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Ionization and fragmentation of some simple molecules in collisions with 40 MeV Ar13+ ions
Nuclear Instruments and Methods in Physics Research B27 (1987) 512-518 North-Holland, Amsterdam
512
IONIZATION AND FRAGMENTATION OF SOME SIMPLE MOLECULES IN COLLISIONS WITH 40 MeV Ar13’ IONS R.J.
MAURER,
C. CAN and R.L. WATSON
The ionization and fragmentation produced in collisions of 40 MeV Ar13+ projectiles with 02, CO, CO*, N,O and SO2 have been identified by time-of-ant mass spectrometry. In each case, the singly ionized molecular ion is observed to have the highest yield, while substantial yields of doubly charged molecular ions appear in the spectra for CO, CO, and SO,. Peaks have been identified for atomic ions up to charge 5 +, and those arising from the dissociation of the two diatomic molecules and the nonlinear triatomic molecule SO, are split into doublets.
Interest in the decomposition of molecules as a result of multiple ionization was first stim~ated by a series of experiments carried out in the mid 1960s by Carlson and his co-workers [l-3]. The creation of an inner-shell vacancy in a heavy atom leads to the generation of a highly charged ion as a result of the Auger cascade which follows. If the heavy atom is contained i11 a molecule, rapid decomposition primarily to atomic ions occurs and the fragments receive considerable kinetic energy because of the large Coulombic repulsion between the different charge centers that are formed. Since the early photoio~~tion work of Carlson et al., numerous measurements of molecular decomposition stimulated by electron impact ionization and photoionization have been performed. Many of these studies have focused on the effects of valence-shell ionization as opposed to inner-shell ionization followed by an Auger cascade. In such cases, singly charged molecular ions are the predominate species observed along with small yields of doubly charged molecular ions and singly charged atomic ions [4,5]. In experiments carried out using beams of I MeV H+ and He’, it has been shown that the the yields of doubly charged molecular ions and more highly charged atomic ions (q d 3 + ) are greatly enhanced in molecular dissociation processes induced by collisions with heavy charged particles j&7]. Work by Cocke [8] on noble gas targets has demonstrated that f&t beams of heavy ions are extremely effective in producing highly charged, low-velocity (atomic) recoil ions. In collisions of 28 to 43 MeV Cl beams with neutral argon atoms, recoil ions were observed up to Ar”’ + having recoil energies estimated to be less than 10 eV. Subsequent work by Gray et al. [9] and Kelbch et al. [lo] has shown that the majority of the yield of highly charged neon 0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
recoil ions (q > 5 + ) are produced in collisions where the projectile captures a Ne K-shell electron. Recent work by numerous other workers on a variety of t~get/projectile systems has provided much new information concerning cross sections for recoil ion production. Only two measurements of the fragmentation patterns of molecular gases produced by collisions of fast heavy ions have been reported. Gray et al. IlO] have studied the ionization and fragmentation of CH, in collisions with 19 MeV fluorine ions. They determined the mass to charge ratios of the positively charged products using a hemispherical electrostatic analyzer and found evidence for the trebly-charged molecular ions CHz+ and CH:“. Tawara et al. [ll] used a double-focusing sector magnet to measure the mass to charge ratios of ions produced by collisions of 40 MeV Ar ions on N,. They observed atomic nitrogen ions with charge states up to the bare ion as well as N: molecular ions. In the present work, the io~zation and fragmentation products produced in collisions of 40 MeV Ar13-(- ions with molecules of 0, CO, CO,, N20 and SO, have been identified by time-of-dint mass spectrometry.
2. ~x~rimen~
methods
A schematic diagram of the system used in the present measurements is shown in fig. 1. The system consisted of a small differentially pumped gas cell mounted above a 2400 l/s diffusion pump and a set of ~croch~nel plates (Chevron design) for ion detection mounted at the end of a 12.8 cm drift tube. A 685 V/cm extraction field was applied across the cell perpendicular to the beam. A second acceleration stage was formed by a focusing lens and a grounded 88% trans-
R.J. Maurer et al. / Ionization andfragwwtation of simple molecules
TO
REGULATED GAS SUPPLY
513
TO MANOMETER I
t
+I200
VOLTS
2mm BEAM -ENTRANCE
INSULATOR
+700
SURFACE BARRIER DETECTOR
I
DIFFERENTIALLY GAS CELL _
I
I
‘Imm
VOLTS
PUMPED
CYCLOTRON
BEAM
COLLIMATOR 2mm
-FLIGHT
/ ’ COLLIMATOR
TUBE
MICROCHANNEL PLATE DETECTOR
TO 2400 DIFFUSION
L/S PUMP
1
Fig. 1. A schematic diagram showing the experimental configuration.
