- Email: [email protected]
Molecular ion collision chemistry using particle accelerators
Nuclear Instruments and Methods in Physics Research BlO/ll North-Holland, Amsterdam
MOLECULAR
ION COLLISION
Tom J. GRAY,
J.C. LEGG
Department
CHEMISTRY
253
(1985) 253-258
USING PARTICLE
ACCELERATORS
and Vincent NEEDHAM
of Physics, J.R. Macdonald Laboratory,
Kansas State University, Manhattan,
KS 66506, USA
A spectroscopy for molecular ions is reported which is based upon the two-dimensional analysis of the time of flight and the ion’s final state using a dispersive electrostatic analyzer. These ions are produced in a recoil ion source by a pulsed fast @’ pump beam. Two-dimensional recoil ion spectra for CH, source gas at pressures of 0.5 mTorr and 4.0 mTorr are reported. Model calculations for spectral distributions provide the identifications of various processes involving specified molecular ion species. The first direct observations of metastable CH*+, CHg+, CH:+, and CH:+ molecular ions are reported and limits on the lifetimes for these ion species are deduced.
1. Introduction
Long-lived multiply-charged molecular ions of the form CHz+ have never been directly observed as products of an electron-bombardment ion source. Spohr et al. (11 reported ESCA measurements indicating that the CH$+ ion was being formed by electron bombardment and had a lifetime of approximately lo-l5 s. Ast et al. [2] reported the observation of CH:+ produced in the charge-stripping reaction, CH: + N, --+CH$+ + R + e, where R and e were undetected residual products. This charge-stripping measurement using a ZAB spectrograph required that CH,*+ have a lifetime 2 3 ps, the ion’s transit time in the instrument. Rabrenovic et al. [3] later reported the observation of the fragmentation products, CH:+ and C*+, resulting from the decay of CH:+ ions formed by charge stripping. Litherland [4] reported the observation of a metastable CH:+ molecule with a lifetime of 10 ps using accelerator mass spectroscopy. Molecular ions with a charge state greater than + 2 were not observed in any of these experiments. The observation of long-lived molecular ions has prompted re-examination of the theoretical structure calculations for these ions. Hanner and Moran [5], Pople et al. [6], and Siegbahn [7] calculated the ground state potential curves for CH$+ allowing a change in molecular symmetry from T,, to D4h, optimizing on the planar geometry. These calculations predicted a minimum in the molecular potential for CH:+ which could account for the suggested experimental lifetimes of 2 3 cs hence ruling out the short-lived state (lo-l5 s) as the sole mode of existence for CH:+ . Pople et al. [6] also computed the molecular potential curves for CH*+, CH;+, and CH;+ with the predicted results that CH*+ had no minima in the potential curves while CH:+ had a very shallow minimum in its potential curve. Hence CH*+ should decay spontaneous because of the repul0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
sive nature of the molecular potential while CH$+ should exhibit a very short lifetime. CH$+ was found to have a deeper minimum in the molecular potential and was thus predicted to have a lifetime of the same order as that observed for CH i’. Heil et al. [8] calculated potential curves for the CH *+ *X+ states which exhibit a shallow minimum at an internuclear distance of approximately 3.2 bohr, which would allow a finite lifetime for the CH*+ molecule. The previous work suggests that there may be unobserved and unpredicted multiply-charged, metastable molecular ions. These ions have not been observed as products of electron bombardment because of the necessity for molecular geometry rearrangement. Electrons do not transfer sufficient momentum in a collision with a heavy molecule to cause the molecular rearrangement required for metastable ion formation. Low-velocity charge-stripping reactions like those reported by Ast et al. [2] and Rabrenovic et al. [3] produce metastable 2 + ions. To date triply-charged molecular ions by chargestripping singly-charged molecular ion beams have not been observed.
