J. Phys. Chem. 1994, 98, 11630-11636
11630
ARTICLES Methylhydrazine Reactions with O+ and Other Ions at Hyperthermal Collision Energies James A. Gardner* PhotoMetrics, Inc., 4 Arrow Dr., Wobum, Massachusetts 01801 -2067
Rainer A. Dressler and Richard H. Salter USAF Phillips Laboratory, PLNSSI, 29 Randolph Rd., Hanscom AFB, Massachusetts 01 731-3010 Received: March 1, 1994; In Final Form: August 29, 1994@
Cross sections have been measured for methylhydrazine (CH3NHNH2) reactions with O+,CO+, C02+, and NO+ at collision energies ranging between 1 and 20 eV (center of mass) in an octopole guided-ion-beam apparatus. As was the case for similar reactions involving hydrazine, charge transfer and dissociative charge transfer occur in each reaction. The total reaction cross section is on the order of 50- 100 A2, and the amount of CH3NHNH2+ fragmentation in each case depends upon both the exothermicity of the charge transfer and the availability of internal degrees of freedom in the incident primary ion. The product ion kinetic energies are primarily thermal or near-thermal in the laboratory frame, as is often found for reactions involving efficient exothermic charge transfer. Charge transfer cross sections are also reported for the CO+ H20 and C02+ H20 systems for the 0.2 IE,, I6 eV range.
+
+
I. Introduction Highly exothermic, hyperthermal charge-transferreactions of large polyatomic systems can result in complex fragmentation patterns, similar to those found in electron impact studies. Because of the long-range, direct nature of the charge transfer in these systems, product states within a narrow range of intemal energies are accessed. When the primary reactant ion is an atomic ion, the internal energy of the initial charge-transfer product is essentially given by the reaction exothermicity. In the case of polyatomic primary ions, the neutralized moiety shares efficiently in the energy partitioning, which lowers the internal energy of the initial ionic product and thereby affects the fragmentation pattern of that species. Our study’ of reactions between various ions and hydrazine found several product ions that were formed by charge transfer followed by fragmentation of the N2&+ principal product. The fragmentation pattern in each reaction was largely independent of the collision energy, and the amount of fragmentation depended upon both the charge-transfer exothermicity and the availability of internal modes in the incident primary ion. Specifically, little N2&+ survived fragmentation when the incident ion was an atomic ion (O+, Kr+, Ar+), but more N2&+ was present in reactions with moelcular primary ions (CO+, C02+). Also, the Ar+ N 2 b reaction produced the greatest amount of fragmentation, as expected because the charge transfer for this reaction pair is about 2 eV more exothermic than for any other pair in that study.’ Studies of O+ chemistry have been stressed because O+ is the dominant ionic component of the earth’s atmosphere at low earth orbital altitudes, where spacecraft such as the space shuttle operate. At 250-km altitude, the density of the neutral ambient atmosphere is on the order of lo9 ~ m - and ~ , the ion density is approximately lo5 ~ m - of ~ which , roughly 98% is O+.2 Several hydrazine species, including hydrazine (N2&), CH3NHNH2, and
+
@
Abstract published in Advance ACS Abstracts, October 15, 1994.
0022-365419412098- 11630$04.50/0
unsymmetrical dimethylhydrazine ((CH~)ZNZH~), are used as fuels for thruster engines aboard spacecraft in low earth orbit. Although most of the fuel is combusted, enough is released in an unbumt state, or as a “dribble out” liquid as the engine is shut down, that the reactions of this fuel with atmospheric constituents must be considered when assessing the importance of fuel products as potential contaminants in the spacecraft en~ironment.39~ Space-bome composition measurements5v6have indicated the presence of several products associated with the firing of space shuttle engines. Several questions remain regarding the chemical sources of observed signals. In particular, a product signal at mlz = 28 is not well understood because the available collision energies are insufficient to produce either CO+ or N2+, which are likely candidate ions at that mass.’ The observed product signal may be attributable to products of exothermic reactions between atmospheric species and contaminant hydrazines. Because ion-neutral reaction cross sections are generally much larger than those for neutralneutral reactions, the ionic reactions may contribute significantly to the overall interaction between atmospheric species and spacecraft effluents at high velocities. Combining the orbital and thruster velocities, collisions between O+ and CH3NHNH2 occur in the 1.1-7.9-eV collision energy range. Efficient exothermic charge-transfer reactions often produce ions with thermal or near-thermal energy, thus charge transfer in this system can convert “fast” ions efficiently into “slow” ions in the spacecraft frame and may therefore be a source of plasma instabilities. We present here a laboratory study of methylhydrazine reactions with each of several primary ions: Of, CO+, C02+, and NO+. The ionization potentials for 0, CO, CO2, NO, and CH3NHNH2 are 13.62, 14.01, 13.77, 9.26, and 7.67 eV, respectively.8 Thus, the charge-transfer exothermicity is quite similar (5.95, 6.34, and 6.10 eV) for the O+,CO’, and C02’ reactions with CH3NHNH2. In each of these reactions, the charge-transfer product is observed along with several frag0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 45, 1994 11631
Methylhydrazine Reactions with O+ and Other Ions
TABLE 1: Exothermic and Slightly Endothermic Reactions between Of and CHJMINHz reaction ionic energy product reaction AE,eV mlz ratio no.
