- Email: [email protected]
Low energy collisions between N2+ ions and N2 molecules
443 and Ion Physics, 16 (1975) 443447 Amsterdam - Printed in The Netherlands
International Journal of Mars Spectrometry Q Ekevier Scientific Publishkg Company,
LOW ENERGY
N. R. DALY Axontic mrst
AND
Weapons
COLLISIONS
BETWEEN
N,+
ICNS AND
N2 MOLECULES
R. E. POWELL Research
received 7 Auwt
Establishment,
Aldermaston
1974; ir final form
11 October
(Gt. Britain) 1974)
ABSTRACT
,4 new technique is described to examine ion-molecule reactions that take Flace in a modified ion source similar to the type described by cermak and Herman. The reaction of N2+ colliding with N, leading to the formation of Nhas been studied. The dependence of the cross-section for the reaction on translation energy and on the energy of the electron-for,ning Nzf was examined. There is an interest in low energy collisions between molecular nitrogen ions and nitrogen molecules that lead to the production of atomic nitrogen ions. This arises because the upper atmosphere above 200 km consists mainly of molecular nitrogen and atomic o;lygen. Free electron de-&t& in these regions are determined by the rates at which atomic and molecular ions recombine with electrons and by the rate of removal cf atomic ions by ion molecular reactions. Recombination of at;-lmic ions is an extremely slow process in relation to the dissociative recombination of molecular ions. Hence any process that leads to the formation of atomic ions is important in determining the steady s-late electron densities at high altitudes. The process that has been studied in this work is N,+ +N,
-+ N+ +N+N,
(1)
This reaction has been studied previously by Lever&al and Friedman [l] using a conventional me of mass spectrometer system. Maier [Z] used a smal1 tandem mass spectrometer to study reaction (1). McGee et al. [3] used a multiple pulse method whereby ion production, ion acceleration and ion reactions occur sequentially within the ionisation chamber of the mass spectrometer and the product ions are separated by time-of-flight methods. The first group reported significantly different results to the other two, with thresholds at much lower energies. This work examines the threshold behaviour of reaction (1) and how the cross-section for it varies with excit&ion of the N, 7 ions.
444 EXPERIMENTAL
Anexperimental technique similar to that of CerrGkand Herman [4] was used in this work. Figure 1 shows the experimental layout of the apparatus employed. A conventional Nier type electron impact ion source A was used to produce the primary ions of nitrogen. A gas cell B was located 2 mm away from the exit slit of the Nier source. GA5 INLET
METASTABLE DETECTOR
ION
Fig. 1. Layout of the experimental apparatus. A represents a Nier type electron impact ion source; B, the gas reaction cell; C, the resolution slit; D. an energy enhancer plate: E. the scintilIator retention plate; W, a glass window.
Ions from the cell A entered the gas cell B through a fine grid. Both the ion source and the gas ccl1 had separate gas systems feeding them with spectrally pure gases Cell B was held at a fixed potential of 6000 V above earth. Cell A could be varied from 0 to 200 V positive with respect to cell B and hence the collision energy for reaction (1) in cell B could be varied. Ions were extracted from cell B, accelerated through a conventional array of focusing plates and analysed by a 12-m radius 90” sector magnet. It is always possible, particularly at low voltages between A and B, that ions formed in cell A will be mixed with ions produced by reactions in cell B. They will then be detected simultaneously with them in any detector located after the resolution slit of the mass spectrometer. To minimise this process an energysensitive ion detector of the type described by Daly et al. [S] was used in this work. After passing through the resolving slit C, ions are accelerated to an electrode D maintained at 4000 V negative to earth. They pass through a slit in D and approach an electrode E that is held at the same potential as the gas reaction cell B. A 1.5-4~1-1
445 diam. disc of scintillator is held in electrode E as shown, and it is vacuum coated with aluminium (60 pg cm-‘) on to which the positive ions can impact. This is done so that the potential of the face of the scintillator can be held at the same potential as the electrode E in which it is located. The operation of the system can now be explained by the way it responds to ions from the two cells A and B. Ions from cell A, which is at a potential positive to B. approach electrode E and impact onto it with an energy equivalent to the potential difference between A and B, since E is at the same potential as B. Since this impact energy is very small, it is insufficient to release photons from the scintilIator on E, and the ions from cell A are therefore not detected_ Ions from B approach very close to E but turn back before they strike it. They strike electrode D with 10 keV energy and release secondary electrons from it. These secondary eIectrons are accelerated into the scintillator at E where they produce light. This light is transmitted through the glass window W which forms part of the vacuum wall and is picked up by the photomultiplier. The photomultiplier can be used to count the ions from cell B or to produce an integrated output_ The small bump on electrode D is to produce an asymmetry in the electric field between D and E and to prevent ions returning down the path towards the magnet. Measurements were carried out to determine the rejection factor for ions formed in A. These showed that when there was 3 V between A and B the rejection factor was 1000 and that it increased with increasing potential between A and B. This system therefore leads to a method of studying ions that are formed in cell B and at the same time rejecting the same type of ioa formed in cell A.
