HF Fluorescence and Ion Production From Infrared Laser Irradiation of SF-Hydrocarbon Mixtures

CO2 laser irradiation of SF6/hydrocarbon mixtures are shown to produce positive ions and infrared fluorescence due to emission by HF. The mechanism for the production of these ions and fluorescence involves the multiple photon dissociation of SF6 and subsequent attack of the free fluorine atom on the hydrocarbon, as well as V-V transfer and thermal ionization reactions. Argon and oxygen are found to increase ion production while quenching the fluorescence. Chemi-ionization of the hydrocarbons by a reaction sequence initiated by the attack of fluorine atoms on trace amounts of an oxygen impurity cannot be disregarded as a channel for ion formation.


INTRODUCTION
Intense irradiation of SF6 and hydrocarbon mixtures by a pulsed CO2 laser tuned to the SF6 absorption band has been shown to give rise to visible chemiluminescence from CH and C2 species, and under appropriate experimental conditions to lead to production of ions as well. 2 The first step in the proposed mechanism for the production of these ions is infrared multiple photon dissociation of SF6 to produce can also be produced by collisional dissociation of vibrationally hot SF6* + SF6* SF6** + SFs* + F (2) Once a fluorine atom is produced, it can abstract a hydrogen atom from the hydrocarbon in a very fast exothermic reaction such as (3)   F+CH4 CH3.+HF* (3)   in which methane is a representative example.'7 The state of the methyl radical is not known.Successive hydrogen abstraction from the hydrocarbon by the fluorine atom can produce radical species which, if highly excited, can lead to ion formation through radical-radical recombination :'8 as shown by reac- tions (4) and (5).CH* + CH3" C2H + e- C+CH3. C3H+e- The * denotes an electronically excited state.
We have investigated ion-producing reactions of SF6 with small hydrocarbons such as methane and present evidence that the initial reaction steps in the mechanism of ion formation follow the reaction sequence proposed by Crim et al.This path is valid only for saturated hydrocarbons.Reactions of atomic fluorine with small unsaturated molecules such as ethylene proceed by different routes, to be discussed later.Our observations are as follows.Firstly infrared luminescence from the vibrationally excited HF produced in reactions such as (3) is observed in the initial decomposition step.Secondly, atomic fluorine scavengers such as H, CO and the nitrogen oxides are found to inhibit ion production, presumably by quenching reaction (3) before attack on the hydrocarbon can take place.Thirdly, when D2 is used as an atomic fluorine scavenger instead of H2 ion production decreases as does the HF* infrared fluorescence with increased Dz partial pressure.This is accompanied by increased infrared fluorescence in the region 2000-3200 cm -, which is attributed to DF*.In addition we see evidence for the presence of competing V V transfer between SF6 and CH4 due to the observation of ion signal under conditions where enough scavenger is added to fully titrate all the fluorine atoms.
EXPERIMENTAL 107 A diagram of the experimental apparatus is shown in Figure 1.A line-tuned Lumonics 801 TEA CO2 laser, operating in the TEMoo mode, is used to generate up to 0.18 J/pulse of infrared energy, depending on the wavelength.The pulse is attenuated by means of a propylene filled glass cell with ZnSe windows.The laser is operated at 10 Hz with a typical pulse-width of 200 ns full-width at half- maximum on the P(24) line of the 10.6 ixm band.The beam is focused by a 20 cm BaF2 lens into a stainless steel cell to which KC1 windows are mounted with viton O-ring seals.Energy deposition is measured by a Gen Tec energy meter placed behind the cell.Ions are collected by three independent parallel sets of 2 cm x 2 cm copper plates.This configuration makes independent biasing and ion collection possible.
