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A plasma desorption mass spectrometry study of CHn+ and C2Hn+ ion formation from frozen organic surfaces
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
and Ion Processes
A plasma desorption mass spectrometry study of CH + and C2Hn+ ion formation from frozen organic surfaces R.L. Betts a, E.F. da Silveira b, E.A. Schweikert a'* aCenter for Chemical Characterization and Analysis, Texas A&M University, College Station, TX 77843-3144, USA bDepartment of Physics, Pontificia Universidade Cat6lica, C.P. 38071, Rio de Janeiro, 22 452-970, Brazil
Received 1 July 1994; accepted 24 March 1995
Abstract
Cyclohexane, cyclohexene, 1,3-cyclohexadiene and benzene frozen targets have been bombarded with 252Cf fission fragments and the desorbed ions have been analyzed by time-of-flight mass spectrometry. The behavior of the desorption yields of the CH + ions (mass 12-16 u) and C2H,+ ion (mass 24-30 u) has been studied as a function of both the target stoichiometry and the fragment molecular structure. Because these results show strong evidence of atomization followed by a recombination process inside the fission fragment track, a quasi-equilibrium plasma model is proposed for the desorption mechanism of these species. Moreover, assuming a local thermodynamic equilibrium, the abundance of neutral and ionic species was determined. It is shown that, for appropriate conditions of temperature and pressure inside the track, these calculations reproduce qualitatively the plasma desorption mass spectra of the above C6 cyclohydrocarbons in the 12 30 u mass range. Keywords." Cycloalkenes; Fragment ion formation; Quasi-equilibrium; Recombination; Thermodynamic modeling
I. Introduction
It is well known that fission fragments, just after penetrating in solids, slow down mainly by interaction with electrons of the solid [1]. In the case of organic targets, the projectile deposits a few kiloelectronvolts of energy per molecular layer in less than 10 -16 S, producing ionization or electronic excitation in hundreds of molecules. Except for less probable direct collisions between the fission fragments and atomic nuclei, all the excited target molecules remain at their positions for about 10 -15* Corresponding author.
10-14s [2]. During this period of time, which is much longer than the projectile dwell time, primarily electronic processes occur such as electronic excitation, ionization, reneutralization, Auger emission, electron gas movement, atomic and molecular bond breakage. The subsequent evolution of the system will depend on how large this region (infratrack) around the projectile track is and on how fast the energy is dissipated into the solid. The formation of a crater implies that thousands of particles are expelled from the solid by Coulomb explosions and by a cascade of molecular collisions [3]. We might then consider several mechanisms such as prompt
0168-1176/95/$09.50 © 1995 Elsevier Science B.V. All'rights reserved SSD1 0168-1176(95)04180-X
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R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
emission from the first molecular layer, plasma emission from the infratrack or shock wave production of large fragments from its peripheral region [4]. In order to analyze the existence or contributions of these mechanisms, it is necessary to determine the abundance, the structure and the velocity and angular distributions of the emitted fragments. Also important is to know how these quantities behave when the mass, structure, velocity, angle and charge of the projectile vary. The production of molecular species which are not likely to be preformed in the target requires chemical recombination. For those, thermal models are appropriate to evaluate pressure and temperature distributions necessary for these fragmentations and reactions to occur. The thermal models may give emphasis to evaporation processes [5 7] or to plasma formation [8-10]. This plasma may be considered in thermal equilibrium during a certain time interval [8] or in an adiabatic expansion process [10,11]. A local thermodynamic equilibrium assumption finds experimental support in the Maxwell-Boltzmann type of initial velocity
II II
11l i n C H
[8,12] or energy distribution [13] of emitted particles. By using a thermalized ion explosion model, Seiberling et al. [8] succeeded in reproducing the velocity spectrum of neutral uranium sputtered from a UF 4 target bombarded by a fluorine beam. While not sufficient, this type of distribution is a necessary condition for emission from an equilibrated plasma. Coulomb explosions on solid surfaces or molecular decay after vibrational excitation may also produce such distributions. In particular, electron beam and fission fragments have been reported to induce Maxwell-Boltzmann type energy distributions from adsorbed hydrogen on metal surfaces [2]. The objective of the current work was to examine to what extent these equilibrium or quasi-equilibrium conditions may be reached on organic compounds during fission fragment track formation. For this, plasma desorption mass spectrometry (PDMS) was used to obtain desorption yields of some frozen C6 cyclic hydrocarbons (structures presented in Fig. 1). After a discussion about the characteristics of the experimental data, a comparison is made with the final concentration
C6H12
1I I1 H ~
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Cyclohexane H.
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Fig. 1. Structures,formulasand molecularweightsof the C6 cyclohydrocarbonsstudied.
R.L. Betts et al./International Journal o f Mass Spectromet 0, and Ion Processes 145 (1995) 9 23
predictions of a plasma model. The model assumes that a hot uncharged plasma of neutral, positive and negative species is in thermal equilibrium inside the track, at constant pressure.
lI
manufacturer. The samples were leaked in at 10min intervals to reduce contamination on the sample surface due to vacuum pump oil and water adsorption. Each sample run lasted 20min and each sample was analyzed in four independent runs. Therefore the nonspectral PDMS data presented are averages of the four independent trials. The pressure during the runs was about 8 x 10-Smbar. These experiments were carried out on the linear time-of-flight instrument described elsewhere [14 16]. The sample was grounded and the acceleration potential was provided by biasing the flight tube to -2800V. A gap of 3.5mm separates the sample and a 90% transmission grid placed at the entrance of an approximately 62cm long flight tube. Spectral resolution was typically on the order of 300.
2. Experimental Cyclohexane, cyclohexene 1,3-cyclohexadiene, (all from Aldrich, Milwaukee, WI), and benzene (Fisher, Fair Lawn, N J) have been bombarded by 252Cf fission fragments and the positive secondary ion (SI) species in the mass range 12-30u, corresponding to the hydrocarbon groups CH + and C2H + (to select a region between the hydrogen ions and large fragments) have been analyzed. All samples were used as received from the
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Fig. 2. Spectrumof cyclohexanewhichis typicalof the resultsobtainedfor all compoundsstudied.
7.5
12
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
1 Yield ratios of the m/z 16 (O+) peak to the m/z 18 (H2O+) and m/z 19 (H3O+) peaks Table
3. Results and discussion As an illustration of a typical spectrum obtained, Fig. 2 shows the major portion of the PDMS spectrum of cyclohexane. The H + peak and the m/z 41, 55 and 83 peaks dominate the spectrum and the molecular ion peak M" + (m/z 84) has a moderate relative yield. The occurrence of water ion peaks in the spectrum is of particular interest. The peak corresponding to m/z 16, which has the smallest intensity of the 12-16 mass region, 2.5
Compound
Y(16)/Y(18)
r(16)/r(19)
Cyclohexane Cyclohexene Benzene
0.17 0.22 0.24 0.25
0.013 0.014 0.018 0.022
Water
0.23
0.010
1,3-Cyclohexadiene
could be due to contributions of CH~- and O + ions. A systematic analysis of the ratio
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m/z Fig. 3. (a) Plasma desorption mass spectrometry relative yields of the CHn+ region for all four compounds studied; (b) plasma desorption spectrometry relative yields from Refs. [11] and [17].
mass
R.L. Betts et al./lnternational Journal of Mass Spectromet O, and Ion Processes 145 (1995) 9-23
30-
cy,~loh~x~,,~~ Cyclohexene
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m/z Fig. 4. (a) Plasma desorption mass spectrometry relative yields of the C_,H,+ regins for all four compounds studied; (b) plasma desorption mass spectrometry relative yields from Refs. [11] and [17].
between the intensities of the m/z 16 peaks with respect to m/z 18 (H2 O+) and m/z 19 (H3 O+) for the compounds studied, shows that they are relatively constant and, within a factor of 2, roughly equal to the same ratio for condensed water vapor (see Table 1). This is a strong indication that the m/z 16 peak is mostly produced by O + species from water contamination. Additional evidence is given by the data of Refs. [10], [11] and [17] for frozen benzene and hexane (reproduced in Figs. 3(b) and 4(b)), in which water peaks are
close to background for frozen benzene; the relative yields of m/z 16, 18 and 19 are very small and comparable. The conclusion is that CH + emission is particularly unfavored for all analyzed compounds.
