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Isotope, molecular and surface effects on hyperthermal surface induced dissociated ionization
InternationalJournalof Mass Spectrometry and Ion Processes 125 (1993) 63-74
63
0168-1 ! 76/93/$06.00 © 1993 - Elsevier Science Publishers B.V., Amsterdam
Isotope, molecular and surface effects on hyperthermal surface induced dissociative ionization A. Danon, A. Amirav* School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel-Aviv 69978, Israel ( R e c e i v e d 2 J a n u a r y 1993; a c c e p t e d 25 J a n u a r y 1993)
Abstract Organic molecules acquired with hyperthermal (1-20eV) kinetic energy undergo efficient surface ionization. This hyperthermal surface ionization (HSI) may produce both positive and negative fragment ions. The dissociative ionization of alkyl halides results in the production of negative ions of the functional group having high electron affinity, and positive ions of the alkyl radical, which can further dissociate into smaller fragments. The effect of the alkyl chain length and the bromine isotope effect on the obtained HSI mass spectra were studied in several alkyl halide molecules. While the negative ion formation yield is found to be independent of the size of the alkyl radical, the positive ion formation yield strongly increases with the size of the CnH2~ + 1 radical for n = 1-4 and then a quasi saturation is observed. The observed radical fragmentation increases with the incident molecular kinetic energy and was affected by the molecular structure and the surface temperature and cleanliness. A considerable (up to 24%) heavy bromine isotope increased ionization is observed at intermediate molecular kinetic energies. Piperidine HSI on a rhenium filament exhibits a single (M - 1) ion while its HSI from an oxidized rhenium filament having a much higher work function is characterized by a much richer fragmentation pattern. The dissociative ionization mechanism is rationalized in terms of a surface-molecule electron transfer followed by an immediate dissociation into a negative halogen ion and an alkyl radical. This radical, which usually has a low ionization potential, can transfer an electron to the surface and scatter as a positive ion. At high kinetic energies, the radicals or positive ions can further dissociate near the surface, and then scatter away as lower mass ions with ion yield which depends on their surface reneutralization probabilities. Thus, the observed fragmentation pattern is governed by surface chemical and electron transfer processes and not by gas phase unimolecular ion dissociation, as found with large polyatomic molecules.
Key words:Surface effects; Ionization processes; Isotope effects.
1. Introduction
e s t a b l i s h e d . I n p r e v i o u s studies w e i d e n t i f i e d t h e hyperthermal
surface ionization (HSI) phenom-
S u r f a c e i o n i z a t i o n is a well k n o w n a n d e s t a b -
e n o n [8-16]. U s i n g a s e e d e d s u p e r s o n i c m o l e c u l a r
lished p h e n o m e n o n [1-7]. It is i n t e r p r e t e d c o n v e n -
beam one can accelerate "heavy" organic molecules to t h e h y p e r t h e r m a l k i n e t i c e n e r g y r a n g e o f
tionally by the Saha-Langmuir
equation, based on
the a s s u m p t i o n t h a t t h e r m a l e q u i l i b r i u m b e t w e e n
1 - 2 0 e V [17-19]. T h i s h y p e r t h e r m a l k i n e t i c e n e r g y
t h e s u r f a c e a n d t h e species d e s o r b e d f r o m it is
effectively b r i d g e s the d i f f e r e n c e b e t w e e n t h e surface w o r k f u n c t i o n a n d t h e i o n i z a t i o n p o t e n t i a l ,
*Corresponding author.
e v e n in m o l e c u l e s w h e r e this e n e r g y g a p is l a r g e
64
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
and cannot otherwise be bridged by the average thermal energy. Thus, virtually all organic molecules can undergo efficient ionization following a hyperthermal collision with a surface. Five main types of HSI processes were observed: (i) Positive ion HSI (PHSI), where positive molecular ions were produced as a result of molecule-surface electron transfer [8, 10]; (ii) Negative ion HSI (NHSI), where negative molecular ions were produced due to surfacemolecule electron transfer [8-10]; (iii) Hyperthermal surface induced dissociative ionization (HSIDI), where both negative and positive fragment ions were produced [8]; (iv) Chemically induced HSI, where the hyperthermal kinetic energy-surface scattering results in the creation of new species with favorable electronic properties that promote efficient HSI [12-15]; (v) Local hot spot HSI was observed in Hg/ Pt(111) scattering where the atom-surface energy transfer induced a transient local hot spot which facilitated a high efficiency thermal surface ionization [16]. Surface-molecule electron transfer processes and their several possible outcomes were theoretically treated by Gadzuk and Holloway [20-23]. However, the physical mechanism responsible for the obtained hyperthermal surface induced dissociative ionization is not yet fully established. Recently, low energy organic ion-surface scattering was studied and is reviewed by Cooks et al. [24]. These ion-surface scattering processes are similar to the second half of the PHSI process where the neutral molecule transfer an electron to the surface and then scatters from it as a low energy ion [25]. While ion-surface scattering can involve kinetic-vibrational energy transfer which may induce dissociation other processes of surfacemolecule electron transfer may control the outcome of HSI and induce dissociation, as will be discussed in this paper.
