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Production of multiply charged ions from CO and CO2 molecules in energetic heavy-ion impact
93
Chemical Physics 121 (1988) 93-98 North-Holland, Amsterdam
PRODUCTION OF MULTIPLY CHARGED IN ENERGETIC HEAVY-ION IMPACT
IONS FROM CO AND CO, MOLECULES
T. MATSUO ‘, T. TONUMA, M. KASE, T. KAMBARA, RIKEN,
H. KUMAGAI
Wako-shi, Saitama 351-01, Japan
and H. TAWARA Institute of Plasma Physics, Nagoya University, Nagoya 464, Japan Received
3 July 1987
A mass-spectroscopic technique has been applied to separate recoiled ions from CO and CO, targets by 1.05 MeV/amu Ar12+ ion impact. It has been found that energies of multiply charged atomic C’+ and O’+ tons from these molecules, except for those of C’+ ions from CO,, shift toward high-energy side, compared with those for recoil ions from single-atom targets such as Ar. These shifts in energy increase roughly with the square of the charge state of the multiply charged atomic ions, indicating that these energies are provided through Coulomb potential of multiply charged molecular ions which are produced first in collisions and dissociate into atomic ions immediately after their production.
1. Introduction This study is one of a series of our investigations which are concerned with production of atomic ions from molecular targets by energetic heavy-ion impact. Since it was recognized that the ionization probabilities of target atoms, in particular production of multiply charged recoil ions, are significantly enhanced under collisions with energetic heavy ions, a number of measurements of the cross sections for their production have been reported [l]. For example, the recoiled ArlEf ions are produced with a cross section of the order of 10-i’ cm* under bombardment of 15.5 MeV/amu U7*+ ions [2]. However, most of these works have been made by using rare gas atoms as targets. Investigations for molecular targets are still scarce. Recently, we have conducted some systematic investigations on production of multiply charged ions from molecular gas targets by energetic Ar 1 Permanent Medical
address: Medical and Dental University,
Research Institute, Tokyo Bunkyo-ku, Tokyo 113
0301-0104/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
ion impact. In the measurement of the N, molecule, multiply charged atomic recoil ions up to N6+ with a weak trace of N7+ have been observed. In addition, the peak positions of multiply charged nitrogen ions (N’+) in mass/charge spectra were found to shift toward the high-energy side, compared with those from single-atom targets such as Ar, and this peak shift increases with increasing charge state of the ions produced [3]. These facts strongly suggest that such multiply charged atomic ions are produced through processes where multiply charged molecular ions are first produced by the impact and, then, dissociate via Coulomb explosions into atomic ions. Also, peak shifts in mass/charge spectra have been observed for other molecular targets such as NO, N,O, NO, and C,H, 143. To obtain further insight into the mechanism and systematics of production of multiply charged ions from molecular targets, we have used a mass-spectroscopic technique to separate various ions from CO and CO, molecules in 1.05 MeV/amu Ar I*+ ion impact. Although a number B.V.
94
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules
of measurements on fragmentation of these molecules by electron and ion impact have been reported so far, no systematic work has been made on production of multiply charged ions, except the observation of Auger electrons from metastable Li-like (1~2~2~ 4P) carbon and oxygen ions produced by 1.4 MeV/amu Arl’+ impact on CH,, CO and CO, by Mann et al. [6].
2. Experimental The measurements have been performed with the apparatus previously reported in detail [7]. In brief, it consists of a crossed-beam collision system and a mass-spectrometer of sector-magnet type. The recoiled ions produced by ion impact are extracted by a weak field (20 V/cm) applied between two parallel plates positioned at the collision center, where a molecular gas target effused from a single-capillary nozzle was bombarded by ions 1.05 MeV/amu Ar12+ ions. The extracted were accelerated up to 1.5 kV, mass/charge analyzed with the double-focusing sector magnet and finally detected by a channeltron multiplier operated at pulse counting mode. The projectile ions were provided by the heavy-ion linear accelerator (RILAC) of RIKEN. At present, no absolute target density has been determined, though the pressure measured at the wall of the vacuum chamber was about 1 X lo-’ Torr ensuring single-collision conditions.