reaction
0' 0'
O+ O+ 0' 0'
-+ CH3-2 + CH3NHNHz + CH3NHNHz + CH3NHNHz + CH3"HZ -
+ CH3-2
+ + + + + O+ + O+ + O+ + 0' + O+ + O+ + 0' + O+ 0' 0' O+ 0'
+ O+ + 0' + Of + 0' + 0' + 0' + Of + O+ +
O+
0'
O+ O+ 0'
O+
O+ Of
+ + + +
+ + +
+
CH3"HZ' 0 + CHZNHZ' NHz 0 CH3NH' NHz +0 HCNHz' NH3 0 CH3NzH' Hz 0 CHz=NH' NH3 CH3NHNHz 0 NzHz+ CHq CH3NHNHz-0 NH3' CH2 = NH CH3NHNHz 0 CHjNz' H CH3NHNH2 0 CHzNz' 2 HZ CHsNHNHz 0 CH3Nz' HZ H C H W H 2 0 N2H3+ CH3 CH3NHNH2 0 HCNH' NH3 H CH3"Hz 0 HCNH+ NHz Hz CH3NHNH2- 0 NzH' CHq H CH3NHNHz + 0 NzH+ CH3 Hz CH3NHNH2 0 CH3NHZ' NH CH3NHNHz-0 CNHz' NH3 H CH3NHNHz 0 CNHz+ NH2 Hz CBNHNHz 0 CH3' NZH3 CH3NHNH2 0 CHzNHz' NH H C H W H z 0 CHq' NzHz CHgNHNHz-0 HNC' NH3 Hz CH3NHNH2-0 HCN' NH3 Hz CH3"HZ 0 f NHz' CHZNHZ CH3NHNHz-0 CH3NH' NH H CH3NHNHz-0 Nz+ CHq HZ CH$GNHz 0 NHz' CH3NH CH3NHNH2 0 HCNHz' NH Hz CH3NHNHz 0 CH2=NHf NH HZ CH3NHNH2 0 CH3NzH' 2H CH3NHNHz 0 HCNHz' NHz H CH3NHNH2 0 CHz=NH+ NHz H CH3NHNH2 0 NzHz' CH3 H 0
--
4
4
4
---
+
--.+
4
-.+
+ +
+
+ + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + +
+ + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
6.0
4.9 4.0 3.9 3.8 3.8 3.6 3.5 3.5 3.2 3.0 3.0 3.0 2.8 2.4 2.4 2.0 1.3 1.2 1.1 0.7 0.6 0.5 0.1 -0.1 -0.2 -0.2 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0
46 30 30 29 44 29 30 17 45 42 43 31 28 28 29 29 31 28 28 15 30 16 27 27 16 30 2a 16 29 29 44 29 29 30
1 2 3 4 5 6 7
a
9 10 11 12 13 14 15
16 17 ia
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
mentation products. The cross section for the nondissociated charge-transfer product follows the primary ion pattern of C02+ > CO+ > O+, as was the case for the same ions reacting with hydrazine.' The reaction of NO+ CH3NHNH2, however, is only 1.59 eV exothermic and simple charge transfer accounts for most of the reaction between these partners. In Table 1, the exothermic and slightly endothermic reaction channels are listed in order of decreasing exothermicity for the O+ C H 3 " H 2 reaction, assuming that the first step in each reaction is charge tran~fer.~A similar number of reaction channels are energetically available that involve the stripping of an H- to initially form OH and CH3N2H2+. The hydride ion stripping reaction is exothermic by 7.9 eV, and the excess energy is expected to produce fragmentation in the ionic product. This exothermicity is also sufficient to yield electronically excited OH (A2E+), which was observed in the O+ 4- N2& reaction.'
