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Positron ionization mass spectrometry. II: ionization by fast positrons
International Journal of Mass Spectrometry and Ion Processes, 97 (1990) 237-252
237
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
POSITRON IONIZATION MASS SPECTROMETRY. II: IONIZATION BY FAST POSITRONS
SCOTT A. McLUCKEY*, HULETT, Jr.
GARY L. GLISH, DAVID L. DONOHUE
and LESTER D.
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN 378314365 (U.S.A.)
(First received 12 June 1989; in final form 2 November 1989)
ABSTRACT The ionization of polyatomic molecules in their interaction with positrons is discussed with respect to the likely mechanisms expected at various positron kinetic energies and the rationale behind the study of these processes. These mechanisms include impact ionization, those that involve the formation of positronium, and those that involve capture of the positron without positronium formation. Data are shown for ionization by fast positrons. These results constitute the first examples of the mass analysis of ions from polyatomic molecules ionized by fast (3-keV) positrons. Mass spectra derived from the bombardment of several different targets with 3-keV positrons, 70-eV electrons, and 2.7-keV electrons are compared. These comparisons support the expectation that the underlying mechanism for ionization in this collision energy region is very similar for electrons and positrons.
INTRODUCTION
The positron, the electron antiparticle, has been known for almost 60 years [l-3]. Despite the experimental difficulties associated with the generation of positrons with sufficient flux, useful analytical applications with positrons have been made, as a result of the unique information provided by their interactions with matter, in analyses of bulk gases, liquids, and solids, and in the analysis of interfaces [4-71. Much of the research with positrons has been aimed at understanding fundamental aspects of interactions of positrons with matter. Nevertheless, much is yet to be learned about details of positronic interactions with molecules. In fact, very little information is available on the fate of molecules that interact .with positrons. Information on single collisions, and sometimes three-body collisions, of positrons with atoms and molecules is available from gas-phase studies. These studies have been almost exclusively of one of two types. The first, referred to * Author to whom correspondence 0168-l 176/90/$03.50
should be addressed.
0 1990 Elsevier Science Publishers B.V.
238
as a lifetime measurement, involves the measurement of the lifetime of positrons emitted into a gas chamber [8,9]. These studies are usually performed at pressures in excess of one atmosphere. The other common experiment is the measurement of the total attenuation cross-section of a positron accelerated into a gas chamber [9,10]. These studies are typically performed under single-collision conditions, i.e. at pressures of a few mTorr or less. The information obtained from these experiments is largely complementary and has provided important insights into how the positron reacts with electrons in atoms and molecules. In these experiments, either a gamma photon from positron-electron annihilation or an unscattered positron is detected. In very few cases have other products from the interaction of positrons with matter been analyzed. We are interested in the ions that are formed from the variety of possible reactions of positrons with molecules in the gas phase. A number of likely ionization mechanisms have been inferred from the cross-section and lifetime studies, and from theoretical calculations as discussed below. However, until the ionic products are analyzed, these mechanisms are largely unconfirmed. Furthermore, the mass spectra obtained from ionization by positrons will reflect the stabilities of and degree of excitation in the species that lead to the final product ions. Essentially no information is available from the gas-phase studies on the partitioning of energy in any of the possible mechanisms for ionization with positrons. Another little-studied aspect of positron interactions with gas-phase molecules is the variation in ionization cross-section with chemical functionality. Ionization by electron capture is known to be highly selective for some compounds, as only a minority of compounds in nature have positive electron affinities [ll]. It remains to be seen, however, if positron capture or some other ionization mechanism involving positrons is highly selective and if conditions can be readily established to optimize the probability for such a reaction. We are currently engaged in a research program to study the ionization of polyatomic molecules by positrons. We refer to the technique herein as positron ionization mass spectrometry, which includes all of the possible ionization mechanisms. In the course of this work we will attempt to characterize each of the important ionization mechanisms. Our ultimate objective is to explore the use of positron ionization mass spectrometry for chemical analysis. Several other groups have also begun to pursue aspects of positron ionization in parallel with our efforts, although with somewhat different approaches [12-141 and, perhaps, with slightly different emphasis. Recently, for example, Passner et al. 1141ahve acquired mass spectra in a Penning trap as a result of the ionization of several different polyatomic molecules by positrons of near-thermal kinetic energy. A beam line has recently been constructed at the Oak Ridge Electron Linear Accelerator (ORELA) which provides IO-20-ns pulses of positrons
239
( 106-10’ positrons per pulse) [ 151. We have added a time-of-flight (TOF) mass spectrometer to this beam line, to acquire the mass spectra of molecules which result from the interactions of positrons at collision energies ranging from a few electron-volts to several kiloelectron-volts. We report here mass spectra acquired by bombardment of a variety of compounds with positrons of 3-keV kinetic energy, and compare these spectra with those obtained by bombardment with electrons of similar kinetic energy. EXPERIMENTAL
SECTION
The ORELA beam line used to supply positrons to the TOF mass spectrometer ion source, the TOF mass spectrometer, the operating conditions of the system, and its performance characteristics are described in a companion paper [ 151. Under normal operating conditions, pulses of lo-20-ns width of 3000 & 5-eV positrons are delivered to the TOF mass spectrometer at 800 Hz. The effective flux of fast positrons that pass through the ionization volume is typically 105- 1O6s- ’ . Gating of the TOF measurement is synchronized with a pulse from the ORELA which signals the imminent arrival of a pulse of fast positrons. After a time delay of a few microseconds, a gate pulse is supplied to the TOF extraction grids to draw ions from the ion source and to start a multi-stop time-to-digital converter [LeCroy (Spring Valley, NY 10977, U.S.A.) model 42081. Stop pulses are provided by ions arriving at the chevron electron multiplier array. Details of the data acquisition system have been described [ 161. Spectra were typically summed for lo5 pulses (data acquisition time of ca. 2 min). Vapors were admitted into the ionization volume via a precision leak valve, typically to an estimated pressure of lo-’ Torr. Dodecane was admitted to an estimated pressure of ca. 2 x 10p6Torr. Pressures were monitored by reading the current gauge of a nearby ion pump. The ion pump current readings were calibrated with an ionization gauge mounted on the ion source housing. All experimental conditions used to obtain electron impact ionization mass spectra were the same as above except where noted here. A heated rhenium filament was used to generate an electron beam of variable energy which was directed into the ionization volume. Data were acquired using 70-eV electrons and 2700-eV electrons. Problems with high-voltage sparks precluded the use of 3000-eV electrons. A different pulse generator was used for the electron impact data which has slightly different rise-time characteristics from the pulsing system for positron impact. This results in a slightly different mass calibration for the ions derived from the two experiments. Electron impact data were acquired by gating the extraction grids at a rate of 10 kHz using an estimated electron flux of a few nanoamperes.