RESULTS
-4ND
DISCUSSION
The excitation function for reaction (1) was obtained by the following method. The N, + ion current from cell A was piotted as a function of the potential difference between cells A and B. This was achieved by raising the potential of electrode E slightly above that of cell A. The N’ ion current from cell 33 was plotted as a function of the potential difference between cells A and B but in this case the potential of- electrode E was the same as that of cell B. The electron energy was held at 20 eV_ The pressure in the gas cell was such that about -1% of the N2 + ions from cell A made collisions in it and produced N+ ions. The N’ ion signal was shown to be linear with pressure of N, in the gas cell. Figure 2 shays the form of the excitation function for reaction (1). It can be seen that the threshold is at about 15 V and that the cross-section rises to a maximum at about 30 eV. Also shown-in Fig. 2 are the results of other measurements made on this reaction_ -AU results have been normalised. Leventhal and Friedman measured a high cross-section, about 2 - lo-l5 cm’, at a very low primary ion kinetic energy, The threshold they observed was at about 1 eV_ McGee et al.
,
5
10
15 N;
ION
ENERGY
20
lV (LAB
,
,
,
25
.
,
,
,
,
,
,
30
1
Fig. 2. Cross-section for reaction N2+-tN2 -+ N’+N+N2 in arbitrary units and normal&d (a) by Leventhal and Friedman, (b) by McGee et al., (c) this work, (d) by Maier.
observed a threshold at 10 eV and a maximum cross-section at 30 eV of lo- ’ 6 cm’_ Maier observed a threshold at about 15 eV and a maximum cross-section of lo-l6 cm2 at 30 eV. The shape of the excitation function that has been measured in this work follows very closely that obtained by Maier. In this work the N,+ ions were produced by 20 eV electrons, whereas Maier’s results refer to an electron ener,7 in the ion source of 19 eV. The primary ion transit time from the ion source to the reaction region is about 1 ps in the apparatus used in this work, and 9 ,YS in Maier’s experiment for the case of a 25 eV ion. In this work N2+ will be formed in the X2x;, A27rU, B2Ci states. Ions in the B2ZT state with a lifetime of about 60 ns will not reach the collision chamber, but N,+ A27rU ions will be transmitted effectively to the chamber since their lifetime is about 12 ,WS.Despite the very significant difference in ion transit time_ from the formation point to the reaction point, between the two experiments, there is close a,oreement on the detailed shape of the excitation function for the reaction. As shown in Fig. 2, very significant differences exist between this work and the work of Leventhal and Friedman. and of McGee et al. It seems that the large cross-section observed at very low energies by the former. and the lower threshold for the reaction observed by the latter workers seem only explicable in terms of the different experimectal techniques used to study the reactionThe variation of the cross-section for the reaction with the energy of the electron beam forming N2 + iors is shown in Fig. 3. No measurements have been made of the spread in electron energy in the Nier type ion source. but it is thought to be about 1 eV. It can be seen that there is a slow rise in the cross-section from just above the ionisation threshold of
447
.