3.1. General features 3.1.1. CO molecule In fig. 1, a number of peaks corresponding to multiply charged atomic ions such as C’+ and Oif as well as a dominant peak of parent molecular CO+ ions are clearly seen. The peak intensity at m/q = 14, corresponding to doubly charged CO’+ ions, is considerably high compared with that by electron impact; the intensity ratio between the CO’+ peak and CO+ peak is about 2.1 x lo-* in this spectrum, while, by 150 eV electron impact, Hille and Mark obtained a value of 0.75 X 1O-2 for the cross section ratio between C02+ and CO+ production [S]. Moreover, they reported the production of metastable (CO*‘)* ions which dissociate spontaneously during transit from the collision region to the entrance of the mass selector. When dissociation of metastable (C02’) * ion
‘”
!
1.05 MeV;amu
1.05 MeVIamu
Ar”’
AI-“+
on COn
on CO
3. Results and discussion In fig. 1 are shown typical mass/charge (m/q) spectra of recoil ions produced from CO and CO, targets by 1.05 MeV/amu Ar12+ ion impact. Both of these spectra are characterized by a number of peaks corresponding to the fragmented atomic ions having various charge states as well as a dominant peak of singly charged molecular ions. Furthermore, the detailed mass/charge spectra of multiply charged atomic ions from these molecules are shown in fig. 2 in comparison with those from Ar and CH, targets.
lo0 0
256 CHANNEL
NUMBER
Fig. 1. Mass/charge spectra of recoil ions from molecular targets, CO and CO,, under 1.05 MeV/amu Ar12+ ion impact.
T. Maisuo et al. / Multiply charged ions from CO and CO, molecules
95
The intensity of Cf ions is slightly higher than that of O+: the intensity ratio Z(C+)/Z(O+) = 1.15. This is understood to be due to the different dissociation processes; namely, the C+ + 0 dissociation process is responsible for C+ production and the O+ + C process for O+ production, with dissociation energies of 8.3 and 10.7 eV, respectively [ 91.
CHANNEL NUMBER Fig. 2. Expanded mass/charge spectra of multiply charged C’+ and O’+ ions from molecular targets, CO and CO,. The vertical dotted lines are drawn to show the shift of peak positions of multiply charged ions from CO and CO, targets with respect to those from Ar and CH, targets.
occurs during the transit, the resultant Cc and O+ ions are expected to appear at m/q = 10.3 and m/q = 18.3, respectively. In our spectrum, signals at m/q = 10.3 seem to be insignificant and those at m/q = 18.3 may be masked by contaminated H20+ ions at m/q = 18.0. It can be also noted that there is no trace at m/q = 9.33 corresponding to C03+ in this spectrum, indicating that triply charged molecular ions are not stable but dissociate immediately after their production.
3.1.2. CO, molecule In addition to a dominant peak of singly charged COT ions, a peak corresponding to doubly charged molecular ions, CO:+, at m/q = 22 is fairly intense. The intensity ratio between doubly charged and singly charged ions, Z(COi+)/ Z(COc), is about 4.0 x lo-* while Mark and Hille reported this intensity ratio to be 1.2 x lo-* by 171.4 eV electron impact [lo]. It should be noted that intensities of C+ and O+ ions are roughly equal, though the number of oxygen atoms is twice as much as that of carbon atom, confirming that molecules cannot be assumed to consist of the independent constituent atoms even at relatively high-energy collisions. In electron impact, the intensity ratio Z(C+)/Z(O+) is reported to be about 0.5 at the impact energy of 300 eV [ll] and Z(C+)/Z(O+) = 0.7 at 1000 eV [12]. In this molecule, a rather broad peak appears around m/q = 35. This is considered to be due to dissociation products of metastable molecular (CO:‘) * ions as reported by Mark and Hille [lo]; CO+ ions from O+ + CO+ dissociation process are expected to appear at m/q = 35.6 (indicated by an arrow in fig. 1). 3.2. Detailed features Fig. 2 is an expanded part of the mass/charge spectra of multiply charged atomic ions from CO and CO, targets, together with those from Ar and CH, targets for comparison. One may easily note that the observed peaks of C’+ and O’+ ions from CO and CO, targets are shifted in position and are considerably broadened, compared with those from Ar and CH, targets. As has been reported previously, peaks of multiply charged carbon ions, Cl+, from CH, targets show neither shift in peak positions nor broadening of peaks with respect to
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules Table 2 Intensity ratio of secondary in each molecule
30 2C
co
co,
ions with the same charge O’+/C’+
o+,c+
02+/C*+
os+,c3+
0.83 0.95
0.70 0.11
> 0.80 > 0.14
a) 04+,c4+ _ _
os+,c5+
7.8 3.4
a) The intensity of C3+ ions is taken to be that of C3+ +04+ ions approximately.