240 RESULTS
AND DISCUSSION
Ionization mechanisms
Several possible mechanisms for ionization of polyatomic molecules by positrons are apparent from lifetime and scattering cross-section data. These mechanisms are discussed briefly below. Impact ionization
This mechanism, written as XY + fast e+--+XY+’
+ faste’ + slow e---,X+
+ Y’
(1)
is expected to be very similar to impact ionization by electrons [ 17,181in which the rapidly changing electric field experienced by the molecule from the passage of the fast positron results in electronic excitation sufficient to liberate an electron in a Franck-Condon transition directly from the neutral molecule to the ion [Fig. l(a)]. This form of ionization is expected to dominate at kinetic energies in excess of the ionization potential of the molecule. At collision energies well in excess of the ionization potential (i.e. more than 100eV) the cross-sections for ionization by positrons and electrons are expected to be similar [19,20], and are generally observed to be so. Any differences between positron impact ionization and electron impact ionization are expected to be most apparent at threshold [21-241. Internal energy depositions are likely to
b)
c)
Fig. 1. Energy diagrams for a hypothetical diatomic molecule that undergoes ionization by positrons by one of several mechanisms: (a) the case where a direct vertical transition occurs from the neutral molecule to an ion, (b) the case where an intermediate is formed involving a positronium atom attached to an ion, and (c) the case where an intermediate is formed by positron attachment without positronium formation. Each diagram represents a special case. A wide range of possibilities can apply for the relative stabilities of the reactants, intermediates, and products, and their degree of overlap.
241
be very similar for electron and positron impact ionization. The interaction time is short, leading to a vertical excitation, and the electric fields experienced by the molecules irradiated by fast leptons are analogous to those obtained by irradiation with white light [ 17,181, leading to a broad distribution of internal energies [25]. Positronium formation In this mechanism, positronium (Ps), the bound positron-electron atom [4,5], is formed as an intermediate leading to the final products of ions, gamma photons from annihilation of the positronium, and neutral fragments if fragmentation occurs. The reaction is written as XY + e+ -
(PsXY+‘)-
XY+’ + hv-
-PsX’+Y+-X.+Y++hv +psx+
+Y’-x+
X+ + Y’
(2) (3)
+Y’+hv
(4)
The positronium atom may or may not be attached to the ion, depending upon the excess energy with which the positronium is formed and the affinity of the positive ion for positronium. Positronium affinities of positive ions are unknown. The probability for positronium formation is expected to be maximized in the so-called Ore gap [26]. The Ore gap is the energy range which falls between the first excited state of the molecule (Ex) and the difference between the ionization potential of the molecule [IP(M)] and that of positronium (6.8eV), i.e. Ex > E > IP(M) - 6.8eV. The Ore gap model for positronium formation has been useful in describing interactions of positrons with atoms in this energy range. Cross-section measurements for the rare gases show a dramatic increase in attenuation cross-section at the Ore gap threshold [27], which is generally interpreted as the result of formation of positronium [lo]. It is unclear how well the model applies to more complex systems. The excited states of molecules are much lower lying than in atoms, which results in a narrow Ore gap. Nevertheless, lifetime studies indicate that positronium is readily formed in at least some molecular gases [28]. Crosssection measurements for the formation of positronium for several molecular gases show a rapid rise in cross-section from zero at the threshold for positronium formation to a maximum value within the Ore gap [29]. For methane the maximum cross-section is 3-4A2. The cross-section does not, however, drop rapidly past the Ore gap but continues to exceed 18?, to a positron kinetic energy of 40eV. Apparently the upper end of the Ore gap does not strictly apply to positronium formation. Both the positronium formation and capture mechanisms (see below) involve the annihilation of an electron by a positron. The products of this annihilation are either two or three gamma photons, depending upon the spin
242
state of the pair. Therefore, the ion need not carry off the energy of the annihilation for the conservation laws to be obeyed. (The cross-section for the interaction of 0.2-OS-MeV gamma rays with molecules composed of low atomic number elements is of the order of 10-‘A2, and by far the most probable interaction in this energy region is Compton scattering [30]. The cross-section for the photo-electric effect is orders of magnitude lower. Therefore, the annihilation photons do not generate ions in this experiment.) The binding energy of the electron can be supplied by the annihilation, i.e. the ionization potential of the molecule can appear as an energy defect in the gamma photons. The point is that ionization by a mechanism which involves annihilation is not likely to be particularly violent, and may be fairly gentle. It is difficult to predict a priori the internal energy deposition in the ion formed after positronium formation and departure. However, several possibilities are clear. If the electron is removed quickly by the positron and forms free positronium, i.e. in a stripping type of mechanism, the ionization is likely to be vertical from the neutral molecule to the ion, as indicated in Fig. l(a). Positron energies required to form positronium for most organic molecules are of the order of 3 eV. A 3-eV positron travels 10 A (molecular dimensions) in lo-” s, which is too fast for the nuclei of the neutral to adjust to the approach of the positron. An intermediate is involved in the overall reaction, however, if the positronium atom remains associated with the nascent ion. This situation is depicted in Fig. l(b), which shows the special case where the structure of the intermediate more closely resembles the ion than does that of the neutral molecule. Provided the lifetime of the intermediate is sufficient, the nuclei can adjust to the new electronic environment before the positronium atom annihilates. The annihilation of the positronium atom, an extremely fast process, should then result in a vertical transition from the intermediate to the ion. The internal energy of the ion should then be governed by the internal energy of the intermediate and the relevant FranckCondon factors. The lifetime of o&o-positronium (spin parallel) in free space is 1.4 x IO-‘s and that of para-positronium (spin paired) is 1.25 x lo-‘OS. Lifetimes are shortened considerably when positronium is associated with matter, because of the increased electron density. However, lifetime studies show that ortho-positronium lifetimes of the order of nanoseconds are not unusual in gases [8], which indicates that intermediates consisting of a bound positronium may indeed have sufficient lifetimes for the nuclei to adjust. The cases discussed above involve ionization before fragmentation, as depicted in Eq. 2. As pointed out by Schrader [12], if the intermediate is sufficiently long-lived, fragmentation from the intermediate may compete with positronium annihilation, as depicted in Eqs. 3 and 4. This is indicated in Fig. l(b) by the presence of an exit channel from the intermediate.