2. Experimental procedure Edwards et al. [9,10] have reported time-of-flight energy spectroscopy for a number of molecular systems. In their method the molecular gas targets are bombarded by a pulsed beam of ions of H, He, or 0 at energies of 1 MeV and charge states of 1-C . The target molecules dissociate into charged fragments which drift out of the interaction region over a 5 cm flight path and then are accelerated prior to entering an electrostatic analyzer where their energy is measured. A similar type of spectroscopy was developed by Cocke [11,12] for the production of low-velocity highly I. ATOMIC
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PHENOMENA
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charged recoil ions by bombardment
of monoatomic gases with high energy heavy ions incident in the energy range of E 2 15 MeV. The details and uses of recoil ions thus produced are discussed in Ref. 12. The main difference between the experimental method of the work of Edwards et al. [9] and the recoil ion spectroscopy [ll] is found in the source design from which the low-velocity ions are derived. The recoil ion source used in the work at Kansas State University is shown in fig. 1. It consists of two grids and a gridded extraction electrode. One grid is at ground potential while the extractor and other grid are set at potentials Vi and V, (Vi > V2), respectively. The pulsed fast heavy ion pump beam from the tandem Van de Graaff accelerator passes between the grids labeled Vi and V,. Thus the ions are extracted from the interaction region and drift over a total flight path of 23.9 cm. This experiment technique has been used extensively in our laboratory to measure charge-changing reactions for low-energy highly charged atomic ions. [12] The time-of-flight of the ions from the recoil ion source is proportional to the square root of the massto-charge ratio of the ions as they are accelerated away from the pump beam. In these experiments the time-offlight varied from 1 to 10 ps. The electrostatic analysis is achieved using a double focusing hemispherical electrostatic analyzer whose voltage was scanned to cover the energy range of - 20 eV to - 550 eV. The analyzer voltage and the time-of-flight of the ions are digitized and stored in a tw~~~en~on~ array referred to as a two-dimensional coincidence spectrum. Computer modeling of the two-dimensional coincidence spectrum was done taking into consideration the geometry of the recoil ion source and analyzer system, the voltages applied to various components in the system, and a representation of the various events that could occur to a given molecular or atomic ion species
between the point of creation and that of detection. Various postulated events which might appear in a two-dimensional spectrum are shown in table 1. A primary ion in the context of this work is any ion created in the initial collision with the fast heavy ion pump beam from the accelerator and arriving at the detector without further interaction or modification. The processes listed as examples in table 1 may take place while the ion is being accelerated in the recoil source or at any subsequent time during motion over the drift length between initial acceleration and final analysis and detection. For specific types of events, general features of the two-dimensional coincidence spectra are as follows: (1) All primary ions are detected at a fixed analyzer voltage which is dependent upon the voltages applied to the recoil ion source and they arrive at the detector at times specified by ( m/q)‘12.
(2) Ions that have undergone charge exchange after acceleration arrive at the detector with the flight time of the parent ion but require higher analyzer voltages which are proportional to the ratio of initial and final charge states, q/q’, (3) Ions that have undergone charge exchange during acceleration wili he on a ~nt~uo~ locus determined by the time-of-flight and analyzer voltage with end points determined by the parent ion and final state ion. (4) Any ion detected as a distinct peak in the primary ion spectrum must have a lifetime, r > the flight time which is typically - 3 +r~s. (5) Molecular ions with shorter lifetimes (T = acceleration time, - 0.2 ps) may be detected as peaks in the spectrum, but such ions are found on the locus of points passing through the species to which they have decayed. Table 1 Examples of postulated events for two-dimensional coincidence spectra for CH, source gas (1) Primary ions are detected Pump Beam+CH, + C2+ +R (2) Single or multiple charge exchange of a primary ion
C9+ +R + @q-m)+ +Rm+ (3) Creation of a molecular ion Pump Beam + CH, 4 CH:+ + 2e (4) Decay of a molecular ion CH;+ -+ CH2+ +H, (5) Coulomb explosion of a short-lived molecular ion mp+l)+ --, R9+ +H; (6) Protonation C+ +CH,+CH+ +R (7) Rick-up of a carbon atom CH;+ +CH, -+ C,H;+ +H, (8) Molecular dissociation through secondary collisions CH2+ 2 +R-+C+ +R’ Fig. 1. Ebqxrimental arrangement
(schematic).
Note: R denotes unidentified residuat components.
T.J. Gray et al. / Molecular
ion collision chemistty
255
3228
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TIWOFFLIGHT
Fig. 2. Model calculationsfor the two-dimensionalcoincidence spectrum of electrostatic analyzer voltage versus ion time of flight. Various types of events are identified in the figure.
(6) Ions that have increased their mass through a pick-up reaction require lower analyzer voltages than the primary ions for passage through the electrostatic analyzer. (7) Ions that are the result of a Coulomb explosion near the site of creation have a characteristic “V-shaped” signature which extends above the primary ion locus and extends to shorter and longer times of flight than the primary ion of the same family of ions. Events of the types discussed above are exhibited in the model calculations shown in fig. 2 for a pump beam bombarding CH,. For clarity only a few representative events are included in fig. 2.
3.
Results ad discussion
Two-dimensional coincidence spectra have been measured for a number of target systems at various source gas pressures. Fig. 3 gives an example for 19 MeV F4+ pumping CH, source gas in the recoil ion source at 0.5 mTorr pressure. The density of points and the size of plotting symbols are proportional to the measured intensity. Representative events are identified in the figure. In fig. 3 the peaks labeled 0: and NC arise from the residual vacuum of - 6 x lo-’ Torr. Fig. 3 is compressed in both dimensions for purposes of display and hence fine details are lost in the compression. However, sufficient spectra resolution remains for the identification of the types of events postulated in table 1. A more detailed analysis of selected areas of fig. 3 for the CH, source gas are discussed below. The richness of this new spectroscopy is evidenced
I 255 TlMECFFLlWT(
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‘4
Fig. 3. BxPerixnentaltwodimensionalcoincidencespectrumfor CH, source gas being pumped by a fast p’ beam. The time-of-flightranges from - 1 ps for H+ ions to - 10 ps for 0: ions. The M/q scale along the top of the figure is proportional to the square of the timsof-flight. Example events are identified in the figure.