+
+
11. Experimental Section The laboratory apparatus consists of a coaxial tandem double mass spectrometer fitted with an octopole ion-guide that has been described in detail previously.lOJ1 Briefly, an ion beam is formed in a low-power electron impact ion source. The ions are accelerated, mass-analyzed 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. The ion beam enters an octopole ion-guide that encompasses a 3.5-cm path-length collision region in which CH3NHNH2 is maintained at a typical pressure of 0.0067 Pa (0.05 mTorr). This low pressure is required to minimize secondary collisions between initial product ions and the neutral CH3NHNH2. Such secondary reactions change the final product fragmentation ratios. A 10-MHz radiofrequency
potential is applied to the octopole rods to trap the ions in the radial direction, eliminating wall collisions and any resulting loss of ions without significantly affecting the ion kinetic en erg^.'^,'^ At the end of the collision region, the ions enter a second octopole, which is capacitively coupled to the first octopole to provide rf phase matching, while allowing for a dc bias potential that is 0.3 V lower than that of the first octopole. The change in the dc bias potential slightly accelerates the ions so that ions formed with only thermal translational energy are able to overcome any small barrier potentials that may be created by surface potential inhomogeneities on the octopole rods. The primary ion beam and the product ions are guided into the entrance of a quadrupole mass filter. The ions are massanalyzed 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. The O+, CO+, and C02+ reactant ions are produced by 40eV electron impact on CO2 (Matheson, 99.8%); NO+ is producted by 30-eV electron impact on N20 (Matheson, 99.99%). The CH3NHNH2 is 98% pure (Johnson Matthey Electronics, natural isotopic abundances, major impurity: H20) and is used without further purification. The impurity H20 forms a methylhydrazine hydrate complex that is observed to also undergo charge transfer and fragmentation. The impact of the hydrate impurity products on the methylhydrazine fragmentation measurements will be discussed. The total natural abundance of I3C, 15N, and *H is 1.94%;14therefore, isotopic impurities do not interfere significantly with the fragmentation patterns. The experimental techniques have been described in detail previo~sly.'~ 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 mlz ranges to collect the primary and product ions. The primary ion energy is ramped in steps, and the ion totals are measured successively at each resulting collision energy step. The spectrometer is first swept at low mass resolution to provide a total products:primary ion ratio, even for large mass differences. The total reaction cross section for the swept range of product masses is determined from this ratio. Next, high mass resolution scans are performed at discrete collision energies to measure the branching ratios for the individual product masses, from which the reaction cross section for each product mass is calculated. The standard error in the cross section measurements is estimated to be f30%. The primary ion energy is measured by a retarding potential analysis. The typical spread in primary ion energy is 0.4-0.5 eV (FWHM). The uncertainty in the collision energy, however, is dominated by thermal broadening and is approximately 1 eV at Ecm= 10 eV. Time-of-flight measurements are performed by pulsing the primary ion beam to provide a 4-6-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. Because the signals for the individual products are low, particularly in the pulsed mode required for measuring times-of-flight, only a select few timeof-flight measurements were performed for products in the O+ CH3NHNH2 system. The results are quite similar to those for the earlier O+ N2H4 study' and may reasonably be considered representative for the other reactions reported here.
+
+
11632 J. Phys. Chem., Vol. 98, No. 45, 1994
I'
"
'
" '
"
* "
Ar'
"
+
"
I
Gardner et al.
''4
CH3NHNH2, = ' 1.8 ' " Ec,m," " eV "
1.
"
I
:I,:,
, ,,,:
:,.:
*,
V
...............................
0
6
I
-
50-
--. /.\, - - - - 7.:.:.:.:.1.: ....................... ,I
10
20
30
40
50
60
...~..-..~.-.=..~..~..~.-?
70
m/z
Figure 1. Pressure dependence of the fragmentation pattern for Ar+ C H 3 " H z collisions at 1.8-eV (center of mass) collision energy. Each spectrum has been normalized for pressure. At higher masses, the 10 times magnification of each signal is inset.
