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Rate constants and branching ratios for the reactions of 18O+ and 18O18O+ ions with 16O16O measured at relative kinetic energies 0.04–eV
Volume 75. number 3
RATE CONSTANTS
AND BRANCHING
FOR THE REACTIONS MEASURED Itzhak
1 November 19gO
CHEMICAL PHYSICS LETTERS
RATIOS
OF l*O+ AND 1*01*0+
AT RELATIVE
KINETIC
IONS WITH 1601’50
ENERGIES
0.04-2
eV
DOTAN
Isotope Departtnent.
The Wemnatm Institute of Science. Reirovot, Israel
Received 2 July 1980, III final form 1 August 1980
Product distribution ured. The study
has
been
and energy dependences of the reacbons of IsO+ and 18CI’80+ ions with *60’60 have been meascarried
out
in a selected-ion
channel all over the energy range studred (0.04-2
flow-drift
I_ Introduction Largely because of its importance in the earth’s ionosphere [I], the reaction of O+ Ions wrth 0, has been studied extensively in many laboratories using a variety of experimental techniques [2--121. The pnmary goal m these investigations has been to measure the rate constant for the loss of Of ions. As a result of these efforts, the rate constant for this reaction 1s now well established as a function of both temperature (80-900 K) and relative kinetic energy (20 04 eV). The use of isotoplcally labelled 0’ ions can cbstmgulsh three different O+ loss processes for th.rs reaction: 180+ + 160160
+. 160160+
+ 180,
(la)
+ 1601*0+
+ 160 ~
(lb)
+ 160+ + 16@30
_
tube.
The
results
show
that
for
both
reactions
the
main
eV center-of-mass kinetic energy) is the charge transfer.
(Id
Such studies can be a useful technique for elucidatmg reaction mechanisms (e.g. ref. [ 131 and references therein) and an investigation of reactions (la)-(lc) would be a complement to the total-rateconstant studies. Some effort in this regard has already been done. Fehsenfeld et al. [ 141 using the flowing afterglow have placed an upper limit on channel (lc) at 300 K. Paulson [ 15 1, using a tandem mass spectrometer measured channels (la) and (lb) as a function of kmet-
rc energy between 0.47 and 9.33 eV center-of-mass kinetic energy. The present study reports the product ratios of three channels of reaction (1) from 300 K to relative kinetic energes up to 2 eV. As a supplement, the d~atomic analogue of reaction (I), 1*(y*(-j+
+ 160160
--f 16016(-y
f 180180,
+ 16(ySo+
+ 16(y80
CW )
(=I
has been examined over the same energy range and the rate constants are also reported here.
2. Experimental
apparatus and techniques
The present measurements were made with a new selected-ion flow-drift tube, which has been described in detail elsewhere [ 161. Basically, the apparatus represents the addrtion of mass-selected ventun intet ion injection [17] to an existing flow-drift tube [IS]. Its operation is summarized as follows. The ‘*O+ and I*OL80+ ions were created in a hollow-cathode kharge source [ 191 containing helium and a trace of heavy oxygen, the latter being from the Weizmann Institute’s enrichment plant. The ion mixture produced in the source was mass analyzed by a quadrupole filter and thence entered the flow tube through a venturi inlet. The critical feature of this mass-selected 509
CHEMICALPHYSKS LETTERS ion source that permits the present study is that no 180180 source gas enters the flow-drift tube, thereby avoiding the compIicat~ng “back reactions” of the product ions of reactions (1) and (2) with t80’80. Once in the flow tube. the helium buffer gas carries the l8O’ or t8Ot80+ ions into the drift-reaction section of the apparatus, where arr axial dc electric field is maintained. The ions rapidly equiljbratc to an average kinetic energy that is greater than thermal and whose magnitude can be calculated accurately from the ion’s mobility in the helium gas [ f 8,20-241. After confirming that the isotopic differences were ne~i~ble for the present purposes, the mobilitics used herein were those measured earlier in the present apparatus for t60” and 160160” in