!
1’ CHARGE
STATE
z
Fig. 3. The observed energy shift of the peaks as a function of the charge state of recoil C’+ and O’+ ions from CO and CO, molecules. Note that no significant energy shift of the peaks is observed for C’+ ions from CO, target.
those of the corresponding Ar’+ ions which gain only a small amount of recoil energy of the order of eV in the present collision energy [4]. Though the present experimental setup does not allow us to measure accurate energies of ions, we have tried to determine the difference in energies of atomic ions from CO and CO* targets, compared with those of ions from Ar targets. In fig. 3 are shown the observed energy shifts of multiply charged atomic ions from CO and CO, targets. From figs. 2 and 3 it can be concluded that the energy shift of C’+ and O’+ ions increases with roughly the square of the charge of ions. The shift for C” ions increases in order of CO > CO,
Table 1 Relative intensities of secondary ions from CO and CO, targets, target; notation m( - n) indicates m x lo-”
> CH,, while that for O’+ increases in order of CO, > CO. Also, the linewidth of C’+ ions increases in the order of CO > CO, > CH, and that of O’+ as CO, > CO. This result is in accordance with that by Mann et al. [6] who observed that, in 1.4 MeV/amu Ar 12-t + CO, CO, and CH, collisions, the line broadening of ls2s2p 4P-ls2 ‘S + e Auger electron transition in carbon ions is increased in order of CO > CO, > CH, and that in oxygen ions in order of CO, > CO. Another important information for understanding the production mechanism of multiply charged atomic ions is the intensity ratios of carbon and oxygen ions in a particular target molecule and those among different molecules. In table 1 are shown relative intensities of multiply charged ions normalized to those of singly charged parent molecular ions and in table 2 intensity ratios between oxygen and carbon ions with the same charge, I(O’+)/I(C’+), in each molecule. Now, we consider the production mechanism of the multiply charged atomic ions for each molecular target.
3.2.1. CO molecule It can be seen that in the case of the CO molecule the intensities of O’+ and C’+ ions (i =
normalized
to those of singly charged
parent
molecules
for each
co:
co:+
co+
co2+
0+
C+
02+
C2+
co
_
_
1
02
1
4(-2)
1(-l)
2.1(-2) 1.7(-3)
6.9( - 2) 1.3(-l)
8.3( - 2) 1.3(-l)
8.5(-3) 4.q - 3)
1.2( -2) 3.9( - 2)
o’+
(Cat
Cd+
0s+
Cs+
06+
1.q - 3) 9.1( - 4)
2.4(-3) 6.5(-3)
35-4) 8.7( - 4)
24-4) 1.8(-4)
3.5( - 5) 5.2(-5)
9.7( - 5) 5.9( - 5)
co co2
+ 04+ )
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules
1-3) are nearly equal, and the energy shift for C’+ and O’+ ions with the same charge are also roughly equal within the limit of the present energy resolution. These facts suggest that, in CO molecules, the dissociation of multiply charged CO’+ molecular ions play a dominant role in production of multiply charged atomic ions. In this dissociation process, CO’+ ions dissociate into CJ+ and Ok+ ions with nearly equal charge (i =j + k, j = k) and, therefore, these atomic ions get relatively large kinetic energies due to Coulomb explosion proportional to jk (= j2). This Coulomb energy is shared in proportion with MO/(&f0 + Mc) for carbon ion and in proportion with M,-/( MO + A4,) for oxygen ion, where MO and MC are the masses of 0 and C ions respectively. As shown in table 1, the intensity of C5+ is much smaller than that of 05+, indicating the shell effect in the ionization cross section; that is, once K-shell electrons begin to play a role in ion production, the intensities of such ions are reduced significantly. 3.2.2. CO, molecule In the case of the CO, molecule, the energy shift of C’+ ions is very small and the width of C’+ peaks is only slightly broadened compared with that of peaks from CH, molecules. On the other hand, the energy shift of O’+ ions from CO, targets was considerably greater than that from CO targets, and also the widths of O’+ peaks are broader in CO, targets. These facts indicate that the multiply charged atomic ions from CO, also originate from multiply charged molecular ions, which dissociate into atomic or molecular co?+, ions immediately after their production. There are three possible processes where multiply charged molecular COT+ ions dissociate into fragment ions: co;+