mission grid attached to the front of the drift tube, across which a 2000 V/cm field was applied. Collisionproduct ions gained 235 eV/q in the extraction stage and an additional 700 eV/q in the second acceleration stage. The microchannel plates were located 0.1 cm from the end for the drift tube and the front plate was biased at - 2100 V. A surface barrier detector was positioned immediately behind the exit port of the gas cell and used to detect the projectile ions. The time-of-flight of the charged ionization and fragmentation products through the spectrometer was measured with a time-to-amplitude converter (TAC). The TAC was started by a timing signal derived from the surface barrier detector upon the detection of a projectile ion and it was stopped with a timing signal generated by the detection of a collision product ion in the set of microchannel plates. The performance of the system was tested by taking time-of-flight (TOF) spectra of monatomic ions produced in collisions of 40 MeV Art3+ with neutral atoms of Ne and Ar. In all of the measurements, the pressure in the gas cell was maintained at 1 mTorr and the projectile ion rate, as measured in the surface barrier
detector, was limited to 2000 ions/s. The Ne and Ar TOF spectra are shown in fig. 2. It is apparent that the resolution of the spectrometer is quite good and that Ne-ion charge states up to 8 + and Ar-ion charge states up to 11 + are readily discernable. The inherent time resolution of the system is estimated to be somewhat less than 1 ns, however the measured fwhm of the Arr+ peak is 12.0 ns. This broadening of the peaks in the TOF spectra in fig. 2 reflects the initial velocity distribution of the recoiling ions and the time spread associated with ion production at different points within the finite cross section of the projectile beam.
3. Results and discussion TOF spectra for collisions of 40 MeV Arr3+ ions with molecular gas targets of 0, and CO are compared in fig. 3. Any molecular ion that does not break up before it has entered the drift tube (- 10e7 s for singly charged ions) should be detected. The species observed to have the highest yields are the singly ionized molecular ions - 0: and CO+. Unfortunately, the peak for
R.J. Maurer et al. / Ionization and fragmentation of simple molecules
514
(al
Ne
"Ne+
TIME-OF-
1
FLIGHT (psec)
Fig. 2. TOF calibration spectra of (a) neon and (b) argon ions produced in collisons with 40 MeV Art3+
the doubly ionized molecular ion Oi+ coincides with the 0’ peak in the oxygen spectrum. However, a peak attributable to CO’-+ is cleanly separated from both the O+ and C+ peaks in the carbon monoxide spectrum. Multiply-charged atomic ions are present in the oxygen spectrum up to 04+ and in the carbon monoxide spectrum up to C3+ and 03+. Compared with the neon spectrum, which displays a maximum charge state of 8 + , the highest charge state observed in the 0, spectrum is lower by a factor of two. This indicates that the
electron loss tends to be shared equally between the two fragments and suggests that electron re~~gement is considerably faster than dissociation, which takes place on a time scale comparable to a vibration period (- lo-r4 s). An unexpected feature of the spectra shown in fig. 3 is the splitting displayed by the O*+S~+.~+ and C2+.3+ peaks. We believe that this splitting is caused by the difference in the times of flight for a fragment whose initial recoil direction is away from the spectrometer
R.J. Maurer et al. / Ionization and fragmentation of simple molecules
515
02
IO’
TIME-OF-
FLIGHT (psec 1
Fig. 3. TOF spectra of the charged products of 40 MeV Arr3* collisions with the diatomic molecules (a) 0, and (b) CO. and one whose initial recoil direction is toward the spectrometer. Because of the Coulomb repulsion between the different charged species, the ions created as a result of the breakup of a diatomic molecule have considerably more recoil energy than an ion produced directly by ionization of a monatomic target. The peak splittings observed in the spectrum for 0, are compared in table 1 with the time differences calculated for the ion of interest assuming initial trajectories of o” and 180” with respect to the direction of the
electric field. The initial velocities were calculated from the Coulomb potential energy assuming the two ions act as point charges separated by a distance equal to the equilibrium bond length (1.21 A). It is apparent that the observed peak splittings for 04+ (4 2 2) agree reasonably well with the those predicted for the most probable contributing reactions. In the case of O+, two possible reaction paths would contribute to a single (unsplit) peak and another pair of reactions would contribute to peaks on both sides. The shape and width of the O+
516
R.J. Maurer et al. / Ionization andfragrnentation of simple molecules
N-N=0
TIME-OF-FLIGHT
(psec)
Fig. 4. TOF spectra of the charged products of 40 MeV Ar13+ collisions with the linear triatomic molecules (a) CO, and (b) NzO.