243
Capture mechanisms (a) Bound state formation. This mechanism involves the capture of a positron, similar to electron capture, i.e. XY + slowe++XYe+-XY+’ -e+X+Y’-+X+
+ hv--,X+ +Y’
+ hv
+ Y’
(5) (6)
which can occur below the threshold for positronium formation. Schrader et al., using semi-empirical molecular orbital calculations, have predicted that some molecules will have a positive positron affinity [31,32]. That is, some molecules might form a bound state with the positron as an intermediate. (These calculations are limited somewhat by the dearth of empirical results upon which to base the positronic molecular orbital. However, the need for such calculations is clear as is the need for more experimental data.) The existence of bound states or resonances (see below) involving positrons has been used for many years to rationalize data acquired with positrons in gases [33]. For example, lifetime data for CH3C1 have indicated the presence of a bound state or resonance with a lifetime of 0.56ns and that the positron capture cross-section is ca. 0.07k [8]. It has been proposed that the lifetime of a bound or resonance state should be in the range of 0. l-l .Ons [34]. Surko et al. have recently confirmed the existence of long-lived resonances in interactions of positrons with polyatomic molecules [35] via lifetime measurements. Capture cross-sections on the order of a few K were indicated for Cl2 and C6 hydrocarbons. More recently, this group acquired low-resolution mass spectra of several polyatomic molecules exposed to positrons with kinetic energies less than 1 eV [ 141. (b) Resonances. This mechanism is very similar to positron capture and the two mechanisms are often not distinguished. A positron of the appropriate kinetic energy may form an unbound state with a molecule with a finite lifetime. That is, the molecule may not have a positive positron affinity but may capture a positron long enough for the positron to annihilate an electron in the molecule, resulting in ionization. In electron spectroscopies these resonances are sometimes classified as shape or Feshbach resonances. (c) Direct annihilation. Even in the absence of a bound or scattering state, a positron may annihilate an electron in free flight. The effective cross-section for this process is given by [9]
where r,, is the classical radius of the electron, c is the velocity of light, u is the velocity of the positron, and Z,, is the “effective” number of electrons in the
244
molecule available for annihilation. Z,, is often larger than the actual number of electrons in a molecule [8], as a result of polarization of the electron density by the positron. However, in some cases, polarization alone cannot account for the empirically observed Z,, values. For example, Z,, for CH,Cl is observed to be 1.5 x lo4 [8] and that for C16H34is observed to be more than lo6 [35]. It is in cases such as these where bound or resonances states are indicated. It should be noted that for values of Z,, that do not exceed the actual number of electrons by orders of magnitude the cross-section for direct annihilation becomes appreciable only for thermal and sub-thermal energy positrons. Internal energy deposition via bound state formation or resonance scattering is expected to be governed by the same factors as discussed above for the case involving a positronium-bound intermediate [see Fig. l(c), which shows the special case in which the neutral molecule has a positive positron affinity and the intermediate is more ion-like than is the neutral molecule]. An intermediate is formed when the positron is captured. Either an ion can be formed by fragmentation of this intermediate as depicted in Eq. 6 or annihilation can occur from the intermediate to form an ion as depicted in Eq. 5. As annihilation is fast on the molecular time-scale (1OP2’s), a vertical transition from the intermediate will occur in the latter case and the internal energy of the ion will be determined by the internal energy distribution of the intermediate and the relevant Franck-Condon factors. The internal energy of the intermediate will include the internal energy initially present in the molecule in addition to the kinetic energy of the positron and the positron affinity. This energy may be sufficient to cause fragmentation of the intermediate before annihilation. It should be recognized that the energy diagrams depicted in Figs. l(b) and I(c) each represent one possible case. A number of other cases may prove to be important. For example, cases are known where electron capture to a repulsive state occurs for some molecules, which results in dissociation of the order of one vibrational period (viz. dissociative electron capture [36]). Dissociation from a bound state can also occur if the internal energy of the system exceeds the dissociation threshold. Further, particularly in cases where the electron affinity is negative, autodetachment can be very rapid. It remains to be seen to what extent positronic analogs exist to the mechanisms associated with electron capture. In the positron case, another reaction channel is available to compete with fragmentation and autodetachment, viz. annihilation. When fragmentation precedes annihilation, the same three processes can then compete in the fragment that contains the positron. Table 1 summarizes the positron kinetic energy regions in which the general mechanisms discussed above are most likely to be important relative to one another. In some cases, mechanisms may be competitive. For example, dissociative positron capture may compete with positronium formation above
245 TABLE
I
Predominant
ionization
mechanisms
e+ kinetic energy
Likely ionization
Thermal and below Thermal to Ore gap Ore gap to ionization
Direct annihilation, e+ capture e+ capture, dissociative e+ capture Positronium formation, dissociative e+ capture Impact ionization
Ionization
potential
potential and above
mechanism(s)
the Ore gap if an excited state is available. Positronium formation is expected to be more universal, however, and should therefore predominate for many species at positron kinetic energies above the Ore gap and below the ionization potential. IONIZATION ELECTRONS
BY 3-keV POSITRONS,
70-eV ELECTRONS,
AND 2.7-keV
Mass spectra were acquired using 3-keV positrons, 70-eV electrons, and 2.7-keV electrons as the ionizing agents for isobutane, dodecane, benzene, toluene, and sulfur hexafluoride. These particular compounds were selected primarily for what they might reveal about ionization by slow positrons. Fast positron ionization data were collected for these compounds so that a complete positron ionization data set could be collected for some of the compounds studied before the instrument was modified for slow positron ionization experiments. The hydrocarbons C,Hio and C,,Hz6 were chosen because the Z,, values for each have been measured to be 1.5 x lo4 [8] and 1.5 x lo6 [35], respectively. The Z,,/Z values for these compounds are roughly 5 x lo* and 1 x 104, respectively, so that a significant difference in ionization crosssection by slow positrons might be anticipated. Benzene and toluene were chosen for study because it has been predicted that benzene will not bind a positron, whereas toluene has a predicted positron affinity of 0.46 eV [3 11. Therefore, significant differences in positron ionization cross-section might be observed for these compounds under conditions where positron capture is optimized. Sulfur hexafluoride was chosen to compare and contrast positron capture with electron capture. The high electron capture rate associated with SF, is well known [37], but recent total cross-section measurements for positrons colliding with SF, show none of the resonances apparent with electrons colliding with SF, [20]. It is therefore anticipated that ionization by positron capture, unlike ionization by electron capture, will be improbable for sulfur hexafluoride. In each case, apart from the poorer signal/noise apparent with the positron
246 ORNL.DWG
66.15592
ISOBUTANE 100 ‘&HI,,
so
mw E 58
60 70 60
(8)
A 1
50
I
I
,
100
I
150
I
200
I
250
I
300
100
90 60 70 60 50
(b)
40 30 1
100
90 60
70 60 50 40
i
30 20 10 L
0 1 0
Fig. 2. Time-of-flight mass spectra obtained for isobutane using (a) 70-eV electron impact, (b) 2.7-keV electron impact, and (c) 3-keV positron impact.