3.1. CH+ region The curves shown in Fig. 3(a) represent the desorption yield of masses 12-16 for all four C6 cyclohydrocarbons. The dependence of the relative yields on the sample composition can
14
R.L. Betts et al./ International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
be seen. The expected formulations of the m/z 12-15 peaks are C +, CH +, CH~- and CH~-. The main trends of the measured desorption yields are as follows. (a) For benzene, the least saturated compound studied, the relative yield decreases as the SI mass increases, i.e. as the number of hydrogen atoms per emitted particle increases. (b) For cyclohexane, the most saturated compound studied, the yield is about the same for m/z 12, 13 and 14, increases by a factor of 3.5 for m/z 15 and practically vanishes for m/z 16. (c) Cyclohexene and 1,3-cyclohexadiene, with intermediate saturation, exhibit, roughly, an intermediate behavior, i.e. their C + and CH~-yields are bounded by the corresponding yields of the two other compounds. An important conclusion of these findings is that the stoichiometry of the target compound has a systematic effect on the desorption yield, implying that the likelihood of observing each species is correlated with the H/C ratio in the native molecule. It is observed that the relative yield of C + changes inversely with the H/C ratio. At m/z 14, the different compounds have about the same yield, while at m/z 15 their yields vary directly with the H/C ratio. This means that for higher masses the yields from the four compounds are arranged in the opposite order to the corresponding C + yields. These data provide strong evidence that the detected ions are formed by direct fragmentation of the sample molecule, followed by a recombination process sensitive to the original stoichiometry. In Fig. 3(b), benzene and hexane data from Ref. [11] are shown. They were renormalized to ours in such a way that the C + yield for benzene is the same. Except for the CH~- yield, the two benzene sets have a reasonable agreement. Also, despite their different structures, the hexane and cyclohexane data are practically proportional to each other, favoring the assumption that a severe fragmentation process had occurred. The
observed trends due to the stoichiometry mentioned for the present results are thus also observed for the Ref. [11] data. The preformed C - H structure in the studied compounds (see Fig. 1) does not favor the CH + yield which was, in fact, observed to be smaller than the C + yield. Also, cyclohexane, which has six preformed - C H 2- sections, presents the same yield for CH~- emission as benzene, which has none. For all samples (except perhaps for the benzene), the CH~yield is higher than the CH + yield, although they do not have any CH3 structure originally. In opposition to the cyclohexane, the hexane has two preformed CH3 structures, but their spectra are very similar to each other in this mass region. These findings support a recombination model for formation of such secondary ions. Because the relative yields follow the saturation of the original molecules, a recombination process would appear to be using the atomic or fragmented species, as would be expected from the stoichiometry of the original molecules. Naturally, if other mechanisms occur, such as the direct emission of ionized fragments or metastable decays, their contribution must be added to that of the atomization/recombination process. It is important to consider also the enthalpy of formation of the generated ions in the analysis of their yields. The ionization potentials for C +, CH +, CH +, CH + and CH~ are ll.26eV, ll.13eV, 10.40eV, 9.83eV and 12.6eV respectively [18]. Because m/z 15 has the highest relative yield for each of the compounds (except benzene), clearly its lower ionization potential makes the formation of the ion more favorable as compared to the other CH + ions. The low ionization potential would account for the increase in the relative yield of m/z 15 from benzene, the stoichiometry of which does not favor formation of the CH + ion. Notice also that the ion in this region with the highest ionization potential from the neutral molecule, CH~-, has the
R.L. Betts et al./lnternational Journal ~[" Mass Spectromet O' and Ion Processes 145 (1995) 9 23
smallest relative yield for all compounds studied, including those of the Ref. [11] measurements (which have less water contamination). Probably due to the very small yields of the CH + fragments, the current electron ionization database [19] of these compounds could not be used in this region for a comparison to the plasma desorption data.
3.2. C2H+ region The ion abundances in the C2 H+ region from the plasma desorption data are shown in Fig. 4(a) for the present data, and in Fig. 4(b) for Ref. [11] data. The relative yield of the C2H~- peak for benzene is the normalization value between the two measurements. Note again the resemblance of the hexane and cyclohexane spectra, as well as a reasonable agreement for the two benzene spectra. The influence of ion stability as well as sample composition is clearly seen in this graph. C2H~- and CzH ~- at m/z 27 and 29 respectively are the two most abundant ions in the C2H + region for all of the compounds, although benzene does not have a prominent m/z 29 peak. Again, strong fragmentation of the original molecule followed by recombination of constituent atoms appears to be the main process of ion formation. The production of even-electron ions has been shown to be a more efficient process than odd-electron ions in both electron ionization and plasma desorption [16,20]. This is probably related to the ionization potential [18] of the CzHn species which favors C2H~and C2H+ yields. The high abundance of these ions suggests that recombination processes producing the more stable ions are more prevalent in this region than a soft fragmentation. The C2 H+ relative abundances from hexane, cyclohexane and cyclohexene are higher than those from the other two compounds for the medium and high mass
15
(26-30u) members of this region. Inversely, the m/z 24 and 25 yields are higher for benzene when compared to hexane. This trend arises partly because of the greater hydrogen density in the hexane, cyclohexane and cyclohexene molecules which increases the chances of forming both medium and high mass members of the C2H + series in a recombination process. For all compounds, the trend of the desorption yields observed for the m/z 28 and 29 peaks is very similar to that of the m/z 14 and 15 peaks. This is actually a general feature seen for all C,,H,+ groups lighter than the molecular group. In particular (see Fig. 2), the yield ratios of peaks 14+/15 +, 27+/29 +, 39+/41 + and 53+/55 + are observed to increase systematically for samples ranging from hexane to benzene [10,11,16]. It reinforces the idea that stoichiometry is quite important in the formation of the desorbed ion; the higher the H/C ratio of the native molecule, the higher the relative yield of the more saturated ions. As discussed below, the stoichiometry also has implications for other properties of the compound, such as density and stopping power. As a consequence, the temperature in the track may be different enough to cause observable modifications in the secondary ion relative yields. For instance, if the infratrack is hotter in benzene than in hexane, dissociation of more saturated species would favor the yield of the m/z 28 and 14 peaks with respect to those of the m/z 29 and 15 peaks, respectively. The yield ratio of peaks m/z 28 and 29 is the most striking difference between the PDMS yields (Fig. 4(b)) and EI yields (Fig. 5). Electron ionization spectra were taken from the N I H / E P A Mass Spectral Database [19]. The -C2H 4- structure is preformed in all compounds studied, except benzene, but the PDMS relative yield of 28 + ions is low. Meanwhile, none of these compounds has a C2H5 structure in the native molecule, but the
16
R,L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
--D--
Cyclohexane
~-
Cyclohexene
10.0
1,3-Cyclohexadiene
~D ¢-4 SII
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25
26
27
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31
m/z Fig. 5. Normalized yields of the C2H,+ fragments produced by electron impact mass spectrometry.
PDMS yield of 29 + is higher than the 28 + yield in all cases. Conversely, in the EI fragmentation patterns, the 28 + yield is higher than the 29 + yield for all compounds. Therefore, it appears that its original structure has been preserved more in the fragmentation from the gaseous samples (EI) than from solid samples (PDMS). It is worthwhile to point out, however, that there are important similarities in the PDMS and EI relative yields. For both techniques, m/z 26 and 30 have small relative yields, while m/z 27 has the highest yield of the C2 H + region. Assuming that these SI are formed by different mechanisms, such a similarity might be explained by their molecular structures. The radical
ions C2H2 + and C2H6 + have relatively high ionization potentials ( l l . 4 e V and l l.6eV respectively), which explains their low yield. The even-electron ion C2 H + , o n the contrary, has one of the smallest ionization energies of the region (8.4eV). The other even-electron ion, C2H~-, has a comparable ionization energy (8.9eV), which explains the higher relative yield in PDMS but does not explain the low relative yield in El. Because there is no preformed C2H 5 structure in the native molecule, the distinct difference in the 27 + and 29 + yield ratios from the two techniques can be attributed to a recombination process occurring in PDMS in opposition to a unimolecular decay occurring in El.
R.L. Betts et al./ International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
The recombination to form low mass desorbed ions, such as that corresponding to the m/z 29 peak, is likely to occur in the fission fragment infratrack where there is a high degree of ionic, atomic, and fragment mixing [21]. The yield of the ions formed is then governed by the relative abundance of the constituent atoms and by the dynamics of chemical reactions in a hot plasma. The plasma temperature can be estimated from initial velocity measurements. For nitrocellulose bombarded by 252Cffission fragments, T ~ 2 6 0 0 0 K ((E,) ~ 4.5eV) has been suggested for H + and H~- SI, while T ~ 4600 K ((Ez) ~ 0.79 eV) was determined for CH~- ions [13]. The above pieces of evidence for an atomization/recombination process in this mass region are the basis and motivation for the theoretical approach presented in the following section.
