J . Phys. Chem. 1992, 96, 4210-4217
4210
Reactions of N,H, with 0' and Other Ions at Suprathermal Energies James A. Gardner,*st Rainer A. Dressler, Richard H. Salter, and Edmond Murad Phillips Laboratory, Spacecraft Interactions Branch, PLIWSVI, Hanscom AFB, Massachusetts 01 731 -5000 (Received: December 2, 1991; In Final Form: January 31, 1992)
Reaction cross sections, product ion kinetic energies, and chemiluminescence have been measured for N2H4 collisions with O+, Ar', Kr', CO+, and C02+at collison energies ranging between 1 and 30 eV (center of mass). Charge transfer and
dissociative charge transfer occur in all cases; however, the product ion distributions are markedly different for reactions involving the atomic vs the molecular primary ions. The difference is attributed to the greater ability of the molecular ions to share in the partitioning of the reaction exothermicity into internal modes. Time-of-flight studies show that the product ions are primarily formed with thermal or near-thermal (5100 meV) forward translational energy in the laboratory frame. Chemiluminescence signals attributable to OH A2Z+ X211 and to N H A311i X3Z- emissions are observed in collisions between O+ and N2H4 at Ec,,, = 4-50 eV. The OH emission intensity decreases with increasing collision energy, typical for an exothermic process. The N H emission intensity increases with increasing collision energy, typical for an endothermic process.
-
-
I. Introduction TABLE I: Exothermic and Slightly Endothermic Reactions between O+ and N2H4with AE and Reaction Number for Each Reaction, The suprathermal reactions of O+with hydrazine (N2H4) and Listed in Order of Decreasing Exothermicity its derivatives are important chemical processes in the plasma environment surrounding spacecraft in low earth orbit. At 250 reaction ionic energy product reaction km altitude, the density of the neutral ambient atmosphere is on reaction AE, eV m / z ratio no. the order of lo9 ~ m - and ~ , the ion density is approximately lo5 ~ m - of~ which , roughly 98% is O+.I Hydrazine is often used for 0' + N2H4 O H + N2H3' -6.6 31 1 0' + N2H4 O H + N2H+ + H2 -6.0 29 2 attitude control jets aboard spacecraft in low earth orbit, and its 0' + N2H4 0 + N2H4' -6 .O 32 3 derivatives monomethylhydrazine (MMH) and unsymmetrical 0' + N,H4 0 + NH4+ + N -3.2 18 4 dimethylhydrazine (UDMH) are used as fuels for thruster engines Of + N2H4 0 + NzH2++ H, -2.7 30 5 aboard the space shuttle as well as other spacecraft in low earth 0 ' + N2H4 O H + N2H2++ H -2.5 30 6 orbit. Spacecraft at such altitudes travel with a nominal orbital 0' + N7H4 0 + N2H3' + H -2.2 31 I velocity of 7.8 km s-I, and the shuttle thruster jets fire at a relative 0' + N2H4 O H + NH3+ + N -1.7 17 s velocity of approximately 3.5 km s-'. Depending upon the ori0++N2H4-+O+N2H++H+H2 -1.6 29 9 entation of the shuttle during thruster firing, the relative collision 0' + N2H4 O H + N2H+ + 2H -1.5 29 10 velocity between the thruster gases and the ambient 0' thus varies O+ + N 2 H 4 0 + NH,+ + N H -1 .o 17 11 0' + N 2 H 4 O H + NH2+ + N H +0.3 16 12 between about 4.3 and 11.3 km s-l. For collisions between 0' 0' + N 2 H 4 0 + NH2+ + N H 2 +0.4 16 13 and hydrazine or MMH (CH3HN2H2),the resulting collision O+ + N2H4 0 + N2+ + 2H2 +1.0 28 14 energy range is 1.0-7.1 or 1.1-7.9 eV, respectively. Although most 28 15 of the fuel is combusted, enough is released in an unburnt ~ t a t e ~ . ~ 0' + N 2 H 4 O H + N,+ + H + H, +1.1 O+ + N,H4 O H + N+ + NH3 +1.1 14 16 that the reactions of these fuels with atmospheric constituents must 0' + N2H4 0 + NH+ + NH3 +2.4 15 17 be considered when assessing the importance of fuel products as 0' + N2H4 O H + NH' + NH2 +2.6 15 18 potential contaminants in the spacecraft environment. Space-borne composition measurements4v5have indicated the presence of a rich ion signal overlaps the significantly weaker product ion signals mixture of products associated with the firing of space shuttle in the mass spectrum. In order to assess the probable hydrazine engines. Because ion-neutral reaction cross sections are generally fragmentation pattern, the reactions of N2H4with Ar', Kr+, CO+, much larger than those for neutral-neutral reactions, it is possible and C02+have also been studied and are reported here. The for the ionic reactions to play a significant role in the phenoexperimental apparatus and techniques are briefly discussed in menology of the interaction between spacecraft effluents and section 11. Results of the cross section, time-of-flight, and atmospheric species at high velocities. chemiluminescence measurements are presented in section 111. We present here a study of reactions between 0' and N2H4. The reaction pathways, possible products in the O'-N2H4 system, The exothermic and moderately endothermic channels for 0+-N2H4 collisions include charge transfer, dissociative charge transfer, H- abstraction, and dissociative H- abstraction, and are ( I ) Handbook of Geophysics and the Space Environment; Jursa, A. S., listed in Table I in order of decreasing exothermicity. Although Ed.; ADA 1670OO;National Technical Information Service: Springfield, VA, it is not possible in our laboratory mass spectrometer to separately 1985. (2) Hoffman, R. J.; Kawasaki, A.; Trinks, H.; Bidermann, I.; Ewering, W. measure the efficiencies of different channels that result in the The CONTAM 3.2 Plume Flowfield Analysis and Contamination Prediction same ionic product (e.g., reactions 2, 8, and 9 all produce N2Hf Computer Program: Analysis Model and Experimental Verification. Pres(mlz = 29)), the total formation cross sections for a given product ented at the AIAA 20th Thermophysics Conference, AIAA-85-0928, Wilion are reported here. A time-of-flight technique is utilized to liamsburg, 1985. (3) Trinks, H.; Hoffman, R. J. Experimental Investigation of Bi-propellant provide information about the kinetic energy and angular disExhaust Plume Flowfield, Heating, and Contamination and Comparison with tribution of the ionic products. Product internal excitation is the CONTAM Computer Model Predictions. In spacecraft Contamination: studied through chemiluminescence measurements using a techSources and Prevention; Roux, J. A., McCay, T. D.,Eds; AIAA: New York, nique described earlier.6 Low resolution chemiluminescence 1984; pp 261-273. (4) Ehlers, H. K. F. An Analysis of Return Flux from the Space Shuttle spectra are reported for the N2H4-atomic ion reactions. Orbiter RCS Engines. Presented at the AIAA 22nd Aerospace Sci. Mtg, Reaction channels that form NH,+ (x = 1-3) products are not AIAA-84-0551, Reno, 1984. observable in the O+-N2H4 system, because the intense primary ( 5 ) Ehlers, H. F. K. J. Spacecraft Rockets 1986, 23, 379. +
4 +
+
-+
-+
--+
+
+
(6) Dressier, R. A,; Gardner, J. A.; Lishawa, C. R.; Salter, R. H.; Murad, E. J . Chem. Phys. 1990, 93, 9189.