.
.
.
-
.
.
. . .
-
.
.
a
.
. .
15
20
25
ELECTRON
BEAM
ENERGY
30 eV
Fig. 3. Cross-section for the production of N + from cell B relative to that for the production of N1+ from cell A as a function of the electron energy in cell A-
nitrogen up to 22 eV. This can be explained and higher vibrational
levels of the X’Z.,f
by assuming
that the ground state
and A3rtU states are being populated
to about 22 eV and that reaction (1) is proceeding
up
faster because of this.
At about 22 eV there is a significant gradient change corresponding to the onset of some new state of N,+ affecting the reaction rate. Electron impact work by Frost and McDowell [6] shows a strong onset for the formation of N,) C’X:,f at abo-ut 23 eV. However an electron in this state can make an ailowed transition to the ground state so its lifetime is probably short and hence It would not reach
the reaction chamber. It therefore seems likely that Nzf ions must be formed in some long-lived metastable states such as the 4C, or 4AU_
1
ACKNOWLEDGEMENTS
We would like to thank the Director of the Atomic Weapons Research Establishment for permission to publish this paper, and Mr. Trevor Hobson for making the measurements.
REFERENCES 1 J. J. Leventbal and L. Friedman, L Chem- Php_,
46 (1967)
997-
2 W. B. Maier II, J. Chem. Phys.. 47 (1967) 859_
3 T. H. McGee, P. F. Fennelly and M. Henchman, Abslrucfs of Papers of Sixth lnternntionol Conference on the Physics of EIectronic and Atomic Collisions, Cambridge, hriarsachusetts, U.S.A., MIT Press, 1969, p_ 321. 4 V. &tik and Z. Herman, Collect. Czech. Chem. Common., 30 (1965) 169. 5 N. R Daly. A. McCormick and R. E. Powell, IrzntL Mars Specfrom. Ion Phys., I1 (1973) 255. 6 D. C. Frost and C. A. McDowell, Proc. Roy. Sot., Ser. A, 232 (1955) 227. 2
International Journal of Mars Spectrometry Q Ekevier Scientific Publishkg Company,
LOW ENERGY
N. R. DALY Axontic mrst
AND
Weapons
COLLISIONS
BETWEEN
N,+
ICNS AND
N2 MOLECULES
R. E. POWELL Research
received 7 Auwt
Establishment,
Aldermaston
1974; ir final form
11 October
(Gt. Britain) 1974)
ABSTRACT
,4 new technique is described to examine ion-molecule reactions that take Flace in a modified ion source similar to the type described by cermak and Herman. The reaction of N2+ colliding with N, leading to the formation of Nhas been studied. The dependence of the cross-section for the reaction on translation energy and on the energy of the electron-for,ning Nzf was examined. There is an interest in low energy collisions between molecular nitrogen ions and nitrogen molecules that lead to the production of atomic nitrogen ions. This arises because the upper atmosphere above 200 km consists mainly of molecular nitrogen and atomic o;lygen. Free electron de-&t& in these regions are determined by the rates at which atomic and molecular ions recombine with electrons and by the rate of removal cf atomic ions by ion molecular reactions. Recombination of at;-lmic ions is an extremely slow process in relation to the dissociative recombination of molecular ions. Hence any process that leads to the formation of atomic ions is important in determining the steady s-late electron densities at high altitudes. The process that has been studied in this work is N,+ +N,
-+ N+ +N+N,
(1)
This reaction has been studied previously by Lever&al and Friedman [l] using a conventional me of mass spectrometer system. Maier [Z] used a smal1 tandem mass spectrometer to study reaction (1). McGee et al. [3] used a multiple pulse method whereby ion production, ion acceleration and ion reactions occur sequentially within the ionisation chamber of the mass spectrometer and the product ions are separated by time-of-flight methods. The first group reported significantly different results to the other two, with thresholds at much lower energies. This work examines the threshold behaviour of reaction (1) and how the cross-section for it varies with excit&ion of the N, 7 ions.
