INVESTIGATION OF O2(C3, v=2) BY NOVEL LASER PHOTOIONIZATION

TECHNIQUE IN AIR AT ATMOSPHERIC PRESSURE

Jonathan D. Umbel,Dr. Steven F. Adams,

Dr. Charles A. DeJoseph, Jr.

Air Force Research LaboratoryWright Patterson AFB, OH

18 Jun 07

Laser Diagnostics Facility for Plasma Studies AFRL Propulsion Directorate , Wright Patterson AFB, OH

Building 450, WPAFB, OH

Introduction

• Resonant-Enhanced Multiphoton Ionization (REMPI) in dry atmospheric pressure air studied for possible low-jitter laser triggering of air spark gap switch

• Interesting REMPI phenomena observed in dry air at 1 atm

– Strong REMPI signal with O2(C3) Rydberg resonance

– Strong N2+ fluorescence at all REMPI transitions

• In this work, we characterize the O2(C3, v=2) state using both fluorescence and traditional REMPI spectra.

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Previous work: O2 REMPIwith ultraviolet laser

Laser P

ho

ton

s

O2(X3g-)

O2+(X2g)

O2(C3g)

e- Ionizatio

n

O(1D)+O(3P)3g

REMPI Band Corresponds to O2(C3g,v=2 → X3g

-)

Two-Photon Resonant Intermediate

R D Johnson, G R Long, and J W Hudgens. J. Chem. Phys., 87 (1987).

Previous REMPI studies with O2(C3g) intermediate found bands with very

diffuse structure due to predissociation.

Only the O2(C3g,v=2) state was rotationally resolved.

Previous Characterization of O2(C3g,v=2) State

Lewis et. al (1999) analyzed O2 REMPI spectra from Sur et. al (1986) and Ogorzalek-Loo (1989, unpublished) and derived spectroscopic

constants for the F1, F2, and F3 sublevels of O2(C3g,v=2).

Total term energy = o + Beff J(J+1) – Dv J2(J+1)2

(For weak spin uncoupling)

cm-1 F1(=0) F2(=1) F3(=2)

o 69366 69445 69550

Beff 1.6 1.65 1.68

Dv - 1.6X10-5 1.3X10-5

B. R. Lewis, S. T. Gibson, J. S. Morrill, M. L. Ginter

J. Chem. Phys., Vol. 111, No. 1, 1 July 1999.

A Sur, C V Ramana, W A Chupka, S D Colson. J. Chem Phys., 84, 1, (1986).

This Work: Laser REMPI / Fluorescence Experimental Setup

YAG Pumped Dye Laser

Laser 280-290 nmLaser 560-580 nm

SHG

Dry Air In

Resonant Enhanced Multi-Photon Ionization (REMPI)

Computer

Al Electrodes

Voltage Source

R

Digital O-scope

ICCD Camera/Spectrometer

391

nm

Fluore

scen

ce

Computer

Post REMPI N2+ Fluorescence

N2+ Fluorescence Signal

in Dry Air at Atmospheric Pressure

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290288286284282280Laser Wavelength (nm)

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290288286284282280Laser Wavelength (nm)

Dry Air at

Atmospheric Pressure

Strong N2+ Fluorescence Band

Corresponding to O2(C,v=2) Resonant Intermediate

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394392390388386Fluorescence Wavelength (nm)

N2+(B 2u

+→ X 2u-)

Laser Excitation Spectrum

Typical Spectrum of N2+ Fluorescence

Coinciding with REMPI Transitions in Dry Air at Atmospheric Pressure

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Proposed Mechanism for N2+ Fluorescence with

O2(C3g) 2 Photon Resonance

N2(X)

N2+(X)

La

se

r Ph

oto

ns

O2(X)

O2+(X)

O2(C)

N2+(B)

Fluorescence

N2(a’)

Collisional Energy Transfer

La

se

r Ph

oto

ns

Experiment:

• Experiment:

–Compare N2+ fluorescence spectrum in air

with traditional O2 REMPI.

–Analyze both spectra for O2(C3g) spectroscopic constants.

Experimental Spectra

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N2+ Fluorescence Spectrum

760 Torr Dry Air

4 mJ Laser Pulse Energy

Traditional REMPI Spectrum25 Torr Pure O2

0.5 mJ Laser Pulse Energy

Note: Fluorescence spectrum is more diffuse

than REMPI spectrum

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Laser Wavelength (nm)

Term Energy Equations Applied to Fit Our Spectral Data

Term Energy Calculations

• Ground State O2(X3, v=0)

F1 = BvJ(J+1) – DvJ2(J+1)2 + (2J+3)Bv – – √((2J+3)2Bv2 + 2 -2Bv) + (J+1)

F2 = BvJ(J+1) – DvJ2(J+1)2

F3 = BvJ(J+1) – DvJ2(J+1)2 + (2J-1)Bv – + √((2J-1)2Bv2 + 2 -2Bv) + J

*Constants for O2(X3, v=0) taken from: R R Laher, F R Gilmore, J. Chem. Phys. Ref. Data, 20, 4, (1991).

*Note: For N = 1, J = 0, the sign in front of the square root was inverted

• Upper State O2(C3, v=2)

S1(=0) = o1 + Beff1 J(J+1) – Dv1 J2(J+1)2 Term energy formula for weak spin uncoupling

S2(=1) = o2 + Beff2 J(J+1) – Dv2 J2(J+1)2

S3(=2) = o3 + Beff3 J(J+1) – Dv3 J2(J+1)2

Constants o, Beff, and Dv for each sublevel, F1(=0), F2(=1), and F3(=2) will be fit to the spectral data

Line Intensities Applied to Generate Simulated Spectrum

Relative Line Intensities for 2-Photon Transitions R G Bray, R M Hochstrasser. Mol. Phys. 31, (1976).