247 OANL-DWG
55-15595
DODECANE 100
C,2H25
so
mwz
170
80 70 60 50
(a)
40 30 20 10 0 0
c iii
70
fi
60
z
10
20
304050
100
150
200
250
50
ii F
40
4 K
30
300
(W
20 10 0 0
10
20
304050
100
150
200
250
300
Fig. 3. Time-of-flight mass spectra obtained for dodecane using (a) 70-eV electron impact, 0) 2.7-keV electron impact, and (c) 3-keV positron impact.
248 ORNL-DWG 88-17053
lOO-
C,H, mw = 78
_ BENZENE 60.
60
(a)
-
40 1 i 20 0-r 0
La
‘I 20
50
100 m I
I
200
Fig. 4. Time-of-flight mass spectra obtained for benzene using (a) 70-eV electron impact, (b) 2.7-keV electron impact, and (c) 3-keV positron impact.
impact mass spectra, the spectra derived from high-energy positron impact are very similar to those obtained by electron impact in terms of the ions that are observed and their relative ‘intensities. For example, Fig. 2 shows the spectra obtained for isobutane. Figure 2(a), the 70-eV electron ionization mass spectrum, is similar to the published spectrum [38] in that it shows m/z 43 (C, HT ) as the base peak with smaller signals at m/z 58 (the molecular ion) and at m/z values in the upper twenties, principally at m/z 29 (C,H: ). Ions of lower intensity are also present within the large fragment ion peaks but are not
249
100
T
TOLUENE
C7HB mw = 92 (a)
c 60 w 3 I- 60 z E 40 a
d K
20
60
60
40 20 0
Fig. 5. Time-of-flight mass spectra obtained for toluene using (a) 70-keV electron 2.7-keV electron impact, and (c) 3-keV positron impact.
impact,
(b)
resolved by our apparatus. Peaks are also observed at masses in the mid to upper teens. These arise primarily from background water and air. The 2.7-keV electron ionization [Fig. 2(b)] and 3-keV positron ionization [Fig. 2(c)] spectra are essentially identical. These same observations can be made for all of the comparisons which are shown in Figs. 3-6. All of the ions that are observed can be rationalized based on an impact mechanism which imparts sufficient energy to some of the ions to cause unimolecular fragmentation. The internal energy distribution of ions formed by 70-eV electron impact
250 ORNLDWG
100
SF6
88.17055
SFBmw=146 (a)
60 60 40 20 0
I
I
(b)
60
I
I
(c)
Fig. 6. Time-of-flight mass spectra obtained for sulfur hexafluoride using (a) 70-eV electron impact, (b) 2.7-keV electron impact, and (c) 3-keV positron impact.
is known to be broad [25]. These distributions are therefore also expected to be broad with 2.7-keV electron and 3-keV positron impact. For this reason, the mass spectra are not expected to reflect any subtle differences in the distributions of ion internal energies that may result from the three experiments. It can only be concluded that the gross features of mass spectra obtained by impact by fast positrons are very similar to those obtained by electron impact at similar energies. The mass spectra shown here are normalized and therefore do not reflect
251
the ionization cross-section. These experiments were not designed for the careful measurement of ionization cross-sections. However, based on the values of positron flux, number densities of the molecules in the ionization volume, path length, efficiency of the mass spectrometer, and numbers of ions detected [15], the cross-sections for ionization by 3000-eV positrons are estimated to be of the order of l-5 k. The ionization cross-sections for 3000-eV positrons and 3000-eV electrons appear to be very similar, as expected. Total attenuation cross-section measurements generally show that the total cross-sections for positrons and electrons converge at high kinetic energy [19,20]. These results, some of which were described recently on a preliminary basis [39], constitute the first example of direct analysis of the ions formed in the track of a fast positron. They indicate, as expected, that ionization of molecules by fast positrons probably proceeds via an impact mechanism very similar to that operative with ionization by fast electrons. We are currently pursuing the study of mechanisms of ionization by slow positrons which do not have an electronic analog. As discussed above, new and interesting observations can be expected if the positronium and/or capture mechanisms are operative and can be studied experimentally. Experiments with slow positrons will indicate the energetics of ionization and if, using the proper kinetic energy, positrons are selective in ionization. ACKNOWLEDGMENT
This research was supported by the U.S. Department of Energy Office of Basic Energy Sciences under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. REFERENCES 1 2 3 4 5 6 7 8 9
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237
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
POSITRON IONIZATION MASS SPECTROMETRY. II: IONIZATION BY FAST POSITRONS
SCOTT A. McLUCKEY*, HULETT, Jr.
