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Mass-selected aluminum anion clusters, Aln–, were reacted with O2. Rate constants (300 K) for 2 < n < 30 and product branching fractions for 2 < n < 17 are reported. Reactivity is strongly anticorrelated to Aln– electron binding energy (EBE). Al13– reacts more slowly than predicted by EBE but notably is not inert, reacting at a measurable 0.05% efficiency (2.5 ± 1.5 × 10–13 cm3 s–1). Al6– is also an outlier, reacting more slowly than expected after accounting for other factors, suggesting that high symmetry increases stability. Implications of observed Al13– reactivity, contributions of both electronic shell-closing and geometric homogeneity to Aln– resistance to O2 etching, and future directions to more fully unravel the reaction mechanisms are discussed.

Rate constants for dissociative electron attachment to N

The kinetics of MgO+ + CH4 was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300-600 K and computationally by running and analyzing reactive atomistic simulations. Rate coefficients and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of k = 5.9 ± 1.5 × 10-10(T/300 K)-0.5±0.2 cm3 s-1. MgOH+ was the dominant product at all temperatures, but Mg+, the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of 0.08 ± 0.03(T/300 K)-0.8±0.7. Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH4]+. Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, in agreement with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate-limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH3OH+ complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH+ + CH3 product channel. An alternative, non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH3OH+ complex thereby allowing a more robust competition between the product channels.

Rate constants for the reactions of C

The kinetics of MgO + + CH 4 was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 − 600 K and computationally by running and analyzing reactive atomistic simula- tions. Rate coefficients and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of k = 5 . 9 ± 1 . 5 × 10 − 10 ( T/ 300 K) − 0 . 5 ± 0 . 2 cm 3 s − 1 . MgOH + was the dominant product at all temperatures, but Mg + , the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of 0 . 08 ± 0 . 03( T/ 300 K) − 0 . 8 ± 0 . 7 . Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH 4 ] + . Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate- limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH 3 OH + complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH + + CH 3 product channel. An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH 3 OH + complex thereby allowing a more robust competition between the product channels .

Coherent microwave Rayleigh scattering for detection resonance-enhanced multiphoton ionization (radar REh^I) has been developed for trace species in air, atomic and radicals in hydrocarbon flames, plasma parameters, and semiconductor inspection in the past decade.

Mass-selected aluminum anion clusters, Al

The kinetics of 4Fe+(CO)n=0–2 + O2 are measured under thermal conditions from 300–600 K using a selected-ion flow tube apparatus. Both the bare metal and n = 2 cations are inert to reaction over this temperature range, but 4Fe+(CO) reacts rapidly (k = 3.2 ± 0.8 × 10−10 cm3 s−1 at 300 K, 52% of the collisional rate coefficient) to form FeO+ + CO2. This is an example of the oxidation of CO by O2 occurring entirely on a single non-noble metal atom. The reaction of the bare metal reaction is known to be endothermic, such that this result is expected; however, the n = 2 reaction has highly exothermic product channels available, such that the lack of reaction is surprising in light of the n = 1 reactivity. Stationary points along all three reaction coordinates are calculated using the TPSSh hybrid functional. These surfaces show that the n = 1 reaction is an example of two-state reactivity; the reaction proceeds initially on the sextet surface over a submerged barrier to a structure with an O–O bond distance longer than that in O2, but must cross to the quartet surface in order to proceed over a second submerged barrier to rearrange to form CO2. The n = 2 reaction does not proceed because, on all spin surfaces, the transition state corresponding to O–O separation is at higher energy than the separated reactants. The difference between the n = 1 and n = 2 reactions is not a result of steric effects, but rather because the O2 is more strongly bound to Fe in the entrance well of the n = 1 case, and that energy is available to overcome the rate-limiting barrier to O–O cleavage. Experimental verification of some of these details are provided by guided ion beam tandem mass spectrometry results. The kinetic energy dependence of the n = 1 reaction shows evidence for a curve crossing and yields relevant thermochemistry for competing reaction channels.

We present 300 K rate constants for the ion-ion reactions of O + a n d N + with the atomic halide anions: C l - , B r - , a n d I - measured using a flowing afterglow – Langmuir probe apparatus. All six rate constants were > 1 × 10 - 8 c m 3 s - 1 , larger than those typical for atomic ion – atomic ion reactions, approaching the rate constants of more complex ion-ion systems. Evaluation of literature atomic ion – atomic ion rate constants shows variation over orders of magnitude, but limited to below the expected value for a polyatomic ion-ion reaction of the same reduced mass, fitting into an expected physical picture. Unlike for polyatomic systems, estimation of rate constant for atomic ion – atomic ion reaction rates by simple means does not seem effective.