helium 1251. The rate constants were obtained in the usual flow-reactor fashion 120,261 by monitoring the decline of the primary ion signal as a function of the t60f60 addition rate, with the electric field strength being varied as a parameter. The speed distribution for 0” ions drifting in a He buffer has been calculated by Lin and Bardsiey [23]. It has beet) found that this speed distribution is quite close to a maxwellian. The Wannier approximation used here for ~~IcuI~tin~ the mean energy [ 171 has been shown cxpcrimentaify 191 and theoretically [20-231 to yield renlarka~;ly accurate (< f 0%)values of the mean energy. With only one type of primary ion in the flow tube, either f*O+ or t*@*o+, the product ratios of channcfs (la)--(lcf could be determined strai~tforwar~y from ion-signal balance, i.e., the decline of the reactantion signal being equal to the increase in the sum of the prlldu~t-ii~n signals. Since ne~jgib~e ~mpling discrimin~rtion occurs hetwcen masses 16 and I8 and between III:ISSCS 32 and 36, ion-signal balance es~blishes for reaction II ):
where A denotes the irlcrease in an ion signal and CYis ill? sc~ught4ffer Silmpiing discrimination belween the Ilphtcr alld ltsavicr nusses. Tflis factor was a function rtf clcitric Ii&l strength arid helium pressure, varying between I .Oand 2.3. Samplingorifice currents and
~our;f-rate ratios 1171 gave e5sentiaIly equal values for a. Secondary reactions occur,between some of the product ions and the neutral reactant t60160: 16o’80+ + IGO
510
-_r16016o+ * lS()lSO,
(4)
160+ + 160160 3 160160+ + 160 .
I November 1980 (5)
These reactions slightly jlter the product-eon signals. Account was made of this small effect (a few percent) by extrapolating the product ratios to zero 1601a0 flow (see ref. [t 71 for examples). The ion source conditions could be set so as to avoid production of excited-state ions, t80~(~D and/or @). These ions are known to be produced in reactive-helium sources [27,283 and, even if present at the several percent level, their fast reaction rates with l6Ot60 [ 1 I ] would aIter the apparent product ratios. Since reaction (i) is known to be relatively slow for ground-state 0” ions, the abssdrz of an apparent fast rate for reaction (1) estabtish~~dthat excited-state O+ ions were absent. Similar tests with an NZ neutral reactant were made for reaction (2) [ 11,291. We estimate that the uncertainty in the branching ratios reported here is 210% and that for the rate constant is t30%.
3. Results For reaction (I), the product ratios were measured the relative ~netic~ner~ range 0.04-Z eV, aud the results are given in fig. I. The major loss process is channel (la), charge transfer. Channels (I b) and (lc), isotopic scrambling and atom transfer, are minor loss processes, being at the few percent level. Channels (I a) and flc) appear to exhibit slight energy dependences. The overall rate constants obtained for reaction (1) (not presented here) agreed with the well-known previous values for reaction (5) over the whole energy range [6,9]. Since channel (fc) does not exist without isotopic labelling, the rate constants of reaction (1) are expected to be about 10% faster than those of reaction 6). The total rate constants obtained for reaction (2) are given in fig. 2 for the relative kinetic energy range 0.04-I .7 eV. The rate constants increase from a 0.04 eV value of 2.7 X iO_to cm3 molecule-t s-I to a value of 8.8 X IO-tocm3 molecule-t s-t at 1.7 eV. Charge transfer was the only channel observed. over
Volume 75, number 3
.o
Present Selecled-Ion
‘; 1+ 60
-
l o 0.30
; z
-
Other
Re
Results:
Tube
,,
0 P Tandem 4O -
Flow-Drlff
iorr He 0 0.45 fort He
.
aD
CHEMICAL PIfYSICS LETTERS
T
0.03
Flowing
0.1
MOSS Spectromeler~ Afterglow
03
I
0.6
3
6
IO
KE,,ieV) 1. Branching ratios of the reaction of 180+ ions with ‘60160 PSa function of relative kinetic energy. Tandem mass spccttometet Corn ref. [ 1.51. Flowingafterglow from ref. [ 141. I;ig.