In electron impact mass spectrometry the fragmentation pattern is described by statistical theories such as QET and RRKM [26-30]. The fragmentation patterns solely depend on the molecular and fragment ion's vibrational energy which may originate from the electron ionization process added to the molecular thermal vibrational energy. In HSI the initial vibrational energy of the molecules is very low due to the supercooling in the supersonic molecular beams [31]. However, vibrational energy can be acquired through kinetic-vibrational energy transfer mediated by the surface or by the initial surface-molecule electron transfer process itself. The fragmentation pattern, however, may be largely influenced by the surface-molecule, surface-fragments and surface-ion interaction potentials. In this paper we present further experimental results that help to establish a more consistent mechanism for the HSIDI of alkyl halides. We describe the sequence of reactions leading from the organic parent molecule colliding with the surface to the observed negative and positive mass spectral fragmentation pattern. Molecules such as alkyl iodides and bromides were chosen because of their high ionization yield, their simple fragmentation pattern, and the availability of higher homologs and isomers that can be examined and compared with the fragmentation pattern of normal alkanes in electron impact. These molecules can be viewed as an electron acceptor halogen group which can facilitate a surface-molecule electron transfer and an alkyl radical "tail" residue, which after dissociation can transfer an electron back to the surface and be observed in the positive ion mass spectra. A central question addressed by this work is whether the ion fragmentation occurs near the surface and is affected by its properties or it is a unimolecular process far away from the surface. Piperidine was also studied as an example of a molecule which can dissociate after a molecule-surface electron transfer on a surface with a controlled workfunction.
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
2. Experimental
2.1. UHV Mass Spectrometric Experiments The U H V mass spectrometer experimental apparatus was described in detail elsewhere [8]. Here we briefly describe the modifications pertinent to the present experiments. The organic molecules were aerodynamically accelerated in a seeded beam of H2 of He. The gas mixture was expanded through a 8 0 # m diameter ceramic nozzle which could be differentially heated up to 1000°C (for detailed description of the nozzle, see ref. 32). The molecular beam was skimmed and collimated through two differentially pumped chambers into a U H V chamber (base pressure 1 x 10 -9 Torr). The beam was square-wave modulated by a mechanical chopper for lock-in amplification, or pulsed for time of flight kinetic energy measurements. In the UHV chamber the beam collided with a single crystal surface or a filament. The beam spot on the surface was 3.2 × 1.2 mm 2. The surfaces used in these experiments were d i a m o n d ( I l l ) , silicon(Ill) and a technical rhenium filament. The diamond surface was mechanically polished and chemically treated by a hot KNO3/H2SO4 solution before insertion into the vacuum. It was also annealed in vacuum at 875°C and checked by helium diffraction as described in ref. 11. The silicon surface was cleaned and annealed in vacuum at 1200°C. The rhenium filament used was a 0.1 mm diameter wire. It was cleaned by heating to 2000°C in vacuum and by oxygen treatment cycles at 1 x 10-6Torr. Two quadrupole mass spectrometer (QMS) heads (UTI 100C) were used as ion mass analyzers and detectors: one for the direct unscattered beam and the second, mounted 45 ° to the molecular beam axis, for the scattered ion detection. This head was modified for external ion collection and negative ion detection (in ion detection measurements the QMS filament was turned off).
65
2.2. Total expansion experiments Another apparatus served us for direct expansion of an unskimmed beam-free jet experiment [33]. We utilized a single stage, small vacuum chamber which was pumped by a rotary pump alone. The surface was placed about 3 - 5 m m downstream from the nozzle. As a result, all or most of the accelerated molecules collided with the surface. This small chamber was coupled to a capillary gas chromatograph (HP5890A) by installing it above the flame ionization detector (FID), without removing the FID. In order to operate this HSI detector, helium was used as a carrier gas in the gas chromatograph column (5 meter megabore 530#m i.d. DB-1 of J&W). Molecular samples for study, calibration or intra-nozzle chemical reactions were introduced through a special opening. The air flow to the FID was shut down and the hydrogen flow was increased to 150-200 ml min -l to achieve high kinetic energies. The total flow was expanded through the nozzle (the same nozzle as in section 2.1). The detector was a simple Faraday cup collector placed at a right angle to the surface. In this apparatus we directly measured the positive and negative total ion currents from the Faraday cup, where the surface was biased at positive or negative voltage respectively.
3. Results Figure 1 shows the PHSI mass spectra of a homologous series of normal alkyl iodides (iodoethane-iodoheptane), scattered from a d i a m o n d ( I l l ) surface. The mass spectra have been redrawn from analog presentations with pen limited noise level. Since adjacent masses were insufficiently resolved, peaks due to natural abundant 13C were omitted. The mass spectra always exhibit the radical alkyl fragments at mass 29 (C2H~-), 43 (C3H~'), 57 (C4H~-), 71 (CsHfi), 85 (C6H+3) and 99 (C7H~-5) which are produced through the dissociative ionization of n-iodoethane to n-iodoheptane molecules respectively. This dissociative ionization also results in
66
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
CnHzn÷l I/DIAMOND POSITIVE IONS
)
r
I
)" ,tb /
//
MASS
I
/' / //
//
/~3H7!
