1. N2 Vibrational Temperature, Gas Temperature,
72nd International Symposium on Molecular Spectroscopy
June 19-23, Champaign-Urbana, Illinois
Nonequilibrium Thermodynamics Laboratory
N2 Vibrational Temperature, Gas Temperature,
and OH Number Density Measurements
in ns Pulse H2-air Plasmas
Introduce people
Caroline Winters, Yichen Hung, Elijah Jans, Kraig Frederickson,
and Igor V. Adamovich

2. Motivation and Objective: Chemical Reactions in Fuel-Air Plasmas
Radical species such as H, O, and OH are critical in low-temperature plasma-assisted combustion
In fuel-air plasmas, a large fraction of discharge input energy goes to N2 vibrational excitation
Question
Is there coupling between N2 vibrational excitation and radical reaction kinetics?
Hypothesis (Starikovskiy and Aleksandrov, 2013):
N2(v=1) + HO2 → N2(v=0) + HO2(v2+v3) → N2 + H + O2 → N2 + OH + O
Energy transfer from N2 breaks up HO2, extends H and OH lifetime.
Dissociation and ionization dominate in (a), vibrational excitation in (b)
Objectives:
Measure [OH], T, and Tv (N2) in plasmas with strong N2 vibrational excitation
Obtain insight into radical reaction kinetics at these conditions using kinetic modeling predictions

3. “Diffuse filament” discharge between hollow spherical electrodes
Discharge Test Cell and Plasma Images
Collinear CARS, axial optical access through hollow electrodes
T0 =300 K, P=100 Torr
Pulse repetition rate = Hz
Peak voltage = kV
Coupled Energy = 5-10 mJ/pulse
Pulse duration = ~100 ns

4. Coherent Anti-Stokes Raman Scattering (CARS) Experimental Schematic
Probe Length ~ 4 mm
Nd: YAG pump/probe beam, 7 mJ/pulse, cm-1 with injection seeding, 0.4 cm-1 without seeding
Broadband dye laser (Stokes beam), 3.2 mJ/pulse, ~ 100 cm-1 FWHM
Access to N2(v=0-3) levels
Spectra taken 800 ns to 14 ms after discharge pulse

5. Typical N2 CARS Spectra ωprobe ωpump ωCARS ωStokes ωvib
T=557±50K
Trot-trans=292±12K
Coherent Anti-Stokes Raman Spectroscopy
T and TV(N2)
Collinear phase matching
CARS Probe length ~4 mm
Plasma filament length ~8 mm
ωp= ωprobe= 532 nm
ωStokes= 607 nm
ωCARS= 473 nm
Sandia National Laboratory, CARSFIT

6. CARS T and Tv(N2) Results in Discharge Afterglow
T0 =300 K, P=100 Torr, E~8 mJ/pulse Tv T Tv T
Tv is much higher than T, indicating strong vibrational non-equilibirum
Tv rise at t= μs due to “downward” N2-N2 V-V exchange, decay at t=200 μs – 1 ms due to V-T relaxation of N2 by O atoms
T increases from 400 K to 500 K at t = μs
After t > 1 ms, both T and Tv drop due to radial diffusion from the filament
Modeling predictions are in fairly good agreement with experimental data
Results in air and H2-air mixtures are similar

7. OH Laser Induced Fluorescence (LIF) Experimental Schematic
308 nm OH
A→X (0,0)
R2(3)
532 nm output of Nd:YAG laser pumps a dye laser, produces 615 nm output
Frequency doubling by a BBO crystal, to 308 nm
LIF quenching rate is measured directly
Fluorescence is collected at 900, calibrated by Rayleigh scattering

8. [OH] LIF line images and signal distribution
OH LIF Images and Signal Distribution
3% H2- Air: T0 =300 K, P=100 Torr, E~8 mJ/pulse
t < 50 µs: OH formed near grounded electrode
t = µs: OH formed near both electrodes
Effect more prominent at higher coupled energy
t > 300 µs: nearly uniform distribution
[OH]
Centerline OH signal intensity distribution
[H]
Ground HV

9. Time resolved [OH] in plasma afterglow
Absolute [OH] in Discharge Afterglow
T0 =300 K, P=100 Torr, E= 8.0 mJ/pulse
[OH] and Tv(N2) peaks coincide in time approximately
Model predictions are in satisfactory agreement with experimental data
However, [OH] peak is not caused by N2(v=1) + HO2 → N2(v=0) + HO2(v2+v3) reaction (adding it reduces [OH])
[OH] peak is not caused by temperature rise induced by N2 vibrational relaxation
In the afterglow, modeling prediction shows [OH] follows [H] and [O]; peak may be caused by diffusion of H atoms from plasma regions with higher density.
[OH] Tv T
[H]

10. Summary
Diffuse filament, ns pulse discharge sustains strong vibrational non- equilibrium in air and H2-air mixtures (up to Tv~2000 K, T~ 500 K) on time scales of up to ~1 ms
CARS measurements of Tv(N2), T show that N2 vibrational excitation air and H2-air mixtures is similar
[OH] in the afterglow follows [H] and [O];
Reaction N2(v=1) + HO2 → N2(v=0) + HO2(v2+v3), temperature rise do not affect [OH] kinetics
Transient rise of [OH] may be caused by H atom transport from other regions in the plasma
Laser H
probe region

11. Thank you Any Questions?

12. Chemical kinetics of vibrationally excited N2 in air
Time resolved “first” level N2 vibrational temperature and rotational-translational temperature in air
“Rapid” heating: N2* quenching by O2
N2(A,B,C)+ O2  N2(X) + O + O + e
[OH]
Downward V-V energy transfer:
N2(w) + N2(v=0)  N2(w-1) + N2(v=1)
V-T relaxation by O atoms, “slow” heating: N2(v=1) + O  N2(v=0) + O
Electron impact, electronic excited N2
Downward V-V energy transfer from us, such that population of v=1 increases and Tv increases.
V-T relaxation of v=1 N2 molecules. This drop of Tv at us after the discharge pulse is not observed in pure N2 case.
The results are consistent with modeling calculation.
[H]
Radial diffusion
Air: T=300 K, P=100 Torr

13. Modeling Prediction of Radical Species in 3% H2-air Plasma
Coupled Energy = 8 mJ/pulse