4. The quasi-equilibrium plasma model and calculations The calculations presented later in this work are based on a model that assumes that desorption occurs in these stages. (a) Ionization regime. Just after the projectile/solid interaction, the track region is left highly charged but the atomized species are still at rest (i.e. at the temperature of the solid before the impact). The electron gas temperature may reach tens of thousands of kelvin. Inside the track, Coulomb repulsion forces create pressures of about 10000atm that are relatively constant in time. This regime is characterized by ionized species with high acceleration but negligible velocities. (b) Thermodynamic quasi-equilibrium regime. The average velocity of species becomes high but the drift velocity is still small because of the high collision rate (the mean free path is of the order of the atomic diameter). The plasma
17
starts to expand toward the vacuum, the pressure drops to about 1000atm and a quasi-equilibrium develops corresponding to temperatures in the 1000-10 000 K range. Under these temperature and pressure conditions, molecular species are formed with concentrations governed by thermodynamic principles. (c) Vacuum free-expansion regime. The drift velocity increases up to the average velocity of species and their acceleration drops to zero. The pressure tends rapidly toward residual gas pressure. This implies that the velocity distribution of emitted particles continues to reflect the average temperature of the previous stage. Also, besides metastable decay, chemical reactions cease so that the relative abundance of ions and neutrals is the same as in the previous stage. Refinements allowing different temperatures for distinct species may be introduced into the model by taking into account that, at the same kinetic energy, lighter particles will reach the free expansion regime before the heavier ones. The H +, CH +, CzH+,... groups sequentially leave a cooling plasma, so that their velocity distributions represent a "clock" and a "thermometer" of the desorption process.
4.1. Thermodynamic modeling The philosophy of this approach is to find a model that takes into account, as completely as possible, the stoichiometry of the original molecules and all the relevant physical quantities related to the structure of the emitted particles. These quantities should include the enthalpy of formation and the entropy for each neutral or ionized final species, and for a wide range of temperature. Nevertheless applied in a quasi-equilibrium framework, for a globally neutral, stationary and isotropic medium, a thermodynamic equilibrium plasma model offers a very convenient structure to host and manipulate
18
R.L. Betts et al./hlternational Journal (71'Mass Spectromet(v and hm Processes 145 (1995) 9-23
those physical quantities. Thus, besides the plasma temperature and pressure, there are no other free parameters in the model. Thermodynamic calculations were performed using the IDEALGAS program (Thermodynamics Research Center, Texas A&M University). Constant pressure was preferred due to the fact that in the second stage of the described model (when the recombinations are assumed to occur) the pressure still does not vary very fast. Also, it is worthwhile to remember that the Gibbs function has a logarithmic dependence on the pressure. Ionization potential values were taken from Ref. [18]. The values of enthalpy of formation, entropy and other thermodynamic data for the hydrogen, carbon and hydrocarbon neutral species are part of the program database. The data for the ions H +, H~-, H +, C +, CH +, C +, H - , C- and C2 are also part of the existing database. Data for ions not contained in the database provided with the program and not available for other sources were estimated from the data for the corresponding neutrals. The range covered by the thermodynamic calculations was from 3000 to 10000K and 10 to 10000atm. The selected conditions 5000 K and 1000atm are consistent with the energy deposition of the projectile, density of the target, size of the desorption site, and some relaxation of the system. The predicted total mole fraction of positive (or negative) species is 3.6 x 10 -4 for these conditions. The program was executed with different sets of input data to analyze the stability of the calculation with respect to some parameters. For instance, all the neutral fragments from hydrogen up to the native molecule were allowed to be formed to study the effect of higher fragment masses on the production of lower masses. In other runs, only fragments up to C2H 6 were considered in order to observe better the influence of the target stoichiometry. In all cases, electrons and positive ions (including H +, H~- and H~-) up to C2 H+ were
taken into account. When formation of H - , C- and C2 negative ions was allowed in the calculations, no significant variation in the final molar concentration at the selected temperature and pressure was observed. Unfortunately, negative ions desorbed in relatively high yield, such as C2H-, C4 Hand C 6 H - [11], were not considered in the calculations (thermodynamic characteristics unknown). The thermodynamic predictions obtained are based on the limiting case of equilibrium among positive and negative ions, electrons and neutral species. Therefore the initial structure (except stoichiometry) of the target compound or the initial ratio between neutral and charged particles does not affect the final molar concentration fraction. Because of the limited knowledge on experimental thermodynamic quantities for most ions, some assumptions needed to be made in order to perform the calculations. On the basis of known data for ions and corresponding neutral species, we assigned heat capacity and entropy values for the other ions. Because these two quantities are primarily based on the number of bonds in the molecule, they may vary in a similar manner in ions and in neutrals. This is seen in the C/C +, C H / C H +, and C2/C + neutral/ion pairs for which experimental data exist. It should be mentioned, however, that a variation of 10% in the entropy values of all the ionized species has a small but visible effect on the relative concentrations. There was no attempt to fit the P D M S spectra by changing these values. Typical IDEALGASresults, corresponding to the benzene stoichiometry, are presented in Figs. 6(a) and 6(b) for the CH + region and C2H + region respectively. The pressure was kept constant and equal to 1000atm. As the plasma temperature decreases, the abundance of all charged particles decreases. At very high temperatures, all molecular species dissociate. As a consequence, the polyatomic ion concen-
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
19
10-1
PDMS Spectra 10-2
-
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-
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--o--
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m/z Fig. 6. C o m p a r i s o n o f the p l a s m a d e s o r p t i o n m a s s s p e c t r o m e t r y relative yields a n d the t h e r m o d y n a m i c the C H + a n d C 2 H + r e g i o n s f r o m c y c l o h e x a n e a n d b e n z e n e .
tration always present a maximum as the plasma temperature increases. The vertical broken line represents a particular temperature (5000 K), selected after the initial velocity measurements [13] for analysis of the stoichiometry's dependence on the concentration. Fig. 7 compares the experimental desorption yields for cyclohexane and benzene (Fig. 7(a)) with the relative con-
p r e d i c t i o n o f t h e m o l e f r a c t i o n in
centrations predicted by the thermodynamic equilibrium model (Fig. 7(b)). A qualitative agreement can be seen between the measured and the predicted spectra. For both compounds, the C2H 2 group has, on average, a higher yield than the CH + group. The relatively high yield of CH +, C2H~- and C2H~peaks is reproduced by the calculations, as well as the relatively low yield of the positive radical
20
R.L. Betts et al./hlternational Journal o/' Mass Spectrometry and Ion Processes 145 (1995) 9 23
1 0 - 4 --
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10-8
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3000
5000
7000
9000
11000
Temperature (K) Fig. 7. Thermodynamic prediction of the mole fractions of the CH,,+ and C2H,+ ions at a constant pressure of 1000 atm.