(u)
Fig. ]. The molecular tail effect on PHS] mass spectra: PHS! mass spectra of normal alkyl iodides (C.H2.+1 !; n = 2-7) scattered from a diamond(l 1l) surface. The surface temperature was 380°C, the nozzle temperature was 480°C and the hydrogen backing pressure was 1000Torr in all the measurements. The mass spectra are normalized and peaks due to t3C were omitted. Note that the mass spectra are mostly dominated by propyl and butyl radical ions.
I- negative ion formation. The molecular ion peaks are missing through all the series in the obtained mass spectra. Lower alkyl ions, which are produced due to sequential fragmentation of the primary alkyl ions, are also observed. In iodoethane, iodopropane and iodobutane molecules, the undissodated ions are the major peaks, while in the larger molecules the dissociation of the alkyl ion is dominant. The unimolecular dissociation of the primary alkyl ion in the lower enthalpy pathway usually forms ethylene or propylene molecules. As an example, the mass spectrum of iodohexane consists of the hexyl ion at 85u and butyl and propyl ions at 57 and 43 u respectively according to the following dissociation pathways of the hexyl ion: C6H~-3 - - * C 4 H ~- -F C 2 H 4
C6H~-3 --* CaH~- + C3H6 These two dissociation pathways of the hexyl ion are the lowest energy unimolecular dissociation pathways [34]. In addition, the peaks at 55 and 41 u are the consequence of hydrogen elimination which is a well known phenomenon in hydrogen-
rich ions. Normally, (under electron impact ionization) the propyl and butyl ions continue to dissociate to form ethyl and methyl ions, and the electron impact mass spectra of these molecules consist of peaks from methyl up to the molecular ion. In the HSIDI, the ethyl ion signal is very weak (except in iodoethane where the ethyl ion is the molecular residue and is the only peak observed), and the methyl ion signal is missing throughout all the series (we could not observe any CH~- ions even from iodomethane). Another notable difference in comparison with similar electron impact ionization spectra is the lack of single hydrogen atom consecutive dissociation peaks and the relative weakness of the molecular hydrogen elimination peaks. This is clearly the outcome of molecular hydrogen iodide elimination in electron impact ionization while in HSI, as will be discussed, the initial dissociation is that of the iodine negative ion and atomic hydrogen consecutive dissociation is unfavorable. In order to compare the relative positive and negative ionization yield of these molecules, their mixture solution should be separated in time, and time integrated using gas chromatography. Figure 2 presents the PHSI and NHSI chromatograms of a mixture of normal iodoethane, iodopropane, iodobutane, iodopentane and iodohexane [33]. The mixture contains 1% of each molecule in ethanol solution. The positive ion chromatogram (upper spectrum) includes all the possible ions emerging from the molecules as shown in Fig. 1. The negative ion mass spectra of these molecules consist of iodine ions alone, meaning that the negative ion peaks are due to the iodine current. Note also that the ethanol solvent peak is unobserved in both traces due to the extreme ionization selectivity which can be encountered in HSI. Table 1 shows the integrated peak areas of the positive and negative chromatographic peaks of Fig. 2. As can be seen from Table 1, the negative ion formation yield of this series is practically constant or decreases slowly with the alkyl chain length (small steric effect). However, the positive ion formation yield of the iodoethane is very low (0.8%)
A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
and the lack of the ethyl and methyl ions in the positive ion mass spectra can be rationalized in terms of ion reneutralization probabilities in their exit trajectories as will be discussed in section 4. Cyclic alkyliodides show similar behavior to the linear alkyliodides, except that they exhibit a relatively more intense cyclic alkyl ion peak than that of the linear molecules. In Fig. 3 the PHSI mass spectra of cyclohexyliodide and 1-iodohexane are compared. It is clearly observed that the relative undissociated alkyl ion (hexyl- ion) is more intense in the cyclic molecule than in the linear one. Moreover, the relative total yield is two orders of magnitude higher in cyclohexyliodide (not shown in the figure). In order to induce fragmentation of the cyclic C6H+I ion, two C - C bonds must be broken and much more energy is
HSI/DIAMONO
C4H9I
CsHnI
POSITIVE IONS C6HI31
C3H71
A C3HTI
NEGATIVE IONS C4HgX CsHIII
C6HI3I
A
I
I
2
0
3
67
I
I
PHSI
TIME(MIN)
I-Iodohexone
Fig. 2. The molecular tail effect on PHSI and NHSI total ionization yields: Positive ion (upper) and negative ion (lower) hyperthermal surface ionization-gas chromatographic detection of a mixture of 1% normal alkyl iodides (C, Hz~+tI; n = 2-6) in ethanol. The mixture was injected into a gas chromatograph with a 5 m long megabore capillary at 50°C. A diamond(Ill) single crystal at 450°C was used as a surface. Note the constant size molecular independent peak area in the negative ion trace, while the positive ion chromatogram exhibits a large peak area increase with the "tail" size.
