The Impact of Vibrational Excitation on Atmospheric Optical Emissions

1. The Impact of Vibrational Excitation on Atmospheric Optical Emissions Jeff Morrill Naval Research Laboratory Washington, DC

2. Outline • N2 Spectroscopy • Processes That Affect N2 Vibrational • Level Populations • Methods of Observation • Spectral Synthesis and Kinetic Models • Examples: Laboratory (Pulsed Discharge) • Examples: Atmosphere (Sprites) • Conclusions

3. N2 Potential Energy Curves and Spectroscopy • Common N2 Band Systems • 1PG: N2(B) → N2(A) • 2PG: N2(C) → N2(B) • 1NG: N2+(B) → N2+(X) • Meinel: N2+(A) → N2+(X) • HIR: N2(C’’) → N2(A’) • W-B: N2(W) ↔ N2(B) • IRA: N2(B’) ↔ N2(B) • VK: N2(A) → N2(X) • LBH: N2(a) → N2(X)

4. N2 First Positive (1PG) Spectrum

5. Processes That Affect N2 Vibrational Populations • Direct electron impact excitation of electronic states • Born-Oppenheimer Approximation • Franck-Condon Principle • N atom recombination – Lewis-Rayleigh Afterglow • Intersystem Collisional Transfer (ICT) • Transitions between nearby energy levels of • overlapping electronic states – Collisions! • Radiative cascade • Vibrational Redistribution - Ground and excited states • Transfer of vibrational energy between molecules • Energy Pooling – Electronic and Vibrational • Transfer of internal energy between molecules

6. Direct Electron Impact Excitation • Born-Oppenheimer Approximation Electrons Move Faster Than Nuclei • Franck-Condon Principle Excited State Vibrational Distribution Governed by Wavefunction Overlap

7. N Atom Recombination:Lewis-Rayleigh Afterglow • N(4S) + N(4S) -> N2(5Σg+) -> N2(B3Πg) • Populates N2(B3Πg) Vibrational Levels Near Dissociation Limit (V=11, 12 & 13) • “Straw Yellow” Afterglow Resonance LTD Lewis-Raleigh Afterglow Source

8. Intersystem Collisional Transfer (ICT) • Couples adjacent vibrational levels • of overlapping electronic states • NOT Quenching – Excitation is not • lost from coupled manifold of levels • Transition rates varies with rotational • level • Propensity rules: • ΔJ ~ 0 and ΔE ~ 200 cm-1 • ΔS = 0 (Triplet <≠> Singlets)

9. Vibrational Redistribution • Low Levels of the Ground State • N2(X, v1) + N2 (X, v = 0) -> • N2 (X, v1-1) + N2 (X, v = 1) • Higher Levels of the Ground State • N2 (X, v) + N2 (X, w) -> • N2 (X, v -1) + N2 (X, w+1) • Levels of the N2 (A) State with v ≥ 2 • N2 (A, v1≥ 2) + N2 (X,v = 0) • -> N2 (A, v1 - 2) + N2 (X,v = 1)

10. Energy Pooling • Electronic Energy Pooling N2(A) + N2 (A) -> N2 (B) + N2 (X, v*) N2 (B) -> hv (1PG) + N2 (A) [N2 (C), N2 (C’’) also] • Vibrational-Electronic Energy Pooling N2 (X, v>4) + N2 (A) -> N2 (B) + N2 (X, v~0) N2 (B) -> hv (1PG) + N2 (A) [N2 (B) only!] • N2 (A) Transfers Energy to N2 (X, v>4) (L. Piper, 1991).

11. Observational Methods • Laboratory Experiments • DC or RF Discharge • Flowing Afterglow • Pusled Discharge • Pulsed Laser • Atmospheric Observations • Aurora and Airglow • Sprites, Jets, …

12. Observational Methods • Normal Steady State Spectroscopy • Time Resolved Spectroscopy • Narrow-Passband Imaging • Broad-Passband High-Speed Imaging All Methods Require Detailed Knowledge of the N2 Spectrum for Analysis

13. Spectral Models • Spectral Synthesis Codes require the calculation of many rotational lines (wavelength and intensity) for a single rovibronic band. • Calculations Include: Energy levels for wavelengths Line strengths and rotational distributions for rotational line intensities • Relative Band Intensities from Transition Probabilities

14. J Spectral Models • Fortrat Diagram – Plot of rotational QN (J) vs • wavelength for a given branch (constant ΔJ) • Rotational Line Energy derived from difference between upper and Lower rotation level energies (hν ~ F(J’) – F(J’’)) • Line Strengths – Function of J

15. Kinetic Models • Kinetic Models require knowledge of Production and Loss Processes and associated rate coefficents • Time-resolved observations can allow reduced number of Processes (e.g. “Glow” vs “Afterglow”) • DC Observations Often Require All (or most) Processes to be Included