.
*
. .
1 November 1980
a function of translational energy in a tandem mass spectrometer. He did not look for channel (1~). His results, as can be seen in fig. 1,are in good agreement with the present data. If the mechanism of reaction (1) where isotopic scrambling via a long-lived complex, then the ratio of channels (la) and (lb) would have been k,,/k,, = l/2. The observed ratios vary from 5/f at the lowest energy to 17/J at the highest energy, indicating the major role played by the charge-transfer mechanism. The isotopicscrambling mechanism does play a small role at the lower energies, but it is almost non-existent at the higher energies. This decrease is consistent with the past interpretation of the energy dependence of the total rate constan ts of the reaction of O+ with 0, [ 181,which show a minimum in the vicinity of 0.25 eV [6,8,9]. This minimum has been interpreted as a decreasing role of complex formation as the relative kinetic energy increases from 0.04 eV. The product ratios of channels (la) and (1 b), with their increases and decreases, respectively, with increasing kinetic energy, support that interpretation. Channel (Jc) shows almost no energy dependence all over the range studied. One may conclude from this kind of behavior that this channel proceeds via some kind of a direct mechanism and does not compete w!th the other two channels. For reaction (2), the fact that only charge tl,ansfer was observed is not unexpected. No isotopic scrambling was reported in studies of similar reactiops envolving N2, CO and CO, [30]. Furthermore, other four-center reactions, such as 0;+N2+NO++N0,.
r?
$ KE,,(eV) Fig. 2. Rate constants
ofthe reaction
160’~0
of relative kinetic energy.
as 3 function
of 180’RO+
ions with
do not occur 1311. despite their exothermicity. In conclusion, wc showed in this study that the use of isotopically labelled reactants in the SIFT-drift tube is a powerful technique for deducing mechanisms ami branching ratios of ion -molecule reactions. Further studies are planned on systems like O+ +NO and O+tCO.
4. Discussion The results of earlier studies of reactiorl (1) are also shown in fig. I. Fehsenfeld et al. [ 141 placed an upper limit of I X 1 O-l2 cm3 molecule-1 s-l for reaction (lc), which is an upper limit of 5% for the product ratio. As indicated, this is consistent with the present study. Paulson [IS] measured reactions (I a) and (1b) as
Acknowledgement This research was supported by a travel grant from the Minna-James-Heinemann-Stiftung, for which J am grateful. J acknowledge the excellent hospitality of Drs. D.L. AJbritton and F.C. Fehsenfeld during my
Volume 75, number
3
stay in Boulder, and their invaluable
CHEMICAL PHYSICS LETTERS
lscussions
and
support during alt stages of tlus work.
References
[21 131 141 t51 161 r71 181 191
1101 111: t121
E.E. Ferguson. EC. Fehsenfeld and D.L Albntton. in Gas phase ion chemistry, Vol. 1, ed hI.T. Bowers (Academic Press, New York, 1979) p_ 45. M.H. Copsey, D. Smith and J. Sayers, Planet Space Sa. 14 (1966) 1047. R.E Stebbings. B.R. Turner and J.A. Rutherford, J. Geophys. Res. 71(1966) 771. D. Srmth and R.A. Fouracre. Planet. Space Set. 16 (1968) 243. E.E. Ferguson, D.K. Bohme. F.C. Fehsenfeld and D.B. Dunkm, J. Chem. Phys. 50 (1969) 5039 R Johnsen and M A. Bromh, J. Chem Phys. 59 (1973) 3504. W. Lindinger, F-C. Fehsenfeld, A.L. Schmeltekopf and E.E. Ferguson, J. Geophys. Res. 79 (1974) 4753. R.G. Kosmider and J.B. Hasted, J. Phys. B 8 (1975) 273. D.L. Albntton, 1. Dotan, W. Liidinger, hf. McFarland, J. Tellinghuisen and F.C. Fehsenfeld, J. Chem. Phys. 66 (1977) 410. D. Srmth, N.G. Adams and T.M. Mffler, J. Chem. Phys. 69 (1978) 308. 3. Glosik, A.B. Rakshit, N.D. Twrddy, N.G. Adams and D. Smith, J. Phys. B 11 (1978) 3365. k chep, R Johnsen and M-A_ Biondi, J. Chem Phys. 69
(1978) 2688. (131 1. Dotan, Z. Karpas and F.S. Klein, intern. J. h1as.s Spectrom
512
Ion Phys., to be pubhshed.