(no PHSI yield in CH3I), and increases dramatically in iodopropane and further in iodobutane and then it slowly saturates. The peak between the iodopropane and iodobutane in the PHSI chromatogram was identified as a tert-butyliodide impurity which possesses a much higher positive ionization yield than that of normal alkyliodides (lower ionization potential of the tertiary ion). This trend in the positive ion formation yield,
Cyclohexyl
I0 Table 1 Integrated peak areas (%) of the PHSI and NHSI peaks of Fig. 2 C2H5I PHSI (integrated) 0.8 NHSI (integrated) 88
C3H7I C4H9I C5HIuI C6HI3I 31.5 100
69 95
1
4
83 91
100 80
I
30
I
iodide
i
50
70
MASS
(U)
I
90
I10
Fig. 3. Molecular structural effects on PHSI mass spectra: PHSI mass spectra of 1-iodohexane (upper) and iodocyclohexane (lower). The nozzle temperature was 450°C, resulting in about 8.5 eV incident molecular kinetic energies for both molecules. The diamond surface temperature was 400°C in both spectra. Note the higher degree of fragmentation of the normal 1-iodohexane.
A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
68
required. In addition, the secondary cyclic ion is more stable than the primary alkyl ion; thus, its relative ionization yield is much higher due to both the higher C6H+I ion kinetic energy and its lower ionization potential as a secondary cation. However, once the ring is broken, the residual ion behaves like ions formed from the linear molecules, giving bunches of ions 14 u apart, due to ethylene or propylene expulsion. The positive and negative ion hyperthermal surface ionization yield and the obtained fragmentation pattern depend on the scattered molecules, their initial kinetic energy, the surface used, its work function and temperature. Figure 4 shows the PHSI mass spectra of piperidine scattered M-!
Piperidine/Re PHSI TN =435"C
POz =1=10" 6 torr TN=435" C
/
M
POz = I. I0 -6 l o r r TN = 8 0 ° C
= I0
i MASS
(U)
I10
Fig. 4. The role of the surface on PHSI mass spectra: PHSI mass spectra of piperidine scattered from Re and ReO surfaces as indicated. The PHSI mass spectra is much richer from the ReO surface which possesses a very high surface work function. Thus, even lower mass fragments are not reneutralized on the surface.
from a rhenium filament with different experimental conditions. Piperidine is a molecule which is efficiently ionized even under thermal surface ionization [35]. Its HSI on rhenium oxide is under current investigation by Kishi and Fujii [36]. The upper mass spectrum is of piperidine at 435°C nozzle temperature (high kinetic energy) scattered from a technical Re filament (about 1100 K filament temperature). It shows only an ( M - 1) peak which is due to molecule-surface H - transfer. This H - transfer is believed to be due to molecule-surface hydrogen atom transfer followed by a radical-surface electron transfer [14]. The addition of 1 × 10-6 Torr oxygen in the vacuum chamber changes the surface properties and turns the Re surface to ReO (unknown stochiometry) which possesses a very high work function of 6.4eV [37]. The middle spectrum shows the obtained PHSI mass spectrum from this ReO surface, at the same beam conditions as those used to obtain the upper mass spectrum. The (M - 1) mass peak intensity is increased by about two orders of magnitude in comparison with its intensity obtained with the technical Re surface. Moreover, extensive fragmentation is now observed and the molecular ion appears as well (in the upper spectrum the peak at the molecular mass is solely due to the 13C contribution of the (M - 1) peak). By reducing the initial kinetic energy, the molecular ion relative intensity increases and the fragmentation to low mass ions decreases as seen in the lower mass spectrum. We note that in scattering from a ReO surface the molecular ion is a major peak (71%), combined with lower mass ions, while in scattering from the Re surface it is not detected at all, and (M - 1) + is the dominant peak. HSI is driven by the incident kinetic energy. In l-iodopropane/diamond scattering the positive and negative ion signal increased by five orders of magnitude between near threshold and 10eV incident kinetic energies [11]. In molecules with rich fragmentation patterns the incident kinetic energy can also largely change the relative height ratio among the various peaks [25]. In Fig. 5 the results of 1-iodopentane scattered
69
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
I - B r o m o p e n l o n e / S ~ (111)
CsHIIIIOIAMOND PHSI
Ts = ~ 3 0 0 ° C
A PHSI
TM,280"C
B
Ts=~800°C
j I
20
I
4o
80 MASS
10
50
50 70 90 MASS (U)
II0
Fig. 5. The incident molecular kinetic energy effect on PHSI mass spectral fragmentation pattern: PHSI mass spectra of 1-iodopentane at three incident kinetic energies. (A) Nozzle temperature, 280°C (low kinetic energy); (B) nozzle temperature, 375°C; (C) nozzle temperature, 455°C. The surface was a diamond surface at 400°C in all the three measurements. Note the tail increased dissociation with the increase of the incident kinetic energy. from diamond(l 11) surface at three different initial kinetic energies are shown. At relatively low incident molecular kinetic energy of 6 eV (Fig. 5(A)) the pentyl (m = 71 u) and propyl ions practically dominate the spectrum and their height ratio is 1 : 1. As the incident molecular kinetic energy increases to ~ 7.2eV (Fig. 5(B)) and 8.1eV (Fig. 5(C)), the dissociation of the pentyl to propyl ion gradually increases and in addition butylene ( m = 55u) and ethyl peaks appear. Similar behavior was found in most of the molecules we have worked with and with all surfaces. Figure 6 shows the fragmentation pattern dependence on the surface temperature in the 1-bromo-
,60
(u)