+
111. Results and Discussion
+
The observed product ions in the X+ CH3NHNH2 reactions fall into three mass ranges: (A) 15-18, (B) 27-31, and (C) 44-47 m u , corresponding to compounds containing a single carbon or nitrogen atom (or H20+, as will be discussed), compounds with either two nitrogens or a carbon and a nitrogen atom, and compounds in which neither the C-N nor the N-N bond has been broken. For each primary ion in this work, there is a mass range for which the product ion signals are at least partially obscured in the apparatus by the presence of the much more intense primary ion beam. For the NO+ reaction, this is not a significant problem; however, for the other primary ions, the total measured cross sections are only lower limits to the actual total reaction cross sections. Because this study deals with the final reaction product fragmentation patterns, the singlecollision requirement is paramount, and any secondary reactions (following the initial charge tranfer) must be minimized or eliminated. In previous work11*16 on the charge transfer from O+ to H20, the following secondary reaction products were observed: H2O+ (symmetric charge transfer at near-thermal collision energy) and H3O+. To determine the total H20+ formation cross section, the H20+ and H3O+ signals were added together, eliminating the effect of the secondary reaction. The secondary reactions occur at near-thermal collision energy and are the result of product ions that have a small forward velocity component in the laboratory frame. Such ions have a very long trajectory length in the collision region before exiting the first octopole. The chief effect of such secondary reactions in the current study would be to produce stable (nondissociated) CH3NHNH2+, either through charge transfer between the initial ionic product and a neutral CH3NHNH2 molecule or through collisional de-excitation of the CH3NHNH2+* internal energy that results from the reaction exothermicity. To test the presence of secondary reactions, the C H 3 " H z pressure dependence of Ar+ CH3NHNH2 was measured at a collision energy of 1.8 eV (center of mass). The argon ion was chosen for this test because it provides 2 eV more exothermicity in the charge-transfer channel than does O+, thus enhancing fragmentation of the charge-transfer product. The low-resolution mass spectra, each representing identical acquisition times, have been normalized for pressure and are shown in Figure 1. The signal corresponding to CH3NHNH2+ is enhanced at the higher pressures, and the signal corresponding to fragmentation
+
products in the range 27 5 mlz I32 is simultaneously reduced. In addition, above about 0.2 mTorr, which is the typical pressure in the previous H2O work, there is a significant signal mlz 64 that corresponds to the hydrate cation, which is formed without structural bond fragmentation in the near-thermal collisions between the initial ionic products and the CH3NHNHyH20 impurity. At pressures below about 0.10 mTorr, the mlz = 64 signal falls below the noise threshold. Additional pressure dependence tests with O+ as the primary ion agreed well with this Ar+ data. To ensure minimal contributions from secondary collisions, all further measurements were performed with the pressure 10.05 mTorr. At this pressure, a significant CH3NHNH2+ signal is observed that corresponds to Penning ionization by metastable neutral Ar* species, which are formed by ion-surface recombination collisions, presumably in the Wien velocity filter. This product signal is measured by deflecting the Ar+ ion beam prior to the collision region, while monitoring the CH3NHNH2+ product ion signal. At a CH3NHNH2 pressure of 0.032 mTorr, this signal accounts for onethird of the total signal observed in the mlz = 44-47 region. Smaller contributions were also observed at the lower product mass ranges. Because the magnitude of this Ar*-related signal is comparable to the actual product signals under the chosen operating conditions, no further measurements were performed with Arf. No products attributable to similar excited neutrals were observed for any of the other primary ion precursor gases used in this study. NO+ CHaHNHz. The charge transfer between NO+ and CH3NHNH2 is exothermic by only 1.59 eV; therefore, longrange collisions between these partners should yield primarily CH3NHNH2+. The cross sections for the observed products are shown in Figure 2, where the nondissociative charge-transfer product is dominant, with relatively minor contributions from mlz = 45 (CH3N2H2+) and from mlz = 47 (CH3Nz&+, product from the hydrate impurity, as discussed later in this section). The appearance of the mlz = 45 product ion below the dissociative charge transfer threshold of 0.86 eV indicates that either (i) a hydride ion abstraction reaction yields HNO -ICH3N2H2+ - this reaction is exothermic by 1.3 eV; or (ii) this product is formed by metastable NO+. It may similarly be possible to form NzH3' (mlz = 31) through net abstraction of CH3- with an exothermicity of 0.3 eV, through reaction with metastable NO+, or in a direct reaction at collision energies above the endothermicity of 1.38 eV. The NzH3' product ion would be masked by the intense primary ion beam signal; however, the large measured cross section for mlz = 45-47
+
J. Phys. Chem., Vol. 98, No. 45, 1994 11633