The time dependent ion signal is displayed on an oscilloscope.Typi- cally, a 75 volt collection potential is impressed across each set of plates.Increasing the bias on the plates above 75 volts reduces the drift time of the ions from the production zone to the plate and causes the time between the laser pulse and the peak of the ion signal to decrease.Decreasing the bias below 75 volts had no observable effect on the signal down to 50 volts below which a decrease in signal intensity is observed down to the point of zero applied potential.This may be due to the loss of gain or ion recombination due to lower drift time.Reversing the bias results in a negative signal comparable in intensity and temporal behavior to that of the positive voltage, indicative of negative ion collection.Changing the bias has no effect on the HF fluorescence.
The focusing lens and cell are positioned so that the maximum ion current is collected on the center plate, with lower but equivalent ion current being collected on the plates on either side of it.This is due to the "dogbone" geometry of the focused CO2 laser beam, which has maximum fluence at the center plates, with equal but lower fluence on either side of this focus.HF fluorescence is observed by a 77 K InSb detector positioned at a right angle to the laser beam.A sapphire window allows infrared luminescence above 2000 cm -1 to pass out of the cell, and a band pass filter isolates the 3200-5000cm -1 region, which includes emission from the three lowest vibrational bands of HF.
Reactants are fed into the cell via a gas manifold and pressures are monitored by an MKS Baratron capacitance manometer mounted directly to it.All gases were UHP grade or better and were used as received.

RESULTS
Figure 2 shows the time dependence of the ion current signal and HF fluorescence for a 50:50 mixture of SF6/CH4 (2 torr total) at an irradiation energy of 80 mJ/pulse, 10 Hz repetition rate.This ion signal, when measured with an electrometer, gave an average current with an order of magnitude of 10 -12 amperes.The ion current increased quadratically with laser fluence while the HF fluorescence increased with fluence as the 1.5 power.When the CO2 laser was tuned to transitions outside the absorption band of SF6 no ion signal or HF fluorescence was observed.With pure SF6 neither ions nor IR emission was observed.It was noted that, with a fresh fill of SF6 a transient ion signal, which disappeared after a few laser pulses was Laser energy 80 mJ/pulse.
observed.No ions or HF fluorescence could be observed with the hydrocarbons alone.Adding 5 torr of argon to the 1 torr SF6/1 torr CH4 mixture increased the magnitude of the ion signal from 0.18 V to 4 V but also shifted the peak of the ion signal later in time.Similar results were observed when diatomic oxygen was substituted for argon.The shift in ion peak to longer times is attributed to slower drift velocity of ions to the collection plate electrode due to the larger number of collisions at the higher total pressure.Applying a higher collection voltage increases the ion drift velocity and can return the ion peak to the original time if desired.Addition of up to 2 torr HE to 1 torr each of CH4 and SF6 caused both ion signal and IR emission to decrease with laser irradiation time until no ion signal was seen.
When ions are no longer detected the HF fluorescence temporal behavior closely resembles that observed from SF6/H2 mixtures at comparable pressures. 8'1 Similar behavior was observed when deuterium was substituted for hydrogen.Although no IR lumines- cence measurements were done under conditions where CO, NO, N20 or NO2 were added as scavengers to the mixture, a decrease in ion signal was observed and is attributed to scavenging of atomic fluorine.Figure 3 shows the ion signal intensity versus time at 10 torr added pressure for various fluorine atom scavengers.At comparable pressures, the absorbed energy was essentially constant for the various scavengers.If the CH4 pressure is reduced below 1 torr, the ion signal and HF fluorescence disappear with irradiation time.This is illustrated by Figures 4 and 5 where peak ion signal and peak fluorescence intensity are plotted versus time, represented by number of laser pulses.The ion signal behavior is exponential while the disappearance of the fluorescence signal is close to linear.Blocking and then unblocking the laser beam during the course of the reaction results in the same values of ion signal and HF* fluorescence as those seen just before the beam was blocked.Such experiments show that the phenomenon we are observing is dependent on the laser and that no appreciable concentration of reactive intermediate is present after the laser pulse to propagate the reaction.Other experiments were performed in which the total pressure was kept constant in order to keep the number of collisions between molecules approximately constant.This was done by adding a certain pressure of scavenger to the 2 torr SF6/CH4 and balancing the total pressure to 12 torr with either argon or oxygen.Table I shows the ion current behavior of various Ar/NO2 mixtures as a function of  FIGURE 4 Ion signal intensity versus time for 1 torr SF6 with 0.5 torr CH4 (O) and 0.5 torr CH4/0.5 torr H2 (&).FIGURE 5 HF fluorescence intensity versus time for torr SF6 with 0.5 torr CH4 (0) and 0.5 torr CH4/0.5 torr HE (,).