'PhotoMetrics, Inc. 0022-3654/92/2096-4210~03.00/0
0 1992 American Chemical Society
Reactions of N2H4 with O+
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4211
dependence of OH emission upon N2H3+cross sections, and implications for the spacecraft environment are discussed in section IV. 11. Experimental Section The laboratory apparatus consists of a coaxial tandem double mass spectrometer in a high-vacuum system that has been described in detail previo~sly.~-~ Briefly, an ion beam is formed either by electron impact or by dc discharge. The ions are accelerated, massanalyzed in a Wien velocity filter, and decelerated to the desired experimental collision energy. The beam can be pulsed by a set of deflector electrodes to conduct time-of-flight measurements. For reaction cross section and time-of-flight measurements, the primary ion beam passes through a 0.27 cm path length collision cell that contains N2H4 at a typical pressure of 0.133 Pa Torr). The primary ion beam and the product ions that emerge from the collision chamber exit aperture are focused onto the entrance of a quadrupole mass filter. The ions are mass-analyzed in the quadrupole, and the selected ions are detected by a channel electron multiplier. The detector output pulses are processed and counted by NIM-module electronics and an 80286-based computer. For chemiluminescence measurements, the collision cell is replaced with a 1.9 cm long cell that is fiber-optically coupled to an optical multichannel analyzer ~ y s t e m . ~ , ~ The O+,CO+, and C02+reactant ions are produced by 40-eV electron impact on C 0 2 (Matheson, 99.8%); Ar+ and Kr+ are produced by 40-eV electron impact on the rare gases (Ar, Matheson, 99.998%; Kr, Air Products,99.99%). At 40-eV electron energy, the contribution of excited O+ to the total O+ beam is less than 5%.1° The N2H4 is 98.5% purity (Johnson Matthey Electronics, major impurity: H 2 0 I1%) and is used without further purification. The experimental techniques have been described in detail Briefly, reaction cross sections are measured by first optimizing the collection efficiencies for the primary and product ions, then sweeping the quadrupole mass spectrometer over the appropriate m/z range to collect the primary and product ions. The spectrometer is first swept at low mass resolution to provide a total products:primary ion ratio, even for large mass differences. Next a high mass resolution sweep is performed to separate the product masses and determine their individual contributions to the total product intensity. The raw cross section (ucxp)is calculated from
where ZprOdand Iprim refer respectively to the signal intensities of the product and primary ions, 1 is the interaction length, and n is the target gas concentration. The number of product ions, and thus the attenuation of the primary ion beam, is always less than 1% of the primary ion number. The linear approximation to Beer's law is therefore sufficient for our analysis. The primary ion energy is measured with a retarding potential scan. The maximum spread in primary ion energy is 0.7 eV (fwhm) above Ec,,,,= 10, 0.5 eV in the range Ec,,,, 5-10 eV, and 0.3 eV below Ec,,,,= 5 eV. The uncertainty in the collision energy, however, is dominated by thermal broadening and is approximately 1 eV at EC,,,,= 10 eV. Time-of-flight measurements are performed by pulsing the primary ion beam to provide a 1-2 ps wide ion pulse, then measuring the times from the ion pulse formation to the arrival of the primary and product ions at the detector. With knowledge of the ion-optical dimensions, the product ion laboratory frame energy is calculated from the flight times and the primary ion beam energy. For the chemiluminescence measurements reported here, an entrance slit and
TABLE I 1 Exothermic and Slightly Endothermic Reactions between Ar+ and N2H4with A E and Reaction Number for Each Reaction, Listed in Order of Decreasing Exothermicity reaction ionic energy product reaction reaction AE,eV m / z ratio no. Ar+ + N2H4 Ar + N2H4+ -8.1 32 19 Ar+ + N2H4 Ar + NH4+ + N -5.3 18 20 Ar+ + N2H4 Ar + N2H2+ + H2 -4.8 30 21 Ar+ + N2H4 Ar + N2H3++ H -4.4 31 22 Ar+ + N2H4 Ar + N2H++ H + H2 -3.8 29 23 Ar+ + N2H4 Ar + NH,' + N H -3.2 17 24 Ar+ + N2H4 Ar + NH2+ + NHl -1.7 16 25 Ar+ + N2H4 Ar + N2+ + 2H2 -1.2 28 26 Ar+ + N2H4 Ar + NH+ + NH, +0.2 15 27
------
TABLE 111: Exothermic and Slightly Endothermic Reactions between Kr+ and N,H, with AE and Reaction Number for Each Reaction, Listed in Order of Decreasing Exothermicity reaction ionic energy product reaction reaction AE,eV m / z ratio no. Kr+ + N2H4 Kr + N2H4+ -6.3 32 28 Kr+ + N2H4 Kr + NH4* N -3.5 18 29 Kr+ + N2H4 Kr + N2H2++ H2 -3.0 30 30 Kr+ + N2H4 Kr + N2H3++ H -2.6 31 31 Kr+ + N2H4 Kr + N2H+ + H + H2 -2.0 29 32 Kr+ + N2H4 Kr + NH,+ + N H -1.4 17 33 Kr+ + N2H4 Kr + NH2+ + NH, +o. 1 16 34 Kr+ + N2H4 Kr + N2+ + 2H2 +0.6 14 35 Kr+ + N,H4 Kr + NH' + NH, +2.0 15 36