+ CO”+ + ok+,
(I)
+cn++o;+,
(2)
-+O j+ + C”’ + Ok+_
(3)
Process (l), where COT+ ions first dissociate into molecular CO”+ and atomic Ok+ ions and, then, ions further dissociate into carbon and co”+
91
oxygen ions, seems to account for the fact that Ok+ ions from CO, target gain more kinetic energy than those from the CO target. However, this fails to explain the fact that C’+ ions have smaller kinetic energies in CO, target than in CO. Process (2) is thought to be less probable, because the CO, molecule has a linear structure with the carbon atom as the center and therefore large deformation of the atomic configuration is needed to form molecular oxygen ions. In fact, the peak intensity at m/q = 16, which might be contaminated by 0: ions directly ionized from 0, molecules in the target impurity gas, is very weak compared with intensities of C+ or 0+ ions (see fig. 1). Process (3), where two oxygen atoms are dissociated in opposite direction, is considered to be most probable because the oxygen ions get strong Coulomb repulsion in the course of the dissociation, whereas the carbon ion, locating at the center of two dissociating oxygen ions, cannot obtain much kinetic energy. Thus, process (3) can well explain the observed facts; that is, the large shift in O’+ peak position and almost no shift in C’+ positions in CO, molecules. It should be noted that C’+ ions (i = 2-4) are much more intense than the corresponding O’+ ions in CO, molecules though the number of oxygen atoms is twice as much as that of carbon atom. Similar results have already been noticed in our previous measurements on N,O targets; the shifts of O’+ peak positions are large while those of N’+ are very small, and the peak intensity of O’+ ions is much weaker than that of N’+ [5]. These results suggest that the nitrogen atom at the center of the N,O molecule tends to be ionized much easier than that at the outside by energetic heavy-ion impact. The similarity in the mass/ charge spectra between CO, and N,O targets indicates that, in dissociation of multiply charged molecular ions from triatomic molecules in linear configuration, electrons tend to be “stolen” by two ions at the outside. In dissociation of multiply charged molecular ions, the geometrical structure. of primary molecules plays an important role in the production of multiply charged atomic ions. This conclusion can be understood from the fact that the collision time in the present work (1.05 MeV/amu Ar’*+ ion impact: order of lOpi7
98
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules
s) and the electron rearrangement time (10-‘6lo-” s) are much shorter than the dissociation time (of the order of lOPi4 s).
powerful structure
method in studying the dynamics of highly charged molecular ions.
and
Acknowledgement 4. Summary In the present work, we have observed the peak shift in mass/charge spectra for multiply charged atomic ions from CO and CO, targets by 1.05 MeV/amu Ar12+ ion impact. The energies of the multiply charged C’+ and O’+ ions from these molecules, except for C’+ ions from CO,, increase roughly with the square of the charge of the atomic ions, indicating that such kinetic energies are provided through the Coulomb potential between the dissociating ions, as has been observed for N,, NO, C,H,, NO, and so on in our previous studies. Almost no shift in peak position of C’+ ions from CO, molecule has been observed and the intensity of the C’+ ions is very high, similar to the case of N,O molecules where the nitrogen atom at the center tends to be ionized easier than N and 0 atoms outside. These results can be understood fairly well by considering the linear configuration of these triatomic molecules. Thus, the present result shows the importance of the atomic configuration of parent molecules in the dissociation of multiply charged molecular ions produced in energetic heavy-ion impact. It has been concluded that the recoil molecular ion spectroscopy (REMIS) in heavy-ion impact would be a
The authors would like to thank Dr. Watanabe and Dr. Y. Awaya for their support the present study.