444 EXPERIMENTAL
Anexperimental technique similar to that of CerrGkand Herman [4] was used in this work. Figure 1 shows the experimental layout of the apparatus employed. A conventional Nier type electron impact ion source A was used to produce the primary ions of nitrogen. A gas cell B was located 2 mm away from the exit slit of the Nier source. GA5 INLET
METASTABLE DETECTOR
ION
Fig. 1. Layout of the experimental apparatus. A represents a Nier type electron impact ion source; B, the gas reaction cell; C, the resolution slit; D. an energy enhancer plate: E. the scintilIator retention plate; W, a glass window.
Ions from the cell A entered the gas cell B through a fine grid. Both the ion source and the gas ccl1 had separate gas systems feeding them with spectrally pure gases Cell B was held at a fixed potential of 6000 V above earth. Cell A could be varied from 0 to 200 V positive with respect to cell B and hence the collision energy for reaction (1) in cell B could be varied. Ions were extracted from cell B, accelerated through a conventional array of focusing plates and analysed by a 12-m radius 90” sector magnet. It is always possible, particularly at low voltages between A and B, that ions formed in cell A will be mixed with ions produced by reactions in cell B. They will then be detected simultaneously with them in any detector located after the resolution slit of the mass spectrometer. To minimise this process an energysensitive ion detector of the type described by Daly et al. [S] was used in this work. After passing through the resolving slit C, ions are accelerated to an electrode D maintained at 4000 V negative to earth. They pass through a slit in D and approach an electrode E that is held at the same potential as the gas reaction cell B. A 1.5-4~1-1
445 diam. disc of scintillator is held in electrode E as shown, and it is vacuum coated with aluminium (60 pg cm-‘) on to which the positive ions can impact. This is done so that the potential of the face of the scintillator can be held at the same potential as the electrode E in which it is located. The operation of the system can now be explained by the way it responds to ions from the two cells A and B. Ions from cell A, which is at a potential positive to B. approach electrode E and impact onto it with an energy equivalent to the potential difference between A and B, since E is at the same potential as B. Since this impact energy is very small, it is insufficient to release photons from the scintilIator on E, and the ions from cell A are therefore not detected_ Ions from B approach very close to E but turn back before they strike it. They strike electrode D with 10 keV energy and release secondary electrons from it. These secondary eIectrons are accelerated into the scintillator at E where they produce light. This light is transmitted through the glass window W which forms part of the vacuum wall and is picked up by the photomultiplier. The photomultiplier can be used to count the ions from cell B or to produce an integrated output_ The small bump on electrode D is to produce an asymmetry in the electric field between D and E and to prevent ions returning down the path towards the magnet. Measurements were carried out to determine the rejection factor for ions formed in A. These showed that when there was 3 V between A and B the rejection factor was 1000 and that it increased with increasing potential between A and B. This system therefore leads to a method of studying ions that are formed in cell B and at the same time rejecting the same type of ioa formed in cell A.
RESULTS
-4ND
DISCUSSION
The excitation function for reaction (1) was obtained by the following method. The N, + ion current from cell A was piotted as a function of the potential difference between cells A and B. This was achieved by raising the potential of electrode E slightly above that of cell A. The N’ ion current from cell 33 was plotted as a function of the potential difference between cells A and B but in this case the potential of- electrode E was the same as that of cell B. The electron energy was held at 20 eV_ The pressure in the gas cell was such that about -1% of the N2 + ions from cell A made collisions in it and produced N+ ions. The N’ ion signal was shown to be linear with pressure of N, in the gas cell. Figure 2 shays the form of the excitation function for reaction (1). It can be seen that the threshold is at about 15 V and that the cross-section rises to a maximum at about 30 eV. Also shown-in Fig. 2 are the results of other measurements made on this reaction_ -AU results have been normalised. Leventhal and Friedman measured a high cross-section, about 2 - lo-l5 cm’, at a very low primary ion kinetic energy, The threshold they observed was at about 1 eV_ McGee et al.