Rate constants for dissociative electron attachment to N2O yielding O− have been measured as a function of temperature from 400 K to 1000 K. Detailed modeling of kinetics was needed to derive the rate constants at temperatures of 700 K and higher. In the 400 K–600 K range, upper limits are given. The data from 700 K to 1000 K follow the Arrhenius equation behavior described by 2.4 × 10−8 e−0.288 eV/kT cm3 s−1. The activation energy derived from the Arrhenius plot is equal to the endothermicity of the reaction. However, calculations at the CCSD(T)/complete basis set level suggest that the lowest energy crossing between the neutral and anion surfaces lies 0.6 eV above the N2O equilibrium geometry and 0.3 eV above the endothermicity of the dissociative attachment. Kinetic modeling under this assumption is in modest agreement with the experimental data. The data are best explained by attachment occurring below the lowest energy crossing of the neutral and valence anion surfaces via vibrational Feshbach resonances.

Mutual neutralization (MN) rate constants k

The kinetics of reactions of the trifluoromethyl radical (CF3) with several simple cations (Ar+, Xe+, O2 +, NO+, CO2 +, C2F5 +) are studied via the variable electron and neutral density attachment mass spectrometry (VENDAMS) technique. CF3 is produced via dissociative electron attachment to CF3I, resulting in radical concentrations that are smaller than produced using pyrolysis methods, but better quantified. Rate constants and product branching fractions are reported, with typical uncertainties of ∼ 30%. Three of the reactions, Ar+, O2 +, and C2F5 + proceed near the calculated collisional rate constant, while the other reactions are between 10% – 30% efficient. The inefficiency of the reactions of Xe+ and CO2 + can be explained by the reactions strictly obeying spin-conservation. The reaction of Ar+ yields predominantly (90%) CF2 + via dissociative charge transfer, with a minor channel to CF3 +. The reaction of C2F5 + yields predominantly CF3 + with a second channel (∼ 25%) yielding C2F4 + and CF4, the only example here of a bond-breaking and bond-making reaction. For the other four reactions, only charge transfer to yield CF3 + is observed. Rate constants for these same ions with CF3I are also reported and found to be rapid with a variety of products formed.

The kinetics of

Rate constants for the reactions of C+ + Cl-, Br-, and I- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of

In this work, coherent microwave Rayleigh scattering for the detection of resonance-enhanced multiphoton ionization (Radar REMPI) was used for the measurement of atomic oxygen in a nonequilibrium pulsed plasma discharge. Specifically, the detection of atomic oxygen is demonstrated in a pulsed discharge at 100 Torr in a molecular oxygen and helium gas mixture. REMPI was conducted using Nd:YAG pumped dye radiation at 618.9 nm mixed with the Nd:YAG 355 nm radiation, generating 225.6 nm laser light, of which two photons match the energy of the 2P33p(3P) electronically excited intermediate state of atomic oxygen. A third photon from the laser source then readily ionizes the atom in the 2 + 1 photon REMPI scheme. The Radar REMPI results are compared to two-photon absorption laser-induced fluorescence measurements from the literature, which shows reasonable agreement.In this work, coherent microwave Rayleigh scattering for the detection of resonance-enhanced multiphoton ionization (Radar REMPI) was used for the measurement of atomic oxygen in a nonequilibrium pulsed plasma discharge. Specifically, the detection of atomic oxygen is demonstrated in a pulsed discharge at 100 Torr in a molecular oxygen and helium gas mixture. REMPI was conducted using Nd:YAG pumped dye radiation at 618.9 nm mixed with the Nd:YAG 355 nm radiation, generating 225.6 nm laser light, of which two photons match the energy of the 2P33p(3P) electronically excited intermediate state of atomic oxygen. A third photon from the laser source then readily ionizes the atom in the 2 + 1 photon REMPI scheme. The Radar REMPI results are compared to two-photon absorption laser-induced fluorescence measurements from the literature, which shows reasonable agreement.