1 November
1980
[14] F-C. Fehsenfeld, D.L. Albrrtton. Y-A. Bush, P.G. Fournier, T-R. Govers and J. Foumier, J. Chem. Phys 61(1974) 2150. [15] J. Paulson, pnvate communicahon (1979). f 16) F. Howork F-C. Fehsenfeld and D-L. Albritton, J. Phys B, to be pubhshed. [ 17) D. Snuth and N.G. Adams, m’ Gas phase Ion chemistry, VoL 1, ed. M T. Bowers (Academic Press. New York, 1979) p. 1. M. McFarland, D.L. Aibritton, F.C Fehsenfeld, E.E. Ferguson and A.L. Schmeltekopf, J. (Them. Phys. 59 (1973) 6610,662O. 6629. 1191 F. Howorka, J. Chem. Phys. 69 (1978) 804. WI H.R. Skullerud, J. Phys. B 6 (1973) 728. tzt1 L.A. Vrehhmd, E.A. Mason and J.H. Whealton, J. Phys. B7 (1974) 2433. WI S-L. L.in and J.N. Bardsley, J. Phys. B 8 (1975) L461. 1231 S.L. Lm and J-N. Bardsley, J. Chem. Phys 66 (1977) 435. reactions r241 D.L. Albritton, in- Kinetics of ion-molecule (Plenum Press, New York, 1979) p_ 119. I251 W. Lmdmger and D-L. Albritton, J. Chem Phys. 62 (1975) 3517. WI E.E. Ferguson, F.C Fehsenfeld and A.L. Schmeltekopf, Advan. At. Mol. Phys. 5 (1969) 1. 1271 G. Mauclarre, R. Derai, S. Fenustein, R. Marx and R. Johnsen. J. Chem. Phys. 70 (1979) 4023. [281 D G. Lister. A-B. Rakshrt, M. Ticky, K. Bukmshaw and N.D. Twrddy. J. Phys. B 12 (1979) 2947. Wl W. Lindmger, D.L. Albritton, hf. hfcFarland, F.C. Fehaeddd, kL SchmeItekopf and EX_ Ferguson. J. Chen 1301
Phys. 62 (1975) 4101. T-B. Mchkahon, P.G. hlaisek
andJ.I_._Beauchan,p,Intern_
I. hfass Spectrom. Ion Phys. 21(1976) 63. [311 E.E. Ferguson, F.C. Fehsenfeld, P.D. Goldan and A.L. Schmeltekopf, J. Geophys Res. 70 (1965) 4323.
RATE CONSTANTS
AND BRANCHING
FOR THE REACTIONS MEASURED Itzhak
1 November 19gO
CHEMICAL PHYSICS LETTERS
RATIOS
OF l*O+ AND 1*01*0+
AT RELATIVE
KINETIC
IONS WITH 1601’50
ENERGIES
0.04-2
eV
DOTAN
Isotope Departtnent.