peak in fig. 3 is entirely consistent with these expectations. The circumstances that give rise to our ability to resolve separate TOF peaks for ions recoiling toward and away from the spectrometer are not yet fully understood. Apparently the focusing elements strongly discriminate against ions with large components of velocity perpendicular to the electric field direction. A detailed analysis of the ion trajectories through the spectrometer is currently .in progress to determine how this dis-
crimination arises and to extract recoil kinetic energies from the measured peak splittings. Shown in fig. 4 are TOF spectra for the linear triatomic molecules CO, and N,O. Here again the species observed with the highest yields are the singly ionized molecular ions CO: and N20e. In the case of CO,, a significant yield of the doubly ionized molecular ion CO,2+ is also observed. Apparently, N202+ is much less stable than CO:+ since its yield in the N,O spectrum is very low. Collisions leading to the removal of a
R.J. Maurer et al. / Ionization
Table 1 Comparison of measured and calculated time-of-flight ferences for the 0, spectrum Peak 0+
Origin 0: +o++o 02+ + o;+ 2 02
2+
o;+
Ox+
3+
Measured At (ns)
0
+
o+
+o+
-+
02+
+o+
-+
02+
+o+
-9
02+
+
02 5+
,03++02+
02+
41.0 58.0 29.0 41.0 50.2
30.7
33.5 41.0 47.3
41.3
35.5 41.0
42.1
of simple molecules
517
Another noteworthy characteristic of the spectrum for CO, is that none of the C-ion peaks are split, while there is clear evidence for splitting of the O-ion peaks (the O+ peak is broadened and the O*+ peak is split). Since CO, is a linear symmetric molecule, this result implies that essentially all collisions leading to the production of atomic C-ions cause both oxygen atoms to dissociate simultaneously. The N,O spectrum is also characterized by the apparent absence of nitrogen ion peak splitting In this case, however, one would expect that the nitrogen on the end of the molecule would gain considerable kinetic energy in the breakup, and hence these ions should display peak splitting. A close examination of the N,O spectrum does reveal low intensity components on the high and low sides of the N*+ and N3+ peaks that could be ascribed to the end nitrogen. The low intensities of these components relative to the main (unsplit) peak may indicate that the spectrometer transmission is much lower for energetic recoil ions than it is for ions that are produced with very little kinetic energy. A spectrum for the nonlinear triatomic molecule SO, is shown in fig. 5. Unlike the spectra for the two linear triatomic molecules just discussed, this spectrum shows clear evidence of peak splitting for the central sulphur atom, as expected because of its nonlinear structure. Another feature of interest in this spectrum is the peak identified as the doubly charged molecular ion fragment
dif-
0
@f
02
Calculated At (ns)
and fragmentation
single oxygen atom or a single nitrogen atom are quite probable, as is indicated by the relatively high yields of the molecular ion fragments CO+, NO+ and N+2. It is also seen that the yields of atomic oxygen ions are much less than those for carbon and nitrogen. This indicates that the oxygen ions retain more electrons than their less electronegative partners during the breakup of the molecule.
-
‘O‘F------
so2
j
t
I
I
I
I
I
2
3
4
TIME-OF-FLIGHT
(psec)
Fig. 5. TOF spectrum of the charged products of 40 MeV Ar13+ collisions with the nonlinear
triatomic
molecule
SO,.