ions. The trends of the stoichiometry effect are also correctly predicted. In the lower mass region of each group, ions emitted from benzene have higher yields than those emitted from cyclohexane, and the inverse occurs at higher masses. Some disagreements should also be noted: (a) the mole fractions of C +, CH + and CH~are too low compared to those of CH~- and the
C2H~ group; (b) the predicted mole fraction ratio for C2 H+ and C2Hf is too high; and (c) the predicted stoichiometry effect is not strong enough. To minimize these discrepancies, inspection of Fig. 7 reveals that, for a benzene temperature greater than 7000 K, C +, CH + and CH~- have about the same abundance. However, under these conditions, the C~- mole fraction
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
becomes higher than that of C2 H+ or C2H~-, in disagreement with the PDMS yield result. A better fit is obtained by choosing 4500K as the temperature only for the C2H + group which also allows the C2 H+ yield to be lower than the CzH ~- yield. The stoichiometry effects are actually underestimated: (liquid) benzene is 33% more dense than hexane, its fission fragment stopping power is about 15% higher and the melting points are respectively 5.5°C and -95°C. Therefore one should expect different temperatures and pressures for the plasma produced inside the target compounds. Other models for material expansion have been reported [10,11]. In particular, PDMS spectra of alkanes have been analyzed by the code CRUNCHERwhich considers a statistically independent breakup of all chemical bonds of the sample molecule [10]. This program treats only neutral species and sums the results of each region containing a certain number of carbon atoms. While this method predicts well the overall trends, the fine structure of fragment ion formation is lost. The IDEALGAS program predicts the abundance of each species individually. The fragment ion intensities are governed primarily by the heat of formation and the concentration of atoms available to form the fragments. This method has the advantage of retaining structural information in the predicted ionic distribution. Fine structure in each carbon region can then be compared with experimental data. 4.2. General considerations
The SI produced by the impact of megaelectronvolt heavy ions on organic samples may be analyzed according to their dominant formation mechanism. Three main groups may be considered: the hydrogen ion group, the low mass CnH + groups and the molecular ion group. The last group includes the quasimolecular ions and the large fragments or clusters having structures similar to the
21
sample molecules. The high initial velocity of positive hydrogen secondary ions indicates that they are, besides electrons, the first emitted particles in a projectile/solid collision [2]. Their energy of emission lies in the tens of electronvolt range [12,13,22], which is higher than for the other secondary ions. Moreover, for atomic projectiles having the same velocity and the same (high) charge state, the H + desorption yield does not depend on their mass or atomic number [23]. This is a strong argument in favor of hydrogen ions originating from the first surface layers. The molecular ion group is expected to be emitted from the surface at a peripheral region of the track (ultratrack). This is so because they could hardly endure the multiple electronic excitations and ionizations occurring very close to the projectile trajectory (infratrack). The description of their desorption process is achieved through models based on shock waves [24,25], multi-hit secondary electrons [26] or molecular dynamics [27]. The emission of the low mass CnH + ions via a plasma process is discussed in this work. Certain observed emission characteristics seem not to fit into this process. It has been reported that the initial radial velocity of large desorbed fragments depends on the projectile's angle of incidence, while low mass contaminant and fragment ions do not exhibit such a dependence (except H + and H~-, for which case there is some) [22]. Recent radial velocity measurements of organic compounds have shown that the angular distribution of low mass Sis is dependent on their degree of hydrogenation [28]. An oblique projectile incident angle generates a charged plasma with a low degree of symmetry which results in asymmetric SI emissions in the incident ion plane. The different radial velocity distributions observed for different CnHm ions [28] may not be incompatible with a plasma desorption model. Rather, these results may be due to the
22
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
fragility of polyatomic species escaping from a non-spherical plasma in expansion. Once formed, more highly saturated molecules may be preferentially destroyed by collisions in the track jet it they move in the projectile direction, while molecules of lower saturation are the main constituents of the jet. On the other hand, negative ions stay trapped by the electric field of the plasma for a certain time and then are emitted with a lower and relatively uniform average kinetic energy.
5. Conclusions We have studied the desorption of low mass CnH + ions through plasma desorption mass spectrometry. By using frozen samples of C6 cyclohydrocarbons, we have obtained evidence for atomization of the sample compounds followed by a recombination process. This implies that most of the information about the target molecule's structure was lost during track formation. The original stoichiometry, preserved throughout the entire process, is shown clearly to affect the relative yield of desorbed ions. PDMS spectra and EI spectra have similarities, as a consequence of the structure of the (same) emitted ions, and differences, attributed to the atomization which occurs only in the former and to the unimolecular decay which dominates the latter. These observations support the assumption of plasma formation in the infratrack in PDMS. A model to describe this process is sketched. Our thermodynamic calculations point towards a high degree of mixing in the desorption site, and suggest that a quasiequilibrium may be achieved. The qualitative agreement of this model's predictions with experimental data shows also that the behavior of desorption yields is very sensitive to the plasma temperature, pressure, stoichiometry of the target compound and
structure of the emitted ions. Thermodynamic data that are based on experiments on ions would enhance the accuracy of predicting, via calculations, the behavior of the desorbed species for a large mass range. As this type of model may, in principle, predict the dependence of desorption yields as a function of the energy deposited by the projectile, future studies may focus on different plasma temperature situations created by varying geometry, projectile characteristics or sample material. The plasma formation mechanism gives a reasonable description of the energy distribution and relative yield of species emitted from the infratrack region. Further research is still necessary to analyze the compatibility of the model with other measurements such as desorption yield dependence on the projectile energy, projectile charge state and angular distribution of the SI.
Acknowledgements We would like to thank D.A. Singleton and C.R. Ponciano for discussions on ion structures and chemical dynamics. We would also like to thank J.W. Bevan for access to the IDEALGAS program. K. Wien is gratefully acknowledged for transmitting to us the benzene and hexane data. Funding for this work was provided through NSF grant CHE-9208185 and CNPq Brazil.
References [1] R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids: Principles and Applications, University of California Press, Berkeley, CA, 1975. [2] E.F. da Silveira, M.G. Blain and E.A. Schweikert, J. Phys., 50 (1989) 79. [3] J. Kopniczky, C.T. Reimann, A. Hall~n, B.U.R. Sundqvist, P. Tengvall and R. Erlandsson, Phys. Rev. B, 49 (1994) 625. [4] C.T. Reimann, Theoretical models for sputtering and desorption of bio-organic molecules under collisional and electronic excitation by ion impact, in P. Sigmuds (Ed.),
R.L. Betts et al./lnternational Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
SPUT92, Matematisk-fysiske Meddelelser, Copenhagen, Denmark, 1992, p. 351. [5] G.H. Vineyard, Radiat. Eft., 29 (1976) 245. [6] R.E. Johnson and R. Evatt, Radiat. Eft., 52 (1980) 187. [7] H. Voit, E. Nieschler, B. Nees, R. Schmidt, C. Schoppmann, P. Beining and J. Scheer, J. Phys., 50 (1989) 237. [8] L.E. Seiberling, J.E. Griffith and T.A. Tombrello, Radiat. Eft., 52 (1980) 201. [9] I. Bitensky, E. Parilis, S. Della-Negra and Y. Le Beyec, Nucl. Instrum. Methods, B72 (1992) 380. [10] E.R. Hilf, W. Tuszynski, B. Curdes, J. Curdes, M. Wagner and K. Wien, Int. J. Mass Spectrom. Ion Processes, 126 (1993) 101. [11] M. Wagner, K. Wien, B. Curdes and E.R. Hilf, Nucl. Instrum. Methods, B82 (1993) 362. [12] N. Furstenau, N. Knippelberg, F.R. Kueger, G. Weiss and K. Wien, Z. Naturforsch, Teil A, 32 (1977) 711. [13] R.D. Macfarlane, J.C. Hill and D.L. Jacobs, J. Trace Microprobe Tech., 4 (1986-87) 281. [14] M.A. Park, E.L. Shirey and E.A. Schweikert, J. Trace Microprobe Tech., 11 (1994) 285. [15] E.L. Shirey, Plasma desorption mass spectrometry of frozen organic compounds, M.S. Thesis, Texas A&M University, 1993. [16] R.L. Betts, M.A. Park, E.L. Shirey and E.A. Schweikert, Org. Mass Spectrom., 29 (1994) 679.
23
[17] K. Wien, personal communication, 1994. [18] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 71st, Edn. CRC Press, Boca Raton, FL, 1990. [19] S.R. Heller and C.W.A. Milne (Eds.), NIH/EPA Mass Spectral Database, National Bureau of Standards, Gaithersburg, MD, 1978. [20] J.H. Beynon, Microchim. Acta, 1 (1956) 437. [21] R.E. Johnson, Int. J. Mass Spectrom. Ion Processes, 126 (1993) 17. [22] D. Feny6, B.U.R. Sundqvist, BR. Karlsson and R.E. Johnson, Phys. Rev. B, 42 (1990) 1895. [23] S. Della-Negra, O. Becker, R. Cotter, Y. Le Beyec, B. Monart, K. Standing and K. Wien, J. Phys., 48 (1987) 261. [24] I.S. Bitensky, A.M. Goldenberg and E.S. Parilis, J. Phys., 50 (1989) 213. [25] R.E. Johnson, B.U.R. Sundqvist, A. Hedin and D. Feny6, Phys. Rev. B, 40 (1989) 49. [26] A. Hedin, P. H~kansson, B.U.R. Sundqvist and R.E. Johnson, Phys. Rev. B, 31 (1985) 1780. [27] D. Feny6, A. Hedin, P. H~kansson and B.U.R. Sundqvist, Int. J. Mass Spectrom. Ion Processes, 100 (1990) 63. [28] R.M. Papalbo, G. Brinkmalm, D. Feny6, J. Eriksson, H.-F. Kammer, P. Demirev, P. H~kansson and B.U.R. Sundqvist, Nucl. Instrum. Methods, B91 (1994) 667.