The kinetics of the FeCO(+) + N2O reaction have been studied at thermal energies (300-600 K) using a variable temperature selected ion flow tube apparatus. Rate constants and product branching fractions are reported. The reaction is modestly inefficient, proceeding with a rate constant of 6.2 × 10(-11) cm(3) s(-1) at 300 K, with a small negative temperature dependence, declining to 4.4 × 10(-11) cm(3) s(-1) at 600 K. Both Fe(+) and FeO(+) products are observed, with a constant branching ratio of approximately 40:60 at all temperatures. Calculation of the stationary points along the reaction coordinate shows that only the ground state quartet surface is initially sampled resulting in N2 elimination; a submerged barrier along this portion of the surface dictates the magnitude and temperature dependence of the total rate constant. The product branching fractions are determined by the behavior of the remaining (4)OFeCO(+) fragment, and this behavior is compared to that found in the reaction of FeO(+) + CO, which initially forms (6)OFeCO(+). Thermodynamic and kinetic arguments are used to show that the spin-forbidden surface crossing in this region is efficient, proceeding with an average rate constant of greater than 10(12) s(-1).

Thermal electron attachment rate coefficients for three interhalogen compounds (ClF, ICl, IBr) have been measured from 300 to 900 K at pressures of 1--2 Torr using a flowing afterglow--Langmuir probe apparatus. ClF attaches somewhat inefficiently ($k=7.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) at 300 K, with the rate coefficient rising to $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 700 K. At higher temperatures the apparent rate coefficient falls steeply; however, this is interpreted as an artifact due to decomposition on the walls of the inlet line. ICl attaches with even lower efficiency ($k=9.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 300 K) and a less steep increase with temperature. Attachment to IBr is too slow to confidently measure with the present experiment, with an upper limit on the rate coefficient of ${10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ from 300 to 600 K. Both ClF and ICl attach dissociatively to yield $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$, likely exclusively, though ${\mathrm{F}}^{\ensuremath{-}}$ or ${\mathrm{I}}^{\ensuremath{-}}$ may be produced with limits of $l2%$ and $l5%$, respectively. The ClF attachment was further explored through ab initio calculation of the ClF and $\mathrm{Cl}{\mathrm{F}}^{\ensuremath{-}}$ potential energy curves and $R$-matrix calculations of the resonance parameters which were used then for calculations of the dissociative attachment cross sections and rate coefficients. While the magnitude of the attachment rate coefficient for ClF is similar to those for both $\mathrm{C}{\mathrm{l}}_{2}$ and ${\mathrm{F}}_{2}$, the calculated cross sections show qualitatively different threshold behavior due to the $s$-wave contribution allowed by the lack of inversion symmetry. The $v=1$ and 2 vibrational modes of ClF attach about three to four times faster than $v=0$ and 3 at energies lower than $\ensuremath{\sim}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The calculated rate coefficients are in good agreement with the experiment at 300 K and increase moderately less steeply with temperature.

A digital holography (DH) surface erosion/deposition diagnostic is being developed for 3D imaging of plasma facing component surfaces in situ and in real time. Digital holography is a technique that utilizes lasers reflected from a material surface to form an interferogram, which carries information about the topology of the surface when reconstructed. As described in this paper, dual CO2 lasers at 9.271 and 9.250 μm wavelengths illuminate the interrogated surface (at a distance of ∼1 m) in a region of ∼1 cm × 1 cm. The surface feature resolution is ∼0.1 mm in the plane of the surface, and the depth resolution ranges from ∼0.0001 to ∼2 mm perpendicular to the surface. The depth resolution lower limit is set by single-laser and detector optical limitations, while the upper limit is determined by 2π phase ambiguity of the dual-laser synthetic wavelength. Measurements have been made "on the bench" to characterize the single-laser and dual-laser DH configurations utilizing standard resolution targets and material targets that were previously exposed to high flux plasmas in either the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) or the electro-thermal (ET) arc source. Typical DH measurements were made with 0.03 ms integration with an IR camera that can be framed at rates approaching 1.5 kHz. The DH diagnostic system is progressing toward in situ measurements of plasma erosion/deposition either on Proto-MPEX or the ET arc source.