------
+
grating are chosen to provide low resolution spectra (4-15 nm fwhm) over a 400-nm wavelength range. 111. Results A. Reaction Cross Section and Time-of-Flight Measurements. In Figure 1, the 0' N2H4 reaction cross sections are plotted as a function of the center-of-mass collision energy. The upper plot shows the cross sections for N2H3+and N2H4+, illustrating the relative difference between the nondissociative charge-transfer product and N2H3+,which is formed in reaction 1 as the coproduct with OH (All = -6.6 eV) and/or in reaction 7 as the ionic product in a dissociative charge-transfer channel (A,? = -2.2 eV). In the lower plot, the N2H+, N2H2+,and N2H4' products are shown on an expanded y-axis scale. Within each data set, the scatter represents the relative experimental error, and the error bars indicate the 2u error in a particular cross section measurement. The absolute error in the cross section measurements is estimated to be the greater value of either f30% or 0.3 A2.* Several reaction channels listed in Table I involve cleavage of the hydrazine N-N bond and result in products in the m/z = 15-18 range. However, the O+signal is approximately 3 orders of magnitude more intense than that of the product ions. Athough single-mass resolution is routinely achieved in the quadrupole, the dynamic range is insufficient to resolve the possible m/z = 15-1 7 products in the presence of the 0' beam. Where m/z = 18 products were observed, the cross section was below 1 X cm2. If the other low mass ions are formed, such fragment ions would most likely be produced in dissociative charge-transfer reactions. The product state distributions in large cross section chargetransfer processes are postulated to be governed by energy resonance and Franck-Condon criteria."-I4 It may therefore be predicted that atomic ions with recombination energies similar to O+would produce similar fragmentation patterns. Therefore, the Ar+-N2H4 and Kr+-N2H4 reaction systems have also been studied. The ionization potential for krypton [14.00 eV (3P3,2), 14.68 eV (3P1,2)]is similar to that for the oxygen atom (13.62
+
(7) Gardner, J. A.; Dressler, R. A,; Salter, R. H.; Murad, E. Geophys. Lab. (AFSC) Tech. Rep. 1989, GL-TR-89-0345.
(8) Dressler, R. A.; Gardner, J. A.; Salter, R. H.; Wodarczyk, F. J.; Murad, E. J . Chem. Phys. 1990, 92, 1117. (9) Dressler, R. A.; Gardner, J A.; Salter, R.H.; Murad, E. J . Chem. Phys.
-.
1992. 96. -, -1062. --~
(10) Murad, E. J. Chem. Phys. 1973, 58, 4374.
(11) Bauer, E.; Fisher, E. R.; Gilmore, F. R. J . Chem. Phys. 1969, 51, 4173. (1 2) Govers, T. R.; Guyon, P. M.; Baer, T.; Cole, K.; Frolich, H.; LavollC, M. Chem. Phys. 1984, 87, 373. (13) Archirel, P.; Levy, B. Chem. Phys. 1986, 106, 51. (14) Parlant, G.; Gislason, E. A. J . Chem. Phys. 1986, 86, 6183.
4212 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 40
I
I
I
I
I
I
I
I
I
I
I
,
,
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,
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Gardner et al.
,
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+ NlH4 o NH?+
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O:
' ' ' '
;' '
' l b ' ' ' '1;'
'
'io' '
I
0
' '215' ' ' '20
Figure 1. Collision energy dependence of the reaction cross sections for the O+-N2H4 system. Upper plot shows the crass sections for the charge transfer (NZH4') and H- abstraction (N2H3+)reaction products; lower plot shows the N2H+,N2H2+,and N2H4+products. Data points for each product ion are connected solely to assist the reader in following a data set; the lines are not interpolations of the cross sections.
n
-
00
10
t NH,+
15
20
N,H+
25
0
Figure 3. Collision energy dependence of the reaction cross sections for four product ions in the Kr+-N2H4 system. The other observed product ions, N2H+and N2H2+,each exhibit cross sections smaller than that for N2H4+at all collision energies studied.
Collision Energy, Eo,m. (eV)
0
10 Energy, 15 20 (eV)25 Collision 5 EC,,,,,
30
C o l l i s i o n E n e r g y , Ec.m. (eV)
Figure 2. Collision energy dependence of the reaction cross sections for the eight product ions in the Ar+-N2H, system.
eV), but the argon IP [15.76 eV (3P3/2),15.93 eV (3Pl/2)]is much higher.Is Thus, if the energy resonance and Franck-Condon criteria apply, the Kr+-N2H4 fragmentation patterns would resemble those for O+-NzH4, and greater fragmentation would be predicted for the Ar+-N2H4 system. The exothermic and moderately endothermic reaction channels for Ar+-N2H4 and Kr+-N2H4 collisions are listed in Tables I1 and 111, respectively. (15) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.;Herron, J . T. J . Phys. Chem. ReJ Dura 1977, 6, Suppl. 1.