,
5
10
15 N;
ION
ENERGY
20
lV (LAB
,
,
,
25
.
,
,
,
,
,
,
30
1
Fig. 2. Cross-section for reaction N2+-tN2 -+ N’+N+N2 in arbitrary units and normal&d (a) by Leventhal and Friedman, (b) by McGee et al., (c) this work, (d) by Maier.
observed a threshold at 10 eV and a maximum cross-section at 30 eV of lo- ’ 6 cm’_ Maier observed a threshold at about 15 eV and a maximum cross-section of lo-l6 cm2 at 30 eV. The shape of the excitation function that has been measured in this work follows very closely that obtained by Maier. In this work the N,+ ions were produced by 20 eV electrons, whereas Maier’s results refer to an electron ener,7 in the ion source of 19 eV. The primary ion transit time from the ion source to the reaction region is about 1 ps in the apparatus used in this work, and 9 ,YS in Maier’s experiment for the case of a 25 eV ion. In this work N2+ will be formed in the X2x;, A27rU, B2Ci states. Ions in the B2ZT state with a lifetime of about 60 ns will not reach the collision chamber, but N,+ A27rU ions will be transmitted effectively to the chamber since their lifetime is about 12 ,WS.Despite the very significant difference in ion transit time_ from the formation point to the reaction point, between the two experiments, there is close a,oreement on the detailed shape of the excitation function for the reaction. As shown in Fig. 2, very significant differences exist between this work and the work of Leventhal and Friedman. and of McGee et al. It seems that the large cross-section observed at very low energies by the former. and the lower threshold for the reaction observed by the latter workers seem only explicable in terms of the different experimectal techniques used to study the reactionThe variation of the cross-section for the reaction with the energy of the electron beam forming N2 + iors is shown in Fig. 3. No measurements have been made of the spread in electron energy in the Nier type ion source. but it is thought to be about 1 eV. It can be seen that there is a slow rise in the cross-section from just above the ionisation threshold of
447
.
.
.
.
-
.
.
. . .
-
.
.
a
.
. .
15
20
25
ELECTRON
BEAM
ENERGY
30 eV
Fig. 3. Cross-section for the production of N + from cell B relative to that for the production of N1+ from cell A as a function of the electron energy in cell A-
nitrogen up to 22 eV. This can be explained and higher vibrational
levels of the X’Z.,f
by assuming
that the ground state
and A3rtU states are being populated
to about 22 eV and that reaction (1) is proceeding
up
faster because of this.
At about 22 eV there is a significant gradient change corresponding to the onset of some new state of N,+ affecting the reaction rate. Electron impact work by Frost and McDowell [6] shows a strong onset for the formation of N,) C’X:,f at abo-ut 23 eV. However an electron in this state can make an ailowed transition to the ground state so its lifetime is probably short and hence It would not reach
the reaction chamber. It therefore seems likely that Nzf ions must be formed in some long-lived metastable states such as the 4C, or 4AU_
1
ACKNOWLEDGEMENTS
We would like to thank the Director of the Atomic Weapons Research Establishment for permission to publish this paper, and Mr. Trevor Hobson for making the measurements.
REFERENCES 1 J. J. Leventbal and L. Friedman, L Chem- Php_,
46 (1967)
997-
2 W. B. Maier II, J. Chem. Phys.. 47 (1967) 859_
3 T. H. McGee, P. F. Fennelly and M. Henchman, Abslrucfs of Papers of Sixth lnternntionol Conference on the Physics of EIectronic and Atomic Collisions, Cambridge, hriarsachusetts, U.S.A., MIT Press, 1969, p_ 321. 4 V. &tik and Z. Herman, Collect. Czech. Chem. Common., 30 (1965) 169. 5 N. R Daly. A. McCormick and R. E. Powell, IrzntL Mars Specfrom. Ion Phys., I1 (1973) 255. 6 D. C. Frost and C. A. McDowell, Proc. Roy. Sot., Ser. A, 232 (1955) 227. 2