Mutual neutralization (MN) rate constants kMN for the reactions of H+ and D+ with the atomic halide anions Cl-, Br-, and I- were measured using the variable electron and neutral density attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. At 300 K, the rate constants for each reaction studied are on the order of 10-8 cm3 s-1. A trend for the rate constants of the systems in this work, kMNCl

Significant reduction of the breakdown threshold in a DC microdischarge via seeding metal nanoparticles has been demonstrated. Compared to standard Paschen curves in dry air, reductions in the breakdown voltage of 5% to 25% were obtained for PD values (the product of pressure and electrode gap distance) ranging from 20 to 40 Torr-cm by seeding aluminum and iron nanoparticles with mean sizes of 75 nm and 80 nm, respectively. No secondary energy source was required to achieve this breakdown threshold reduction. From high-speed chemiluminescence imaging of the discharge evolution, breakdown was shown to be initiated at reduced voltages. Following breakdown, the increase in temperature ignited some of the nanoparticles near the cathode. Results suggest that possible charging of the nanoparticles within the gap may reduce the effective transient distance, leading to the threshold reduction.

A novel technique is described for the measurement of rate constants and product branching fractions of thermal reactions between cation and radical species. The technique is a variant of the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, employing a flowing afterglow-Langmuir probe apparatus. A radical species is produced in situ via dissociative electron attachment to a neutral precursor; this allows for a quantitative derivation of the radical concentration and, as a result, a quantitative determination of rate constants. The technique is applied to the reactions of Ar(+) and O2(+) with CH3 at 300 K. The Ar(+) + CH3 reaction proceeds near the collisional rate constant of 1.1 × 10(-9) cm(3) s(-1) and has three product channels: → CH3(+) + Ar (k = 5 ± 2 × 10(-10) cm(3) s(-1)), → CH2(+) + H + Ar (k = 7 ± 2 × 10(-10) cm(3) s(-1)), → CH(+) + H2 + Ar (k = 5 ± 3 × 10(-11) cm(3) s(-1)). The O2(+) + CH3 reaction is also efficient, with direct charge transfer yielding CH3(+) as the primary product channel. Several results needed to support these measurements are reported, including the kinetics of Ar(+) and O2(+) with CH3I, electron attachment to CH3I, and mutual neutralization of CH3(+) and CH2(+) with I(-).

In this work, we present findings of significant reduction of the breakdown threshold in a DC microdischarge via seeding metal nanoparticles. Reductions in the breakdown voltage were found to be between 5 to 25% for PD values (the product of pressure and electrode gap distance) of 20 to 40 Torr-cm. These reductions were achieved be seeding aluminum and iron nanoparticles with mean diameters of 75 and 80 nm, respectively. This technique required no secondary energy source to achieve this breakdown threshold reduction. Through high-speed chemiluminescence imaging of the discharge evolution it was found that the increase in temperature following breakdown ignited some of the nanoparticles near the cathode. It is postulated that the breakdown threshold reduction is achieved by an effective reduction in the electrode gap distance due charging of the nanoparticles.

This paper presents non-intrusive measurements of sodium-argon and sodium-nitrogen cluster ion recombination with electrons based on coherent microwave scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI). The number density of sodium, in a mixture of sodium vapor with argon or nitrogen buffer gas, was determined by direct absorption measurement. Sodium was resonantly ionized by a tunable laser beam in the 2+1 REMPI process. Since the plasma is mainly made of sodium ions in the mixture, the sodium ion clusters are thus formed by the Chaperone process. The dissociative recombination rate of sodium-argon and sodium-nitrogen and the neutral stabilized recombination rate of sodium ions were determined from a least-squares Monte Carlo algorithm (LSM) that fitted the plasma dynamic model to the direct measurement of the total electron number in the laser-induced plasma. At 300 o C, and , the dissociative recombination rate of Na + ∙(Ar) was determined to be and the neutral stabilized recombination rate was determined to be . At 300C, and , the dissociative recombination rate of Na + ∙(N2) was determined to be and the neutral stabilized recombination rate was determined to be .The method can be easily extended to measurements of other cluster ion species over a wide range of temperatures and pressures in various mixture compositions, such as Mg and Ca with N2 and CO2. The results are of fundamental importance to the plasma chemistry field and hypersonic research.

Here we present quantitative measurements of total electron numbers in laser-induced air breakdown at pressures ranging from atmospheric to 40 barg by 10 ns and 100 ps laser pulses. A quantifiable definition for the laser-induced breakdown threshold is identified by a sharp increase in the measurable total electron numbers via dielectric-calibrated coherent microwave scattering. For the 10 ns laser pulse, the threshold of laser-induced breakdown in atmospheric air is defined as the total electron number of ∼106. This breakdown threshold decreases with an increase of pressure and laser photon energy (shorter wavelength), which is consistent with the theory of initial multiphoton ionization and subsequent avalanche processes. For the 100 ps laser pulse cases, a clear threshold is not present and only marginal pressure effects can be observed, which is due to the short pulse duration leading to stronger multiphoton ionization and minimal collisional avalanche ionization.