The Ar+-N2H4 cross sections for the product ions NH+, NH2+, NH3+, NH4+, N2H+, N2H2+,N2H3+, and N2H4+ are plotted versus collision energy in Figure 2. The charge transfer between Ar+ and N2H4 has an exothermicity of AE = -8.1 eV, which is approximately 2 eV more exothermic than for the other reaction pairs in this study. The Ar+-N2H4 system exhibits the largest dissociation product cross sections obtained in this study, consistent with the increased exothermicity being partitioned to the N2H4+ internal modes. It is noteworthy that no N2+ product ions are detected in any of the reaction systems studied. The Ar+-N2H4 system represents the only case in which the formation of N2+ is an exothermic process. As no counts above the noise level are measured for m / z = 28, the upper limit to the cross section for N2+production in reaction 35 is 3 X lo-'* cm2. In the Kr+-N2H4 system, six products ions are observed with cross sections above 1 X cm2. The collision energy dependence of the reaction cross section is shown in Figure 3 for four of these ions (NH2+,NH3+, N2H3+, and N2H4'). The other observed product ions, N2H+ and N2HZ+,each exhibit cross sections smaller than those for N2H4' at all collision energies studied. The relatively high cross sections observed at the highest collision energy studied, Ec,m,= 21 eV, are likely to result from discrimination against the 76 eV Kr+ beam in the quadrupole. The total reactive cross section for the Kr+-N2H4 collision system is less than 10 A2 a t all energies studied. The Kr+-N2H4 cross sections are generally the smallest cross sections in this study, regardless of the product ion. This trend to relatively small cross sections for Kr+, even in exothermic reactions, has been observed previously both in the Kr+-NH3 charge transfer systemI6 and in the hydrogen atom abstraction from H2, where the reaction cross sections are about 1 order of magnitude smaller for Kr+ than for Ar+." To further test the importance of primary ion recombination energy in determining product fragmentation, reaction cross sections have been measured for CO+-N2H4 and C0,+-N2H4 collisions. The ground-state ionization potentials for CO (14.0 eV) and C 0 2 (1 3.8 eV) are both similar to the 0 and Kr ionization potentials,I5 and thus the reaction energetics are similar for all of these species. The product ion distribution for the molecular primary ions, however, is markedly different than for the atomic primary ions. The collision energy dependence of the reaction cross section for six product ions in the CO+-N2H4 system is shown in Figure cm2 at 4. The NH4+product cross section was below 3 X ~~~~~~~~~~~
~
~~
(16) Kemper, P. R.; Bowers, M. T.; Parent, D. C.; Mauclaire, G.; Derai, R.; Marx, R. J . Chem. Phys. 1983, 79, 160. (17) Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1985, 83, 166; J . Chem. Phys. 1986, 85, 6380.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4213
Reactions of N2H4 with O+
TABLE IV: Product Ion Laboratory Frame Energies Obtained from Time-of-Flight Measurements" ion E,,;, E,, mfz 15 mfz 16 mfz 17 m/z 18 mfz29 mlz30 0.07 O+ 3.3 2.2 0.07 0.05 0.1 1 O+ 23.4 15.6
O+
33.6
17.9
0.06
0.05
O+
47.6
25.4
0.11
0.04 9.3*
0.04
0.05
0.06
0.08
0.08
0.08
0.10 5.2'
0.14 6.2*
0.02 5.7'
0.04
0.02
0.05
0.04
CO+
47.8
25.5
0.05 4.2*
0.05 5.1'
0.09
14.4
C02+
54.5
22.9
Ar+
54.9
24.4
0.07 2.8 (4%)
0.05 6.5*
0.10 4.1 (5%) 0.07 6.5* 13.8 (5%) 0.09 6.8*
0.10
0.07
0.08
0.09
0.13 7.2*
0.19 7.6'
0.06 7.9 (25%) 0.17 23.2 (15%) 0.09 5.8'
0.09
0.05 4.2*
12.1
34.2
0.08
mi233 0.02 0.02
6.3*
0.08 5.1' 0.07 4.9*
22.7
C02+
mlr32 0.03 0.08 0.04
0.04 3.7. 0.05 4.9'
Cot
6.3*
mi231 0.02 0.12 0.05 5.3'
0.02
"Listed are the primary ion, energy of the primary ion (Eprim), collision energy (Ec,,,,),and the product ion energies obtained. Product ion energies labeled with an asterisk contribute less than 0.5%of the total cross section signal for that ion. Percentage labels indicate the relative contribution to the total cross section for those products. Unlabeled energies are the majority product ions. All energies are in eV.
o NH~+
oN~H~+
:i,
N~H,'
'0
.-
0
20
'0 20
A N ~ H ~ *
o I
0
N ~ H ~ + N~H,+
-
A
L
J
5
Collision Energy, Ec,,. (eV) Figure 4. Collision energy dependence of the reaction cross sections for six product ions in the CO+-N2H4 system. The product m/z = 18 cross section was below 3 X lo-'' cm2 at all energies studied. The other likely product ion, N2Ht (m/z = 29), is not observable because of interference from the primary ion beam at m/z = 28.
all energies studied. The other likely product ion (NzH+,m / z = 29) is not observable because of interference from the intense primary ion beam at m / z = 28. There are two significant differences between the product ion distribution for CO+ and those for the atomic ions: (i) the large cross section for the nondissociative CO+-NzH4charge transfer; and (ii) the significant cross section for NH+ formation in the CO+-N2H4case. In the C02+-N2H4system, the N2H3+ and NzH4+ products dominate the reaction, as shown in Figure 5 where the reaction cross sections for NH2+,N2H3+, and N2H4' are plotted as a function of the collision energy. The other observed product ions (NH+, NH3+,NzH+, and NzH2+)each exhibit cross sections smaller than that for NH2+a t all collision energies studied. The product laboratory frame ion energies obtained from time-of-flight measurements are summarized in Table IV. The product ion energies are listed along with the energy of the primary and the center-of-mass collision energy (Ec,m,).The ion (Eprim) time-of-flight spectra show nearly all product ions to be formed at or near thermal energy in the laboratory frame. In fact, all product ions listed in Table IV as having energies less than 0.2 eV may be assumed to be thermal ions, as a result of the instrument geometry and specific applied potentials that greatly reduce the sensitivity (less than 4% collection efficiency) to thermal
10
15
20
25
30
Collision E n e r g y , Ec,m, (eV) Figure 5. Collision energy dependence of the reaction cross sections for three product ions in the C02+-N2H4system. The other observed product ions, NH+, NH3+,N2Ht, and N2H2+,each exhibit cross sections smaller than that for NH2+ at all collision energies studied.
ions in the time-of-flight mode.7,8J8 Thus, the product ions are essentially scattered isotropically in the laboratory frame. In the crass section measurement mode, however, the collection efficiency has been determined to be 20.8%731sfor thermal and near-thermal product ions, based on the apparatus geometry. The cross sections reported here have therefore been corrected for the 20.8% instrumental collection efficiency (i.e., the raw data has been multiplied by a factor of 4.8) and are reported as integral reaction cross sections. Many product ions also exhibit very small (less than 0.5% of the total cross section-marked in Table IV with asterisks) contributions from products formed with velocities comparable to the center-of-mass velocity. The extremely small contribution of these ions to the cross sections, coupled with the high translational laboratory-frame kinetic energy of these ions, indicates that they are formed in small impact parameter collisions, where considerable translational energy is transferred into internal excitation. The C02+-N2H4system is unique in that three product ions (NH+, NH2+,and N2H+)include a relatively large fractional contribution from such fast products. However, the absolute contribution of small impact parameter collisions to the total cross (18) Lishawa, C. R.; Dressler, R. A.; Gardner, J. A,; Salter, R. H.; Murad,
E. J . Chem. Phys. 1990, 93, 3196.
4214
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Gardner et al.