International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
and Ion Processes
A plasma desorption mass spectrometry study of CH + and C2Hn+ ion formation from frozen organic surfaces R.L. Betts a, E.F. da Silveira b, E.A. Schweikert a'* aCenter for Chemical Characterization and Analysis, Texas A&M University, College Station, TX 77843-3144, USA bDepartment of Physics, Pontificia Universidade Cat6lica, C.P. 38071, Rio de Janeiro, 22 452-970, Brazil
Received 1 July 1994; accepted 24 March 1995
Abstract
Cyclohexane, cyclohexene, 1,3-cyclohexadiene and benzene frozen targets have been bombarded with 252Cf fission fragments and the desorbed ions have been analyzed by time-of-flight mass spectrometry. The behavior of the desorption yields of the CH + ions (mass 12-16 u) and C2H,+ ion (mass 24-30 u) has been studied as a function of both the target stoichiometry and the fragment molecular structure. Because these results show strong evidence of atomization followed by a recombination process inside the fission fragment track, a quasi-equilibrium plasma model is proposed for the desorption mechanism of these species. Moreover, assuming a local thermodynamic equilibrium, the abundance of neutral and ionic species was determined. It is shown that, for appropriate conditions of temperature and pressure inside the track, these calculations reproduce qualitatively the plasma desorption mass spectra of the above C6 cyclohydrocarbons in the 12 30 u mass range. Keywords." Cycloalkenes; Fragment ion formation; Quasi-equilibrium; Recombination; Thermodynamic modeling
I. Introduction
It is well known that fission fragments, just after penetrating in solids, slow down mainly by interaction with electrons of the solid [1]. In the case of organic targets, the projectile deposits a few kiloelectronvolts of energy per molecular layer in less than 10 -16 S, producing ionization or electronic excitation in hundreds of molecules. Except for less probable direct collisions between the fission fragments and atomic nuclei, all the excited target molecules remain at their positions for about 10 -15* Corresponding author.
10-14s [2]. During this period of time, which is much longer than the projectile dwell time, primarily electronic processes occur such as electronic excitation, ionization, reneutralization, Auger emission, electron gas movement, atomic and molecular bond breakage. The subsequent evolution of the system will depend on how large this region (infratrack) around the projectile track is and on how fast the energy is dissipated into the solid. The formation of a crater implies that thousands of particles are expelled from the solid by Coulomb explosions and by a cascade of molecular collisions [3]. We might then consider several mechanisms such as prompt
0168-1176/95/$09.50 © 1995 Elsevier Science B.V. All'rights reserved SSD1 0168-1176(95)04180-X
10
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
emission from the first molecular layer, plasma emission from the infratrack or shock wave production of large fragments from its peripheral region [4]. In order to analyze the existence or contributions of these mechanisms, it is necessary to determine the abundance, the structure and the velocity and angular distributions of the emitted fragments. Also important is to know how these quantities behave when the mass, structure, velocity, angle and charge of the projectile vary. The production of molecular species which are not likely to be preformed in the target requires chemical recombination. For those, thermal models are appropriate to evaluate pressure and temperature distributions necessary for these fragmentations and reactions to occur. The thermal models may give emphasis to evaporation processes [5 7] or to plasma formation [8-10]. This plasma may be considered in thermal equilibrium during a certain time interval [8] or in an adiabatic expansion process [10,11]. A local thermodynamic equilibrium assumption finds experimental support in the Maxwell-Boltzmann type of initial velocity
II II
11l i n C H
[8,12] or energy distribution [13] of emitted particles. By using a thermalized ion explosion model, Seiberling et al. [8] succeeded in reproducing the velocity spectrum of neutral uranium sputtered from a UF 4 target bombarded by a fluorine beam. While not sufficient, this type of distribution is a necessary condition for emission from an equilibrated plasma. Coulomb explosions on solid surfaces or molecular decay after vibrational excitation may also produce such distributions. In particular, electron beam and fission fragments have been reported to induce Maxwell-Boltzmann type energy distributions from adsorbed hydrogen on metal surfaces [2]. The objective of the current work was to examine to what extent these equilibrium or quasi-equilibrium conditions may be reached on organic compounds during fission fragment track formation. For this, plasma desorption mass spectrometry (PDMS) was used to obtain desorption yields of some frozen C6 cyclic hydrocarbons (structures presented in Fig. 1). After a discussion about the characteristics of the experimental data, a comparison is made with the final concentration
C6H12
1I I1 H ~
1t
MW=84
I1 ~ , , ~
H
Cyclohexene
Cyclohexane H.
H~
MW=82
[1 H
H 1t
H
C6HIo
H
u",,,,,,~'n
C6H8
H ~ I t
MW=78
MW=80
|1 1,3-Cyclohexadiene
C6H6
t! Benzene
Fig. 1. Structures,formulasand molecularweightsof the C6 cyclohydrocarbonsstudied.
R.L. Betts et al./International Journal o f Mass Spectromet 0, and Ion Processes 145 (1995) 9 23
predictions of a plasma model. The model assumes that a hot uncharged plasma of neutral, positive and negative species is in thermal equilibrium inside the track, at constant pressure.
lI
manufacturer. The samples were leaked in at 10min intervals to reduce contamination on the sample surface due to vacuum pump oil and water adsorption. Each sample run lasted 20min and each sample was analyzed in four independent runs. Therefore the nonspectral PDMS data presented are averages of the four independent trials. The pressure during the runs was about 8 x 10-Smbar. These experiments were carried out on the linear time-of-flight instrument described elsewhere [14 16]. The sample was grounded and the acceleration potential was provided by biasing the flight tube to -2800V. A gap of 3.5mm separates the sample and a 90% transmission grid placed at the entrance of an approximately 62cm long flight tube. Spectral resolution was typically on the order of 300.
2. Experimental Cyclohexane, cyclohexene 1,3-cyclohexadiene, (all from Aldrich, Milwaukee, WI), and benzene (Fisher, Fair Lawn, N J) have been bombarded by 252Cf fission fragments and the positive secondary ion (SI) species in the mass range 12-30u, corresponding to the hydrocarbon groups CH + and C2H + (to select a region between the hydrogen ions and large fragments) have been analyzed. All samples were used as received from the
1000 H+
Cyclohexane I
27
800" H3 O+ >a
4.a °~..~
29
H2 +
600"
~D
4,-a
400 12 200
0
k_,
0
ILl
1.5
II
3 Flight Time
I
4.5
6 (~ts)
Fig. 2. Spectrumof cyclohexanewhichis typicalof the resultsobtainedfor all compoundsstudied.
7.5
12
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
1 Yield ratios of the m/z 16 (O+) peak to the m/z 18 (H2O+) and m/z 19 (H3O+) peaks Table
3. Results and discussion As an illustration of a typical spectrum obtained, Fig. 2 shows the major portion of the PDMS spectrum of cyclohexane. The H + peak and the m/z 41, 55 and 83 peaks dominate the spectrum and the molecular ion peak M" + (m/z 84) has a moderate relative yield. The occurrence of water ion peaks in the spectrum is of particular interest. The peak corresponding to m/z 16, which has the smallest intensity of the 12-16 mass region, 2.5
Compound
Y(16)/Y(18)
r(16)/r(19)
Cyclohexane Cyclohexene Benzene
0.17 0.22 0.24 0.25
0.013 0.014 0.018 0.022
Water
0.23
0.010
1,3-Cyclohexadiene
could be due to contributions of CH~- and O + ions. A systematic analysis of the ratio
Cyclohexane ......o---
Q
Cyclohexene "'-4'--"
2.0-
1,3-Cyclohexadiene ---o--1.5-
i
Benzene
x
- -
--
H/C Ratio /
~ - . . . . . . . . . . . 2.00
oS
1.0re?
0.5-
73 0.0 2.5 o~,,I >
~
- -1.00
i
i
i
i
"1"
®
Hexane --.-o..--
2.0-
Benzene
A
4-a
1.5c~
H/C Ratio 1.0-
~
0.5-
11
~
t.r-'--
Fll
0.0
%
12
~
~
i
~ ' i iII" II. ..2.33
-A...~.~
1 13
~
..~.
i
i
14
15
".1.00
~'" 16
7
m/z Fig. 3. (a) Plasma desorption mass spectrometry relative yields of the CHn+ region for all four compounds studied; (b) plasma desorption spectrometry relative yields from Refs. [11] and [17].
mass
R.L. Betts et al./lnternational Journal of Mass Spectromet O, and Ion Processes 145 (1995) 9-23
30-
cy,~loh~x~,,~~ Cyclohexene
13
(~
......