R.J. Maurer et al. / Ionization and fragmentation of simple molecules
518
S02+. The spectrum for CO, does not contain an analogous peak for C02+ even though it displays a high yield of CO+. The C02+ molecular ion is stable enough to be seen in the spectrum for CO, and hence its absence from the CO, spectrum must be a unique characteristic of the dissociation mechanism for multiply ionized CO, molecular ions.
4. Conclusion This survey of the ionization and fragmentation products of 40 MeV Ar13+ collisions with some diatomic and triatomic molecules has demonstrated that a wide variety of molecular and atomic ions are produced in fast collisions of heavy, highly stripped ions. Furthermore, it has been shown that time-of-flight mass spectrometry is capable of providing useful information concerning the identities of the products and their yields. It is likely that a detailed analysis of such spectra will provide new insight on molecular structure, molecular bonding, and the dynamics of the excitation and dissociation processes. This work is supported
by the Division
of Chemical
Sciences of the U.S. Department of Energy, the Robert A. Welch Foundation and the Texas A&M Center for Energy and Mineral Resources.
References 111 T.A. Carlson and R.M. White, J. Chem. Phys. 44 (1966) 4510. PI T.A. Carlson and R.M. White, J. Chem. Phys. 48 (1968) 5191. 131 T.A. Carlson and M.O. Krause, J. Chem. Phys. 56 (1972) 3206. and M.J. Van der Wiel, J. Phys. B12 [41 A.P. Hitchcock (1979) 2153. M.C. Nelson, S.L. Anderson and D.M. [51 J. Murakami, Hanson, J. Chem. Phys. 85 (1986) 5755. [61 A.K. Edwards, R.M. Wood and M.F. Steuer, Phys. Rev. Al5 (1977) 48. [71 M.F. Steuer, R.M. Wood and A.K. Edwards, Phys. Rev. Al6 (1977) 1873. PI C.L. Cocke, Phys. Rev. A20 (1979) 749. Phys. Rev. A22 [91 T.J. Gray, CL. Cocke and E. Justiniano, (1980) 849. J. Ullrich, R. Schuch, E. PO1 S. Kelbch, H. Schmidt-Backing, Justiniano, H. Ingwersen and CL. Cocke, Z. Phys. A317 (1984) 9.
512
IONIZATION AND FRAGMENTATION OF SOME SIMPLE MOLECULES IN COLLISIONS WITH 40 MeV Ar13’ IONS R.J.
MAURER,
C. CAN and R.L. WATSON
The ionization and fragmentation produced in collisions of 40 MeV Ar13+ projectiles with 02, CO, CO*, N,O and SO2 have been identified by time-of-ant mass spectrometry. In each case, the singly ionized molecular ion is observed to have the highest yield, while substantial yields of doubly charged molecular ions appear in the spectra for CO, CO, and SO,. Peaks have been identified for atomic ions up to charge 5 +, and those arising from the dissociation of the two diatomic molecules and the nonlinear triatomic molecule SO, are split into doublets.
Interest in the decomposition of molecules as a result of multiple ionization was first stim~ated by a series of experiments carried out in the mid 1960s by Carlson and his co-workers [l-3]. The creation of an inner-shell vacancy in a heavy atom leads to the generation of a highly charged ion as a result of the Auger cascade which follows. If the heavy atom is contained i11 a molecule, rapid decomposition primarily to atomic ions occurs and the fragments receive considerable kinetic energy because of the large Coulombic repulsion between the different charge centers that are formed. Since the early photoio~~tion work of Carlson et al., numerous measurements of molecular decomposition stimulated by electron impact ionization and photoionization have been performed. Many of these studies have focused on the effects of valence-shell ionization as opposed to inner-shell ionization followed by an Auger cascade. In such cases, singly charged molecular ions are the predominate species observed along with small yields of doubly charged molecular ions and singly charged atomic ions [4,5]. In experiments carried out using beams of I MeV H+ and He’, it has been shown that the the yields of doubly charged molecular ions and more highly charged atomic ions (q d 3 + ) are greatly enhanced in molecular dissociation processes induced by collisions with heavy charged particles j&7]. Work by Cocke [8] on noble gas targets has demonstrated that f&t beams of heavy ions are extremely effective in producing highly charged, low-velocity (atomic) recoil ions. In collisions of 28 to 43 MeV Cl beams with neutral argon atoms, recoil ions were observed up to Ar”’ + having recoil energies estimated to be less than 10 eV. Subsequent work by Gray et al. [9] and Kelbch et al. [lo] has shown that the majority of the yield of highly charged neon 0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