Mass 33 product 5 N
E
Ar+ A Kr'
0 co+ A
&--A--s
5
co2+
______ 15
10
20
Collision E n e r g y ,
25
30
Ec.,,,. (eV)
Figure 6. Collision energy dependent plot of the m / z = 33 product for each of the five ion-N2H4 reaction systems. Cross sections are calculated assuming that this product is formed directly in the ion-NzH4 reactions, rather than being formed in the secondary channel (reaction 57).
sections for these products is no larger than for the other ions. For all five reaction pairs, a small signal is measured at m / z = 33. The possible reactions leading to this signal are nondissociative charge transfer to an N2H3Dimpurity,I9 dissociative charge transfer to an N2H4*H20impurity, or via a secondary reaction channel:
N2H4+ + N2H4
+
N2H5+ + N2H3
-
X
+ N2H5+ + OH
0
Wavelength (nanometers)
Figure 7. Chemiluminescence measurements for O+-N2H4 collisions at
Ec,,,, = 4-50 eV.
(37)
Time-of-flight studies were performed on the m / z = 33 product for O+ and Ar+ reactions. In each case, the product ions were found to be formed at thermal energy in the laboratory frame (see Table IV). The integral reaction cross sections, calculated by assuming the products to be formed directly in the ion-N2H4 collision rather than through the secondary channel, are shown in Figure 6. The cross sections are similar in the O+,Ar+, and C02+reactions, with the CO+ reaction exhibiting a slightly larger cross section. The Kr+ reaction cross section is approximately to that of the O+ reaction, similar to the above Kr+ cross-section data. Apart from the lower Kr+ data, there is no significant difference between the atomic ions and the molecular ions in forming m / z = 33. This is in sharp contrast to the nondissociative charge-transfer product, N2H4+. Therefore, the m / z = 33 product cannot result from nondissociative charge transfer to N2H3D,nor from secondary reactions. The m / z = 33 signal is therefore attributed to reactions of the form
X+ + N2H4.HzO
2
(38)
For X = Kr, this reaction is exothermic by 4.3 eV, given an enthalpy of formation20for N2H5+of 184 kcal mol-' and assuming a typical value of 0.22 eV for the hydrogen bond in N2H4.H20.21 The presence of the N2H4.H20impurity implies that OH+ and H20+may also appear in these measurements, interfering with the measurements for NH3+and NH4+, respectively. For Kr+ + N2H4.H20,the formation of H20+is exothermic by 1.2 eV and OH+ formation is endothermic by 3.9 eV. Each of these products might therefore be formed at the collision energies studied here, (19) Ordinarily, the magnitude of the cross section for m / z = 33 would have disqualified char e transfer to N2H,D as a possible mechanism since the natural abundance of ?H is too small to result in these cross sections relative to the N2H4+cross sections. The experiments immediately preceding this work, however, involved the use of D20as a target gas. While every precaution was taken to clear the D20from the system prior to introducing N2H4, the existence of the m / r = 33 channel required that contamination be considered as an explanation. (20) Lias, S.G.;Liebman, J. F.; Levin, R. D. J . Phys. Chem. ReJ Dura 1984, 13, 695. (21) Morrison, R. T.; Boyd, R. N . Organic Chemistry, 3rd ed.; Allyn and Bacon: Boston, 1973; p 27.
0
5
10
15
20
25
30
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Collision Energy, Ec,,,,, (eV)
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Figure 8. Collision energy and velocity dependence of the O H and N H bands integrated emission intensities normalized to ion current, target gas pressure, and acquisition time. Error bars show integration error, based on limits selected for the signals.
although energetically the product ratio should follow the form N2H5+ >> HzO+>> OH+. This trend is clearly visible for the m / z = 33:18 ratio, and it is possible that the m / z = 18 signal is entirely due to H20+. Although OH+ formation is possible above about 4 eV (for Kr+, CO+,and C02+;above 2 eV for Ar+), the energy dependence of the m / z =: 17 signal is clearly representative of an exothermic reaction. Furthermore, comparison of the m/z = 17 signal to the m / z = 18 signal suggests that the contribution of OH+ to the m / z = 17 signal is negligible. The m / z = 17 signal is therefore attributed to NH3+. B. Chemiluminescence Measurements. Chemiluminescence measurements have been performed for the first time on the O+-N2H4 system, over the collision energy range from 4 to 50 eV. The only emissions detected in the 250-850-nm range are OH A22+ X2n(306-nm band head) and NH A%, X32(336-nm band head) emissions. The 15-nm (fwhm) resolution, 200-600-nm range data are shown in Figure 7. The emission intensities for both the OH and NH bands have been integrated and normalized to the O+ ion current, the N2H4 target gas pressure, and the data accumulation time. These normalized integral emission intensities are plotted as a function of collision energy in Figure 8. Two H- abstraction channels (reactions 1 and 2) are sufficiently exothermic to produce OH in the A22+
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The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4215
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Figure 9. Chemiluminescence measurements for the Ar+-N2H4 system at three collision energies. Kr'
200
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Figure 10. Chemiluminescence measurements for the Kr+-N2H4 system at two collision energies.
electronically excited state, which lies 4.0 eV above ground-state OH. In contrast, formation of the N H A state is an endothermic process (AI3 = 2.7 eV). The OH emissions decrease in intensity as the collision energy is increased. This behavior is typical for an exothermic process. The N H emission intensity increases with increasing collision energy, typical for an endothermic process. Chemiluminescence measurements have also been performed for the first time on the Ar+-N2H4 (Figure 9) and KP-NZH4 (Figure 10) collision systems, where the thresholds for the endothermic production of NH A state products are 0.6 and 2.3 eV, respectively. The time-of-flight measurement shows the NH3+ in the Ar+-N2H4 case to be slightly above thermal energy, conceivably corresponding to a translational-to-internal energy transfer, Q,that permits population of the N H A state. In the absence of a measurement of the two neutral product velocities, however, the value of Q remains indeterminate for this reaction.22 In the chemiluminescence data, the NH A X band is the main feature for both of these reaction systems. In Figure 9, the highest energy spectrum includes a X10 magnification that shows a weak band at 435 nm. This feature is identified as the (0,O)vibrational band of the NH+ BzA X2nt r a n ~ i t i o n . ~ ~
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(22) Maier, W. B.; Murad, E. J . Chem. Phys. 1971, 55, 2307. (23) Kusunoki, I.; Ottinger, Ch. J . Chem. Phys. 1984, 80, 1872.