The Wemnatm Institute of Science. Reirovot, Israel
Received 2 July 1980, III final form 1 August 1980
Product distribution ured. The study
has
been
and energy dependences of the reacbons of IsO+ and 18CI’80+ ions with *60’60 have been meascarried
out
in a selected-ion
channel all over the energy range studred (0.04-2
flow-drift
I_ Introduction Largely because of its importance in the earth’s ionosphere [I], the reaction of O+ Ions wrth 0, has been studied extensively in many laboratories using a variety of experimental techniques [2--121. The pnmary goal m these investigations has been to measure the rate constant for the loss of Of ions. As a result of these efforts, the rate constant for this reaction 1s now well established as a function of both temperature (80-900 K) and relative kinetic energy (20 04 eV). The use of isotoplcally labelled 0’ ions can cbstmgulsh three different O+ loss processes for th.rs reaction: 180+ + 160160
+. 160160+
+ 180,
(la)
+ 1601*0+
+ 160 ~
(lb)
+ 160+ + 16@30
_
tube.
The
results
show
that
for
both
reactions
the
main
eV center-of-mass kinetic energy) is the charge transfer.
(Id
Such studies can be a useful technique for elucidatmg reaction mechanisms (e.g. ref. [ 131 and references therein) and an investigation of reactions (la)-(lc) would be a complement to the total-rateconstant studies. Some effort in this regard has already been done. Fehsenfeld et al. [ 141 using the flowing afterglow have placed an upper limit on channel (lc) at 300 K. Paulson [ 15 1, using a tandem mass spectrometer measured channels (la) and (lb) as a function of kmet-
rc energy between 0.47 and 9.33 eV center-of-mass kinetic energy. The present study reports the product ratios of three channels of reaction (1) from 300 K to relative kinetic energes up to 2 eV. As a supplement, the d~atomic analogue of reaction (I), 1*(y*(-j+
+ 160160
--f 16016(-y
f 180180,
+ 16(ySo+
+ 16(y80
CW )
(=I
has been examined over the same energy range and the rate constants are also reported here.
2. Experimental
apparatus and techniques
The present measurements were made with a new selected-ion flow-drift tube, which has been described in detail elsewhere [ 161. Basically, the apparatus represents the addrtion of mass-selected ventun intet ion injection [17] to an existing flow-drift tube [IS]. Its operation is summarized as follows. The ‘*O+ and I*OL80+ ions were created in a hollow-cathode kharge source [ 191 containing helium and a trace of heavy oxygen, the latter being from the Weizmann Institute’s enrichment plant. The ion mixture produced in the source was mass analyzed by a quadrupole filter and thence entered the flow tube through a venturi inlet. The critical feature of this mass-selected 509
CHEMICALPHYSKS LETTERS ion source that permits the present study is that no 180180 source gas enters the flow-drift tube, thereby avoiding the compIicat~ng “back reactions” of the product ions of reactions (1) and (2) with t80’80. Once in the flow tube. the helium buffer gas carries the l8O’ or t8Ot80+ ions into the drift-reaction section of the apparatus, where arr axial dc electric field is maintained. The ions rapidly equiljbratc to an average kinetic energy that is greater than thermal and whose magnitude can be calculated accurately from the ion’s mobility in the helium gas [ f 8,20-241. After confirming that the isotopic differences were ne~i~ble for the present purposes, the mobilitics used herein were those measured earlier in the present apparatus for t60” and 160160” in helium 1251. The rate constants were obtained in the usual flow-reactor fashion 120,261 by monitoring the decline of the primary ion signal as a function of the t60f60 addition rate, with the electric field strength being varied as a parameter. The speed distribution for 0” ions drifting in a He buffer has been calculated by Lin and Bardsiey [23]. It has beet) found that this speed distribution is quite close to a maxwellian. The Wannier approximation used here for ~~IcuI~tin~ the mean energy [ 171 has been shown cxpcrimentaify 191 and theoretically [20-231 to yield renlarka~;ly accurate (< f 0%)values of the mean energy. With only one type of primary ion in the flow tube, either f*O+ or t*@*o+, the product ratios of channcfs (la)--(lcf could be determined strai~tforwar~y from ion-signal balance, i.e., the decline of the reactantion signal being equal to the increase in the sum of the prlldu~t-ii~n signals. Since ne~jgib~e ~mpling discrimin~rtion occurs hetwcen masses 16 and I8 and between III:ISSCS 32 and 36, ion-signal balance es~blishes for reaction II ):
where A denotes the irlcrease in an ion signal and CYis ill? sc~ught4ffer Silmpiing discrimination belween the Ilphtcr alld ltsavicr nusses. Tflis factor was a function rtf clcitric Ii&l strength arid helium pressure, varying between I .Oand 2.3. Samplingorifice currents and
~our;f-rate ratios 1171 gave e5sentiaIly equal values for a. Secondary reactions occur,between some of the product ions and the neutral reactant t60160: 16o’80+ + IGO
510
-_r16016o+ * lS()lSO,
(4)
160+ + 160160 3 160160+ + 160 .