T. to
References 111C.L. Cocke, Phys. Rev. A 20 (1979) 749. PI J. Ullrich, S. Kelbch, R. Mann, P. Richard and H. Schmit-Backing, J. Phys. B 17 (1984) L785. [31 H. Tawara, T. Tonuma, H. Shibata, M. Kase, T. Kambara, S.H. Be, H. Kumagai and I. Kohno, Phys. Rev. A 33 (1986) 1385. 141 H. Tawara, T. Tonuma, H. Shibata, S.H. Be, H. Kumagai, M. Kase, T. Kambara and I. Kohno, Z. Physik D 2 (1986) 239. 151 H. Tawara, T. Tonuma, K. Baba, T. Matsuo, M. Kase, T. Kambara, H. Kumagai and M. Kohno, Nucl. Instr. Methods B 23 (1987) 203. [61 R. Mann, F. FoIkmann, R.S. Peterson, Gy. Szabo and K.O. Groeneveld, J. Phys. B 11 (1978) 3045. [71 T. Tonuma, H. Shibata, S.H. Be, H. Kumagai, M. Kase, T. Kambara, I. Kohno, A. Ohsaki and H. Tawara, Phys. Rev. A 33 (1986) 3047. PI E. Hille and T.D. Mark, J. Chem. Phys. 69 (1978) 4600. 191 R. Locht, Chem. Phys. 22 (1977) 13. WI T.D. Mark and E. Hille, J. Chem. Phys. 69 (1987) 2492. 1111 A. Crowe and J.W. McConky, J. Phys. B 7 (1974) 349. J. WI K.E. McCulloh, T.E. Sharp and H.M. Rosenstcck, Chem. Phys. 42 (1965) 349.
Chemical Physics 121 (1988) 93-98 North-Holland, Amsterdam
PRODUCTION OF MULTIPLY CHARGED IN ENERGETIC HEAVY-ION IMPACT
IONS FROM CO AND CO, MOLECULES
T. MATSUO ‘, T. TONUMA, M. KASE, T. KAMBARA, RIKEN,
H. KUMAGAI
Wako-shi, Saitama 351-01, Japan
and H. TAWARA Institute of Plasma Physics, Nagoya University, Nagoya 464, Japan Received
3 July 1987
A mass-spectroscopic technique has been applied to separate recoiled ions from CO and CO, targets by 1.05 MeV/amu Ar12+ ion impact. It has been found that energies of multiply charged atomic C’+ and O’+ tons from these molecules, except for those of C’+ ions from CO,, shift toward high-energy side, compared with those for recoil ions from single-atom targets such as Ar. These shifts in energy increase roughly with the square of the charge state of the multiply charged atomic ions, indicating that these energies are provided through Coulomb potential of multiply charged molecular ions which are produced first in collisions and dissociate into atomic ions immediately after their production.
1. Introduction This study is one of a series of our investigations which are concerned with production of atomic ions from molecular targets by energetic heavy-ion impact. Since it was recognized that the ionization probabilities of target atoms, in particular production of multiply charged recoil ions, are significantly enhanced under collisions with energetic heavy ions, a number of measurements of the cross sections for their production have been reported [l]. For example, the recoiled ArlEf ions are produced with a cross section of the order of 10-i’ cm* under bombardment of 15.5 MeV/amu U7*+ ions [2]. However, most of these works have been made by using rare gas atoms as targets. Investigations for molecular targets are still scarce. Recently, we have conducted some systematic investigations on production of multiply charged ions from molecular gas targets by energetic Ar 1 Permanent Medical
address: Medical and Dental University,
Research Institute, Tokyo Bunkyo-ku, Tokyo 113
0301-0104/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
ion impact. In the measurement of the N, molecule, multiply charged atomic recoil ions up to N6+ with a weak trace of N7+ have been observed. In addition, the peak positions of multiply charged nitrogen ions (N’+) in mass/charge spectra were found to shift toward the high-energy side, compared with those from single-atom targets such as Ar, and this peak shift increases with increasing charge state of the ions produced [3]. These facts strongly suggest that such multiply charged atomic ions are produced through processes where multiply charged molecular ions are first produced by the impact and, then, dissociate via Coulomb explosions into atomic ions. Also, peak shifts in mass/charge spectra have been observed for other molecular targets such as NO, N,O, NO, and C,H, 143. To obtain further insight into the mechanism and systematics of production of multiply charged ions from molecular targets, we have used a mass-spectroscopic technique to separate various ions from CO and CO, molecules in 1.05 MeV/amu Ar I*+ ion impact. Although a number B.V.
94
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules
of measurements on fragmentation of these molecules by electron and ion impact have been reported so far, no systematic work has been made on production of multiply charged ions, except the observation of Auger electrons from metastable Li-like (1~2~2~ 4P) carbon and oxygen ions produced by 1.4 MeV/amu Arl’+ impact on CH,, CO and CO, by Mann et al. [6].