Fig. 6. The surface temperature effect on PHSI mass spectra: PHSI mass spectra of 1-bromopentane scattered from Si(111) surface at the indicated surface temperatures. The tail dissociation may be affectedby the temperature of the surface,probably due to a change in its cleanliness. pentane/Si(111) system. The bromopentane P H S I mass spectra consist mostly of pentyl and propyl ions. As seen from the spectra, the ratio between these two peaks depends on the surface temperature. The surface temperature affects the surface cleanliness during the scattering. It can also change the surface reactivity, its work function, the adsorption probabilities, the energy transfer to the molecules and the reneutralization probabilities of the different fragments. The Si surface at 300°C is covered by atomic hydrogen which can emerge from the carrier hydrogen gas or from the scattered molecules. Thus, atomic hydrogen or other contamination coverage on the surface can change the ionization probabilities and the fragmentation pattern. In contrast with the Si surface, on a diamond surface we could not observe such a surface temperature dependence of the fragmentation pattern. The ionization yield also depends on the molecular weight and exhibits an isotope effect. Figure 7 shows the N H S I mass spectra of 1,2-dibromopen-
A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 53-?4
70
I, 2-DIBROMOPROPANE/DIAMOND
\. Ek:6e
I
77
I
79 MASS
I
81 (U)
the natural isotope abundance ratio. This means that at kinetic energies where the ionization yield dependence on the molecular kinetic energy is stiff, a considerable (up to 1.24) isotope effect is observed which is largely reduced at higher kinetic energies. We note that the results shown in Fig. 7 are with pen limited noise due to extensive mass spectra averaging with Le-Croy 9400 signal averager so that the isotope effect can be accurately extracted. At lower incident kinetic energies the isotope effect is presumably higher but the ion statistical noise prohibits its precise measurement. As the kinetic energy is increased the isotope effect is gradually reduced. A similar effect of heavy ion enrichment at intermediate kinetic energies was observed with chlorine isotopes and with the positive fragment ions.
1
83
Fig. 7. Isotope effectin HSI: NHSI of 1,2-dibromopropanenear the bromineisotopemasses at two differentkineticenergies.The two spectra were taken at the same surface conditions. Each mass spectrum shown was averaged over 500 times with a Le Croy 9400signalaverager.A substantial isotopeeffectis demonstrated with increasedheavy isotope ionization at intermediate kinetic energies. tane scattered from diamond, near the two bromine isotope masses, at two different incident molecular kinetic energies. The upper mass spectrum was obtained at 9 eV incident kinetic energy of the bromopentane molecules. At this kinetic energy the height ratio of the two bromine isotopes is close to their natural isotope abundance (50.5% :49.5% to 79Br and SIBr respectively) as has been also measured in electron impact ionization at the same QMS head and experimental conditions. However, when the kinetic energy is lowered to the range of 5-6 eV, (near the practical threshold) it is clearly observed (also in PHSI) that the low mass bromine isotope relative height decreases (lower spectrum), while the high mass bromine isotope height increases. At this incident kinetic energy the height ratio between the two isotopes is 45.2% :54.8%o, while in the electron impact ionization mass spectrum the ratio is still
4. Discussion The available experimental results shown here allow us to present an improved qualitative picture of the HSIDI mechanism. Accordingly, in all those molecules which contain an atom or a group with a high electron affinity, the first step in the HSIDI process is an electron transfer from the surface to the molecule to form a negative molecular ion [8,9]. In the alkyliodide (or bromide) molecules, in which the difference between the electron affinity of the iodine atom and that of the molecule is larger than the carbon-iodine bond strength in the negative ion, a consecutive spontaneous dissociation occurs. As a result, the initial outcome of the surface-alkyliodide electron transfer is a dissociative ionization into I - and an alkyl radical residue. This process can be further enhanced by the possible radical-diamond (or another) surface reactions, which can also lower the threshold energy. Both diamond and silicon are semiconductors so that the electron transfer from the valence band forms a single hole near the negative ion. The existence of only a single positive "image" charge near the negative ion reduces its reneutralization and substantially enhances the negative ion forma-
A. Danon and A. Amirav/ lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
tion which was observed from semiconductor surfaces in contrast with metal surfaces [8]. Above a certain kinetic energy the radical can also scatter from the surface. As radicals usually possess a low ionization potential, they can effectively transfer an electron back to the surface (to the single hole formed by the initial surface-molecule electron transfer) and scatter as positive ions. Thus, the mass spectra obtained will consist of negative iodine (or bromine) ions and positive ions of the alkyl residue. Therefore, the kinetic energy dependence and the threshold energy and ionization yield of the negative iodine and of the positive alkyl ions will be different. The fragmentation of the alkyl ions into lower mass ions, in the positive mass spectra, can take place by two possible mechanisms. One possibility is that the primary alkyl radical dissociates near the surface to form a lower mass homologous radical and a neutral alkene molecule. Then the lower alkyl radical transfers an electron to the surface and scatters as a positive ion. The other possibility is that the dissociation occurs in the primary alkyl ion after the scattering from the surface, unimolecularly during the flight time to the