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The Impact of Vibrational Excitation on Atmospheric Optical Emissions. Jeff Morrill Naval Research Laboratory Washington, DC. Outline. N 2 Spectroscopy Processes That Affect N 2 Vibrational Level Populations Methods of Observation Spectral Synthesis and Kinetic Models

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The impact of vibrational excitation on atmospheric optical emissions l.jpg

The Impact of Vibrational Excitation on Atmospheric Optical Emissions

Jeff Morrill

Naval Research Laboratory

Washington, DC


Outline l.jpg
Outline Emissions

  • 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


N 2 potential energy curves and spectroscopy l.jpg
N Emissions2 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)


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N Emissions2 First Positive (1PG) Spectrum


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Processes That Affect N Emissions2 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


Direct electron impact excitation l.jpg
Direct Electron Impact Excitation Emissions

  • Born-Oppenheimer Approximation

    Electrons Move Faster Than

    Nuclei

  • Franck-Condon Principle

    Excited State Vibrational

    Distribution Governed by Wavefunction Overlap


N atom recombination lewis rayleigh afterglow l.jpg
N Atom Recombination: EmissionsLewis-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


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Intersystem Collisional Transfer (ICT) Emissions

  • 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)


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Vibrational Redistribution Emissions

  • 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)


Energy pooling l.jpg
Energy Pooling Emissions

  • 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).


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Observational Methods Emissions

  • Laboratory Experiments

  • DC or RF Discharge

  • Flowing Afterglow

  • Pusled Discharge

  • Pulsed Laser

  • Atmospheric Observations

  • Aurora and Airglow

  • Sprites, Jets, …


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Observational Methods Emissions

  • 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


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Spectral Models Emissions

  • 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


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J Emissions

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


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Kinetic Models Emissions

  • 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


Model n 2 triplet populations of the aurora l.jpg
Model N Emissions2 Triplet Populations of the Aurora


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Large Volume Pulsed Discharge Emissions

Volume ~ 40,200 cm3

Diameter

~ 16 cm

Length ~ 2 meters


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Computer Emissions

Time-Resolved Spectroscopy:Large Volume Pulse Discharge

Experimental Setup


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Large Volume Pulsed Discharge: Emissions

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


Pulsed discharge changes in 1pg spectrum glow vs afterglow l.jpg

“Glow” Emissions

“Afterglow”

Pulsed Discharge:Changes in 1PG Spectrum – “Glow” vs “Afterglow”

“Glow”

N2(B) Vibrational Distributions

“Afterglow”


Pulsed discharge emission curves l.jpg
Pulsed Discharge: EmissionsEmission Curves

  • “Glow” Emisson Governed by Lifetime

1PG: τ ~ 5-10 µs

1NG: τ ~ 50 ns

2PG: τ ~ 50 ns


Pulsed discharge vibrational temperature decay glow l.jpg
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


Pulsed discharge vibrational temperature decay glow23 l.jpg
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


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Relative Energy/Molecule “Glow”

Pulsed Discharge: Vibrational Temperature Decay – “Glow”

Vibrational Energy vs Collisions Between Consecutive Discharge Pulses


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Pulsed Discharge: “Glow”Emission Curves

  • “Afterglow” Emission Continues Beyond Lifetime Limits

N2(B,v=5) Emission

Curve at Various Pressures


Pulsed discharge changes in 1pg spectrum afterglow l.jpg
Pulsed Discharge: Changes in 1PG Spectrum - “Afterglow” “Glow”

  • Presence of HIR Bands Indicate Energy Pooling

  • Enhancement in V’ = 10 Bands Implies Energy

  • Transfer from N2(A’) to N2(B)


Pulsed discharge changes in 1pg spectrum afterglow27 l.jpg
Pulsed Discharge: Changes in 1PG Spectrum - “Afterglow” “Glow”

  • N2(A, v=0,1) & Total N2(B) Population in the afterglow


Pulsed discharge vibrational distribution in the afterglow l.jpg
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?


Pulsed discharge vibrational distribution in the afterglow29 l.jpg
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


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Pulsed Discharge: Conclusions “Afterglow”

  • 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



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Sprite Spectroscopy “Afterglow”

  • Video Spectrograph

  • Spectra at 53 & 57 km

  • Possible N2+ Meinel Emission

  • Vibrational Distribution Indicative of Energy Pooling


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Sprite Spectroscopy “Afterglow”

  • 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


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Sprite Spectroscopy “Afterglow”

  • Vibrational Distributions

  • Population Similar to

  • Energy Pooling Afterglow

  • 53 km Spectrum

  • No Significant

  • Intensity near 8000 A

  • Spectrum of Body


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Sprite Spectroscopy “Afterglow”


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High Time-Resolution Sprite Spectroscopy “Afterglow”

  • N2(B) varies with altitude

  • Lower alt. similar to earlier work

  • 1ms still not fast enough!!


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Sprite Spectroscopy: Conclusions “Afterglow”

  • 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.


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