1,3-Cyclcohexadiene
- - -O- - -
13enzene
A
20 l
H/C, Ratk 2.00
,"-"
10
1.77
¢'¢3
1.33 1.00
,......4 *~ml
30
@ Itexane
"~+.a~ 20
Benzene
~9
A " ~ A
A
x 1/2
H/C Ratio
,o
//>\/
\v 0~,'
24
~
,
~
'
'
"
25
26
27
28
29
30
31
m/z Fig. 4. (a) Plasma desorption mass spectrometry relative yields of the C_,H,+ regins for all four compounds studied; (b) plasma desorption mass spectrometry relative yields from Refs. [11] and [17].
between the intensities of the m/z 16 peaks with respect to m/z 18 (H2 O+) and m/z 19 (H3 O+) for the compounds studied, shows that they are relatively constant and, within a factor of 2, roughly equal to the same ratio for condensed water vapor (see Table 1). This is a strong indication that the m/z 16 peak is mostly produced by O + species from water contamination. Additional evidence is given by the data of Refs. [10], [11] and [17] for frozen benzene and hexane (reproduced in Figs. 3(b) and 4(b)), in which water peaks are
close to background for frozen benzene; the relative yields of m/z 16, 18 and 19 are very small and comparable. The conclusion is that CH + emission is particularly unfavored for all analyzed compounds.
3.1. CH+ region The curves shown in Fig. 3(a) represent the desorption yield of masses 12-16 for all four C6 cyclohydrocarbons. The dependence of the relative yields on the sample composition can
14
R.L. Betts et al./ International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
be seen. The expected formulations of the m/z 12-15 peaks are C +, CH +, CH~- and CH~-. The main trends of the measured desorption yields are as follows. (a) For benzene, the least saturated compound studied, the relative yield decreases as the SI mass increases, i.e. as the number of hydrogen atoms per emitted particle increases. (b) For cyclohexane, the most saturated compound studied, the yield is about the same for m/z 12, 13 and 14, increases by a factor of 3.5 for m/z 15 and practically vanishes for m/z 16. (c) Cyclohexene and 1,3-cyclohexadiene, with intermediate saturation, exhibit, roughly, an intermediate behavior, i.e. their C + and CH~-yields are bounded by the corresponding yields of the two other compounds. An important conclusion of these findings is that the stoichiometry of the target compound has a systematic effect on the desorption yield, implying that the likelihood of observing each species is correlated with the H/C ratio in the native molecule. It is observed that the relative yield of C + changes inversely with the H/C ratio. At m/z 14, the different compounds have about the same yield, while at m/z 15 their yields vary directly with the H/C ratio. This means that for higher masses the yields from the four compounds are arranged in the opposite order to the corresponding C + yields. These data provide strong evidence that the detected ions are formed by direct fragmentation of the sample molecule, followed by a recombination process sensitive to the original stoichiometry. In Fig. 3(b), benzene and hexane data from Ref. [11] are shown. They were renormalized to ours in such a way that the C + yield for benzene is the same. Except for the CH~- yield, the two benzene sets have a reasonable agreement. Also, despite their different structures, the hexane and cyclohexane data are practically proportional to each other, favoring the assumption that a severe fragmentation process had occurred. The
observed trends due to the stoichiometry mentioned for the present results are thus also observed for the Ref. [11] data. The preformed C - H structure in the studied compounds (see Fig. 1) does not favor the CH + yield which was, in fact, observed to be smaller than the C + yield. Also, cyclohexane, which has six preformed - C H 2- sections, presents the same yield for CH~- emission as benzene, which has none. For all samples (except perhaps for the benzene), the CH~yield is higher than the CH + yield, although they do not have any CH3 structure originally. In opposition to the cyclohexane, the hexane has two preformed CH3 structures, but their spectra are very similar to each other in this mass region. These findings support a recombination model for formation of such secondary ions. Because the relative yields follow the saturation of the original molecules, a recombination process would appear to be using the atomic or fragmented species, as would be expected from the stoichiometry of the original molecules. Naturally, if other mechanisms occur, such as the direct emission of ionized fragments or metastable decays, their contribution must be added to that of the atomization/recombination process. It is important to consider also the enthalpy of formation of the generated ions in the analysis of their yields. The ionization potentials for C +, CH +, CH +, CH + and CH~ are ll.26eV, ll.13eV, 10.40eV, 9.83eV and 12.6eV respectively [18]. Because m/z 15 has the highest relative yield for each of the compounds (except benzene), clearly its lower ionization potential makes the formation of the ion more favorable as compared to the other CH + ions. The low ionization potential would account for the increase in the relative yield of m/z 15 from benzene, the stoichiometry of which does not favor formation of the CH + ion. Notice also that the ion in this region with the highest ionization potential from the neutral molecule, CH~-, has the
R.L. Betts et al./lnternational Journal ~[" Mass Spectromet O' and Ion Processes 145 (1995) 9 23
smallest relative yield for all compounds studied, including those of the Ref. [11] measurements (which have less water contamination). Probably due to the very small yields of the CH + fragments, the current electron ionization database [19] of these compounds could not be used in this region for a comparison to the plasma desorption data.
3.2. C2H+ region The ion abundances in the C2 H+ region from the plasma desorption data are shown in Fig. 4(a) for the present data, and in Fig. 4(b) for Ref. [11] data. The relative yield of the C2H~- peak for benzene is the normalization value between the two measurements. Note again the resemblance of the hexane and cyclohexane spectra, as well as a reasonable agreement for the two benzene spectra. The influence of ion stability as well as sample composition is clearly seen in this graph. C2H~- and CzH ~- at m/z 27 and 29 respectively are the two most abundant ions in the C2H + region for all of the compounds, although benzene does not have a prominent m/z 29 peak. Again, strong fragmentation of the original molecule followed by recombination of constituent atoms appears to be the main process of ion formation. The production of even-electron ions has been shown to be a more efficient process than odd-electron ions in both electron ionization and plasma desorption [16,20]. This is probably related to the ionization potential [18] of the CzHn species which favors C2H~and C2H+ yields. The high abundance of these ions suggests that recombination processes producing the more stable ions are more prevalent in this region than a soft fragmentation. The C2 H+ relative abundances from hexane, cyclohexane and cyclohexene are higher than those from the other two compounds for the medium and high mass
15
(26-30u) members of this region. Inversely, the m/z 24 and 25 yields are higher for benzene when compared to hexane. This trend arises partly because of the greater hydrogen density in the hexane, cyclohexane and cyclohexene molecules which increases the chances of forming both medium and high mass members of the C2H + series in a recombination process. For all compounds, the trend of the desorption yields observed for the m/z 28 and 29 peaks is very similar to that of the m/z 14 and 15 peaks. This is actually a general feature seen for all C,,H,+ groups lighter than the molecular group. In particular (see Fig. 2), the yield ratios of peaks 14+/15 +, 27+/29 +, 39+/41 + and 53+/55 + are observed to increase systematically for samples ranging from hexane to benzene [10,11,16]. It reinforces the idea that stoichiometry is quite important in the formation of the desorbed ion; the higher the H/C ratio of the native molecule, the higher the relative yield of the more saturated ions. As discussed below, the stoichiometry also has implications for other properties of the compound, such as density and stopping power. As a consequence, the temperature in the track may be different enough to cause observable modifications in the secondary ion relative yields. For instance, if the infratrack is hotter in benzene than in hexane, dissociation of more saturated species would favor the yield of the m/z 28 and 14 peaks with respect to those of the m/z 29 and 15 peaks, respectively. The yield ratio of peaks m/z 28 and 29 is the most striking difference between the PDMS yields (Fig. 4(b)) and EI yields (Fig. 5). Electron ionization spectra were taken from the N I H / E P A Mass Spectral Database [19]. The -C2H 4- structure is preformed in all compounds studied, except benzene, but the PDMS relative yield of 28 + ions is low. Meanwhile, none of these compounds has a C2H5 structure in the native molecule, but the
16
R,L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
--D--
Cyclohexane
~-
Cyclohexene
10.0
1,3-Cyclohexadiene
~D ¢-4 SII
7.5 -
©
•~N
5.0-
©
Z
,,,.~
2.5
0.0
!
24
25
26
27
28
29
30
31
m/z Fig. 5. Normalized yields of the C2H,+ fragments produced by electron impact mass spectrometry.
PDMS yield of 29 + is higher than the 28 + yield in all cases. Conversely, in the EI fragmentation patterns, the 28 + yield is higher than the 29 + yield for all compounds. Therefore, it appears that its original structure has been preserved more in the fragmentation from the gaseous samples (EI) than from solid samples (PDMS). It is worthwhile to point out, however, that there are important similarities in the PDMS and EI relative yields. For both techniques, m/z 26 and 30 have small relative yields, while m/z 27 has the highest yield of the C2 H + region. Assuming that these SI are formed by different mechanisms, such a similarity might be explained by their molecular structures. The radical
ions C2H2 + and C2H6 + have relatively high ionization potentials ( l l . 4 e V and l l.6eV respectively), which explains their low yield. The even-electron ion C2 H + , o n the contrary, has one of the smallest ionization energies of the region (8.4eV). The other even-electron ion, C2H~-, has a comparable ionization energy (8.9eV), which explains the higher relative yield in PDMS but does not explain the low relative yield in El. Because there is no preformed C2H 5 structure in the native molecule, the distinct difference in the 27 + and 29 + yield ratios from the two techniques can be attributed to a recombination process occurring in PDMS in opposition to a unimolecular decay occurring in El.