A laser-ignition (LI) method is presented that utilizes a high-repetition-rate (HRR) nanosecond laser to reduce minimal ignition energies of individual pulses by ∼10 times while maintaining comparable total energies. The most common LI employs a single nanosecond-laser pulse with energies on the order of tens of millijoules to ignite combustible gaseous mixtures. Because of the requirements of high energy per pulse, fiber coupling of traditional LI systems is difficult to implement in real-world systems with limited optical access. The HRR LI method demonstrated here has an order of magnitude lower per-pulse energy requirement than the traditional single-pulse LI technique, potentially allowing delivery through standard commercial optical fibers. Additionally, the HRR LI approach significantly increases the ignition probability of lean combustible mixtures in high-speed flows while maintaining low individual pulse energies.

This paper demonstrates non-intrusive measurements of sodium argon ion cluster recombinations with electrons, based on coherent microwave scattering (Radar) from REMPI (Resonance Enhanced Multi-Photon Ionization). The sodium number density in the sodium argon mixture was determined by the direct absorption measurement. A tunable laser beam resonantly ionizes the sodium atoms by 2+1 REMPI process in the mixture of sodium and argon at various pressures of 1 Torr to atmospheric pressure and temperatures of 300K to ~800K. Since the plasma is mainly made of sodium ions in the mixture, the sodium argon ion clusters are thus formed by the Chaperone process. The sodium argon ion cluster recombinations with electrons are inferred from the direct measurement of the total

Nonintrusive spatially resolved rotational temperature measurements in an atmospheric air microdischarge are presented. The measurements were based on coherent microwave Rayleigh scattering (Radar) from resonance-enhanced multiphoton ionization of molecular oxygen. The open air DC microdischarge source operated in a stable “normal-glow” mode and pin-to-pin electrodes spaced 1.3 mm apart. The second harmonic of a tunable dye laser beam was focused between the two electrodes and scanned between 286 and 288 nm. Coherent microwave Rayleigh scattering was used to collect the two-photon rotational spectra of O2 at C3Π(v = 2)←X3Σ(v′ = 0) transitions. The Boltzmann plots from analyses of the O2 rotational lines determined local rotational temperatures at various axial locations between the electrodes. The molecular oxygen rotational temperature varied from ∼1150 K to ∼1350 K within the discharge area. The measurements had an accuracy of ∼±50 K.

Here we report nonintrusive local rotational temperature measurements of molecular oxygen, based on coherent microwave scattering (radar) from resonance-enhanced multiphoton ionization (REMPI) in room air and hydrogen/air flames. Analyses of the rotational line strengths of the two-photon molecular oxygen C(3)Π(v=2)←X(3)Σ(v'=0) transition have been used to determine the hyperfine rotational state distribution of the ground X(3)Σ(v'=0) state. Rotationally resolved 2+1 REMPI spectra of the molecular oxygen C(3)Π(v=2)←X(3)Σ(v'=0) transition at different temperatures were obtained experimentally by radar REMPI. Rotational temperatures have been determined from the resulting Boltzmann plots. The measurements in general had an accuracy of ~±60 K in the hydrogen/air flames at various equivalence ratios. Discussions about the decreased accuracy for the temperature measurement at elevated temperatures have been presented.

In this paper, microwave Mie scattering from a laser-induced plasma in atmospheric air is computed. It shows that the scattered microwave transitions from coherent Rayleigh scattering to Mie scattering based on the relative transparency of the laser-induced plasma at the microwave frequency. The microwave penetration in the plasma alters from total transparency to partial shielding due to the sharp increase of the electron number density within the avalanche ionization phase. The transition from Rayleigh scattering to Mie scattering is verified by both the temporal evolution of the scattered microwave and the homogeneity of polar scattering plots.

In this paper phenomena seen in microwave scattering experiements from a laser-induced plasma in atmospheric air will be explained via Mie scattering theory. The relative transparency of the laser-induced plasma at microwave frequency is shown to lead to a transition from coherent Rayleigh scattering to Mie scattering. A sharp increase in conductivity during the avalanche ionization phase of the spark evolution alters the microwave penetration from total transparency to partial shielding. Temporal and polar scatterings plots will be presented to show the effects of the transition from Rayleigh scattering to Mie scattering and change in transparency.