recoil ions (q > 5 + ) are produced in collisions where the projectile captures a Ne K-shell electron. Recent work by numerous other workers on a variety of t~get/projectile systems has provided much new information concerning cross sections for recoil ion production. Only two measurements of the fragmentation patterns of molecular gases produced by collisions of fast heavy ions have been reported. Gray et al. IlO] have studied the ionization and fragmentation of CH, in collisions with 19 MeV fluorine ions. They determined the mass to charge ratios of the positively charged products using a hemispherical electrostatic analyzer and found evidence for the trebly-charged molecular ions CHz+ and CH:“. Tawara et al. [ll] used a double-focusing sector magnet to measure the mass to charge ratios of ions produced by collisions of 40 MeV Ar ions on N,. They observed atomic nitrogen ions with charge states up to the bare ion as well as N: molecular ions. In the present work, the io~zation and fragmentation products produced in collisions of 40 MeV Ar13-(- ions with molecules of 0, CO, CO,, N20 and SO, have been identified by time-of-dint mass spectrometry.
2. ~x~rimen~
methods
A schematic diagram of the system used in the present measurements is shown in fig. 1. The system consisted of a small differentially pumped gas cell mounted above a 2400 l/s diffusion pump and a set of ~croch~nel plates (Chevron design) for ion detection mounted at the end of a 12.8 cm drift tube. A 685 V/cm extraction field was applied across the cell perpendicular to the beam. A second acceleration stage was formed by a focusing lens and a grounded 88% trans-
R.J. Maurer et al. / Ionization andfragwwtation of simple molecules
TO
REGULATED GAS SUPPLY
513
TO MANOMETER I
t
+I200
VOLTS
2mm BEAM -ENTRANCE
INSULATOR
+700
SURFACE BARRIER DETECTOR
I
DIFFERENTIALLY GAS CELL _
I
I
‘Imm
VOLTS
PUMPED
CYCLOTRON
BEAM
COLLIMATOR 2mm
-FLIGHT
/ ’ COLLIMATOR
TUBE
MICROCHANNEL PLATE DETECTOR
TO 2400 DIFFUSION
L/S PUMP
1
Fig. 1. A schematic diagram showing the experimental configuration.
mission grid attached to the front of the drift tube, across which a 2000 V/cm field was applied. Collisionproduct ions gained 235 eV/q in the extraction stage and an additional 700 eV/q in the second acceleration stage. The microchannel plates were located 0.1 cm from the end for the drift tube and the front plate was biased at - 2100 V. A surface barrier detector was positioned immediately behind the exit port of the gas cell and used to detect the projectile ions. The time-of-flight of the charged ionization and fragmentation products through the spectrometer was measured with a time-to-amplitude converter (TAC). The TAC was started by a timing signal derived from the surface barrier detector upon the detection of a projectile ion and it was stopped with a timing signal generated by the detection of a collision product ion in the set of microchannel plates. The performance of the system was tested by taking time-of-flight (TOF) spectra of monatomic ions produced in collisions of 40 MeV Art3+ with neutral atoms of Ne and Ar. In all of the measurements, the pressure in the gas cell was maintained at 1 mTorr and the projectile ion rate, as measured in the surface barrier
detector, was limited to 2000 ions/s. The Ne and Ar TOF spectra are shown in fig. 2. It is apparent that the resolution of the spectrometer is quite good and that Ne-ion charge states up to 8 + and Ar-ion charge states up to 11 + are readily discernable. The inherent time resolution of the system is estimated to be somewhat less than 1 ns, however the measured fwhm of the Arr+ peak is 12.0 ns. This broadening of the peaks in the TOF spectra in fig. 2 reflects the initial velocity distribution of the recoiling ions and the time spread associated with ion production at different points within the finite cross section of the projectile beam.