by the spectral features exhibited in fig. 3. Previous spectroscopies such as magnetic and/or electrostatic or time-of-flight mass spectroscopy have been restricted to one-dimensional scans of the mass spectrum. Hence many relevant small-probability events were lost in the background. Furthermore electron bombardment has been the dominant mode of system excitation. Through the combination of time-of-flight mass spectroscopy with final state selection through dispersive analysis and the utilization of a recoil ion source for excitation the formation and decay of multiple-charged molecular ions have become observable in the present work. In fig. 3 for CH, we observe a peak at M/q = 28 on the loci of CH: ions. These data indicate the formation of heavier hydrocarbon ion species such as C, Hi, C,H; , and CIH: which decay into lighter CH: ion species. Work is in progress to improve the system resolution to allow a more precise identification of the final decay products for these heavier ion species. We now restrict our attention to a two-dimensional segment of the recoil ion spectra covering the range from the CH: primary ion to the H+ primary ion. By so doing we can expand the scale of the data to bring out spectral features of interest. The data for CH, as the source gas at a pressure of 0.5 mTorr are presented in fig. 4. We note the Coulomb explosion signatures for the Hl and H+ groups identified in fig. 4. We further expand the data by the definition of a two-dimensional I. ATOMIC PHYSICS/RELATED PHENOMENA
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T.J. Gray et al. / Molecular ion collision chemistry
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Fig. 4. An experimentaltwo-dimensionalcoincidencespectrum for a F4+ beam pumping CH, at a source pressure of 0.5 mTorr. Various events are identifiedin the figure. We define two windowsfor expansionto allowa more detailed display of
64
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the spectral features. Only window 1 is indicated in the figure. Window 2 has the same m/q boundaries as are given for window 1 but window 2 is for a higher source gas pressure (P = 4.0 mTorr).
window specified as window 1 in the figure. This window encompasses the region including the Cz+ and C3+ primary ions. Although not shown explicitly, we have also defined a second window, i.e. window 2, with the same m/q boundaries as window 1 for data taken at a CH, pressure of 4.0 mTorr in the recoil ion source. Both windows 1 and 2 are shown in fig. 5. On the expanded scale of window 1, the detail of this new spectroscopy becomes apparent. We identify eight peaks in this spectrum as indicated in the figure. These peaks result from the detection of CH:+ , CHi+ , CHg+ , CH2+ C2+ CH;+, CH:+ , and C3+ ions. The C2+ and C’+ are’stable atomic primary ions from the recoil source. The current measurements present the first direct observation of the CH:+ molecular ion without doing charge-stripping as reported by Ast et al. [2] The fact that we see CH:+ and CH:+ as peaks in the timeof-flight dimension indicates that their lifetimes are 7 > 3 gs, the instrumental flight time for these ions. Similarly these measurements present the first observation of CH2+ molecular ions. As in the case of the CHi+ and CHz+ ions the CH2+ species is estimated to have a lifetime, 7 & 3 gs. Lying below the primary ion locus are two peaks identified as CH$+ and CHs+. These data represent the first observations of these species. There is essentially no intensity on the primary
04.
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3'2
4.8
6
TIME OF FLIGHT
Fig. 5. Window 1 and window 2 are displayed. Window 1 is for a source pressure of 0.5 mTorr while window 2 is for a source pressure of 4.0 mTorr. The solid lines define event loci from which we obtain projections of the data contained in the particular locus of interest for display in fig. 6.