Methylhydrazine Reactions with O+ and Other Ions 140
(a) O*
C
0
+
CHaNHNHs
- 55)
in Figure 3b, where the high mass resolution individual cross sections for mass range C are plotted along with the data for mlz = 18. The only observed masses in the mlz = 42-48 range are mlz = 45, 46, and 47. The sudden increase of the mlz = 47 product cross section with collision energy indicates increased formation of products from the hydrate impurity, as discussed below. Alternately, the increase may be an experimental artifact that is caused by the trapping of very slow product ions in the reaction zone octopole. The time-of-flight measurements for this reaction system show the mlz = 47 product to have the smallest observed forward translational energy in the laboratory frame, and the product velocity may be insufficient to overcome a surface potential barrier within the collision zone. The range B high mass resolution data are plotted in Figure 3c for mlz = 28, 29, 30, and 31 individual cross sections. By comparison, mlz = 28, 31, 18, and 47 have similar large cross sections. The mlz = 30 signal is about one-half that of the mlz = 3 1 product; the mlz = 29 signal is about one-quarter that of the mlz = 3 1; and the m/z = 45 and 46 cross sections are very small. At collision energies less than 3 eV, the energy-dependent decrease in the individual cross sections is apparent, but above about 3 eV the cross sections are nominally independent of collision energy. Time-of-flight measurements were performed at several collision energies to determine qualitatively the product ion laboratory frame forward kinetic energy. The measurements were performed at collision energies of 3.4, 5.9, and 13.4 eV, and product ions measured were at mlz = 18,28-31, and 4547. In all cases where the product ion formation cross section was significant, the time-of-flight measurements showed that '90% of the product ions were formed with thermal or nearthermal kinetic energy, as is typical for efficient reactions that proceed at long range (e.g., exothermic electron-transfer reaction). The product ion signals are inherently small, because of both the pulsed mode that is required for this type of experiment and the small individual product cross sections; therefore, a detailed product kinetic energy analysis is not performed here. CO+ CHsNHNH2. The cross sections for the observable mass ranges in the CO+ CH3NHNH.2 reaction are plotted along with their sum in Figure 4a. The CO+ beam prevents observation of likely products at mlz = 28 and 29. The mlz = 30 and 3 1 cross sections were measured in a series of maximum quadrupole resolution scans and therefore exhibit much greater noise than the other standard-scan data. In particular, above about 5-eV collision energy the mlz = 30 and 3 1 data both show a significant increase that is more likely to be in error, because of the very low signal-to-noise ratio, than it is to be a true enhancement of those cross sections. In Figure 4b, the observed products in mass range C are plotted with the measured mlz = 18 signal, which is attributed to H20+ and is a remnant of the hydrate impurity. For this reaction pair, the nondissociated charge-transfer product is significant. In Figure 4c the mass range A products and the mlz = 30 and 31 products are plotted. Interestingly, mlz = 17 is a major product, being similar to the mlz = 31 and 46 cross sections, while the mlz = 16 and 30 cross sections are smaller, and the mlz = 45 signal is the smallest significant observed product signal. The mlz = 17 product ion is attributed to NH3+, which is likely to be formed through an intermediate isomer CH3N-NH3+. In the hydrazine case, the corresponding NH-NH3+ isomer is found at 1.2-eV higher energy than the D2h HzN-NHz+ ground-state configuration." Although no calculations appear in the literature for CH3" H 2 + isomers, the analogous rearrangement to CH3N-NH3+ is plausible. The other possible product at mlz = 17 is OH+; however, its formation is endothermic in the reaction of CO+
.E 1 u(emu 27
~ ~ ~ " 5" " ' " " 10 " " ' "15' ' ' " 20
Colltslon Energy, E..,
25
(rV)
+
W
C
0
E
y 20
w
n I $10
o ~ " " " " 5 " " " " "10" " " '
15
Collision Energy, E.,
20
25
(rV)
+
Figure 3. Energy dependence of the O+ CH3NHNH2 (a) low and (b, c) high mass resolution cross sections.
makes it unlikely that the mlz = 31 ion, if formed, contributes significantly to the total reaction cross section. O+ CHJNHNH?. Measurements for the O+ +CH3NHNH2 system show a large overall reaction cross section and the formation of several ionic products. The total observed cross sections for the mass ranges B (mlz = 27-31) and C (mlz = 44-47) are plotted with their sum in Figure 3a. Products in the mass range of mlz = 15-17 were not observable because of the intense O+ ion signal. The range B masses dominate the total cross section; Le., the majority of products are formed through cleavage of the N-N or the C-N bond. A prime feature in the range C mass data is the large upward spike near 1 eV. This spike is attributable to the mlz = 47 signal, as shown
+
+
Gardner et al.
11634 J. Phys. Chem., Vol. 98, No. 45,1994
Total obrrnrd e 40
3 10
5
15
Collision Energy, E.,
20
k
..
\-.
25
(eV)
10 r n
~ ~ ~ " 5' " " " 10" " " ' 15' ' ' ' *20 Collision Energy, E..,,. (eV)
+
I
+
Figure 4. Energy dependence of the CO+ CH3NHNH2 (a) low and (b, c) high mass resolution cross sections.