3. Results and discussion TOF spectra for collisions of 40 MeV Arr3+ ions with molecular gas targets of 0, and CO are compared in fig. 3. Any molecular ion that does not break up before it has entered the drift tube (- 10e7 s for singly charged ions) should be detected. The species observed to have the highest yields are the singly ionized molecular ions - 0: and CO+. Unfortunately, the peak for
R.J. Maurer et al. / Ionization and fragmentation of simple molecules
514
(al
Ne
"Ne+
TIME-OF-
1
FLIGHT (psec)
Fig. 2. TOF calibration spectra of (a) neon and (b) argon ions produced in collisons with 40 MeV Art3+
the doubly ionized molecular ion Oi+ coincides with the 0’ peak in the oxygen spectrum. However, a peak attributable to CO’-+ is cleanly separated from both the O+ and C+ peaks in the carbon monoxide spectrum. Multiply-charged atomic ions are present in the oxygen spectrum up to 04+ and in the carbon monoxide spectrum up to C3+ and 03+. Compared with the neon spectrum, which displays a maximum charge state of 8 + , the highest charge state observed in the 0, spectrum is lower by a factor of two. This indicates that the
electron loss tends to be shared equally between the two fragments and suggests that electron re~~gement is considerably faster than dissociation, which takes place on a time scale comparable to a vibration period (- lo-r4 s). An unexpected feature of the spectra shown in fig. 3 is the splitting displayed by the O*+S~+.~+ and C2+.3+ peaks. We believe that this splitting is caused by the difference in the times of flight for a fragment whose initial recoil direction is away from the spectrometer
R.J. Maurer et al. / Ionization and fragmentation of simple molecules
515
02
IO’
TIME-OF-
FLIGHT (psec 1
Fig. 3. TOF spectra of the charged products of 40 MeV Arr3* collisions with the diatomic molecules (a) 0, and (b) CO. and one whose initial recoil direction is toward the spectrometer. Because of the Coulomb repulsion between the different charged species, the ions created as a result of the breakup of a diatomic molecule have considerably more recoil energy than an ion produced directly by ionization of a monatomic target. The peak splittings observed in the spectrum for 0, are compared in table 1 with the time differences calculated for the ion of interest assuming initial trajectories of o” and 180” with respect to the direction of the
electric field. The initial velocities were calculated from the Coulomb potential energy assuming the two ions act as point charges separated by a distance equal to the equilibrium bond length (1.21 A). It is apparent that the observed peak splittings for 04+ (4 2 2) agree reasonably well with the those predicted for the most probable contributing reactions. In the case of O+, two possible reaction paths would contribute to a single (unsplit) peak and another pair of reactions would contribute to peaks on both sides. The shape and width of the O+
516
R.J. Maurer et al. / Ionization andfragrnentation of simple molecules
N-N=0
TIME-OF-FLIGHT
(psec)
Fig. 4. TOF spectra of the charged products of 40 MeV Ar13+ collisions with the linear triatomic molecules (a) CO, and (b) NzO.
peak in fig. 3 is entirely consistent with these expectations. The circumstances that give rise to our ability to resolve separate TOF peaks for ions recoiling toward and away from the spectrometer are not yet fully understood. Apparently the focusing elements strongly discriminate against ions with large components of velocity perpendicular to the electric field direction. A detailed analysis of the ion trajectories through the spectrometer is currently .in progress to determine how this dis-
crimination arises and to extract recoil kinetic energies from the measured peak splittings. Shown in fig. 4 are TOF spectra for the linear triatomic molecules CO, and N,O. Here again the species observed with the highest yields are the singly ionized molecular ions CO: and N20e. In the case of CO,, a significant yield of the doubly ionized molecular ion CO,2+ is also observed. Apparently, N202+ is much less stable than CO:+ since its yield in the N,O spectrum is very low. Collisions leading to the removal of a
R.J. Maurer et al. / Ionization
Table 1 Comparison of measured and calculated time-of-flight ferences for the 0, spectrum Peak 0+