detector as was observed with large polyatomic molecules such as cholesterol [25]. In this case the dissociation will be to a lower mass positive alkyl ion and a residue according to statistical dissociation rates. Here we will try to determine which of the two mechanisms is more appropriate for the alkyl iodide molecules. The total measured ionization yield depends on the molecule-surface electron transfer yield and the reneutralization probabilities of each radical and ion. The reneutralization probabilities depend on the surface, the molecular properties and the kinetic energy of the ions which scatter from the surface. When an ion dissociates, its kinetic energy is distributed among the resulting fragments approximately according to their mass (momentum conservation). In C3H7I , for example, 75% of the available kinetic energy goes to the iodine atom upon dissociation while only 25% goes to the propyl radical [8]. Since ions near the surface are attracted to it due to its image potential, ions with
7l
low kinetic energy will be trapped near the surface and will be reneutralized on it. Accordingly, if the dissociation of the primary positive alkyl ion occurs on the surface, the low mass ions (methyl and ethyl), which carry a low portion of the molecular ion kinetic energy, will not be able to escape from the image potential of the surface and will be reneutralized. Additionally the ionization potentials of the radicals CH3, C2H3 and Call 7 are 9.83, 8.4 and 8.1 eV respectively [38]. Thus, ethyl and especially methyl ions are expected to be effectively neutralized on the surface even if radicalsurface electron transfer Occurred and only propyl radical exhibits efficient ion formation. However, if the dissociation occurs far away from the surface, then the ratio between the different alkyl ions' height will be given by the intramolecular dissociation theories alone, and the resulting fragmentation pattern will be similar to that obtained in electron, impact ionization. In Fig. 1 we show that the fragmentation pattern of the alkyl halide molecules consists mostly of high mass alkyl ions while the low mass methyl and ethyl ions are missing. This observation suggests that the primary alkyl ion dissociation occurs near the surface and therefore the methyl and ethyl ions which are effectively neutralized on the surface are not detected. The same arguments explain the PHSI and NHSI yields of the gas chromatograms shown in Fig. 2. In the negative ion chromatogram the measured current was from negative iodine ions alone through all the series. Since the iodine atom is heavier than the alkyl chain, it will carry most of the kinetic energy which is weakly dependent on the alkyl chain length. In addition, because all components of the mixture have the same velocity in the beam, the kinetic energy in the beam increases linearly with the molecular mass. In the experiments presented in Fig. 1, the molecular kinetic energy of iodopropane was 7.2 eV and the kinetic energies of the other alkyl iodides were linearly dependent on the molecular weight. Iodohexane for example, contains 30% more initial kinetic energy than iodoethane as the same nozzle
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A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
conditions. While the incident kinetic energy increases as the molecular alkyl chain increases, the heavier radical residue obtains the excess energy and the iodine velocity and kinetic energy remains unchanged. Therefore according to the proposed mechanism, the kinetic energy of the iodine ions from all the series in the gas chromatogram will be similar and one can expect the same NHSI yield to all the series (see Table 1). Only minor differences are expected due to steric effects on the electron transfer probabilities. However, the opposite is true for positive ion formation. In iodomethane the ionization yield is very low and we could not detect any positive ions. The positive ionization yield of iodoethane is also low because the mass spectrum contains only the low mass ethyl ion. The ionization yield increases in iodopropane and iodobutane and saturates in the longer alkyl iodide molecules. The increase in iodopropane and iodobutane molecules is because the positive ion mass spectra of these molecules consist of higher mass propyl and butyl ions (also lower ionization potentials). With larger molecules the molecular alkyl radical dissociates to form mostly propyl and butyl ions and thus the positive ionization yield of these molecules is expected to saturate. Again, we assume here that the molecular alkyl radical dissociates near the surface, then the lower mass radicals transfer back an electron to the surface and scatter as ions. The mechanism discussed is also consistent with the observed differences between the normal 1-iodohexane and cyclohexyliodide mass spectra. Since the mass spectrum of cyclohexyliodide is dominated by the heavier hexyl ions which also possess a lower ionization potential (secondary cation), its neutralization probability is low and the ionization yield is much higher. It is important to note that the mechanism of the molecular ion dissociation near the surface is appropriate for small molecules in which the vibrational energy redistribution rate is very fast. However, in large molecules such as cholesterol, the dissociation time scale is expected to be much longer and the fragmentation will be unimolecular