the relative NO2 pressure.Note that the ion signal drops by a factor of 20 as the NO2 pressure is increased to 10 torr.Other scavengers (NO, N20, CO) gave qualitatively similar results.Although most of our experiments involve using methane as the hydrocarbon probe, some investigations of ethylene and ethane under comparable condi- tions were done.Relative to methane, ethane showed only a very slight ion signal but comparable HF fluorescence; ethylene showed no ion current and less HF emission.These results suggest that under our experimental conditions the first step in the proposed mechanism is dissociation of SF6 by multiple photon absorption of infrared photons.The observations of HF luminescence and positive ion currents are consistent with the hypothesis of Crim and co-workers, who postulated that multiple photon dissociation of SF6 occurs first, followed by exothermic reac- tion of the fluorine atoms with hydrocarbons to form HF and free- radicals which subsequently undergo chemi-ionization if energetically allowed.Those authors tabulated a partial list of allowed energetics for CH and C2 radicals which presumably are involved here. 2 The rate constant for the abstraction reaction of hydrogen from methane by free fluorine has been measured in a fast flow reactor system with mass spectrometric detection to be 3.3 1014 exp (-1150/RT) cm3-mol--s-1. 9Similar values of this rate con- stant obtained by different methods are well documented.Under our experimental conditions, the rate of HF formation is 1.8 107 s-1.This rate constant is sufficiently large that one would expect HF to be formed within a few microseconds with subsequent ion reactions occurring later in time.The fluorescence signal reaches a maximum in < 10 Is, while the ion signal does not reach its maximum until approximately 20 Is later.Both fluorescence and ion signal follow single exponential behavior, with tl/2 lifetimes of 25 and 50 Is respectively.Simultaneous measurements of HF fluorescence and ion signal are limited by the fact that the diagnostics monitor different reaction channels.The HF fluorescence is a direct measure of the first and second steps of the reaction mechanism, SF6 dissociation and alkyl radical formation by atomic fluorine attack on the saturated hydrocarbon, respectively.On the other hand, the ion current is an integrated measure of the chemistry occurring following radical for- mation.The results shown in Figure 2 support this claim.No correc- tions have been made for ion drift time, which are necessary to directly probe the different ion chemistry channels, i.e., what ions are pro- duced.Experiments with a mass spectrometric probe to identify the ions produced in the various stages of this process are now under design.
From beam profile measurements, we estimate an irradiation volume of 1.5 10 -2 cm3.At pressures of 1 torr each SF6 and CH4 a maximum of 5.0 x 10 TM SF6 molecules can be dissociated, 11 assuming 100% SF6 dissociation yield, which means that there would be at least 5.0x 10 TM fluorine atoms present.At most 3.0x 1015 fluorine atoms would be present if the SF6 was completely stripped of its fluorine atoms.Assuming that all ions produced are collected, the number of ions produced per laser pulse is then calculated to be <6 x l07 per pulse for 1 torr SF6/1 torr CH4 irradiated with 80mJ at a 10 Hz frequency.Therefore, the ratio of the number of ions produced to the number of SF6 molecules dissociated is about 10 .7 per pulse.