Figure 5. Energy dependence of the COzf CH3NHNHz (a) low and (b, c) high mass resolution cross sections.
with CH3NHNHyH20; while NH3+ formation is 3.9 eV exothermic in the CO+ CH3NHNH2 reaction. C02+ CHaHNH2. The cross sections for the observable mass ranges in the C02+ CH3NHNH2 reaction are plotted along with their sum in Figure 5a. The CO2+ ion beam obscured the probable mlz = 45 products. The mlz = 46 and 47 cross sections were measured in a series of maximum quadrupole resolution scans and therefore exhibit much greater uncertainty than the low mass resolution data. Figure 5b contains the high mass resolution data for mlz = 17, 18, 46, and 47. In Figure Sc, the observed products in mass range B are plotted. Once again, the mlz = 31 signal has the highest formation cross section for product ions in this mass range. Comparing this
data, the nondissociated charge-transfer product, the two hydrate products (mlz = 18 and 47), and the m/z = 31 product are seen to each have relatively large formation cross sections. The remaining ions are formed with decreasing cross sections in the order mlz = 28 > 30 =- 17 > 29. The m/z = 17 cross section is smaller by a factor of 2 than in the CO+ case and is consistent with both the generally lower degree of fragmentation in the CO2+ case and a decreased efficiency in accessing the higher energy intermediate state required to produce NH3+. Finally, very small signals are seen for m/z = 16 and 15, but these cross sections are smaller than the m/z = 29 cross sections. Methylhydrazine Hydrate Impurity. A series of experiments was performed with a methylhydrazine sample with a high water
+
+
+
Methylhydrazine Reactions with O+ and Other Ions
J. Phys. Chem., Vol. 98, No. 45, 1994 11635 N
E 250 V
-
0
W
200
C 0
.c
b
+
CH3NHNH2
I
.
X+
6: 150
v, v) u)
E
100
-0
>
:50 n 0
28
29
30
31
45
46
50
47
mh Figure 6. Bar graph of the average cross sections for O+ CH3NHNH2 in the 2-10 eV collision energy range with high and low hydrate impurity levels.
+
b
0
~
.
.
.
.
o
.
.
.
5
l
.
.
.
.
10
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,
.
.
.
,
15
,
20
,
.
,
]
25
Collision Energy, Ec.m. (eV)
Figure 8. Comparison of the total observed cross sections for each of the primary ions in this work. TABLE 2: Comparison of Electron Impact Mass Spectrum (EIMS)of CHJNHNH~with the Observed Abundances of Reaction Products for O+, CO+, Cot+,and NO+ Reactiond abundances % obsd signal
-100 N
5
-
80
I
0 W
a u 40
t...
I
1
I
.
.
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I
.
.
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.
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.
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.
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.
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1
.
,
5
4
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.
I
Collision Energy, Ec.m. (eV)
Figure 7. Charge exchange cross sections for CO+ + H20 and C02+
+ H20 reactions at collision energies of 0.2 < E,
< 7 eV.
contamination (%20% mole fraction). The average cross sections in the O+ CH3-2 reaction over the 2 < Ecm < 10 eV range are shown in Figure 6 for the low and high hydrate impurity levels. As the hydrate level increases, the only measured product ion that shows a significant cross section increase is CH3N2I&+ ( d z = 47). Similar increases are seen for the m/z = 18 peak in both the C02+ and the C02+ cases; thus, the peak is assigned as H2O+.l8 Formation of H20+ from CH3NHNH2*H20 requires charge transfer coupled with hydrogen bond cleavage in the hydrated complex. Because the ionization potential of CH3I"H2 (7.67 eV) is significantly lower than the ionization potential of H20 (12.61 eV),8 one would anticipate preferential ionization of the CH3NHNH2. If that occurred exclusively, then the observed H20+ signals could result from charge transfer to free water vapor in the collision cell. To ascertain whether the observed H20+ signals are direct remnants of the methylhydrazine hydrate or, rather, charge transfer to free water vapor, the cross sections for charge transfer from O+,CO+, and C02+ to H20 are considered. The O+ H20 charge exchange has been extensively studied in this laboratory and has a reaction cross section of 30-32 A2 over the collision energy range 1.5 < Ecm < 10 eV.19 The CO+ H20 and C02+ H20 charge exchange reactions have been measured and are reported here for the first time. The cross sections are plotted in Figure 7 for these reactions over the collision energy range 0.2 < Ecm < 7 eV. The CO+ H20 reaction cross section is approximately the same value as.observedlgin the O+ H20 reaction, and the
+
+
+
+
+
+
mJz
EMS
O+
CO+
C02+
NO+
46 45 44 43 42 41 31 30 29 28 27 17 16 15
30.3 20.1 0.8 2.2 1.3 0.4 12.8 4.8 7.1 17.6 1.2
7.3 4.4
18.7 5.3
31.1 ? ? ? ?
84.7 15.3
---
---
29.0 9.4 4.2 19.7
---
? ? ? ? ?