ion locus for these two groups. Model calculations indicate that these two ion species were formed in primary collisions with the pump beam and accelerated out of the ion source. Their lifetimes are short compared with the instrumental ffight time of - 3 ps; the CH:+ and CHz+ decay in flight to CH2+ and C’+, respectively. Because they are peaks in the time-of-flight dimension, their lifetimes must be comparable with their respective acceleration times in the recoil ion source. Hence we set a limit of T 2 206 ns for the lifetimes of CH:+ and CH:+ . Also seen in window 1 above the primary locus
T.J. Gray et al. / Molecular ion collision chemistv
are three continua, one of which contains a peak corresponding to CH:+ decaying in flight to C2+. The lifetime for the CH:+ ion is approximately the acceleration time, 7 = 200 ns. Projections taken from the two-dimensional plots of fig. 5 for Windows 1 and 2 are shown in fig. 6. On the primary ion locus and CH 2+ locus for the 0.5 mTorr data, the peaks associated with CH:+, CH:+, CH:+, and CHZ+ are clearly observed. It is worthwhile to note that the number of coincidence counts stored in fig. 3 for the main spectrum is > 4 X lo6 counts. The projections in fig. 6 show that the intensity in the peaks of interest (CHf+ , CH :+, CH$+, CH2+) are an extremely small part of the total coincidence intensity. If one performed a one-dimensional standard dispersive analysis, such small signal intensities would be lost in the noise. Window 2 in fig. 5 presents measurements of the spectra at a source pressure of 4.0 mTorr for the same m/q boundaries as window 1. Comparison of windows 1 and 2 show substantial differences in spectral features. At 4.0 mTorr all intensity below the primary ion locus is absent in contrast to the features noted for window 1 at the lower source pressure. Furthermore, the CH:+ , CH:+, and CH;+ ions have been displaced from the window to the C+ locus which is outside the boundaries of the window. This displacement is the result of secondary collisions at the higher source pressure. The peak corresponding to the CH:+ ions seen below the primary ion locus in window 1 is absent in window 2. However, we see strong intensities for peaks corresponding to CH:+ and CHz+ in the C2+ locus indicated in window 2. These two molecular ion species are clearly seen in the 4.0 mTorr C2+ locus of fig. 6. We do not detect them as CH:+ and CH:+ , but see the dissociation products of the reactions CH:++R+C2++R’
and
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where R and R’ are unidentified components. The fact that CH:+ is a sharp peak as shown in the projection of the C2+ locus as given in fig. 6 indicates that the lifetime for this species is comparable with the acceleration time, T = 200 ns. This lifetime limit could be increased as there is competition between the dissociation rate and the natural decay rate at this source pressure. The same conclusions can be made for the CHz+ ion species in view of the data of windows 1 and 2. As shown by the differences between window 1 (0.5 mTorr source gas pressure) and window 2 (4.0 mTorr source. gas pressure) there are very large pressure dependences which dominate the spectral features observed in the two-dimensional coincidence spectroscopy. Work is in progress to study the pressure dependence in detail over selected regions of the full spectrum. Through such studies the formation and collisional history of each
TIMEOF FLIGHT
Fig. 6. Projections of the primary ion locus, CH*+ ion locus for P = 0.5mTorr, and C2+ ion locus for P = 4.0 mTorr. The molecular ion species CH:+, CH$+, CH$+, CH’+, CH:+ and CHi+ are identified in the figure. The m/q scale at the top of the figure does not apply to the C? ion locus data at the bottom of the figure.
identifiable molecular ion species may be further characterized. Ion species such as CH 2+ CH:+ and CH:+ exhibit lifetimes > 200 ns. previous LoretiLl molecular structure calculations by Pople et al. [6] have suggested that CH2+ had no bound states and that CH:+ should have an extremely short lifetime. Our data for CH2+ indicate I. ATOMIC PHYSICS/RELATED PHENOMENA
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T.J. Gray et al. / Molecular ion collision chemistry
otherwise. Recent theoretical work by Heil et al. [8] for CH’+ shows a shallow minimum for a diabatic 2X+ state, but no lifetime estimates were made. No theoretical calculations for the lifetimes of CHi+ and CH:+ exist. This work was supported by the Division of Chemical Sciences, US Department of Energy. The authors wish to express their appreciation to Mr. Bob Krause and Mr. Mike Wells for their contributions to accelerator modifications which provided the beam stability required for this work.
References T. Bergmark, N. Magnusson, L.O. Werme, C. Nordhng and K. Siegbahn, Phys. Scripta 2 (1970) 31. [Z] T. Ast, C.J. Porter, C.J. Proctor and J.H. Beynon, Chem.
[l) R. Spohr,
Phys. Lett. 78 (1981) 439. [3] M. Rabrenovic, A.G. Brenton, and J.H. Beynon, Mass Spectr. Ion Phys. 52 (1983) 175.
Int. J.
[4] A.E. Litherland, Ann. Rev. Nucl. Part. Sci. 30 (1980) 437. [5] A.W. Hanner and T.F. Moran, Organic Mass Spectr. 16 (1981) 512. [6] J.A. Pople, B. Tidor and P. von Ragut Schleyer, Chem. Phys. L&t. 88 (1982) 533. [7] P.E.M. Siegbahn, Chem. Phys. 66 (1982) 443. [8] T.G. Heil, S.E. Butler and A. DaIgamo, Phys. Rev. A27 (1983) 2365. [9] A.K. Edwards, R.M. Wood and M.F. Steuer, Phys. Rev. Al5 (1977) 48. [lo] A.K. Edwards, R.M. Wood and M.F. Steuer, Ann. Hr. Phys. Sot. 4 (1980) 171 and references therein. [ll] T.J. Gray and CL. Cocke, IEEE Trans. Nucl. Sci. NS-30 (1983) 937. [12] C.L. Cocke, T.J. Gray, E. Justiniano, C. Can, B. Waggoner, S.L. Varghese and R. Mann, Phys. Scripta 13 (1983) 75; C.L. Cocke, R. Dubois, T.J. Gray, E. Justiniano and C. Can, Phys. Rev. Lett. 46 (1981) 1671; Ed. Justiniano, C.L. Cocke, T.J. Gray, R.D. Dubois and C. Can, Phys. Rev. A24 (1981) 2953; T.J. Gray, C.L. Cocke and E. Justiniano, Phys. Rev. A22 (1980) 849; C.L. Cocke, Phys. Rev. A20 (1979) 749.