Origin 0: +o++o 02+ + o;+ 2 02
2+
o;+
Ox+
3+
Measured At (ns)
0
+
o+
+o+
-+
02+
+o+
-+
02+
+o+
-9
02+
+
02 5+
,03++02+
02+
41.0 58.0 29.0 41.0 50.2
30.7
33.5 41.0 47.3
41.3
35.5 41.0
42.1
of simple molecules
517
Another noteworthy characteristic of the spectrum for CO, is that none of the C-ion peaks are split, while there is clear evidence for splitting of the O-ion peaks (the O+ peak is broadened and the O*+ peak is split). Since CO, is a linear symmetric molecule, this result implies that essentially all collisions leading to the production of atomic C-ions cause both oxygen atoms to dissociate simultaneously. The N,O spectrum is also characterized by the apparent absence of nitrogen ion peak splitting In this case, however, one would expect that the nitrogen on the end of the molecule would gain considerable kinetic energy in the breakup, and hence these ions should display peak splitting. A close examination of the N,O spectrum does reveal low intensity components on the high and low sides of the N*+ and N3+ peaks that could be ascribed to the end nitrogen. The low intensities of these components relative to the main (unsplit) peak may indicate that the spectrometer transmission is much lower for energetic recoil ions than it is for ions that are produced with very little kinetic energy. A spectrum for the nonlinear triatomic molecule SO, is shown in fig. 5. Unlike the spectra for the two linear triatomic molecules just discussed, this spectrum shows clear evidence of peak splitting for the central sulphur atom, as expected because of its nonlinear structure. Another feature of interest in this spectrum is the peak identified as the doubly charged molecular ion fragment
dif-
0
@f
02
Calculated At (ns)
and fragmentation
single oxygen atom or a single nitrogen atom are quite probable, as is indicated by the relatively high yields of the molecular ion fragments CO+, NO+ and N+2. It is also seen that the yields of atomic oxygen ions are much less than those for carbon and nitrogen. This indicates that the oxygen ions retain more electrons than their less electronegative partners during the breakup of the molecule.
-
‘O‘F------
so2
j
t
I
I
I
I
I
2
3
4
TIME-OF-FLIGHT
(psec)
Fig. 5. TOF spectrum of the charged products of 40 MeV Ar13+ collisions with the nonlinear
triatomic
molecule
SO,.
R.J. Maurer et al. / Ionization and fragmentation of simple molecules
518
S02+. The spectrum for CO, does not contain an analogous peak for C02+ even though it displays a high yield of CO+. The C02+ molecular ion is stable enough to be seen in the spectrum for CO, and hence its absence from the CO, spectrum must be a unique characteristic of the dissociation mechanism for multiply ionized CO, molecular ions.
4. Conclusion This survey of the ionization and fragmentation products of 40 MeV Ar13+ collisions with some diatomic and triatomic molecules has demonstrated that a wide variety of molecular and atomic ions are produced in fast collisions of heavy, highly stripped ions. Furthermore, it has been shown that time-of-flight mass spectrometry is capable of providing useful information concerning the identities of the products and their yields. It is likely that a detailed analysis of such spectra will provide new insight on molecular structure, molecular bonding, and the dynamics of the excitation and dissociation processes. This work is supported
by the Division
of Chemical
Sciences of the U.S. Department of Energy, the Robert A. Welch Foundation and the Texas A&M Center for Energy and Mineral Resources.
References 111 T.A. Carlson and R.M. White, J. Chem. Phys. 44 (1966) 4510. PI T.A. Carlson and R.M. White, J. Chem. Phys. 48 (1968) 5191. 131 T.A. Carlson and M.O. Krause, J. Chem. Phys. 56 (1972) 3206. and M.J. Van der Wiel, J. Phys. B12 [41 A.P. Hitchcock (1979) 2153. M.C. Nelson, S.L. Anderson and D.M. [51 J. Murakami, Hanson, J. Chem. Phys. 85 (1986) 5755. [61 A.K. Edwards, R.M. Wood and M.F. Steuer, Phys. Rev. Al5 (1977) 48. [71 M.F. Steuer, R.M. Wood and A.K. Edwards, Phys. Rev. Al6 (1977) 1873. PI C.L. Cocke, Phys. Rev. A20 (1979) 749. Phys. Rev. A22 [91 T.J. Gray, CL. Cocke and E. Justiniano, (1980) 849. J. Ullrich, R. Schuch, E. PO1 S. Kelbch, H. Schmidt-Backing, Justiniano, H. Ingwersen and CL. Cocke, Z. Phys. A317 (1984) 9.