IV. Discussion X+-N2H4Reaction Dynamics. At the collision energies investigated in this work, ion-molecule reactions proceed primarily through a direct mechanism." Direct processes exhibit large cross sections if they proceed efficiently at large impact parameters, Little momentum is transferred in such collisions, and thus large cross sections are only observed for the transfer of light particles (e.g., e-, H, H+, H-) in exothermic processes where the reaction exothermicity is channeled into internal excitation of the produ c t ~ . ~Disregarding ,~~ the large reaction exothermicities, these considerations would predict the nondissociative charge transfer and the H- abstraction products, N2H4+and N2H3+,to be the dominant ionic hydrazine products in the X+-NzH4 reactions studied here. For charge-transfer reactions, it has been postulated that the magnitude of the state-to-state cross sections is a function both of the Franck-Condon overlap between the reactant and product vibronic wave functions and of the energy gap between the reactant and product energy Thus, reactions of a particular species with various ions that have similar ionization potentials can populate states of similar energy in the product ion. This has been demonstrated in the Nz+-H20 and Ar+-HzO chargetransfer systems. There, chemiluminescence studiesg have shown the large suprathermal cross sections t? result from the population of highly vibrationally excited H 2 0 + A state levels that are near-resonant with the reactant energy level. Such arguments predict that O+ and Kr+ produce NzH4+levels of similar energy. The significantly smaller Kr+-N2H4reaction cross sections indicate a smaller vibronic coupling in the krypton system. This is presumably due to the different electronic interaction in the two systems, since the NzH4-NzH4+ FranckCondon overlap is independent of the primary atomic ion. The small total cross sections for the Kr+ case are consistent with the relative behavior of Kr+ in other systems, as mentioned earlier. The other predicted channel, H- abstraction, is not available in the Ar+-NzH4 and Kr+-NzH4 systems, as neutral ArH and KrH are not stable species. Therefore, charge transfer is the only possible reaction that is efficient at large impact parameters for these primary ions. It has also been suggestedz6that hydrogen abstraction frequently proceeds via a twestep mechanism: charge transfer followed by hydrogen transfer. This suggests that even the channels that form OH may begin with charge transfer. Thus, the products in all cases may be primarily formed by charge transfer between the ion and N2H4, followed by the exothermicity-driven dissociation required to reach a particular product, and by H transfer where appropriate. The time-of-flight studies reported here show the hydrazine product ions to be formed with near-thermal translational energy in the laboratory frame. For the charge-transfer product, NzH,+, the time-of-flight studies confirm that the charge-transfer proctss involves little momentum transfer and is therefore a long-range process. The near-thermal energy observed for the dissociation products indicates isotropic scattering (laboratory frame) for the product ions. This is consistent with a two-step process of charge transfer with little-to-no momentum transfer followed by dissociation with little kinetic energy appearing in the ionic dissociation prod~ct.2~ In contrast, a single-step reaction would result in products scattered isotropically in the center-of-mass frame resulting in significant laboratory frame translational energies. The NZH4+ fragmentation pattern is governed by both the exothermicity and the partitioning of the exothermicity between the primary particle and NzH4' internal modes, and therefore differs for the various primary ions. In the atomic primary ion reactions, assuming the atomic products to be formed in their (24) Mahan, B. H. An Analysis of Direct Ion-Molecule Reactions. In Interactions Between Ions and Molecules; Ausloos, P., Ed.;Plenum Press: New York, 1975; p 75. (25) Dressler, R. A.; Gardner, J. A.; Cooke, D. L.; Murad, E. J . Geophys. Res. 1991, 96A, 13,795. (26) Suzuki, S. J . Chem. Phys. 1990, 93,4102. (27) Masson, A. J.; Birkinshaw, K.; Henchman, M. J. J . Chem. Phys. 1969, 50, 41 12.