MOLECULAR
ION COLLISION
Tom J. GRAY,
J.C. LEGG
Department
CHEMISTRY
253
(1985) 253-258
USING PARTICLE
ACCELERATORS
and Vincent NEEDHAM
of Physics, J.R. Macdonald Laboratory,
Kansas State University, Manhattan,
KS 66506, USA
A spectroscopy for molecular ions is reported which is based upon the two-dimensional analysis of the time of flight and the ion’s final state using a dispersive electrostatic analyzer. These ions are produced in a recoil ion source by a pulsed fast @’ pump beam. Two-dimensional recoil ion spectra for CH, source gas at pressures of 0.5 mTorr and 4.0 mTorr are reported. Model calculations for spectral distributions provide the identifications of various processes involving specified molecular ion species. The first direct observations of metastable CH*+, CHg+, CH:+, and CH:+ molecular ions are reported and limits on the lifetimes for these ion species are deduced.
1. Introduction
Long-lived multiply-charged molecular ions of the form CHz+ have never been directly observed as products of an electron-bombardment ion source. Spohr et al. (11 reported ESCA measurements indicating that the CH$+ ion was being formed by electron bombardment and had a lifetime of approximately lo-l5 s. Ast et al. [2] reported the observation of CH:+ produced in the charge-stripping reaction, CH: + N, --+CH$+ + R + e, where R and e were undetected residual products. This charge-stripping measurement using a ZAB spectrograph required that CH,*+ have a lifetime 2 3 ps, the ion’s transit time in the instrument. Rabrenovic et al. [3] later reported the observation of the fragmentation products, CH:+ and C*+, resulting from the decay of CH:+ ions formed by charge stripping. Litherland [4] reported the observation of a metastable CH:+ molecule with a lifetime of 10 ps using accelerator mass spectroscopy. Molecular ions with a charge state greater than + 2 were not observed in any of these experiments. The observation of long-lived molecular ions has prompted re-examination of the theoretical structure calculations for these ions. Hanner and Moran [5], Pople et al. [6], and Siegbahn [7] calculated the ground state potential curves for CH$+ allowing a change in molecular symmetry from T,, to D4h, optimizing on the planar geometry. These calculations predicted a minimum in the molecular potential for CH:+ which could account for the suggested experimental lifetimes of 2 3 cs hence ruling out the short-lived state (lo-l5 s) as the sole mode of existence for CH:+ . Pople et al. [6] also computed the molecular potential curves for CH*+, CH;+, and CH;+ with the predicted results that CH*+ had no minima in the potential curves while CH:+ had a very shallow minimum in its potential curve. Hence CH*+ should decay spontaneous because of the repul0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
sive nature of the molecular potential while CH$+ should exhibit a very short lifetime. CH$+ was found to have a deeper minimum in the molecular potential and was thus predicted to have a lifetime of the same order as that observed for CH i’. Heil et al. [8] calculated potential curves for the CH *+ *X+ states which exhibit a shallow minimum at an internuclear distance of approximately 3.2 bohr, which would allow a finite lifetime for the CH*+ molecule. The previous work suggests that there may be unobserved and unpredicted multiply-charged, metastable molecular ions. These ions have not been observed as products of electron bombardment because of the necessity for molecular geometry rearrangement. Electrons do not transfer sufficient momentum in a collision with a heavy molecule to cause the molecular rearrangement required for metastable ion formation. Low-velocity charge-stripping reactions like those reported by Ast et al. [2] and Rabrenovic et al. [3] produce metastable 2 + ions. To date triply-charged molecular ions by chargestripping singly-charged molecular ion beams have not been observed.