R.L. Betts et al./ International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
The recombination to form low mass desorbed ions, such as that corresponding to the m/z 29 peak, is likely to occur in the fission fragment infratrack where there is a high degree of ionic, atomic, and fragment mixing [21]. The yield of the ions formed is then governed by the relative abundance of the constituent atoms and by the dynamics of chemical reactions in a hot plasma. The plasma temperature can be estimated from initial velocity measurements. For nitrocellulose bombarded by 252Cffission fragments, T ~ 2 6 0 0 0 K ((E,) ~ 4.5eV) has been suggested for H + and H~- SI, while T ~ 4600 K ((Ez) ~ 0.79 eV) was determined for CH~- ions [13]. The above pieces of evidence for an atomization/recombination process in this mass region are the basis and motivation for the theoretical approach presented in the following section.
4. The quasi-equilibrium plasma model and calculations The calculations presented later in this work are based on a model that assumes that desorption occurs in these stages. (a) Ionization regime. Just after the projectile/solid interaction, the track region is left highly charged but the atomized species are still at rest (i.e. at the temperature of the solid before the impact). The electron gas temperature may reach tens of thousands of kelvin. Inside the track, Coulomb repulsion forces create pressures of about 10000atm that are relatively constant in time. This regime is characterized by ionized species with high acceleration but negligible velocities. (b) Thermodynamic quasi-equilibrium regime. The average velocity of species becomes high but the drift velocity is still small because of the high collision rate (the mean free path is of the order of the atomic diameter). The plasma
17
starts to expand toward the vacuum, the pressure drops to about 1000atm and a quasi-equilibrium develops corresponding to temperatures in the 1000-10 000 K range. Under these temperature and pressure conditions, molecular species are formed with concentrations governed by thermodynamic principles. (c) Vacuum free-expansion regime. The drift velocity increases up to the average velocity of species and their acceleration drops to zero. The pressure tends rapidly toward residual gas pressure. This implies that the velocity distribution of emitted particles continues to reflect the average temperature of the previous stage. Also, besides metastable decay, chemical reactions cease so that the relative abundance of ions and neutrals is the same as in the previous stage. Refinements allowing different temperatures for distinct species may be introduced into the model by taking into account that, at the same kinetic energy, lighter particles will reach the free expansion regime before the heavier ones. The H +, CH +, CzH+,... groups sequentially leave a cooling plasma, so that their velocity distributions represent a "clock" and a "thermometer" of the desorption process.
4.1. Thermodynamic modeling The philosophy of this approach is to find a model that takes into account, as completely as possible, the stoichiometry of the original molecules and all the relevant physical quantities related to the structure of the emitted particles. These quantities should include the enthalpy of formation and the entropy for each neutral or ionized final species, and for a wide range of temperature. Nevertheless applied in a quasi-equilibrium framework, for a globally neutral, stationary and isotropic medium, a thermodynamic equilibrium plasma model offers a very convenient structure to host and manipulate
18
R.L. Betts et al./hlternational Journal (71'Mass Spectromet(v and hm Processes 145 (1995) 9-23
those physical quantities. Thus, besides the plasma temperature and pressure, there are no other free parameters in the model. Thermodynamic calculations were performed using the IDEALGAS program (Thermodynamics Research Center, Texas A&M University). Constant pressure was preferred due to the fact that in the second stage of the described model (when the recombinations are assumed to occur) the pressure still does not vary very fast. Also, it is worthwhile to remember that the Gibbs function has a logarithmic dependence on the pressure. Ionization potential values were taken from Ref. [18]. The values of enthalpy of formation, entropy and other thermodynamic data for the hydrogen, carbon and hydrocarbon neutral species are part of the program database. The data for the ions H +, H~-, H +, C +, CH +, C +, H - , C- and C2 are also part of the existing database. Data for ions not contained in the database provided with the program and not available for other sources were estimated from the data for the corresponding neutrals. The range covered by the thermodynamic calculations was from 3000 to 10000K and 10 to 10000atm. The selected conditions 5000 K and 1000atm are consistent with the energy deposition of the projectile, density of the target, size of the desorption site, and some relaxation of the system. The predicted total mole fraction of positive (or negative) species is 3.6 x 10 -4 for these conditions. The program was executed with different sets of input data to analyze the stability of the calculation with respect to some parameters. For instance, all the neutral fragments from hydrogen up to the native molecule were allowed to be formed to study the effect of higher fragment masses on the production of lower masses. In other runs, only fragments up to C2H 6 were considered in order to observe better the influence of the target stoichiometry. In all cases, electrons and positive ions (including H +, H~- and H~-) up to C2 H+ were
taken into account. When formation of H - , C- and C2 negative ions was allowed in the calculations, no significant variation in the final molar concentration at the selected temperature and pressure was observed. Unfortunately, negative ions desorbed in relatively high yield, such as C2H-, C4 Hand C 6 H - [11], were not considered in the calculations (thermodynamic characteristics unknown). The thermodynamic predictions obtained are based on the limiting case of equilibrium among positive and negative ions, electrons and neutral species. Therefore the initial structure (except stoichiometry) of the target compound or the initial ratio between neutral and charged particles does not affect the final molar concentration fraction. Because of the limited knowledge on experimental thermodynamic quantities for most ions, some assumptions needed to be made in order to perform the calculations. On the basis of known data for ions and corresponding neutral species, we assigned heat capacity and entropy values for the other ions. Because these two quantities are primarily based on the number of bonds in the molecule, they may vary in a similar manner in ions and in neutrals. This is seen in the C/C +, C H / C H +, and C2/C + neutral/ion pairs for which experimental data exist. It should be mentioned, however, that a variation of 10% in the entropy values of all the ionized species has a small but visible effect on the relative concentrations. There was no attempt to fit the P D M S spectra by changing these values. Typical IDEALGASresults, corresponding to the benzene stoichiometry, are presented in Figs. 6(a) and 6(b) for the CH + region and C2H + region respectively. The pressure was kept constant and equal to 1000atm. As the plasma temperature decreases, the abundance of all charged particles decreases. At very high temperatures, all molecular species dissociate. As a consequence, the polyatomic ion concen-
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
19
10-1
PDMS Spectra 10-2
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Thermodynamic Prediction
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m/z Fig. 6. C o m p a r i s o n o f the p l a s m a d e s o r p t i o n m a s s s p e c t r o m e t r y relative yields a n d the t h e r m o d y n a m i c the C H + a n d C 2 H + r e g i o n s f r o m c y c l o h e x a n e a n d b e n z e n e .
tration always present a maximum as the plasma temperature increases. The vertical broken line represents a particular temperature (5000 K), selected after the initial velocity measurements [13] for analysis of the stoichiometry's dependence on the concentration. Fig. 7 compares the experimental desorption yields for cyclohexane and benzene (Fig. 7(a)) with the relative con-
p r e d i c t i o n o f t h e m o l e f r a c t i o n in
centrations predicted by the thermodynamic equilibrium model (Fig. 7(b)). A qualitative agreement can be seen between the measured and the predicted spectra. For both compounds, the C2H 2 group has, on average, a higher yield than the CH + group. The relatively high yield of CH +, C2H~- and C2H~peaks is reproduced by the calculations, as well as the relatively low yield of the positive radical
20
R.L. Betts et al./hlternational Journal o/' Mass Spectrometry and Ion Processes 145 (1995) 9 23
1 0 - 4 --
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10-8
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Temperature (K) Fig. 7. Thermodynamic prediction of the mole fractions of the CH,,+ and C2H,+ ions at a constant pressure of 1000 atm.