2. Experimental The measurements have been performed with the apparatus previously reported in detail [7]. In brief, it consists of a crossed-beam collision system and a mass-spectrometer of sector-magnet type. The recoiled ions produced by ion impact are extracted by a weak field (20 V/cm) applied between two parallel plates positioned at the collision center, where a molecular gas target effused from a single-capillary nozzle was bombarded by ions 1.05 MeV/amu Ar12+ ions. The extracted were accelerated up to 1.5 kV, mass/charge analyzed with the double-focusing sector magnet and finally detected by a channeltron multiplier operated at pulse counting mode. The projectile ions were provided by the heavy-ion linear accelerator (RILAC) of RIKEN. At present, no absolute target density has been determined, though the pressure measured at the wall of the vacuum chamber was about 1 X lo-’ Torr ensuring single-collision conditions.
3.1. General features 3.1.1. CO molecule In fig. 1, a number of peaks corresponding to multiply charged atomic ions such as C’+ and Oif as well as a dominant peak of parent molecular CO+ ions are clearly seen. The peak intensity at m/q = 14, corresponding to doubly charged CO’+ ions, is considerably high compared with that by electron impact; the intensity ratio between the CO’+ peak and CO+ peak is about 2.1 x lo-* in this spectrum, while, by 150 eV electron impact, Hille and Mark obtained a value of 0.75 X 1O-2 for the cross section ratio between C02+ and CO+ production [S]. Moreover, they reported the production of metastable (CO*‘)* ions which dissociate spontaneously during transit from the collision region to the entrance of the mass selector. When dissociation of metastable (C02’) * ion
‘”
!
1.05 MeV;amu
1.05 MeVIamu
Ar”’
AI-“+
on COn
on CO
3. Results and discussion In fig. 1 are shown typical mass/charge (m/q) spectra of recoil ions produced from CO and CO, targets by 1.05 MeV/amu Ar12+ ion impact. Both of these spectra are characterized by a number of peaks corresponding to the fragmented atomic ions having various charge states as well as a dominant peak of singly charged molecular ions. Furthermore, the detailed mass/charge spectra of multiply charged atomic ions from these molecules are shown in fig. 2 in comparison with those from Ar and CH, targets.
lo0 0
256 CHANNEL
NUMBER
Fig. 1. Mass/charge spectra of recoil ions from molecular targets, CO and CO,, under 1.05 MeV/amu Ar12+ ion impact.
T. Maisuo et al. / Multiply charged ions from CO and CO, molecules
95
The intensity of Cf ions is slightly higher than that of O+: the intensity ratio Z(C+)/Z(O+) = 1.15. This is understood to be due to the different dissociation processes; namely, the C+ + 0 dissociation process is responsible for C+ production and the O+ + C process for O+ production, with dissociation energies of 8.3 and 10.7 eV, respectively [ 91.
CHANNEL NUMBER Fig. 2. Expanded mass/charge spectra of multiply charged C’+ and O’+ ions from molecular targets, CO and CO,. The vertical dotted lines are drawn to show the shift of peak positions of multiply charged ions from CO and CO, targets with respect to those from Ar and CH, targets.
occurs during the transit, the resultant Cc and O+ ions are expected to appear at m/q = 10.3 and m/q = 18.3, respectively. In our spectrum, signals at m/q = 10.3 seem to be insignificant and those at m/q = 18.3 may be masked by contaminated H20+ ions at m/q = 18.0. It can be also noted that there is no trace at m/q = 9.33 corresponding to C03+ in this spectrum, indicating that triply charged molecular ions are not stable but dissociate immediately after their production.
3.1.2. CO, molecule In addition to a dominant peak of singly charged COT ions, a peak corresponding to doubly charged molecular ions, CO:+, at m/q = 22 is fairly intense. The intensity ratio between doubly charged and singly charged ions, Z(COi+)/ Z(COc), is about 4.0 x lo-* while Mark and Hille reported this intensity ratio to be 1.2 x lo-* by 171.4 eV electron impact [lo]. It should be noted that intensities of C+ and O+ ions are roughly equal, though the number of oxygen atoms is twice as much as that of carbon atom, confirming that molecules cannot be assumed to consist of the independent constituent atoms even at relatively high-energy collisions. In electron impact, the intensity ratio Z(C+)/Z(O+) is reported to be about 0.5 at the impact energy of 300 eV [ll] and Z(C+)/Z(O+) = 0.7 at 1000 eV [12]. In this molecule, a rather broad peak appears around m/q = 35. This is considered to be due to dissociation products of metastable molecular (CO:‘) * ions as reported by Mark and Hille [lo]; CO+ ions from O+ + CO+ dissociation process are expected to appear at m/q = 35.6 (indicated by an arrow in fig. 1). 3.2. Detailed features Fig. 2 is an expanded part of the mass/charge spectra of multiply charged atomic ions from CO and CO, targets, together with those from Ar and CH, targets for comparison. One may easily note that the observed peaks of C’+ and O’+ ions from CO and CO, targets are shifted in position and are considerably broadened, compared with those from Ar and CH, targets. As has been reported previously, peaks of multiply charged carbon ions, Cl+, from CH, targets show neither shift in peak positions nor broadening of peaks with respect to
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules Table 2 Intensity ratio of secondary in each molecule
30 2C
co
co,
ions with the same charge O’+/C’+
o+,c+
02+/C*+
os+,c3+
0.83 0.95
0.70 0.11
> 0.80 > 0.14
a) 04+,c4+ _ _
os+,c5+
7.8 3.4
a) The intensity of C3+ ions is taken to be that of C3+ +04+ ions approximately.