GARY L. GLISH, DAVID L. DONOHUE
and LESTER D.
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN 378314365 (U.S.A.)
(First received 12 June 1989; in final form 2 November 1989)
ABSTRACT The ionization of polyatomic molecules in their interaction with positrons is discussed with respect to the likely mechanisms expected at various positron kinetic energies and the rationale behind the study of these processes. These mechanisms include impact ionization, those that involve the formation of positronium, and those that involve capture of the positron without positronium formation. Data are shown for ionization by fast positrons. These results constitute the first examples of the mass analysis of ions from polyatomic molecules ionized by fast (3-keV) positrons. Mass spectra derived from the bombardment of several different targets with 3-keV positrons, 70-eV electrons, and 2.7-keV electrons are compared. These comparisons support the expectation that the underlying mechanism for ionization in this collision energy region is very similar for electrons and positrons.
INTRODUCTION
The positron, the electron antiparticle, has been known for almost 60 years [l-3]. Despite the experimental difficulties associated with the generation of positrons with sufficient flux, useful analytical applications with positrons have been made, as a result of the unique information provided by their interactions with matter, in analyses of bulk gases, liquids, and solids, and in the analysis of interfaces [4-71. Much of the research with positrons has been aimed at understanding fundamental aspects of interactions of positrons with matter. Nevertheless, much is yet to be learned about details of positronic interactions with molecules. In fact, very little information is available on the fate of molecules that interact .with positrons. Information on single collisions, and sometimes three-body collisions, of positrons with atoms and molecules is available from gas-phase studies. These studies have been almost exclusively of one of two types. The first, referred to * Author to whom correspondence 0168-l 176/90/$03.50
should be addressed.
0 1990 Elsevier Science Publishers B.V.
238
as a lifetime measurement, involves the measurement of the lifetime of positrons emitted into a gas chamber [8,9]. These studies are usually performed at pressures in excess of one atmosphere. The other common experiment is the measurement of the total attenuation cross-section of a positron accelerated into a gas chamber [9,10]. These studies are typically performed under single-collision conditions, i.e. at pressures of a few mTorr or less. The information obtained from these experiments is largely complementary and has provided important insights into how the positron reacts with electrons in atoms and molecules. In these experiments, either a gamma photon from positron-electron annihilation or an unscattered positron is detected. In very few cases have other products from the interaction of positrons with matter been analyzed. We are interested in the ions that are formed from the variety of possible reactions of positrons with molecules in the gas phase. A number of likely ionization mechanisms have been inferred from the cross-section and lifetime studies, and from theoretical calculations as discussed below. However, until the ionic products are analyzed, these mechanisms are largely unconfirmed. Furthermore, the mass spectra obtained from ionization by positrons will reflect the stabilities of and degree of excitation in the species that lead to the final product ions. Essentially no information is available from the gas-phase studies on the partitioning of energy in any of the possible mechanisms for ionization with positrons. Another little-studied aspect of positron interactions with gas-phase molecules is the variation in ionization cross-section with chemical functionality. Ionization by electron capture is known to be highly selective for some compounds, as only a minority of compounds in nature have positive electron affinities [ll]. It remains to be seen, however, if positron capture or some other ionization mechanism involving positrons is highly selective and if conditions can be readily established to optimize the probability for such a reaction. We are currently engaged in a research program to study the ionization of polyatomic molecules by positrons. We refer to the technique herein as positron ionization mass spectrometry, which includes all of the possible ionization mechanisms. In the course of this work we will attempt to characterize each of the important ionization mechanisms. Our ultimate objective is to explore the use of positron ionization mass spectrometry for chemical analysis. Several other groups have also begun to pursue aspects of positron ionization in parallel with our efforts, although with somewhat different approaches [12-141 and, perhaps, with slightly different emphasis. Recently, for example, Passner et al. 1141ahve acquired mass spectra in a Penning trap as a result of the ionization of several different polyatomic molecules by positrons of near-thermal kinetic energy. A beam line has recently been constructed at the Oak Ridge Electron Linear Accelerator (ORELA) which provides IO-20-ns pulses of positrons
239
( 106-10’ positrons per pulse) [ 151. We have added a time-of-flight (TOF) mass spectrometer to this beam line, to acquire the mass spectra of molecules which result from the interactions of positrons at collision energies ranging from a few electron-volts to several kiloelectron-volts. We report here mass spectra acquired by bombardment of a variety of compounds with positrons of 3-keV kinetic energy, and compare these spectra with those obtained by bombardment with electrons of similar kinetic energy. EXPERIMENTAL
SECTION
The ORELA beam line used to supply positrons to the TOF mass spectrometer ion source, the TOF mass spectrometer, the operating conditions of the system, and its performance characteristics are described in a companion paper [ 151. Under normal operating conditions, pulses of lo-20-ns width of 3000 & 5-eV positrons are delivered to the TOF mass spectrometer at 800 Hz. The effective flux of fast positrons that pass through the ionization volume is typically 105- 1O6s- ’ . Gating of the TOF measurement is synchronized with a pulse from the ORELA which signals the imminent arrival of a pulse of fast positrons. After a time delay of a few microseconds, a gate pulse is supplied to the TOF extraction grids to draw ions from the ion source and to start a multi-stop time-to-digital converter [LeCroy (Spring Valley, NY 10977, U.S.A.) model 42081. Stop pulses are provided by ions arriving at the chevron electron multiplier array. Details of the data acquisition system have been described [ 161. Spectra were typically summed for lo5 pulses (data acquisition time of ca. 2 min). Vapors were admitted into the ionization volume via a precision leak valve, typically to an estimated pressure of lo-’ Torr. Dodecane was admitted to an estimated pressure of ca. 2 x 10p6Torr. Pressures were monitored by reading the current gauge of a nearby ion pump. The ion pump current readings were calibrated with an ionization gauge mounted on the ion source housing. All experimental conditions used to obtain electron impact ionization mass spectra were the same as above except where noted here. A heated rhenium filament was used to generate an electron beam of variable energy which was directed into the ionization volume. Data were acquired using 70-eV electrons and 2700-eV electrons. Problems with high-voltage sparks precluded the use of 3000-eV electrons. A different pulse generator was used for the electron impact data which has slightly different rise-time characteristics from the pulsing system for positron impact. This results in a slightly different mass calibration for the ions derived from the two experiments. Electron impact data were acquired by gating the extraction grids at a rate of 10 kHz using an estimated electron flux of a few nanoamperes.