ions. The trends of the stoichiometry effect are also correctly predicted. In the lower mass region of each group, ions emitted from benzene have higher yields than those emitted from cyclohexane, and the inverse occurs at higher masses. Some disagreements should also be noted: (a) the mole fractions of C +, CH + and CH~are too low compared to those of CH~- and the
C2H~ group; (b) the predicted mole fraction ratio for C2 H+ and C2Hf is too high; and (c) the predicted stoichiometry effect is not strong enough. To minimize these discrepancies, inspection of Fig. 7 reveals that, for a benzene temperature greater than 7000 K, C +, CH + and CH~- have about the same abundance. However, under these conditions, the C~- mole fraction
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
becomes higher than that of C2 H+ or C2H~-, in disagreement with the PDMS yield result. A better fit is obtained by choosing 4500K as the temperature only for the C2H + group which also allows the C2 H+ yield to be lower than the CzH ~- yield. The stoichiometry effects are actually underestimated: (liquid) benzene is 33% more dense than hexane, its fission fragment stopping power is about 15% higher and the melting points are respectively 5.5°C and -95°C. Therefore one should expect different temperatures and pressures for the plasma produced inside the target compounds. Other models for material expansion have been reported [10,11]. In particular, PDMS spectra of alkanes have been analyzed by the code CRUNCHERwhich considers a statistically independent breakup of all chemical bonds of the sample molecule [10]. This program treats only neutral species and sums the results of each region containing a certain number of carbon atoms. While this method predicts well the overall trends, the fine structure of fragment ion formation is lost. The IDEALGAS program predicts the abundance of each species individually. The fragment ion intensities are governed primarily by the heat of formation and the concentration of atoms available to form the fragments. This method has the advantage of retaining structural information in the predicted ionic distribution. Fine structure in each carbon region can then be compared with experimental data. 4.2. General considerations
The SI produced by the impact of megaelectronvolt heavy ions on organic samples may be analyzed according to their dominant formation mechanism. Three main groups may be considered: the hydrogen ion group, the low mass CnH + groups and the molecular ion group. The last group includes the quasimolecular ions and the large fragments or clusters having structures similar to the
21
sample molecules. The high initial velocity of positive hydrogen secondary ions indicates that they are, besides electrons, the first emitted particles in a projectile/solid collision [2]. Their energy of emission lies in the tens of electronvolt range [12,13,22], which is higher than for the other secondary ions. Moreover, for atomic projectiles having the same velocity and the same (high) charge state, the H + desorption yield does not depend on their mass or atomic number [23]. This is a strong argument in favor of hydrogen ions originating from the first surface layers. The molecular ion group is expected to be emitted from the surface at a peripheral region of the track (ultratrack). This is so because they could hardly endure the multiple electronic excitations and ionizations occurring very close to the projectile trajectory (infratrack). The description of their desorption process is achieved through models based on shock waves [24,25], multi-hit secondary electrons [26] or molecular dynamics [27]. The emission of the low mass CnH + ions via a plasma process is discussed in this work. Certain observed emission characteristics seem not to fit into this process. It has been reported that the initial radial velocity of large desorbed fragments depends on the projectile's angle of incidence, while low mass contaminant and fragment ions do not exhibit such a dependence (except H + and H~-, for which case there is some) [22]. Recent radial velocity measurements of organic compounds have shown that the angular distribution of low mass Sis is dependent on their degree of hydrogenation [28]. An oblique projectile incident angle generates a charged plasma with a low degree of symmetry which results in asymmetric SI emissions in the incident ion plane. The different radial velocity distributions observed for different CnHm ions [28] may not be incompatible with a plasma desorption model. Rather, these results may be due to the
22
R.L. Betts et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 9-23
fragility of polyatomic species escaping from a non-spherical plasma in expansion. Once formed, more highly saturated molecules may be preferentially destroyed by collisions in the track jet it they move in the projectile direction, while molecules of lower saturation are the main constituents of the jet. On the other hand, negative ions stay trapped by the electric field of the plasma for a certain time and then are emitted with a lower and relatively uniform average kinetic energy.
5. Conclusions We have studied the desorption of low mass CnH + ions through plasma desorption mass spectrometry. By using frozen samples of C6 cyclohydrocarbons, we have obtained evidence for atomization of the sample compounds followed by a recombination process. This implies that most of the information about the target molecule's structure was lost during track formation. The original stoichiometry, preserved throughout the entire process, is shown clearly to affect the relative yield of desorbed ions. PDMS spectra and EI spectra have similarities, as a consequence of the structure of the (same) emitted ions, and differences, attributed to the atomization which occurs only in the former and to the unimolecular decay which dominates the latter. These observations support the assumption of plasma formation in the infratrack in PDMS. A model to describe this process is sketched. Our thermodynamic calculations point towards a high degree of mixing in the desorption site, and suggest that a quasiequilibrium may be achieved. The qualitative agreement of this model's predictions with experimental data shows also that the behavior of desorption yields is very sensitive to the plasma temperature, pressure, stoichiometry of the target compound and
structure of the emitted ions. Thermodynamic data that are based on experiments on ions would enhance the accuracy of predicting, via calculations, the behavior of the desorbed species for a large mass range. As this type of model may, in principle, predict the dependence of desorption yields as a function of the energy deposited by the projectile, future studies may focus on different plasma temperature situations created by varying geometry, projectile characteristics or sample material. The plasma formation mechanism gives a reasonable description of the energy distribution and relative yield of species emitted from the infratrack region. Further research is still necessary to analyze the compatibility of the model with other measurements such as desorption yield dependence on the projectile energy, projectile charge state and angular distribution of the SI.
Acknowledgements We would like to thank D.A. Singleton and C.R. Ponciano for discussions on ion structures and chemical dynamics. We would also like to thank J.W. Bevan for access to the IDEALGAS program. K. Wien is gratefully acknowledged for transmitting to us the benzene and hexane data. Funding for this work was provided through NSF grant CHE-9208185 and CNPq Brazil.
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SPUT92, Matematisk-fysiske Meddelelser, Copenhagen, Denmark, 1992, p. 351. [5] G.H. Vineyard, Radiat. Eft., 29 (1976) 245. [6] R.E. Johnson and R. Evatt, Radiat. Eft., 52 (1980) 187. [7] H. Voit, E. Nieschler, B. Nees, R. Schmidt, C. Schoppmann, P. Beining and J. Scheer, J. Phys., 50 (1989) 237. [8] L.E. Seiberling, J.E. Griffith and T.A. Tombrello, Radiat. Eft., 52 (1980) 201. [9] I. Bitensky, E. Parilis, S. Della-Negra and Y. Le Beyec, Nucl. Instrum. Methods, B72 (1992) 380. [10] E.R. Hilf, W. Tuszynski, B. Curdes, J. Curdes, M. Wagner and K. Wien, Int. J. Mass Spectrom. Ion Processes, 126 (1993) 101. [11] M. Wagner, K. Wien, B. Curdes and E.R. Hilf, Nucl. Instrum. Methods, B82 (1993) 362. [12] N. Furstenau, N. Knippelberg, F.R. Kueger, G. Weiss and K. Wien, Z. Naturforsch, Teil A, 32 (1977) 711. [13] R.D. Macfarlane, J.C. Hill and D.L. Jacobs, J. Trace Microprobe Tech., 4 (1986-87) 281. [14] M.A. Park, E.L. Shirey and E.A. Schweikert, J. Trace Microprobe Tech., 11 (1994) 285. [15] E.L. Shirey, Plasma desorption mass spectrometry of frozen organic compounds, M.S. Thesis, Texas A&M University, 1993. [16] R.L. Betts, M.A. Park, E.L. Shirey and E.A. Schweikert, Org. Mass Spectrom., 29 (1994) 679.
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[17] K. Wien, personal communication, 1994. [18] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 71st, Edn. CRC Press, Boca Raton, FL, 1990. [19] S.R. Heller and C.W.A. Milne (Eds.), NIH/EPA Mass Spectral Database, National Bureau of Standards, Gaithersburg, MD, 1978. [20] J.H. Beynon, Microchim. Acta, 1 (1956) 437. [21] R.E. Johnson, Int. J. Mass Spectrom. Ion Processes, 126 (1993) 17. [22] D. Feny6, B.U.R. Sundqvist, BR. Karlsson and R.E. Johnson, Phys. Rev. B, 42 (1990) 1895. [23] S. Della-Negra, O. Becker, R. Cotter, Y. Le Beyec, B. Monart, K. Standing and K. Wien, J. Phys., 48 (1987) 261. [24] I.S. Bitensky, A.M. Goldenberg and E.S. Parilis, J. Phys., 50 (1989) 213. [25] R.E. Johnson, B.U.R. Sundqvist, A. Hedin and D. Feny6, Phys. Rev. B, 40 (1989) 49. [26] A. Hedin, P. H~kansson, B.U.R. Sundqvist and R.E. Johnson, Phys. Rev. B, 31 (1985) 1780. [27] D. Feny6, A. Hedin, P. H~kansson and B.U.R. Sundqvist, Int. J. Mass Spectrom. Ion Processes, 100 (1990) 63. [28] R.M. Papalbo, G. Brinkmalm, D. Feny6, J. Eriksson, H.-F. Kammer, P. Demirev, P. H~kansson and B.U.R. Sundqvist, Nucl. Instrum. Methods, B91 (1994) 667.