This agrees with Crim and co-workers who also found 10 -7 fractional ionization ratios in their experiments.Ion formation is therefore a minorityprocess.This suggests that not all fluorine atoms are involved in ion production.Although the addition of as much as 5 torr of oxygen or argon to the above mixture does not appreciably change the amount of infrared laser energy absorbed by the system 12 the number of ions formed increases by a factor of 6 for oxygen and a factor of 4 for argon.It has been shown that buffer gases with vibrational frequencies close to '3(SF6) will reduce the degree of SF6 dissociation due to V V exchange. 13The strong V -+ T relaxation due to added argon will also reduce the dissociation probability, 15 although the increase in collisions leads to an abundance of vibra- tionally hot SF6.Under these conditions, the dominant pathway for atomic fluorine production is reaction (2).Similar behavior is observed for oxygen, but in addition to reducing the SF6 multiphoton dissoci- ation yield due to V-+ V exchange, it can participate in chemi- ionization reactions.Once atomic fluorine is produced in reaction (1) it can react with diatomic oxygen in a combustion reaction (6) 1 F+O2 ---+ FO.+O. ( or a combination reaction (7)  F+ 02 FO2 (7)   If reaction (6) takes place, the nascent oxygen produced can further react with ell4 or CH3. to produce radicals which would lead to increased ion signa,s while simultaneously decreasing HF # emission.
On the other hand, if ( 7) is dominating under conditions where oxygen is present, both ion signal and HF fluorescence would be expected to decrease since a product molecule is formed, i.e., oxygen is acting as a fluorine scavenger.With 5 torr oxygen added to I torr each SF6 and CH4, the fractional ionization rises by a factor of 6, which supports the presence of reaction (6).HF is still observed under these reaction conditions and, therefore, it is probable that oxygen is acting as a collisional partner in a manner similar to that of argon rather than as a reactant.Since trace oxygen is a common contaminant in metal systems as well as bottled gases, one cannot preclude that some or possibly even all of the ionization seen with only SF6 and hydrocarbons is not due to trace amounts of this gas.At the present time we have not resolved this question.
Reaction "quenchers" scavenge the free fluorine atoms before reaction with a hydrocarbon.Reactions of the types shown in Table II are predicted to be the most thermodynamically favored and are both feasible and probable under the reaction conditions reported here. 1If only free radical chemi-ionization occurs one would expect all of the ion signal to be quenched in situations where all of the free fluorine would be scavenged.This is not observed, as shown in Figure 3, where the effect of various scavengers on the ion signal of SF6/CH4 is shown.Even when 10 torr scavenger is added an ion signal can be detected.This suggests that yet another mechanism which leads to ion formation is occurring independent of atomic fluorine production.If the hydrocarbon has vibrational levels nearly resonant with the excited SF6 molecule, then efficient V V energy transfer can take place.Successive collisions between the excited hydrocarbon and the excited SF6 can lead to dissociation of the latter species and subsequent ion formation by reaction (4).For such a reaction to be significant its rate must be competitive with that of SF6 dissociation.
Using deuterium as a probe of F atom production under the same experimental conditions as those involving hydrogen, we were able to monitor DF* fluorescence from reaction (9)   F+D2 DF*+D The DF fluorescence is sufficiently red-shifted from HF not to be passed by the HF filters in front of the IR detector.
Diatomic hydrogen behaves differently from all other scavengers, including deuterium.When 10tort H2 is added to the SF6/CH4 mixture, no ion signal is observed, presumably due to scavenging of all free fluorine atoms.The fluorescence lifetime of HF* is observed to increase with increasing hydrogen partial pressure.At H2 pressures where all ion signal was quenched the temporal behavior of HF fluorescence closely resembled that of a pure SF6/H2 mixture. 4Two mechanisms can be proposed to account for such behavior.(1) Ther- mal pyrolysis of the hydrocarbon to elemental carbon and hydrogen CH4 C 4-2H2 (10)   and/or (2) radical recombinations producing stable saturated hydro- carbons such as ethane, CH3"+CH3" C2H6 (11)   whose larger radicals do not possess the energetics necessary for ion formation.2 The first mechanism is unlikely as the temperature in the reaction zone is not high enough for complete pyrolysis to occur.