)12)(1(15

))(2)()(1(

JJJ

JJJJ

P branch*

)1)(1(30

)12)()(1( 2

JJJ

JJJ

Q branch

)32)(12)(1(10

))(12()12)(1( 2

JJJJ

JJJ

R branch

)2)(1(30

)2)(2)(1( 2

JJJ

JJJ

S branch

)32)(2)(1(15

)1)(3)(2)(1(

JJJ

JJJJ

*Note: The P branch equation was modified through a change in sign to correspond with symmetry seen in the models. However, there is no current published acclimation of this error.

O branch

Theoretical Linewidth Effect Due to O2(C3, v=2) Pre-Dissociation

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Laser P

ho

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s

O2(X3g-)

O2+(X2g)

O2(C3g)

O(1D)+O(3P)3g

N2+(X)

Fluorescence

N2(a’)

La

se

r Ph

oto

ns

Li et. al (1996) calculated O2(C3, v=2) linewidths for various J values

due to pre-dissociation

Y. Li, D. Petsalakis, H-P Liebermann, G. Hirsch, R. Buenker, J.Chem. Phys. 106 (3), 15 January 1997.

We used a function fits to these values to determine the widths for each F(J) for our simulated spectra

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F1 (theory) F2 (theory) F3 (theory) F1 (fit) F2 (fit) F3 (fit)

Simulated SpectrumUsing Constants and Linewidths from Literature

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Experiment Simulated Spectrum

Experiment Simulated Spectrum•Initial Simulated Spectrum:

•Constants derived by Lewis et. al•Linewidths calculated by Li et. al

Entire simulated spectrum is red-shifted compared to both experimental spectra

Linewidths are not well matched, especially in the fluorescence case

REMPI in O2

N2+ Fluorescence in Air

Simulated Spectra with Adjusted Constants and Linewidths

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Linewidth Adjustment

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F1 (theory) F2 (theory) F3 (theory) F1 (adjusted fit) F2 (adjusted fit) F3 (adjusted fit)

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Linewidth Adjustment

REMPI in O2

N2+ Fluorescence in Air

Spectroscopic Constants

cm-1 F1 () F2 () F3 ()

o 69369(2)69366

69449(1)69445

69552(1)69550

Beff 1.61(5)1.6

1.65(1)1.65

1.69(1)1.68

Dv 1.9(5)x10-5

_

1.6(2)x10-5

1.6X10-5

1.9(2)x10-5

1.3X10-5

• Our improved fits required an increase in o term energies by 2-3 cm-1

over the published constants.

• REMPI data provided more precise fit than Fluorescence data

• Higher signal-to-noise

• Less line broadening

* Previous constants

Summary of O2(C3, v=2) Investigation

by Novel Laser Photoionization• Fluorescence phenomenon introduced for REMPI transitions in atmospheric pressure air

• O2(C3g,v=2) state characterized using fluorescence and REMPI spectra

• Derived spectroscopic constants differ slightly from literature

• Fluorescence linewidths in atmospheric air are significantly broadened

• Likely due to pressure broadening and laser power broadening

Plasma Diagnostics Research Team

Back-up Slide References

1) M Aldén, W Wendt. Optics Communications, 69, 1, (1988).

2) A Sur, L Nguyen, N Nikoi. J. Chem. Phys., 96, 9, (1992).

3) A Sur, C V Ramana, W A Chupka, S D Colson. J. Chem Phys., 84, 1, (1986).

4) P H Krupenie. J. Chem. Phys. Ref. Data, 1, 2, (1972).

5) G. Herzberg. Molecular Spectra and Molecular Structure, Krieger Publishing: Malabar, Florida. (1989).

6) W Demtroder. Laser Spectroscopy, Springer-Verlag: Berlin, Germany. (1982).

7) Y Li, I D Petsalakis, H Liebermann, G Hirsch, R Buenker. J. Chem. Phys., 106, 3, (1997).

8) R R Laher, F R Gilmore. J. Chem. Phys. Ref. Data, 20, 4, (1991).

9) National Institute of Standards and Technology. “Diatomic Spectral Database.” http://physics.nist.gov/PhysRefData/MolSpec/Diatomic/Html/sec3

10) Kwok, S., and Volk, K.: 1985, `On the Energetics of High-Velocity Molecular Flows', Astrophys.J.Lett., 299, L191. http://www.iras.ucalgary.ca/~volk/o2lev.gif

11) R D Johnson, G R Long, and J W Hudgens. J. Chem. Phys., 87 (1987).

12) I N Levine. Quantum Chemistry, 4th Ed., Prentice Hall: Englewood Cliffs, NJ. (1991).

13) R G Bray, R M Hochstrasser. Mol. Phys. 31, (1976).

14) W Kaiser, C G Garrett. Phys. Rev. Lett., 7, 6, (1961).

Back-up SlideCalibration with N2

• For energy calibration, the laser was scanned over the N2(a 1g → X 1g+)

intermediate transition and the resulting fluorescence signal was fit with a simulated spectrum using the accepted N2 constants.

N2(a1400

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Back-up SlideCollisional O2 - N2 Energy Exchange

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O2(C32g,v=2)

O2(C31g,v=2)

O2(C30g,v=2)

N2(a'1u

-,v=1)

Term Energies of O2(C3g) and N2(a’ 1u-) are nearly coincident

N2

O2*

O2(C3g) + N2(X) → O2(X) + N2(a’ 1u-)

is energetically probable