2. Experimental procedure Edwards et al. [9,10] have reported time-of-flight energy spectroscopy for a number of molecular systems. In their method the molecular gas targets are bombarded by a pulsed beam of ions of H, He, or 0 at energies of 1 MeV and charge states of 1-C . The target molecules dissociate into charged fragments which drift out of the interaction region over a 5 cm flight path and then are accelerated prior to entering an electrostatic analyzer where their energy is measured. A similar type of spectroscopy was developed by Cocke [11,12] for the production of low-velocity highly I. ATOMIC
PHYSICS/RELATED
PHENOMENA
254
T.J. Gray et al, / Molecular ion collision chemistry
charged recoil ions by bombardment
of monoatomic gases with high energy heavy ions incident in the energy range of E 2 15 MeV. The details and uses of recoil ions thus produced are discussed in Ref. 12. The main difference between the experimental method of the work of Edwards et al. [9] and the recoil ion spectroscopy [ll] is found in the source design from which the low-velocity ions are derived. The recoil ion source used in the work at Kansas State University is shown in fig. 1. It consists of two grids and a gridded extraction electrode. One grid is at ground potential while the extractor and other grid are set at potentials Vi and V, (Vi > V2), respectively. The pulsed fast heavy ion pump beam from the tandem Van de Graaff accelerator passes between the grids labeled Vi and V,. Thus the ions are extracted from the interaction region and drift over a total flight path of 23.9 cm. This experiment technique has been used extensively in our laboratory to measure charge-changing reactions for low-energy highly charged atomic ions. [12] The time-of-flight of the ions from the recoil ion source is proportional to the square root of the massto-charge ratio of the ions as they are accelerated away from the pump beam. In these experiments the time-offlight varied from 1 to 10 ps. The electrostatic analysis is achieved using a double focusing hemispherical electrostatic analyzer whose voltage was scanned to cover the energy range of - 20 eV to - 550 eV. The analyzer voltage and the time-of-flight of the ions are digitized and stored in a tw~~~en~on~ array referred to as a two-dimensional coincidence spectrum. Computer modeling of the two-dimensional coincidence spectrum was done taking into consideration the geometry of the recoil ion source and analyzer system, the voltages applied to various components in the system, and a representation of the various events that could occur to a given molecular or atomic ion species
between the point of creation and that of detection. Various postulated events which might appear in a two-dimensional spectrum are shown in table 1. A primary ion in the context of this work is any ion created in the initial collision with the fast heavy ion pump beam from the accelerator and arriving at the detector without further interaction or modification. The processes listed as examples in table 1 may take place while the ion is being accelerated in the recoil source or at any subsequent time during motion over the drift length between initial acceleration and final analysis and detection. For specific types of events, general features of the two-dimensional coincidence spectra are as follows: (1) All primary ions are detected at a fixed analyzer voltage which is dependent upon the voltages applied to the recoil ion source and they arrive at the detector at times specified by ( m/q)‘12.
(2) Ions that have undergone charge exchange after acceleration arrive at the detector with the flight time of the parent ion but require higher analyzer voltages which are proportional to the ratio of initial and final charge states, q/q’, (3) Ions that have undergone charge exchange during acceleration wili he on a ~nt~uo~ locus determined by the time-of-flight and analyzer voltage with end points determined by the parent ion and final state ion. (4) Any ion detected as a distinct peak in the primary ion spectrum must have a lifetime, r > the flight time which is typically - 3 +r~s. (5) Molecular ions with shorter lifetimes (T = acceleration time, - 0.2 ps) may be detected as peaks in the spectrum, but such ions are found on the locus of points passing through the species to which they have decayed. Table 1 Examples of postulated events for two-dimensional coincidence spectra for CH, source gas (1) Primary ions are detected Pump Beam+CH, + C2+ +R (2) Single or multiple charge exchange of a primary ion
C9+ +R + @q-m)+ +Rm+ (3) Creation of a molecular ion Pump Beam + CH, 4 CH:+ + 2e (4) Decay of a molecular ion CH;+ -+ CH2+ +H, (5) Coulomb explosion of a short-lived molecular ion mp+l)+ --, R9+ +H; (6) Protonation C+ +CH,+CH+ +R (7) Rick-up of a carbon atom CH;+ +CH, -+ C,H;+ +H, (8) Molecular dissociation through secondary collisions CH2+ 2 +R-+C+ +R’ Fig. 1. Ebqxrimental arrangement
(schematic).
Note: R denotes unidentified residuat components.
T.J. Gray et al. / Molecular
ion collision chemistty
255
3228
I I
%
18
12
884
21
I
I
III
II
lecn,
--cl+
TIWOFFLIGHT
Fig. 2. Model calculationsfor the two-dimensionalcoincidence spectrum of electrostatic analyzer voltage versus ion time of flight. Various types of events are identified in the figure.
(6) Ions that have increased their mass through a pick-up reaction require lower analyzer voltages than the primary ions for passage through the electrostatic analyzer. (7) Ions that are the result of a Coulomb explosion near the site of creation have a characteristic “V-shaped” signature which extends above the primary ion locus and extends to shorter and longer times of flight than the primary ion of the same family of ions. Events of the types discussed above are exhibited in the model calculations shown in fig. 2 for a pump beam bombarding CH,. For clarity only a few representative events are included in fig. 2.
3.
Results ad discussion
Two-dimensional coincidence spectra have been measured for a number of target systems at various source gas pressures. Fig. 3 gives an example for 19 MeV F4+ pumping CH, source gas in the recoil ion source at 0.5 mTorr pressure. The density of points and the size of plotting symbols are proportional to the measured intensity. Representative events are identified in the figure. In fig. 3 the peaks labeled 0: and NC arise from the residual vacuum of - 6 x lo-’ Torr. Fig. 3 is compressed in both dimensions for purposes of display and hence fine details are lost in the compression. However, sufficient spectra resolution remains for the identification of the types of events postulated in table 1. A more detailed analysis of selected areas of fig. 3 for the CH, source gas are discussed below. The richness of this new spectroscopy is evidenced
I 255 TlMECFFLlWT(
I
I
I
‘4
Fig. 3. BxPerixnentaltwodimensionalcoincidencespectrumfor CH, source gas being pumped by a fast p’ beam. The time-of-flightranges from - 1 ps for H+ ions to - 10 ps for 0: ions. The M/q scale along the top of the figure is proportional to the square of the timsof-flight. Example events are identified in the figure.