However, Woodin and Kajkowski TM have shown that SF6 sensitized laser pyrolysis of methane under similar experimental conditions results in ethane production.Their data show methyl radical formation in the first step A CH4 CH3.4-H (12)   followed by reaction (11).Therefore, the second mechanism postu- lated above is felt to be more likely in our experiments.
Experiments in which the hydrocarbon pressure was reduced below 1 torr are shown in Figures 4 and 5.At 1 torr SF6 and 0.5 torr CH4, the intensity of the ion signal is not stable with time but rather decays following first-order kinetics with a rate constant of --4 x 10 3 s-.Adding 0.5 torr of hydrogen to this mixture results in a faster decay of the ion signal.The decay under such conditions is not unimolecular, as a plot of In ion signal peak versus time is not linear.Such a falloff is more accurately described by a bimolecular rate expression. dt In contrast to the ion signal, the decrease in HF* intensity of the same reaction mixtures (Figure 5) is linear over 10 minutes and also dependent on CO2 laser pulses.
We had previously shown that at least 5.0x 1014F atoms are produced per laser pulse under our experimental conditions.At 0.5 torr CH4 there are 1.5 x 1019 molecules present in the volume of our cell (900 cm3).From the ion signals for these experiments, we calculate that 2.5 x 101 ions are formed per pulse initially.The fractional ionization is, therefore, two orders of magnitude greater than seen with equal pressures of SF6 and CH4.This implies that ion formation is a more efficient process under these conditions.However, the following conclusions are reached" (1) Not all fluorine atoms attack the hydrocarbon, (2) not all hydrocarbon radicals form ions, and (3) more than one hydrocarbon radical is needed for ion for- mation.
'15'16 This trend is the reverse order of the exothermicity and Gibb's free energy of the reaction of scavenger with atomic fluorine.Apparently, the best fluorine scavenging reactions, defined here as those which decrease ion production the most, have the largest positive heats of formation.Therefore, liberating a large amount of heat into the area of the reaction results in a relatively small decrease in ion signal compared with no scavenger at all.A higher temperature in the reaction zone can enhance thermal ionization.The best fluorine scavenging reaction is also the least spontaneous.This reverse behavior is possibly due to the high electronegativity of the fluorine atom, which is highly reactive.
The reaction of ethane with atomic fluorine is expected to proceed via the same mechanism as that of methane C2H6 + F C2H5" + HF* HF fluorescence from reaction (10) is observed to have comparable intensity and lifetime to that found in methane.The ion signal resulting from the reaction of ethane with fluorine is a factor of two smaller than that observed with methane.This may be because excited radicals (CH*, CH2*, CH3*) are more difficult to produce from the larger ethyl radical.'2 Also, ethane and atomic fluorine react in an eight-step sequence to yield ethylene as well as HF. 17 -CEH + HF (15) Other end products include C4Ho, CHaF, CEHsF and CaH7F.These larger molecules also will not form small radicals, hence ions, easily.
For this reason, production of ethylene from ethane is expected to result in low HF emission and little, if any, ion production.This is due to the fact that, unlike the alkanes, which undergo a hydrogen abstraction reaction to form radicals, ethylene will add fluorine across the double bond. 7 F + C2H --CEH3F + H (16) Experiments of the type previously described in which ethylene was used as the hydrocarbon support this claim.The HF * fluorescence was a factor of three smaller than that of methane and no ion production was observed.This last discussion will address the possible importance of negative charge carriers, which are also observed.It is probable that SF is the predominant negative species, formed by electron attachment to neutral SF6 in a nondissociative process.SF6 + e-SF (17)   The metastable sulfur hexafluoride anion has a long lifetime with respect to autodetachment.TM However, if it is vibrationally hot from

TABLE II
Calculated from JANAF Thermochemical Tables, Second Edition, NBS Publications. b