4216 The Journal of Physical Chemistry, Vol. 96,No. 11 1992
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ground electronic states, the exothermicity is channeled to internal excitation of the charge-transfer product N2H4'. In the reactions between N2H4 and the molecular ions, the CO and C 0 2 charge-transfer products can also share in the partitioning of the exothermicity into internal modes. This is reflected in the higher nondissociative charge-transfer cross sections for these ions. The greater enhancement in N2H4+ production in the C02+-N2H4case relative to the CO+-N2H4 case is related to the larger number of available internal modes in the triatomic C02 relative to the diatomic CO. Although determining the precise partitioning of energy into specific N2H4+ modes is beyond the scope of this work, consideration of the N2H4 and N2H4+ geometry could provide information on the dynamical reaction pathways. The structures of the hydrazine molecule and radical cation are well-known. The N2H4 molecule has only C2 symmetry with bond lengths d(N-N) = 1.45 A and d(N-H) - 1.02 A, bond angles LHNH = 106' 0' and LNNH = 112' 0', and a torsional angle x = 90' 2'.28,29The stable N2H4' cation has a preference for a planar [H2N=NH2 configuration with DZhsymmetry for which d(N=N) = 1.291 , d(N-H) = 1.002 A, LHNH = 120.8', and LNNH = 119.6O have been calculated.M The cation nitrogen atoms are sp2 hybridized, unlike the nitrogen atoms of the neutral molecule, which are sp3 hybridized. The shortened N=N bond distance in the ion results from the three-electron A bond containing two electrons in the bonding and one electron in the antibonding a orbitals. It is noteworthy that the resulting N-N bond strength is approximately 5.5 eV, as opposed to 2.5 eV for the N-N bond in neutral N2H4.I5 This may partially explain the NH2+ cross sections being small relative to the N2H3+cross sections: although NH2+ formation is nearly thermoneutral for the ion-N2H, reactions being studied here (except Ar+-N2H4), nearly all of the exothermicity must be partitioned into the N-N bond (presumably the N-N stretch mode) for NH2+to be formed. Conversely, N2H3+is formed by cleaving any one of the four N-H bonds, for which bond strengths are approximately 3.2 eV in N2H4 but only 2.3 eV in N2H4+.I5Thus, cleavage of an N-H bond is energetically favored over cleavage of the N=N bond in N2H4+, which may account for the dominance of N2H3+formation that is observed for these reactions. Although the [H2N=NH2]+ isomer is the lowest energy configuration, the C, symmetry [H3N-NH]+ isomer is also possible, at an energy that is 1.2 eV higher than the DZhconfiguration.31 In the [H3N-NH]+ configuration, d(N-N) = 1.437, d(N-H) = 1.015 f 0.02 A, LNNH = 105.7', the nitrogens remain sp3 hybridized, the N-N single bond strength is approximately 3.3 eV, the positive charge resides on the H3N- nitrogen, and the unpaired electron is on the -NH n i t r ~ g e n . This ~ ~ ? configuration ~~ is energetically accessible in the ion-neutral reactions under study here, and is likely to be an intermediate in the formation of both NH+ and NH3+. The preference for NH3+ over NH+ production ([NH,+]:[NH+] is L2:l in the Kr+-N2H4, Ar+-N2H4, and CO+-N2H4 systems) indicates that the N-N bond tends to cleave homolytically. Indeed, this is consistent with the energetics as NH3+ production is favored over NH+ production by 3.4 eV (compared reactions 33 and 36 in Table 111). The only ion in the low mass range that has a consistently small cross section is NH4+, which is energetically the most favored product ion in this range. Even if the [H3N-NH]+ isomer is an intermediate, the fourth hydrogen must then substitute into the N-N bond, with a loss of one nitrogen atom. It appears that this dynamic requirement hinders the formation of NH4+. As mentioned earlier, it is also possible that the weak m / z = 18 signal is due to H20+from the N2H4.H20impurity. Experiments are planned in which ion-N2D4 reactions will be studied to further identify the possibility of NH4+ (ND4+) formation.
Possible O+-N2H4Low-Mass Products. The reactions of Ar+, Kr+, CO+,and C02+with N2H4 are studied here to provide insight into the feasibility of forming low-mass products ( m / z = 15-17) in the O+-N2H4 case. The emission data for 0+-N2H4 clearly show the production of N H as a product, implying that NH3+ is formed to some extent, as reaction 1 1 is the most energetically favorable reaction in which N H may be formed. In measuring the low mass cross sections for N2H4 collisions with the atomic ions Ar+ and Kr+, both NH2+ and NH3+ were found to be significant products, with minor amounts of NH+ and NH4+ also being formed. The NH2+and NH3+cross sections for Ar+ and Kr+ account for approximately onethird of the total reaction cross sections observed. It seems reasonable to expect the O+-N2H4 system to also produce low-mass ions in approximately this same proportion. Although the [NH2']:[NH3+] product ratios are significantly different for the Ar+-N2H4 (=1 :2) and Kr+-N2H4 (=1: 1) cases, the extra exothermicity in the Ar+-N2H4 case may increase the production of the [H3N-NH]+ isomer, thus yielding more NH3+product. For both the CO+-N2H4 and C02+-N2H4 systems, however, NH2+ is the dominant low-mass ion. The above-mentioned planned experiments on the ion-N2D4 system will provide direct data on the O+ system products. OH Production. If OH production in reaction 1 occurs via a two-step process in which the first step is charge transfer, as suggested above, and the efficiency of O H A state formation is constant, then the energy dependence of the OH emission data and the N2H3+ cross sections for O+-N2H4 should be similar. However, comparison of the normalized O H emission intensities in Figure 8 with the N2H3+ cross section data for O+-N2H4 collisions (Figure 1) shows the observed OH emissions to decrease more rapidly with energy (a Ec,m,4,75) than do the N2H3+cross sections ( a Ec,,,4,30).This difference may indicate that an energy dependence exists for the fraction of reactions that form O H in its A state while producing N2H3+. Alternately, the faster falloff for the OH emissions may be caused by a decrease in optical collection efficiency for OH emissions at higher collision energy. The OH laboratory kinetic energy is assumed to be described by a spectator stripping type m e c h a n i ~ m , *with ~ ~ ~the - ~O ~ H velocity equal to 16/17 times that of Of. This assumption is supported by the time-of-flight studies that show the coproduct N2H3+to be formed with near-thermal energy. At 4 eV (Ec,,,), the O H velocity is therefore 8 km s-l. The OH consequently attains a velocity sufficient to exit the viewing region during its emission lifetime. For such a case, Ottinger has determined an expression for the fractional light loss, p, in terms of the emitting species mean flight distance from the point of formation, Xo, to be38 p = [Ao/([, - I,)][e-'1/b - e - ' l / ' ~ ]
(28) Morino, Y.; Iijima, T.; Murata, Y. Bull. Chem. SOC.Jpn. 1960, 33, 46. (29) Kasuya, T. Sei. Pap. Inst. Phys. Chem. Res. 1962, 56, 1 . (30) Drewello, T.; Lebrilla, C. B.; Schwarz, H.; Stahl, D. Int. J . Mass Spectrom. Ion Processes 1987, 77, R3. (31) Frisch, M. J.; Raghavachari, K.; Pople, J . A.; Bouma, W. J.; Radom, L. Chem. Phys. 1983, 75. 323.
1967, 71, 596.