4.2 0.9 1.5
---
---
---
---
31.6 17.7 9.0 29.9
30.1 15.1 ? ? ? 19.5 9.8 1.5
---
---
? ?
1.5
?
---
* Values for this work are averaged over &.,,. = 2-15 eV after eliminating the hydrate products at mlz = 18 and 47. Abundances are in percent of total observed signal; - - - signifies that no products were observed at a mJz value; and ? denotes product masses that were obscured by the intense primary ion beam.
+
C02+ H20 reaction cross section is one-half to two-thirds that of the CO+ H20 reaction above 1 eV. In contrast, the m/z = 18 formation cross sections for O+, CO+, and C02+ reactions with CH3NHNH2 (averaged over the collision ener y range 1 < Em < 10 eV) are approximately 20, 3, and 7 respectively. The lack of correlation between the two sets of cross sections shows that the majority of the m/z = 18 signal cannot be attributed to free water vapor in the CH3NHNH2 sample and target cell. The large (20 A*) value for the O+ CH3"H2 case, compared with the smaller values for the molecular ion cases, suggests that charge transfers to (along with fragmentation of) the methylhydrazine hydrate moiety is responsible for the majority of the m/z = 18 signal in the O+ case. Methylhydrazine Ion Fragmentation. The total cross sections for CH3"H2 reactions with NO+, O+, CO+, and C02+ are plotted together in Figure 8. The fragmentation pattern of the CH3NHNH2+ charge-transfer product is highly dependent on the identity of the primary ion. The patterns for the ion-CH3NHNH2 reactions are compared in Table 2 with the 70-eV electron-impact mass spectrometer (EIMS) data for CH3NHNH2 As is the case for the EIMS that was reported by Dibeler et data, the fragmentation patterns in the ion-CH3NHNH2 reactions are consistent with Rosenstock's model2' for the initial
+
f2, +
Gardner et al.
11636 J. Phys. Chem., Vol. 98, No. 45,1994 formation of an excited molecular ion, which lives at least several vibrational periods and possibly several microseconds, and distribution of the excess energy into several internal modes prior to fragmentation. For Table 2, the mlz = 18 and 47 hydrate products were eliminated, and the abundances reported are the percent of observed product signal, averaged over the collision energy range 2 5 E,, 5 15 eV, for the ions that are fragmentation products of CH3NHNH2+. Dashes indicate that no product was observed for a particular mass; question marks indicate those potential product masses that were obscured by the intense primary ion signal in each case. Although the incomplete data sets make a full comparison impossible, there are some clear trends in the fragmentation data for the different ions. First is the stabilization of the CH3NHNH2+ parent ion with increasing number of internal degrees of freedom in the primary ion. This effect was also observed in the hydrazine case: the greater number of internal modes in the molecular primary ions allow these species to share in the partitioning of the reaction exothermicity, Le., the electron transfer occurs into an internally excited CO+* (C02+*) state. Similar charge transfer involving highly excited molecular products has been seen in charge transfer to HzO, where electronically and highly vibrationally excited HzO+ is formed.15 In the current work, the excess energy imparted to the initial CH3NHNHz*, along with the resulting fragmentation, is greatest for the O+ reaction and least for the CO2+ reaction. The prominence of the mlz = 28 and 31 peaks is also clear in each of the data sets, including the EIMS data. Both peaks also exhibit a collision energy dependence that is typical of an exothermic reaction. The mlz = 28 peak is most likely HCNH+, which is easily formed through an H2 elimination from the intermediate CH3NH+. This reaction channel has an overall exothermicity of 2.8 eV in the O+ CH3NHNH2 reaction. In contrast, the other candidate ion at mlz = 28 is N2+, which can only be formed by cleaving the C-N bond and all three N-H bonds. Even if this is accomplished by elimination of H2 and CH4, this pathway is slightly endothermic in the O+ CH3NHNH2 reaction. The lack of a significant mlz = 28 product in the ion-hydrazine reactions’ corroborates the assignment of the mlz = 28 peak to HCNH+. The mlz = 31 peak is assignable to N2H3+, with the CH3 radical being the initial neutral coproduct.22 Comparing this peak with the CH3NzH2’ (mlz = 45) signal, one observes that the mlz = 45 peak is larger in the EIMS data but much smaller for the O+ and CO+ cases; Le., breaking the C-N bond is strongly favored over the cleavage of just one hydrogen bond. The prominence in the current data of the mlz = 30 signal, which includes the CH3NH+ NH2 product channel, suggests that breaking either of the structural bonds is preferred over loss of a single hydrogen.
