicals
EthO2
Reference
kobs,
10-13 cm3/s
P.D. Lightfoot, Atmos.Environ., 26A, 10, 1805 (1992)
1.08(34)
T.J.Wallington et al, Chem. Rev, 92, 667 (1992)
0.91(23)
R.Atkinson, J. Phys. Chem. Ref. Data, 26, 217 (1997)
1.03(29)
F.F.Fenter et al, J. Phys. Chem., 97, 3530 (1993)
1.29(7)
A.C.Noell et al, J. Phys. Chem. A, 114, 6983, (2010)
1.42(7)
D.B.Atkinson and J. W. Hudgens, JPCA. A, 101, p (1997)
1.24(41)
D. Melnik and T.A.Miller, JCP, 139, (2013)
0.966(44)
These measurements (avg)
1.09(9)
Reference
kobs,
10-13 cm3/s
D. A. Parkes et al, CPL, 23, p425 (1973)
3.3(1.1)
D. A. Parkes, IJCK, 9, p 451 (1977)
5.2(9)
C.J. Hochanadel et. al., JPC, 81, p3 (1977)
3.8(5)
C. K. Kan, IJCK, 11, p921 (1979)
4.2(5)
H. Adachi et al., IJCK, 12, p949 (1980)
5.8(5)
K. McAdam et al., CPL, 133, p39 (1987)
5.9(10)
M. E. Jenkin et al., JCSFT2, 84, p913 (1988)
4.7(5)
F.-G. Simon, IJCK, 22, p791 (1990)
4.8(5)
R. Atkinson, JPCRD, 26, p215 (1997)
4.9(11)
These measurements
5.14(48)
MethO2

15. Summary
The use of heat signature for determination of the absolute concentrations
has been successfully demonstrated by measuring the effective rate constants
of methyl and ethyl peroxy self reaction.
The developed optical calorimetry (OC) technique is a non-resonant technique
which does not require accurate frequency monitoring or stabilization
of the probe lasers.
The accuracy and precision of the method is fundamentally limited by those of
thermochemical data, which are generally higher than spectroscopic absorption
data of potential “reporter” species.
The OC technique requires careful monitoring of relatively large number of
parameters, however is potentially useful for measurement of broad range of
species

16. Acknowledgements
Colleagues:
Prof. T. A. Miller
Dr. Mourad Roudjane
Dr. Rabi Chhantyal Pun
Dr. Neal Kline
Terrance Codd,
Meng Huang
Henry Tran
OSU
DOE

17. Methyl peroxy peak absorption cross-section for 1211 band
Reference
s, x 1020 cm2
D. D. Atkinson and J. L. Spillman, JPCA, 106, p8891 (2001)
1.5(8)
M. Pushkarsky, S. Zalyubovsky and T. A. Miller, JCP, 112, p10695 (2001)
~ 1.0 (2)
E. Farago, C. Fittshen et. al, JPCA, 117, p12802 (2013)
3.40(68)
This work
1.20(11)