far away from the surface and will not be affected by it [25]. The high positive ionization yield of the ReO surface and the different fragmentation pattern on it can be explained by its high work function and by assuming that the ion fragmentation occurs near or on the surface. The reduced ionization yield on the bare unoxidized rhenium filament originates from the lower work function of rhenium (5.1 eV) in comparison with that of rhenium oxide (~ 6.4eV). This means that on a Re surface the molecule-surface electron transfer probability is smaller. Again those ions which possess low kinetic energy will have lower survival probability on climbing the image potential barrier. Thus on technical rhenium, only ions with relatively high mass and low ionization potential will be observed, while on ReO the degree of selection on ion's survival probabilities due to their reneutralization is considerably smaller because of the much higher surface work function, and the obtained mass spectrum is much richer. The results o f Fig. 4 serve as a strong support for the role of the surface on the observed mass spectral fragmentation pattern. These results are interpreted in terms of radical dissociation near the surface even though i o n surface interaction can also influence the final outcome. Note that complete dissociation only to ( M - 1 ) + can not be achieved at any kinetic energy on ReO. This result also strongly indicates that the observed differences in ion dissociation pathways are not due to differences in vibrational excitation but rather due to differences in fragment ionization and or ion reneutralization near the surface. Thus the observed mass spectra are not affected by unimolecular dissociation and are determined near or on the surface. Actually these results are in contrast with models based on unimolecular dissociation and thus provide the strongest support for the role of the surface on the ion fragmentation in small polyatomic molecules. Up to now the ReO surface seems to be the best surface for PHSI. The ionization yield of molecules with low ionization potential reaches ~ 10% on
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
this surface. In addition, simple organic molecules with high ionization yield such as normal alkanes are also chemically ionized with l0 -3-10 -6 ionization yield on this surface [31]. Furthermore, a major advantage of this surface for practical applications is its on-line cleaning by the continuous flow of the oxygen, which successfully survives under the hydrogen carrier gas scattering. However, it should be mentioned that unlike with other surfaces a slight thermal vacuum background is observed from rhenium oxide. Thus, with rhenium oxide, HSI can serve as a universal ion source for organic mass spectrometry with only a few exceptions such as CO, CO2 and CH 4. The ionization efficiency can be high and uniform for molecules with low ionization potential such as polycyclic aromatic hydrocarbons or amines while for aliphatic molecules the ionization efficiency strongly depends on the incident molecular kinetic energy which can be controlled and facilitate a "tunable" HSI selectivity. The demonstrated isotope effect in the bromine NHSI mass spectra (Fig. 7) is also in accord with the proposed mechanism. The increased yield of the heavier bromine isotope at low incident kinetic energies is due to its lower reneutralization probability. Since molecules in the beam contain the same velocity, the incident kinetic energy of molecules containing the st Br isotope is higher than that of those containing the 79Br isotope. In addition, in the dissociation process the bromine negative ion retains kinetic energy proportional to its mass. Thus, at near threshold incident kinetic energies, the SlBr ion will carry 2.5% more kinetic energy than the 79Br ion and its survival probability upon escaping the surface image potential will be exponentially increased. At higher incident kinetic energies, since the produced negative bromine ions contain much more kinetic energy, the isotope effect is reduced. This heavy isotope enrichment is anticipated to be a universal characteristic of HSIDI at near threshold and intermediate kinetic energies.
73
Acknowledgements This work was supported by a grant from the United States Army through its European Research office and by the Wolfson Research Awards administered by the Israel Academy of Sciences and Humanities.
References 1 E.Y. Zandberg, and N.I. Ionov, in Surface Ionization, Israel Program for Scientific Translation, Jerusalem, 197 I. 2 N.F. Ramsey, Molecular Beams, Oxford University Press, London, 1956. 3 T. Fujii and T. Kitai, Anal. Chem., 59 (1987) 379. 4 T. Fujii and Y. Kitai, Int. J. Mass. Spectrom. Ion Processes, 71 (1986) 129. 5 T. Fujii, Int. J. Mass. Spectrom. Ion Processes, 57 (1984) 63. 6 E.Ya. Zandberg and U. Kh. Rasulev, Russ. Chem. Rev., 51 (1982) 819. 7 U. Kh. Rasulev and E. Ya Zandberg, Prog. Surf. Sci., 28 (1988) 182. 8 A. Danon and A. Amirav, J. Phys. Chem., 93 (1989) 5549. 9 A. Danon and A. Amirav, Phys. Rev. Lett., 61 (1988) 2961. l0 A. Danon and A. Amirav, Israel J. Chem., 29 (1988) 443. 11 A. Danon, E. Kolodney and A. Amirav, Surf. Sci., 193 (1988) 132. 12 A. Danon and A. Amirav, J. Chem. Phys., 92 (1990) 6968. 13 E. Kuipers, A. Vardi, A. Danon and A. Amirav, Phys. Rev. Lett., 66 (1991) 116. 14 E. Kuipers, A. Vardi, A. Danon and A. Amirav, Surf. Sci., 261 (1992) 299. 15 A. Vardi, A. Danon and A. Amirav, J. Chem. Phys., 93 (1991) 7506. 16 A. Danon, A. Vardi and A. Amirav, Phys. Rev. Lett., 65 (1991) 2038. 17 E. Kolodney and A. Amirav, Chem. Phys., 82 (1983) 269. 18 H. Pauly and J.P. Toennies, in B. Belesson and W. Fite (Eds.), Methods of Experimental Physics, Vol. 7, Academic Press, New York, 1968, p. 227. 19 J.B. Anderson, R.P. Andress and J.B. Fenn, Adv. Chem. Phys., l0 (1965) 275. 20 J.W. Gadzuk and S. HoUoway, Chem. Phys. Lett., 114 (1985) 314. 21 S. Holloway and J.W. Gadzuk, Surf. Sci., 152 (1985) 833. 22 J.W. Gadzuk and S. Holloway, J. Chem. Phys., 84 (1986) 3502. 23 J.W. Gadzuk, J. Chem. Phys., 86 (1987) 5196. 24 R.G. Cooks, T. Ast and Md. A. Mabud, Int. J. Mass. Spectrom. Ion. Processes, 100 (1990) 209 (and references cited therein). 25 S. Dagan, A. Danon and A. Amirav, Int. J. Mass. Spectrom. Ion Processes, ll3 (1992) 157.