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+
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IV. Conclusion Reaction cross sections are reported for hyperthermal collisions of methylhydrazine with several ions. In each case, several product ions are formed via charge exchange followed by fragmentation that is governed by the exothermicity and the ability of the neutralized primary moiety to share in the energy partitioning. The Of CH3NHNH2 reaction is important in the plasma environment of low earth orbital spacecraft. The
+
large total reaction cross section also shows that this reaction efficiently converts “fast” atmospheric O+ to “slow” product ions in the orbital environment, thus increasing the spacecraft local plasma ion density. In addition, this reaction produces a significant mlz = 28 product signal, and is therefore a possible source of the mlz = 28 product ions that have been reported in the space shuttle environment.
Acknowledgment. This work is supported by the United States Air Force Office of Scientific Research under Task 2303EP2. We thank E. Murad for helpful discussions. References and Notes (1) Gardner, J. A.; Dressler, R. A.; Salter, R. H.; Murad, E. J . Phys. Chem. 1992, 96, 4210. (2) Handbook of Geophysics and the Space Environment; Jursa, A. S., Ed.; ADA 167000; National Technical Information Service: Springfield, VA, 1985. (3) Hoffman, R. J.; Kawasaki, A.; Trinks, H.; Bidermann, I.; Ewering, W. The CONTAM 3.2 Plume Flowfield Analysis and Contamination Prediction Computer Program: Analysis Model and Experimental Vesication. In AIAA 20th Thermophysics Conference, AIAA-85-0928, Williamsburg, 1985. (4) Trinks, H.; Hoffman, R. J. Experimental Investigation of Bipropellant Exhaust Plume Flowfield, Heating, and Contamination and Comparison with the CONTAM Computer Model Predictions. In Spacecrafl Contamination: Sources and Prevention; Roux, J. A., McCay, T. D., Eds.; AIM: New York, 1984; pp 261-273. ( 5 ) Ehlers, H. K. F. An Analysis of R e m Flux from the Space Shuttle Orbiter RCS Engines. A I M 22nd Aerospace Sci. Mtg., AIAA-84-0551, Reno, 1984. (6) Ehlers, H. F. K. J . Spacecrafi Rockets 1986, 23, 379. (7) Machuzak, J. S.; Burke, W. J.; Retterer, J. M.; Hunton, D. E.; Jasperse, J. R.; Smiddy, M. J . Geophys. Res. 1993, 98A, 1513. (8) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref. Data 1988, 17, Suppl. 1. (9) Exothermicity calculated from data in ref 8 and from: Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Re$ Data 1977, 6, Suppl. 1. (10) Gardner, J. A,; Dressler, R. A.; Salter, R. H.; Murad, E. Geophysics Laboratory (AFSC) Technical Report 1989, GL-TR-89-0345. (11) Dressler, R. A,; Salter, R. H.; Murad, E. J . Chem. Phys. 1993, 99, 1159. (12) Teloy, E.; Gerlich, D. Chem. Phys. 1974, 4 , 417. (13) Gerlich, D. In State-selected and State-to-state Ion-molecule Reaction Dynamics, Part I; Baer, M., Ng, C. Y., Eds. Chem. Phys., 1992, 82. (14) CRC Handbook of Chemistry and Physics, 60th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1980. (15) Dressler, R. A,; Gardner, J. A.; Salter, R. H.; Murad, E. J . Chem. Phys. 1992, 96, 1062. (16) Dressler, R. A.; Salter, R. H.; Murad, E. Chem. Phys. Lett. 1993, 204, 111. (17) Frisch, M. J.; Raghavachari, K.; Pople, J. A.; Bouma, W. J.; Radom, L. Chem. Phys. 1983, 75, 323. (18) The other candidate ion at m/z = 18 is N&+, which can only be formed via endothermic pathways, the most favorable of which requires a minimum T E transfer of 0.9 eV. The m/z = 16:18 separation in this data set was insufficient to determine a significant change in the H20+ signal for the O+ CHsNHNH2 reaction. (19) Dressler, R. A.; Salter, R. H.; Murad, E. Planet. Space Sci. 1992, 40, 1695. (20) Dibeler, V. H.; Franklin, J. L.; Reese, R. M. J. Am. Chem. Soc. 1959, 81, 68. (21) Rosenstock, H. M.; Wallenstein, M. B.; Wahraftig, A. L.; Eyring, H. Proc. Natl. Acad. Sei. U.S. A. 1952, 38, 667. (22) The other possible product at m/z = 31 is CH3NH2+, which may be formed via an intemal rearrangement accompanied by cleavage of the N-N bond. In the previous work with hydrazine,’ such rearrangement products could be unambiguously assigned but were found to be minor products.
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