240 RESULTS
AND DISCUSSION
Ionization mechanisms
Several possible mechanisms for ionization of polyatomic molecules by positrons are apparent from lifetime and scattering cross-section data. These mechanisms are discussed briefly below. Impact ionization
This mechanism, written as XY + fast e+--+XY+’
+ faste’ + slow e---,X+
+ Y’
(1)
is expected to be very similar to impact ionization by electrons [ 17,181in which the rapidly changing electric field experienced by the molecule from the passage of the fast positron results in electronic excitation sufficient to liberate an electron in a Franck-Condon transition directly from the neutral molecule to the ion [Fig. l(a)]. This form of ionization is expected to dominate at kinetic energies in excess of the ionization potential of the molecule. At collision energies well in excess of the ionization potential (i.e. more than 100eV) the cross-sections for ionization by positrons and electrons are expected to be similar [19,20], and are generally observed to be so. Any differences between positron impact ionization and electron impact ionization are expected to be most apparent at threshold [21-241. Internal energy depositions are likely to
b)
c)
Fig. 1. Energy diagrams for a hypothetical diatomic molecule that undergoes ionization by positrons by one of several mechanisms: (a) the case where a direct vertical transition occurs from the neutral molecule to an ion, (b) the case where an intermediate is formed involving a positronium atom attached to an ion, and (c) the case where an intermediate is formed by positron attachment without positronium formation. Each diagram represents a special case. A wide range of possibilities can apply for the relative stabilities of the reactants, intermediates, and products, and their degree of overlap.
241
be very similar for electron and positron impact ionization. The interaction time is short, leading to a vertical excitation, and the electric fields experienced by the molecules irradiated by fast leptons are analogous to those obtained by irradiation with white light [ 17,181, leading to a broad distribution of internal energies [25]. Positronium formation In this mechanism, positronium (Ps), the bound positron-electron atom [4,5], is formed as an intermediate leading to the final products of ions, gamma photons from annihilation of the positronium, and neutral fragments if fragmentation occurs. The reaction is written as XY + e+ -
(PsXY+‘)-
XY+’ + hv-
-PsX’+Y+-X.+Y++hv +psx+
+Y’-x+
X+ + Y’
(2) (3)
+Y’+hv
(4)
The positronium atom may or may not be attached to the ion, depending upon the excess energy with which the positronium is formed and the affinity of the positive ion for positronium. Positronium affinities of positive ions are unknown. The probability for positronium formation is expected to be maximized in the so-called Ore gap [26]. The Ore gap is the energy range which falls between the first excited state of the molecule (Ex) and the difference between the ionization potential of the molecule [IP(M)] and that of positronium (6.8eV), i.e. Ex > E > IP(M) - 6.8eV. The Ore gap model for positronium formation has been useful in describing interactions of positrons with atoms in this energy range. Cross-section measurements for the rare gases show a dramatic increase in attenuation cross-section at the Ore gap threshold [27], which is generally interpreted as the result of formation of positronium [lo]. It is unclear how well the model applies to more complex systems. The excited states of molecules are much lower lying than in atoms, which results in a narrow Ore gap. Nevertheless, lifetime studies indicate that positronium is readily formed in at least some molecular gases [28]. Crosssection measurements for the formation of positronium for several molecular gases show a rapid rise in cross-section from zero at the threshold for positronium formation to a maximum value within the Ore gap [29]. For methane the maximum cross-section is 3-4A2. The cross-section does not, however, drop rapidly past the Ore gap but continues to exceed 18?, to a positron kinetic energy of 40eV. Apparently the upper end of the Ore gap does not strictly apply to positronium formation. Both the positronium formation and capture mechanisms (see below) involve the annihilation of an electron by a positron. The products of this annihilation are either two or three gamma photons, depending upon the spin
242
state of the pair. Therefore, the ion need not carry off the energy of the annihilation for the conservation laws to be obeyed. (The cross-section for the interaction of 0.2-OS-MeV gamma rays with molecules composed of low atomic number elements is of the order of 10-‘A2, and by far the most probable interaction in this energy region is Compton scattering [30]. The cross-section for the photo-electric effect is orders of magnitude lower. Therefore, the annihilation photons do not generate ions in this experiment.) The binding energy of the electron can be supplied by the annihilation, i.e. the ionization potential of the molecule can appear as an energy defect in the gamma photons. The point is that ionization by a mechanism which involves annihilation is not likely to be particularly violent, and may be fairly gentle. It is difficult to predict a priori the internal energy deposition in the ion formed after positronium formation and departure. However, several possibilities are clear. If the electron is removed quickly by the positron and forms free positronium, i.e. in a stripping type of mechanism, the ionization is likely to be vertical from the neutral molecule to the ion, as indicated in Fig. l(a). Positron energies required to form positronium for most organic molecules are of the order of 3 eV. A 3-eV positron travels 10 A (molecular dimensions) in lo-” s, which is too fast for the nuclei of the neutral to adjust to the approach of the positron. An intermediate is involved in the overall reaction, however, if the positronium atom remains associated with the nascent ion. This situation is depicted in Fig. l(b), which shows the special case where the structure of the intermediate more closely resembles the ion than does that of the neutral molecule. Provided the lifetime of the intermediate is sufficient, the nuclei can adjust to the new electronic environment before the positronium atom annihilates. The annihilation of the positronium atom, an extremely fast process, should then result in a vertical transition from the intermediate to the ion. The internal energy of the ion should then be governed by the internal energy of the intermediate and the relevant FranckCondon factors. The lifetime of o&o-positronium (spin parallel) in free space is 1.4 x IO-‘s and that of para-positronium (spin paired) is 1.25 x lo-‘OS. Lifetimes are shortened considerably when positronium is associated with matter, because of the increased electron density. However, lifetime studies show that ortho-positronium lifetimes of the order of nanoseconds are not unusual in gases [8], which indicates that intermediates consisting of a bound positronium may indeed have sufficient lifetimes for the nuclei to adjust. The cases discussed above involve ionization before fragmentation, as depicted in Eq. 2. As pointed out by Schrader [12], if the intermediate is sufficiently long-lived, fragmentation from the intermediate may compete with positronium annihilation, as depicted in Eqs. 3 and 4. This is indicated in Fig. l(b) by the presence of an exit channel from the intermediate.