I November 1980 (5)
These reactions slightly jlter the product-eon signals. Account was made of this small effect (a few percent) by extrapolating the product ratios to zero 1601a0 flow (see ref. [t 71 for examples). The ion source conditions could be set so as to avoid production of excited-state ions, t80~(~D and/or @). These ions are known to be produced in reactive-helium sources [27,283 and, even if present at the several percent level, their fast reaction rates with l6Ot60 [ 1 I ] would aIter the apparent product ratios. Since reaction (i) is known to be relatively slow for ground-state 0” ions, the abssdrz of an apparent fast rate for reaction (1) estabtish~~dthat excited-state O+ ions were absent. Similar tests with an NZ neutral reactant were made for reaction (2) [ 11,291. We estimate that the uncertainty in the branching ratios reported here is 210% and that for the rate constant is t30%.
3. Results For reaction (I), the product ratios were measured the relative ~netic~ner~ range 0.04-Z eV, aud the results are given in fig. I. The major loss process is channel (la), charge transfer. Channels (I b) and (lc), isotopic scrambling and atom transfer, are minor loss processes, being at the few percent level. Channels (I a) and flc) appear to exhibit slight energy dependences. The overall rate constants obtained for reaction (1) (not presented here) agreed with the well-known previous values for reaction (5) over the whole energy range [6,9]. Since channel (fc) does not exist without isotopic labelling, the rate constants of reaction (1) are expected to be about 10% faster than those of reaction 6). The total rate constants obtained for reaction (2) are given in fig. 2 for the relative kinetic energy range 0.04-I .7 eV. The rate constants increase from a 0.04 eV value of 2.7 X iO_to cm3 molecule-t s-I to a value of 8.8 X IO-tocm3 molecule-t s-t at 1.7 eV. Charge transfer was the only channel observed. over
Volume 75, number 3
.o
Present Selecled-Ion
‘; 1+ 60
-
l o 0.30
; z
-
Other
Re
Results:
Tube
,,
0 P Tandem 4O -
Flow-Drlff
iorr He 0 0.45 fort He
.
aD
CHEMICAL PIfYSICS LETTERS
T
0.03
Flowing
0.1
MOSS Spectromeler~ Afterglow
03
I
0.6
3
6
IO
KE,,ieV) 1. Branching ratios of the reaction of 180+ ions with ‘60160 PSa function of relative kinetic energy. Tandem mass spccttometet Corn ref. [ 1.51. Flowingafterglow from ref. [ 141. I;ig.
.
*
. .