by the spectral features exhibited in fig. 3. Previous spectroscopies such as magnetic and/or electrostatic or time-of-flight mass spectroscopy have been restricted to one-dimensional scans of the mass spectrum. Hence many relevant small-probability events were lost in the background. Furthermore electron bombardment has been the dominant mode of system excitation. Through the combination of time-of-flight mass spectroscopy with final state selection through dispersive analysis and the utilization of a recoil ion source for excitation the formation and decay of multiple-charged molecular ions have become observable in the present work. In fig. 3 for CH, we observe a peak at M/q = 28 on the loci of CH: ions. These data indicate the formation of heavier hydrocarbon ion species such as C, Hi, C,H; , and CIH: which decay into lighter CH: ion species. Work is in progress to improve the system resolution to allow a more precise identification of the final decay products for these heavier ion species. We now restrict our attention to a two-dimensional segment of the recoil ion spectra covering the range from the CH: primary ion to the H+ primary ion. By so doing we can expand the scale of the data to bring out spectral features of interest. The data for CH, as the source gas at a pressure of 0.5 mTorr are presented in fig. 4. We note the Coulomb explosion signatures for the Hl and H+ groups identified in fig. 4. We further expand the data by the definition of a two-dimensional I. ATOMIC PHYSICS/RELATED PHENOMENA
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Fig. 4. An experimentaltwo-dimensionalcoincidencespectrum for a F4+ beam pumping CH, at a source pressure of 0.5 mTorr. Various events are identifiedin the figure. We define two windowsfor expansionto allowa more detailed display of
64
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the spectral features. Only window 1 is indicated in the figure. Window 2 has the same m/q boundaries as are given for window 1 but window 2 is for a higher source gas pressure (P = 4.0 mTorr).
window specified as window 1 in the figure. This window encompasses the region including the Cz+ and C3+ primary ions. Although not shown explicitly, we have also defined a second window, i.e. window 2, with the same m/q boundaries as window 1 for data taken at a CH, pressure of 4.0 mTorr in the recoil ion source. Both windows 1 and 2 are shown in fig. 5. On the expanded scale of window 1, the detail of this new spectroscopy becomes apparent. We identify eight peaks in this spectrum as indicated in the figure. These peaks result from the detection of CH:+ , CHi+ , CHg+ , CH2+ C2+ CH;+, CH:+ , and C3+ ions. The C2+ and C’+ are’stable atomic primary ions from the recoil source. The current measurements present the first direct observation of the CH:+ molecular ion without doing charge-stripping as reported by Ast et al. [2] The fact that we see CH:+ and CH:+ as peaks in the timeof-flight dimension indicates that their lifetimes are 7 > 3 gs, the instrumental flight time for these ions. Similarly these measurements present the first observation of CH2+ molecular ions. As in the case of the CHi+ and CHz+ ions the CH2+ species is estimated to have a lifetime, 7 & 3 gs. Lying below the primary ion locus are two peaks identified as CH$+ and CHs+. These data represent the first observations of these species. There is essentially no intensity on the primary
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Fig. 5. Window 1 and window 2 are displayed. Window 1 is for a source pressure of 0.5 mTorr while window 2 is for a source pressure of 4.0 mTorr. The solid lines define event loci from which we obtain projections of the data contained in the particular locus of interest for display in fig. 6.