8'
where lI and l2 are the distances along the flight path from the cell entrance to the beginning and end of the fiber-optic viewing region, respectively. To consider the impact of this effect on the OH emission measurements, p has been calculated as a function of collision energy for the geometry of the chemiluminescence collision cell, with the above assumptions. The OH normalized integral signals from Figure 8 have been adjusted to correct for the p factor and re-plotted in Figure 11 . The solid line in Figure 1 1 is the power best fit to the adjusted OH emission data (a P.42)-interestingly, For the C02+-N2H4charge transfer cross section data is a (32) Lacmann, K.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1965,69, 292. (33) Henglein, A. Adu. Mass Spectrom. 1966, 3, 33 1. (34) Herman, Z.; Kerstetter, J.; Rose, T.; Wolfgang, R. Discuss. Faraday SOC.1967, 44, 123. (35) Gentry, W. R.; Gislason, E. A.; Lee, Y. T.; Mahan, B. H.; Tsao, C. W. Discuss. Faraday SOC.1967, 44, 137. (36) Ding, A.; Lacmann, K.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. (37) Henglein, A. In Proceedings of the International School of Physics *Enrrco Fermi," C. XLIV, Mol. Beams and Kinetics; Schlier, Ch., Ed.; Academic: New York, 1970. (38) Ottinger, Ch. Electronically Chemiluminescent Ion-Molecule Exchange Reactions. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academy: New York, 1984; Vol. 3, p 262.
4217
J. Phys. Chem. 1992, 96,4217-4219 Collision Velocity (km s-')
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Figure 11. Plot of OH emission intensity adjusted for the loss of emissions due to OH motion out of the viewing region as a function of collision energy and velocity. Solid line is the power best fit to the adjusted data. Short dashed line is proportional to the power best fit to the m / z 31 cross section, scaled to this plot's limits. Long dashed line is proportional to the power fit to the OH emission data prior to the p-factor
adjustment.
comparison, the shapes of the power best fits to the N2H3+ cross sections for O+ N2H4 (short dashed) and the OH emission data from Figure 8 (long dashed) are shown in Figure 11. Both dashed lines have been arbitrarily scaled to show the relative falloff rates for the appropriate data sets. The shape of the fit to the adjusted data is in reasonable qualitative agreement with the shape of the
+
cross sections fit, indicating that the faster drop-off in the OH emissions is due to the A state products exiting the viewing region before emitting. Additionally, the energy dependence of the corrected data appears to be consistent with the mechanism in which OH is formed in a two-step process, beginning with charge transfer. Implications for the Spacecraft Environment. These measurements show that the overall reaction between O+and N2H4 is very efficient a t energies relevant in the low earth orbital environment. The O+-N2H4 high-mass product reaction cross sections sum to 15-20 A2. The Kr+-N2H4 and Art-N2H, data suggest that the total cross section for O+-N2H4 may be a p proximately 50% higher than this, as a result of low-mass-product formation. A total cross section of 20-30 A2in this energy range is similar to that for the charge transfer between Ot and H20,39 which is another significant reaction in the spacecraft environment. The chemiluminescence data shows that both OH A X and NH A X emissions are produced at the relevant orbital energies, with the OH emissions being significantly more intense than the NH emissions.
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Acknowledgment. This work was supported by the Air Force Office of Scientific Research under Task 2303G2. Registry NO. N2H4, 302-01-2; O ', 14581-93-2; Art, 14791-69-6;Kr', 16915-28-9; CO', 12144-04-6;C02', 12181-61-2; NzH-,', 37369-93-0; NZH4", 20771-51-1; N2H', 12357-66-3; NZHS', 63986-30-1; NH', 19067-62-0; NH2'. 15194-15-7; NH,', 19496-55-0;NHI', 14798-03-9. (39) Heninger, M.; Fenistein, S.; Mauclaire, G.; Marx, R.; Geophys. Res. 1989, 16, 139.
Murad, E. J .
The 1,l-Elimination of HCN and Formation of Triplet Vinylidene from the Photolysis of Acrylonitrile Askar Fahr* Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
and Allan H. Laufer* Chemical Sciences Division, Office of Basic Energy Sciences, US.Department of Energy, Washington, D.C. 20585 (Received: December 19, 1991)
The single-photon photodissociation of acrylonitrile-1-d has been investigated by flash photolysis and kinetic absorption spectroscopy in the vacuum-ultraviolet region. Both electronically excited triplet vinylidene and DCN were observed. The result provides further evidence that a "vinylidene" type structure with an extended lifetime exists. Alternative primary processes were considered, particularly those involving vinyl radical or molecular acetylene formation, but they were not observed.
Introduction The chemistry, spectroscopy, and structure of hydrocarbon free radicals are of interest in diverse areas ranging from the atmospheres of the outer planets to combustion. In particular, unsaturated species are of interest both theoretically, for investigation of tunneling phenomena, and experimentally, for kinetic comparison to simple alkyl radicals. The simplest unsaturated carbene is vinylidene (H2C=C). An absorption spectrum at 137 nm attributed to the long-lived electronically excited triplet (3B2) vinylidene radical has been observed following vacuum-UV photolysis of a variety of precursor molecules including acetylene,l ethylene,2 and vinyl ~ h l o r i d e . ~The triplet state also has been (1) Laufer, A. H. J . Chem. Phys. 1980, 73,49. (2) Fahr, A.; Laufer, A. H. J. Phorochem. 1986, 34, 221. (3) Fahr, A.; Laufer, A. H. J . Phys. Chem. 1985, 89, 2906.
observed recently by mass spectrometry! The ground-state singlet (XIA,) surface lies about 48 kcal/mol below the a(3B2) state of ~inylidene.~The barrier to isomerization on the singlet surface has been calculated to be about 3 kcal/mol. Tunneling of the H atom through this barrier is suggested to lead to a short lifetime, a fraction of a picosecond.6 The short-lived ground-state singlet has been observed by ultraviolet photoelectron ~pectroscopy.~ The formation of vinylidene from either C2H4 or C2H3Clinvolves the 1,l-elimination of H2or HCl, respectively. The propensity of molecules to undergo either a 1,l- or 1,Zphotochemical (4) Sulzle, D.; Schwarz, H. Chem. Phys. Len. 1989, 156, 397. ( 5 ) Burnett, S. M.; Stevens, A. E.; Feigerle, C. S.; Lineberger, W. C. Chem. Phys. Lett. 1983, 100, 124. Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989, 91, 5974. (6) Gallo, M. M.; Hamilton, T. P.; Schaefer 111, H. F.J . Am. Chem. Soc. 1990, 112, 8714 and references therein.
This article not subject to U.S.Copyright. Published 1992 by the American Chemical Society