1 November 1980
a function of translational energy in a tandem mass spectrometer. He did not look for channel (1~). His results, as can be seen in fig. 1,are in good agreement with the present data. If the mechanism of reaction (1) where isotopic scrambling via a long-lived complex, then the ratio of channels (la) and (lb) would have been k,,/k,, = l/2. The observed ratios vary from 5/f at the lowest energy to 17/J at the highest energy, indicating the major role played by the charge-transfer mechanism. The isotopicscrambling mechanism does play a small role at the lower energies, but it is almost non-existent at the higher energies. This decrease is consistent with the past interpretation of the energy dependence of the total rate constan ts of the reaction of O+ with 0, [ 181,which show a minimum in the vicinity of 0.25 eV [6,8,9]. This minimum has been interpreted as a decreasing role of complex formation as the relative kinetic energy increases from 0.04 eV. The product ratios of channels (la) and (1 b), with their increases and decreases, respectively, with increasing kinetic energy, support that interpretation. Channel (Jc) shows almost no energy dependence all over the range studied. One may conclude from this kind of behavior that this channel proceeds via some kind of a direct mechanism and does not compete w!th the other two channels. For reaction (2), the fact that only charge tl,ansfer was observed is not unexpected. No isotopic scrambling was reported in studies of similar reactiops envolving N2, CO and CO, [30]. Furthermore, other four-center reactions, such as 0;+N2+NO++N0,.
r?
$ KE,,(eV) Fig. 2. Rate constants
ofthe reaction
160’~0
of relative kinetic energy.
as 3 function
of 180’RO+
ions with
do not occur 1311. despite their exothermicity. In conclusion, wc showed in this study that the use of isotopically labelled reactants in the SIFT-drift tube is a powerful technique for deducing mechanisms ami branching ratios of ion -molecule reactions. Further studies are planned on systems like O+ +NO and O+tCO.
4. Discussion The results of earlier studies of reactiorl (1) are also shown in fig. I. Fehsenfeld et al. [ 141 placed an upper limit of I X 1 O-l2 cm3 molecule-1 s-l for reaction (lc), which is an upper limit of 5% for the product ratio. As indicated, this is consistent with the present study. Paulson [IS] measured reactions (I a) and (1b) as
Acknowledgement This research was supported by a travel grant from the Minna-James-Heinemann-Stiftung, for which J am grateful. J acknowledge the excellent hospitality of Drs. D.L. AJbritton and F.C. Fehsenfeld during my
Volume 75, number
3
stay in Boulder, and their invaluable
CHEMICAL PHYSICS LETTERS
lscussions
and
support during alt stages of tlus work.
References
[21 131 141 t51 161 r71 181 191
1101 111: t121
E.E. Ferguson. EC. Fehsenfeld and D.L Albntton. in Gas phase ion chemistry, Vol. 1, ed hI.T. Bowers (Academic Press, New York, 1979) p_ 45. M.H. Copsey, D. Smith and J. Sayers, Planet Space Sa. 14 (1966) 1047. R.E Stebbings. B.R. Turner and J.A. Rutherford, J. Geophys. Res. 71(1966) 771. D. Srmth and R.A. Fouracre. Planet. Space Set. 16 (1968) 243. E.E. Ferguson, D.K. Bohme. F.C. Fehsenfeld and D.B. Dunkm, J. Chem. Phys. 50 (1969) 5039 R Johnsen and M A. Bromh, J. Chem Phys. 59 (1973) 3504. W. Lindinger, F-C. Fehsenfeld, A.L. Schmeltekopf and E.E. Ferguson, J. Geophys. Res. 79 (1974) 4753. R.G. Kosmider and J.B. Hasted, J. Phys. B 8 (1975) 273. D.L. Albntton, 1. Dotan, W. Liidinger, hf. McFarland, J. Tellinghuisen and F.C. Fehsenfeld, J. Chem. Phys. 66 (1977) 410. D. Srmth, N.G. Adams and T.M. Mffler, J. Chem. Phys. 69 (1978) 308. 3. Glosik, A.B. Rakshit, N.D. Twrddy, N.G. Adams and D. Smith, J. Phys. B 11 (1978) 3365. k chep, R Johnsen and M-A_ Biondi, J. Chem Phys. 69
(1978) 2688. (131 1. Dotan, Z. Karpas and F.S. Klein, intern. J. h1as.s Spectrom
512
Ion Phys., to be pubhshed.
1 November
1980
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