ion locus for these two groups. Model calculations indicate that these two ion species were formed in primary collisions with the pump beam and accelerated out of the ion source. Their lifetimes are short compared with the instrumental ffight time of - 3 ps; the CH:+ and CHz+ decay in flight to CH2+ and C’+, respectively. Because they are peaks in the time-of-flight dimension, their lifetimes must be comparable with their respective acceleration times in the recoil ion source. Hence we set a limit of T 2 206 ns for the lifetimes of CH:+ and CH:+ . Also seen in window 1 above the primary locus
T.J. Gray et al. / Molecular ion collision chemistv
are three continua, one of which contains a peak corresponding to CH:+ decaying in flight to C2+. The lifetime for the CH:+ ion is approximately the acceleration time, 7 = 200 ns. Projections taken from the two-dimensional plots of fig. 5 for Windows 1 and 2 are shown in fig. 6. On the primary ion locus and CH 2+ locus for the 0.5 mTorr data, the peaks associated with CH:+, CH:+, CH:+, and CHZ+ are clearly observed. It is worthwhile to note that the number of coincidence counts stored in fig. 3 for the main spectrum is > 4 X lo6 counts. The projections in fig. 6 show that the intensity in the peaks of interest (CHf+ , CH :+, CH$+, CH2+) are an extremely small part of the total coincidence intensity. If one performed a one-dimensional standard dispersive analysis, such small signal intensities would be lost in the noise. Window 2 in fig. 5 presents measurements of the spectra at a source pressure of 4.0 mTorr for the same m/q boundaries as window 1. Comparison of windows 1 and 2 show substantial differences in spectral features. At 4.0 mTorr all intensity below the primary ion locus is absent in contrast to the features noted for window 1 at the lower source pressure. Furthermore, the CH:+ , CH:+, and CH;+ ions have been displaced from the window to the C+ locus which is outside the boundaries of the window. This displacement is the result of secondary collisions at the higher source pressure. The peak corresponding to the CH:+ ions seen below the primary ion locus in window 1 is absent in window 2. However, we see strong intensities for peaks corresponding to CH:+ and CHz+ in the C2+ locus indicated in window 2. These two molecular ion species are clearly seen in the 4.0 mTorr C2+ locus of fig. 6. We do not detect them as CH:+ and CH:+ , but see the dissociation products of the reactions CH:++R+C2++R’
and
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where R and R’ are unidentified components. The fact that CH:+ is a sharp peak as shown in the projection of the C2+ locus as given in fig. 6 indicates that the lifetime for this species is comparable with the acceleration time, T = 200 ns. This lifetime limit could be increased as there is competition between the dissociation rate and the natural decay rate at this source pressure. The same conclusions can be made for the CHz+ ion species in view of the data of windows 1 and 2. As shown by the differences between window 1 (0.5 mTorr source gas pressure) and window 2 (4.0 mTorr source. gas pressure) there are very large pressure dependences which dominate the spectral features observed in the two-dimensional coincidence spectroscopy. Work is in progress to study the pressure dependence in detail over selected regions of the full spectrum. Through such studies the formation and collisional history of each
TIMEOF FLIGHT
Fig. 6. Projections of the primary ion locus, CH*+ ion locus for P = 0.5mTorr, and C2+ ion locus for P = 4.0 mTorr. The molecular ion species CH:+, CH$+, CH$+, CH’+, CH:+ and CHi+ are identified in the figure. The m/q scale at the top of the figure does not apply to the C? ion locus data at the bottom of the figure.
identifiable molecular ion species may be further characterized. Ion species such as CH 2+ CH:+ and CH:+ exhibit lifetimes > 200 ns. previous LoretiLl molecular structure calculations by Pople et al. [6] have suggested that CH2+ had no bound states and that CH:+ should have an extremely short lifetime. Our data for CH2+ indicate I. ATOMIC PHYSICS/RELATED PHENOMENA
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otherwise. Recent theoretical work by Heil et al. [8] for CH’+ shows a shallow minimum for a diabatic 2X+ state, but no lifetime estimates were made. No theoretical calculations for the lifetimes of CHi+ and CH:+ exist. This work was supported by the Division of Chemical Sciences, US Department of Energy. The authors wish to express their appreciation to Mr. Bob Krause and Mr. Mike Wells for their contributions to accelerator modifications which provided the beam stability required for this work.
References T. Bergmark, N. Magnusson, L.O. Werme, C. Nordhng and K. Siegbahn, Phys. Scripta 2 (1970) 31. [Z] T. Ast, C.J. Porter, C.J. Proctor and J.H. Beynon, Chem.
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Phys. Lett. 78 (1981) 439. [3] M. Rabrenovic, A.G. Brenton, and J.H. Beynon, Mass Spectr. Ion Phys. 52 (1983) 175.
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[4] A.E. Litherland, Ann. Rev. Nucl. Part. Sci. 30 (1980) 437. [5] A.W. Hanner and T.F. Moran, Organic Mass Spectr. 16 (1981) 512. [6] J.A. Pople, B. Tidor and P. von Ragut Schleyer, Chem. Phys. L&t. 88 (1982) 533. [7] P.E.M. Siegbahn, Chem. Phys. 66 (1982) 443. [8] T.G. Heil, S.E. Butler and A. DaIgamo, Phys. Rev. A27 (1983) 2365. [9] A.K. Edwards, R.M. Wood and M.F. Steuer, Phys. Rev. Al5 (1977) 48. [lo] A.K. Edwards, R.M. Wood and M.F. Steuer, Ann. Hr. Phys. Sot. 4 (1980) 171 and references therein. [ll] T.J. Gray and CL. Cocke, IEEE Trans. Nucl. Sci. NS-30 (1983) 937. [12] C.L. Cocke, T.J. Gray, E. Justiniano, C. Can, B. Waggoner, S.L. Varghese and R. Mann, Phys. Scripta 13 (1983) 75; C.L. Cocke, R. Dubois, T.J. Gray, E. Justiniano and C. Can, Phys. Rev. Lett. 46 (1981) 1671; Ed. Justiniano, C.L. Cocke, T.J. Gray, R.D. Dubois and C. Can, Phys. Rev. A24 (1981) 2953; T.J. Gray, C.L. Cocke and E. Justiniano, Phys. Rev. A22 (1980) 849; C.L. Cocke, Phys. Rev. A20 (1979) 749.