!
1’ CHARGE
STATE
z
Fig. 3. The observed energy shift of the peaks as a function of the charge state of recoil C’+ and O’+ ions from CO and CO, molecules. Note that no significant energy shift of the peaks is observed for C’+ ions from CO, target.
those of the corresponding Ar’+ ions which gain only a small amount of recoil energy of the order of eV in the present collision energy [4]. Though the present experimental setup does not allow us to measure accurate energies of ions, we have tried to determine the difference in energies of atomic ions from CO and CO* targets, compared with those of ions from Ar targets. In fig. 3 are shown the observed energy shifts of multiply charged atomic ions from CO and CO, targets. From figs. 2 and 3 it can be concluded that the energy shift of C’+ and O’+ ions increases with roughly the square of the charge of ions. The shift for C” ions increases in order of CO > CO,
Table 1 Relative intensities of secondary ions from CO and CO, targets, target; notation m( - n) indicates m x lo-”
> CH,, while that for O’+ increases in order of CO, > CO. Also, the linewidth of C’+ ions increases in the order of CO > CO, > CH, and that of O’+ as CO, > CO. This result is in accordance with that by Mann et al. [6] who observed that, in 1.4 MeV/amu Ar 12-t + CO, CO, and CH, collisions, the line broadening of ls2s2p 4P-ls2 ‘S + e Auger electron transition in carbon ions is increased in order of CO > CO, > CH, and that in oxygen ions in order of CO, > CO. Another important information for understanding the production mechanism of multiply charged atomic ions is the intensity ratios of carbon and oxygen ions in a particular target molecule and those among different molecules. In table 1 are shown relative intensities of multiply charged ions normalized to those of singly charged parent molecular ions and in table 2 intensity ratios between oxygen and carbon ions with the same charge, I(O’+)/I(C’+), in each molecule. Now, we consider the production mechanism of the multiply charged atomic ions for each molecular target.
3.2.1. CO molecule It can be seen that in the case of the CO molecule the intensities of O’+ and C’+ ions (i =
normalized
to those of singly charged
parent
molecules
for each
co:
co:+
co+
co2+
0+
C+
02+
C2+
co
_
_
1
02
1
4(-2)
1(-l)
2.1(-2) 1.7(-3)
6.9( - 2) 1.3(-l)
8.3( - 2) 1.3(-l)
8.5(-3) 4.q - 3)
1.2( -2) 3.9( - 2)
o’+
(Cat
Cd+
0s+
Cs+
06+
1.q - 3) 9.1( - 4)
2.4(-3) 6.5(-3)
35-4) 8.7( - 4)
24-4) 1.8(-4)
3.5( - 5) 5.2(-5)
9.7( - 5) 5.9( - 5)
co co2
+ 04+ )
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules
1-3) are nearly equal, and the energy shift for C’+ and O’+ ions with the same charge are also roughly equal within the limit of the present energy resolution. These facts suggest that, in CO molecules, the dissociation of multiply charged CO’+ molecular ions play a dominant role in production of multiply charged atomic ions. In this dissociation process, CO’+ ions dissociate into CJ+ and Ok+ ions with nearly equal charge (i =j + k, j = k) and, therefore, these atomic ions get relatively large kinetic energies due to Coulomb explosion proportional to jk (= j2). This Coulomb energy is shared in proportion with MO/(&f0 + Mc) for carbon ion and in proportion with M,-/( MO + A4,) for oxygen ion, where MO and MC are the masses of 0 and C ions respectively. As shown in table 1, the intensity of C5+ is much smaller than that of 05+, indicating the shell effect in the ionization cross section; that is, once K-shell electrons begin to play a role in ion production, the intensities of such ions are reduced significantly. 3.2.2. CO, molecule In the case of the CO, molecule, the energy shift of C’+ ions is very small and the width of C’+ peaks is only slightly broadened compared with that of peaks from CH, molecules. On the other hand, the energy shift of O’+ ions from CO, targets was considerably greater than that from CO targets, and also the widths of O’+ peaks are broader in CO, targets. These facts indicate that the multiply charged atomic ions from CO, also originate from multiply charged molecular ions, which dissociate into atomic or molecular co?+, ions immediately after their production. There are three possible processes where multiply charged molecular COT+ ions dissociate into fragment ions: co;+