243
Capture mechanisms (a) Bound state formation. This mechanism involves the capture of a positron, similar to electron capture, i.e. XY + slowe++XYe+-XY+’ -e+X+Y’-+X+
+ hv--,X+ +Y’
+ hv
+ Y’
(5) (6)
which can occur below the threshold for positronium formation. Schrader et al., using semi-empirical molecular orbital calculations, have predicted that some molecules will have a positive positron affinity [31,32]. That is, some molecules might form a bound state with the positron as an intermediate. (These calculations are limited somewhat by the dearth of empirical results upon which to base the positronic molecular orbital. However, the need for such calculations is clear as is the need for more experimental data.) The existence of bound states or resonances (see below) involving positrons has been used for many years to rationalize data acquired with positrons in gases [33]. For example, lifetime data for CH3C1 have indicated the presence of a bound state or resonance with a lifetime of 0.56ns and that the positron capture cross-section is ca. 0.07k [8]. It has been proposed that the lifetime of a bound or resonance state should be in the range of 0. l-l .Ons [34]. Surko et al. have recently confirmed the existence of long-lived resonances in interactions of positrons with polyatomic molecules [35] via lifetime measurements. Capture cross-sections on the order of a few K were indicated for Cl2 and C6 hydrocarbons. More recently, this group acquired low-resolution mass spectra of several polyatomic molecules exposed to positrons with kinetic energies less than 1 eV [ 141. (b) Resonances. This mechanism is very similar to positron capture and the two mechanisms are often not distinguished. A positron of the appropriate kinetic energy may form an unbound state with a molecule with a finite lifetime. That is, the molecule may not have a positive positron affinity but may capture a positron long enough for the positron to annihilate an electron in the molecule, resulting in ionization. In electron spectroscopies these resonances are sometimes classified as shape or Feshbach resonances. (c) Direct annihilation. Even in the absence of a bound or scattering state, a positron may annihilate an electron in free flight. The effective cross-section for this process is given by [9]
where r,, is the classical radius of the electron, c is the velocity of light, u is the velocity of the positron, and Z,, is the “effective” number of electrons in the
244
molecule available for annihilation. Z,, is often larger than the actual number of electrons in a molecule [8], as a result of polarization of the electron density by the positron. However, in some cases, polarization alone cannot account for the empirically observed Z,, values. For example, Z,, for CH,Cl is observed to be 1.5 x lo4 [8] and that for C16H34is observed to be more than lo6 [35]. It is in cases such as these where bound or resonances states are indicated. It should be noted that for values of Z,, that do not exceed the actual number of electrons by orders of magnitude the cross-section for direct annihilation becomes appreciable only for thermal and sub-thermal energy positrons. Internal energy deposition via bound state formation or resonance scattering is expected to be governed by the same factors as discussed above for the case involving a positronium-bound intermediate [see Fig. l(c), which shows the special case in which the neutral molecule has a positive positron affinity and the intermediate is more ion-like than is the neutral molecule]. An intermediate is formed when the positron is captured. Either an ion can be formed by fragmentation of this intermediate as depicted in Eq. 6 or annihilation can occur from the intermediate to form an ion as depicted in Eq. 5. As annihilation is fast on the molecular time-scale (1OP2’s), a vertical transition from the intermediate will occur in the latter case and the internal energy of the ion will be determined by the internal energy distribution of the intermediate and the relevant Franck-Condon factors. The internal energy of the intermediate will include the internal energy initially present in the molecule in addition to the kinetic energy of the positron and the positron affinity. This energy may be sufficient to cause fragmentation of the intermediate before annihilation. It should be recognized that the energy diagrams depicted in Figs. l(b) and I(c) each represent one possible case. A number of other cases may prove to be important. For example, cases are known where electron capture to a repulsive state occurs for some molecules, which results in dissociation of the order of one vibrational period (viz. dissociative electron capture [36]). Dissociation from a bound state can also occur if the internal energy of the system exceeds the dissociation threshold. Further, particularly in cases where the electron affinity is negative, autodetachment can be very rapid. It remains to be seen to what extent positronic analogs exist to the mechanisms associated with electron capture. In the positron case, another reaction channel is available to compete with fragmentation and autodetachment, viz. annihilation. When fragmentation precedes annihilation, the same three processes can then compete in the fragment that contains the positron. Table 1 summarizes the positron kinetic energy regions in which the general mechanisms discussed above are most likely to be important relative to one another. In some cases, mechanisms may be competitive. For example, dissociative positron capture may compete with positronium formation above
245 TABLE
I
Predominant
ionization
mechanisms
e+ kinetic energy
Likely ionization
Thermal and below Thermal to Ore gap Ore gap to ionization
Direct annihilation, e+ capture e+ capture, dissociative e+ capture Positronium formation, dissociative e+ capture Impact ionization
Ionization
potential
potential and above
mechanism(s)
the Ore gap if an excited state is available. Positronium formation is expected to be more universal, however, and should therefore predominate for many species at positron kinetic energies above the Ore gap and below the ionization potential. IONIZATION ELECTRONS
BY 3-keV POSITRONS,
70-eV ELECTRONS,
AND 2.7-keV
Mass spectra were acquired using 3-keV positrons, 70-eV electrons, and 2.7-keV electrons as the ionizing agents for isobutane, dodecane, benzene, toluene, and sulfur hexafluoride. These particular compounds were selected primarily for what they might reveal about ionization by slow positrons. Fast positron ionization data were collected for these compounds so that a complete positron ionization data set could be collected for some of the compounds studied before the instrument was modified for slow positron ionization experiments. The hydrocarbons C,Hio and C,,Hz6 were chosen because the Z,, values for each have been measured to be 1.5 x lo4 [8] and 1.5 x lo6 [35], respectively. The Z,,/Z values for these compounds are roughly 5 x lo* and 1 x 104, respectively, so that a significant difference in ionization crosssection by slow positrons might be anticipated. Benzene and toluene were chosen for study because it has been predicted that benzene will not bind a positron, whereas toluene has a predicted positron affinity of 0.46 eV [3 11. Therefore, significant differences in positron ionization cross-section might be observed for these compounds under conditions where positron capture is optimized. Sulfur hexafluoride was chosen to compare and contrast positron capture with electron capture. The high electron capture rate associated with SF, is well known [37], but recent total cross-section measurements for positrons colliding with SF, show none of the resonances apparent with electrons colliding with SF, [20]. It is therefore anticipated that ionization by positron capture, unlike ionization by electron capture, will be improbable for sulfur hexafluoride. In each case, apart from the poorer signal/noise apparent with the positron
246 ORNL.DWG
66.15592
ISOBUTANE 100 ‘&HI,,
so
mw E 58
60 70 60
(8)
A 1
50
I
I
,
100
I
150
I
200
I
250
I
300
100
90 60 70 60 50
(b)
40 30 1
100
90 60
70 60 50 40
i
30 20 10 L
0 1 0
Fig. 2. Time-of-flight mass spectra obtained for isobutane using (a) 70-eV electron impact, (b) 2.7-keV electron impact, and (c) 3-keV positron impact.