16. Model N2 Triplet Populations of the Aurora

17. Large Volume Pulsed Discharge Volume ~ 40,200 cm3 Diameter ~ 16 cm Length ~ 2 meters

18. Computer Time-Resolved Spectroscopy:Large Volume Pulse Discharge Experimental Setup

19. Large Volume Pulsed Discharge: I-V Traces at Various Pressures Light Source Parameters Voltage (peak) ~ 10 to 12 kV Current (peak) ~ 600 – 1000 A Pulse Width ~ 4μs Frequency ~ 1 - 500 Hz Pressure ~ 30 - 500 mT Primary Spectral Observations Time Resolution: 0.2 μs Pressures: 50, 200, 400 mT Frequency: 5, 15, 32, 50 Hz Observation Times and Spacing ~ 0 - 10 μs: Δt = 1 μs ~ 10 - 20 μs: Δt = 2 μs ~ 20 -200 μs Δt = 20 μs

20. “Glow” “Afterglow” Pulsed Discharge:Changes in 1PG Spectrum – “Glow” vs “Afterglow” “Glow” N2(B) Vibrational Distributions “Afterglow”

21. Pulsed Discharge:Emission Curves • “Glow” Emisson Governed by Lifetime 1PG: τ ~ 5-10 µs 1NG: τ ~ 50 ns 2PG: τ ~ 50 ns

22. Pulsed Discharge: Vibrational Temperature Decay – “Glow” • Electron impact produces significant ground state vibrational excitation • N2(B) vibrational distribution shows evidence of enhanced N2(X) vibrational distribution • Vibrational temperature decays with number of collisions • Anharmonic nature of N2(X) yields non-thermal vibrational distributions

23. Pulsed Discharge: Vibrational Temperature Decay – “Glow” Shifts in N2(B) Vibrational Distribution Corresponds to Increased Vibrational Temperature N2(B) Initial Vibrational Distribution Shifts to Higher V-Levels With Increasing Frequency

24. Relative Energy/Molecule Pulsed Discharge: Vibrational Temperature Decay – “Glow” Vibrational Energy vs Collisions Between Consecutive Discharge Pulses

25. Pulsed Discharge:Emission Curves • “Afterglow” Emission Continues Beyond Lifetime Limits N2(B,v=5) Emission Curve at Various Pressures

26. Pulsed Discharge: Changes in 1PG Spectrum - “Afterglow” • Presence of HIR Bands Indicate Energy Pooling • Enhancement in V’ = 10 Bands Implies Energy • Transfer from N2(A’) to N2(B)

27. Pulsed Discharge: Changes in 1PG Spectrum - “Afterglow” • N2(A, v=0,1) & Total N2(B) Population in the afterglow

28. Pulsed Discharge: Vibrational Distribution in the “Afterglow” • Remove electronic energy pooling component • Residual is due to N2(X,v) + N2(A) • Use know rates to calculate N2(X,v) from N2(B) • Can this be done with Sprites?

29. Pulsed Discharge: Vibrational Distribution in the “Afterglow” • N2(X) Vibrational Distribution Derived from Vibrational Energy Pooling Fits Treanor-Type Distribution • Implications for N2(B) Populations Observed in Sprites

30. Pulsed Discharge: Conclusions • Significant differences between glow and afterglow spectra • N2(B) afterglow distributions indicate several energy pooling processes • Significant portion of N2(B) afterglow due to vibrational-electronic energy pooling • N2(A) distribution from HIR allows calculation of N2(X,v) distribution

31. Electron Energy Estimate in a Sprite:1NG/2PG Ratio 1NG/2PG 1NG 2PG

32. Sprite Spectroscopy • Video Spectrograph • Spectra at 53 & 57 km • Possible N2+ Meinel Emission • Vibrational Distribution Indicative of Energy Pooling

33. Sprite Spectroscopy • Several Possible • Vibrational Distributions • V = 1 Population Low • Due to Calibration • 57 km Spectrum • Intensity near 8000 A • Fit as N2+ Meinel • Spectrum of Tendril

34. Sprite Spectroscopy • Vibrational Distributions • Population Similar to • Energy Pooling Afterglow • 53 km Spectrum • No Significant • Intensity near 8000 A • Spectrum of Body

35. Sprite Spectroscopy

36. High Time-Resolution Sprite Spectroscopy • N2(B) varies with altitude • Lower alt. similar to earlier work • 1ms still not fast enough!!

37. Sprite Spectroscopy: Conclusions • Improved time and spatial resolution spectra of Sprites is required to distinguish between “Glow” and “Afterglow.” • Only require moderate to low spectral resolution to model band shapes. • Kinetic models must be expanded to include additional processes such as energy pooling, ground state vibrational redistribution. • N2(X,v) distribution appears to play an important role in Sprite emission spectrum.