+ CO”+ + ok+,
(I)
+cn++o;+,
(2)
-+O j+ + C”’ + Ok+_
(3)
Process (l), where COT+ ions first dissociate into molecular CO”+ and atomic Ok+ ions and, then, ions further dissociate into carbon and co”+
91
oxygen ions, seems to account for the fact that Ok+ ions from CO, target gain more kinetic energy than those from the CO target. However, this fails to explain the fact that C’+ ions have smaller kinetic energies in CO, target than in CO. Process (2) is thought to be less probable, because the CO, molecule has a linear structure with the carbon atom as the center and therefore large deformation of the atomic configuration is needed to form molecular oxygen ions. In fact, the peak intensity at m/q = 16, which might be contaminated by 0: ions directly ionized from 0, molecules in the target impurity gas, is very weak compared with intensities of C+ or 0+ ions (see fig. 1). Process (3), where two oxygen atoms are dissociated in opposite direction, is considered to be most probable because the oxygen ions get strong Coulomb repulsion in the course of the dissociation, whereas the carbon ion, locating at the center of two dissociating oxygen ions, cannot obtain much kinetic energy. Thus, process (3) can well explain the observed facts; that is, the large shift in O’+ peak position and almost no shift in C’+ positions in CO, molecules. It should be noted that C’+ ions (i = 2-4) are much more intense than the corresponding O’+ ions in CO, molecules though the number of oxygen atoms is twice as much as that of carbon atom. Similar results have already been noticed in our previous measurements on N,O targets; the shifts of O’+ peak positions are large while those of N’+ are very small, and the peak intensity of O’+ ions is much weaker than that of N’+ [5]. These results suggest that the nitrogen atom at the center of the N,O molecule tends to be ionized much easier than that at the outside by energetic heavy-ion impact. The similarity in the mass/ charge spectra between CO, and N,O targets indicates that, in dissociation of multiply charged molecular ions from triatomic molecules in linear configuration, electrons tend to be “stolen” by two ions at the outside. In dissociation of multiply charged molecular ions, the geometrical structure. of primary molecules plays an important role in the production of multiply charged atomic ions. This conclusion can be understood from the fact that the collision time in the present work (1.05 MeV/amu Ar’*+ ion impact: order of lOpi7
98
T. Matsuo et al. / Multiply charged ions from CO and CO, molecules
s) and the electron rearrangement time (10-‘6lo-” s) are much shorter than the dissociation time (of the order of lOPi4 s).
powerful structure
method in studying the dynamics of highly charged molecular ions.
and
Acknowledgement 4. Summary In the present work, we have observed the peak shift in mass/charge spectra for multiply charged atomic ions from CO and CO, targets by 1.05 MeV/amu Ar12+ ion impact. The energies of the multiply charged C’+ and O’+ ions from these molecules, except for C’+ ions from CO,, increase roughly with the square of the charge of the atomic ions, indicating that such kinetic energies are provided through the Coulomb potential between the dissociating ions, as has been observed for N,, NO, C,H,, NO, and so on in our previous studies. Almost no shift in peak position of C’+ ions from CO, molecule has been observed and the intensity of the C’+ ions is very high, similar to the case of N,O molecules where the nitrogen atom at the center tends to be ionized easier than N and 0 atoms outside. These results can be understood fairly well by considering the linear configuration of these triatomic molecules. Thus, the present result shows the importance of the atomic configuration of parent molecules in the dissociation of multiply charged molecular ions produced in energetic heavy-ion impact. It has been concluded that the recoil molecular ion spectroscopy (REMIS) in heavy-ion impact would be a
The authors would like to thank Dr. Watanabe and Dr. Y. Awaya for their support the present study.
T. to
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