247 OANL-DWG
55-15595
DODECANE 100
C,2H25
so
mwz
170
80 70 60 50
(a)
40 30 20 10 0 0
c iii
70
fi
60
z
10
20
304050
100
150
200
250
50
ii F
40
4 K
30
300
(W
20 10 0 0
10
20
304050
100
150
200
250
300
Fig. 3. Time-of-flight mass spectra obtained for dodecane using (a) 70-eV electron impact, 0) 2.7-keV electron impact, and (c) 3-keV positron impact.
248 ORNL-DWG 88-17053
lOO-
C,H, mw = 78
_ BENZENE 60.
60
(a)
-
40 1 i 20 0-r 0
La
‘I 20
50
100 m I
I
200
Fig. 4. Time-of-flight mass spectra obtained for benzene using (a) 70-eV electron impact, (b) 2.7-keV electron impact, and (c) 3-keV positron impact.
impact mass spectra, the spectra derived from high-energy positron impact are very similar to those obtained by electron impact in terms of the ions that are observed and their relative ‘intensities. For example, Fig. 2 shows the spectra obtained for isobutane. Figure 2(a), the 70-eV electron ionization mass spectrum, is similar to the published spectrum [38] in that it shows m/z 43 (C, HT ) as the base peak with smaller signals at m/z 58 (the molecular ion) and at m/z values in the upper twenties, principally at m/z 29 (C,H: ). Ions of lower intensity are also present within the large fragment ion peaks but are not
249
100
T
TOLUENE
C7HB mw = 92 (a)
c 60 w 3 I- 60 z E 40 a
d K
20
60
60
40 20 0
Fig. 5. Time-of-flight mass spectra obtained for toluene using (a) 70-keV electron 2.7-keV electron impact, and (c) 3-keV positron impact.
impact,
(b)
resolved by our apparatus. Peaks are also observed at masses in the mid to upper teens. These arise primarily from background water and air. The 2.7-keV electron ionization [Fig. 2(b)] and 3-keV positron ionization [Fig. 2(c)] spectra are essentially identical. These same observations can be made for all of the comparisons which are shown in Figs. 3-6. All of the ions that are observed can be rationalized based on an impact mechanism which imparts sufficient energy to some of the ions to cause unimolecular fragmentation. The internal energy distribution of ions formed by 70-eV electron impact
250 ORNLDWG
100
SF6
88.17055
SFBmw=146 (a)
60 60 40 20 0
I
I
(b)
60
I
I
(c)
Fig. 6. Time-of-flight mass spectra obtained for sulfur hexafluoride using (a) 70-eV electron impact, (b) 2.7-keV electron impact, and (c) 3-keV positron impact.
is known to be broad [25]. These distributions are therefore also expected to be broad with 2.7-keV electron and 3-keV positron impact. For this reason, the mass spectra are not expected to reflect any subtle differences in the distributions of ion internal energies that may result from the three experiments. It can only be concluded that the gross features of mass spectra obtained by impact by fast positrons are very similar to those obtained by electron impact at similar energies. The mass spectra shown here are normalized and therefore do not reflect
251
the ionization cross-section. These experiments were not designed for the careful measurement of ionization cross-sections. However, based on the values of positron flux, number densities of the molecules in the ionization volume, path length, efficiency of the mass spectrometer, and numbers of ions detected [15], the cross-sections for ionization by 3000-eV positrons are estimated to be of the order of l-5 k. The ionization cross-sections for 3000-eV positrons and 3000-eV electrons appear to be very similar, as expected. Total attenuation cross-section measurements generally show that the total cross-sections for positrons and electrons converge at high kinetic energy [19,20]. These results, some of which were described recently on a preliminary basis [39], constitute the first example of direct analysis of the ions formed in the track of a fast positron. They indicate, as expected, that ionization of molecules by fast positrons probably proceeds via an impact mechanism very similar to that operative with ionization by fast electrons. We are currently pursuing the study of mechanisms of ionization by slow positrons which do not have an electronic analog. As discussed above, new and interesting observations can be expected if the positronium and/or capture mechanisms are operative and can be studied experimentally. Experiments with slow positrons will indicate the energetics of ionization and if, using the proper kinetic energy, positrons are selective in ionization. ACKNOWLEDGMENT
This research was supported by the U.S. Department of Energy Office of Basic Energy Sciences under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. REFERENCES 1 2 3 4 5 6 7 8 9
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