Experimental measurements of multiphoton enhanced air breakdown by a subthreshold intensity excimer laser

Journal Article · Thu Oct 15 00:00:00 EDT 2009 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.3245332· OSTI ID:21361882

Way, Jesse; Hummelt, Jason; Scharer, John ^[1]

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1. Department of Electrical and Computer Engineering, University of Wisconsin, Madison, Wisconsin 53706 (United States)

This work presents density, spectroscopic temperature, and shockwave measurements of laser induced breakdown plasma in atmospheric air by subthreshold intensity (5.5x10{sup 9} W/cm{sup 2}) 193 nm laser radiation. Using molecular spectroscopy and two-wavelength interferometry, it is shown that substantial ionization (>10{sup 16} cm{sup -3}) occurs that is not predicted by collisional cascade (CC) breakdown theory. While the focused laser irradiance is three orders of magnitude below the theoretical collisional breakdown threshold, the substantial photon energy at 193 nm (6.42 eV/photon) compared with the ionization potential of air (15.6 eV) significantly increases the probability of multiphoton ionization effects. By spectroscopically monitoring the intensity of the N{sub 2}{sup +} first negative system (B {sup 2}SIGMA{sub u}{sup +}-X {sup 2}SIGMA{sub g}{sup +}) vibrational bandhead (v{sup '}=0,v{sup ''}=0) at low pressure (20 Torr) where multiphoton effects are dominant, it is shown that two photon excitation, resonant enhanced multiphoton ionization is the primary mechanism for quantized ionization of N{sub 2} to the N{sub 2}{sup +}(B {sup 2}SIGMA{sub u}{sup +}) state. This multiphoton effect then serves to amplify the collisional breakdown process at higher pressures by electron seeding, thereby reducing the threshold intensity from that required via CC processes for breakdown and producing high density laser formed plasmas.

• OSTI ID: 21361882
• Journal Information: Journal of Applied Physics, Vol. 106, Issue 8; Other Information: DOI: 10.1063/1.3245332; (c) 2009 American Institute of Physics; ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
AIR
ELECTRONS
EXCIMER LASERS
EXCITATION
INTERFEROMETRY
IONIZATION POTENTIAL
LASER RADIATION
MULTI-PHOTON PROCESSES
PHOTOIONIZATION
PHOTONS
PLASMA
PLASMA DIAGNOSTICS
PLASMA PRODUCTION
PROBABILITY
RADIANT FLUX DENSITY
SHOCK WAVES
SPECTROSCOPY
WAVELENGTHS
BOSONS
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
ENERGY-LEVEL TRANSITIONS
FERMIONS
FLUIDS
FLUX DENSITY
GAS LASERS
GASES
IONIZATION
LASERS
LEPTONS
MASSLESS PARTICLES
RADIATIONS