Major pathway for multiphoton air ionization at 248 nm laser wavelength

Abstract

Multiphoton ionization mechanisms and ionization rates of atmospheric air and constituent gases are studied at the 248-nm KrF laser wavelength within a laser pulse intensity range of 108–1013 W/cm2 using both long 25-ns and short 160-fs pulses. We have experimentally shown that it is the photoionization of water vapor naturally contained in atmospheric air that acts as the dominant process of air ionization. (2 + 1) Resonance-Enhanced Multiphoton Ionization (REMPI) occurs through 2-photon resonant excitation of water molecules, which results in a quadratic dependence of electron density on laser intensity at lower laser intensities of 108–1010 W/cm2 in the long pulse and in a cubic dependence at higher intensities of 1010–1013 W/cm2 in the short pulse. Direct 3-photon ionization and (3 + 1) REMPI take place in pure O2 and N2, respectively, and their contributions to air ionization are in the ratio of 5:3. The total ionization rate of O2 and N2 in atmospheric air is about an order of magnitude less than that of water vapor. Relevant ionization coefficients (effective multiphoton ionization cross sections) have been measured and that for the H2O molecule is more than 2–3 orders of magnitude larger than the others.

Numerical Investigation on the Influence of Water Vapor Ionization on the Dynamic and Energy Deposition of Femtosecond Ultraviolet Laser Filamentation in Air

“…The solid blue and green lines represent the results for pulses in dry and wet air conditions, respectively. The multiphoton ionization cross section σ K for N 2 , O 2 , and H 2 O are taken from the measured results by Shutov [17], as listed in Table 1. The black dash line represents the simulated results for dry air when the cross section σ K for N 2 and O 2 were obtained from the theoretical formula by Couairon and Mysyrowicz [9].…”

“…In other words, the density of electrons tends to saturate in high humidity conditions. This saturation is attributed to the nonlinear energy loss in the multiphoton gas ionization by Shutov [17]. The differences in nonlinear energy loss can be clearly seen in Figure 7b.…”

“…The blue and green solid lines represent the simulated results in dry and wet air conditions, respectively. The cross section σ K for N 2 , O 2 , and H 2 O is obtained from the measured results by Shutov [17]. The dry air is assumed to only consist of N 2 and O 2 in a ratio of 0.78 to 0.21.…”

“…The results are obtained from the calculated multiphoton absorption (MPA) parameters in Reference [9]. The solid lines in both figures are the results from the measured multiphoton absorption (MPA) parameters in Reference [17], which are clearly indicated in the legends, respectively. (a) Figure 2a, the blue dash line is the results for O 2 gas, the red dash line denotes the results for N 2 gas, and the black dash line represents the results for dry air gas.…”

“…Simulation studies also showed that physical effects like dispersion and linear absorption become more and more important in the formation and propagation dynamics of 10 µm filaments with the increase of relative humidity [15]. More recently, Shutov et al [17] experimentally identified that the photoionization of water vapor acts as the dominant ionization process in atmospheric air for femtosecond laser pulses at a 248-nm central wavelength and the relevant ionization coefficients were also measured. With increasing air humidity, the plasma channels generated by ultraviolet laser pulses in air become longer and wider [18].…”