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26 H.M. Rosenstock and M. Krauss, in F.M. McLafferty (Ed.), Mass Spectrometry of Organic Ions, Academic Press, New York, 1963. 27 H.M. Rosenstock, M.B. Wahrhattig, A. Los and H. Eyring, Proc. Natl. Acad. Sci., U.S., 38 (1957) 667. 28 H.M. Rosenstock and M. Krauss, in R.M. Elliott (Ed.), Adv. Mass Spectrom. Vol. 2, Pergamon, Oxford, 1963. 29 W. Forst, Theory of Unimolecular Reactions, Academic Press, New York, 1973. 30 C. Lifshitz, Adv. Mass Spectrom., 11 (1989) 713. 31 A. Amirav, Org. Mass Spectrom., 26 (1991) 1. 32 A. Danon and A. Amirav, Rev. Sci. Instrum., 58 (1987) 1724.
33 A. Danon and A. Amirav, Int. J. Mass. Spectrom. Ion. Processes, 96 (1990) 139. 34 J. Silberstein, N. Ohmichi and R.D. Levine, J. Phys. Chem., 89 (1986) 5606. 35 T. Fujii and H. Jimba, Int. J. Mass. Spectrom. Ion. Processes, 79 (1987) 221. 36 H. Kishi and T. Fujii, Proc. 40th ASMS Conference on Mass Spectrometry and Allied Topics, Washington, DC, 1992. 37 T. Fujii, Int. J. Mass. Spectrom. Ion. Processes, 57 (1987) 221. 38 D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1990.
63
0168-1 ! 76/93/$06.00 © 1993 - Elsevier Science Publishers B.V., Amsterdam
Isotope, molecular and surface effects on hyperthermal surface induced dissociative ionization A. Danon, A. Amirav* School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel-Aviv 69978, Israel ( R e c e i v e d 2 J a n u a r y 1993; a c c e p t e d 25 J a n u a r y 1993)
Abstract Organic molecules acquired with hyperthermal (1-20eV) kinetic energy undergo efficient surface ionization. This hyperthermal surface ionization (HSI) may produce both positive and negative fragment ions. The dissociative ionization of alkyl halides results in the production of negative ions of the functional group having high electron affinity, and positive ions of the alkyl radical, which can further dissociate into smaller fragments. The effect of the alkyl chain length and the bromine isotope effect on the obtained HSI mass spectra were studied in several alkyl halide molecules. While the negative ion formation yield is found to be independent of the size of the alkyl radical, the positive ion formation yield strongly increases with the size of the CnH2~ + 1 radical for n = 1-4 and then a quasi saturation is observed. The observed radical fragmentation increases with the incident molecular kinetic energy and was affected by the molecular structure and the surface temperature and cleanliness. A considerable (up to 24%) heavy bromine isotope increased ionization is observed at intermediate molecular kinetic energies. Piperidine HSI on a rhenium filament exhibits a single (M - 1) ion while its HSI from an oxidized rhenium filament having a much higher work function is characterized by a much richer fragmentation pattern. The dissociative ionization mechanism is rationalized in terms of a surface-molecule electron transfer followed by an immediate dissociation into a negative halogen ion and an alkyl radical. This radical, which usually has a low ionization potential, can transfer an electron to the surface and scatter as a positive ion. At high kinetic energies, the radicals or positive ions can further dissociate near the surface, and then scatter away as lower mass ions with ion yield which depends on their surface reneutralization probabilities. Thus, the observed fragmentation pattern is governed by surface chemical and electron transfer processes and not by gas phase unimolecular ion dissociation, as found with large polyatomic molecules.
Key words:Surface effects; Ionization processes; Isotope effects.
1. Introduction
e s t a b l i s h e d . I n p r e v i o u s studies w e i d e n t i f i e d t h e hyperthermal
surface ionization (HSI) phenom-
S u r f a c e i o n i z a t i o n is a well k n o w n a n d e s t a b -
e n o n [8-16]. U s i n g a s e e d e d s u p e r s o n i c m o l e c u l a r
lished p h e n o m e n o n [1-7]. It is i n t e r p r e t e d c o n v e n -
beam one can accelerate "heavy" organic molecules to t h e h y p e r t h e r m a l k i n e t i c e n e r g y r a n g e o f
tionally by the Saha-Langmuir
equation, based on
the a s s u m p t i o n t h a t t h e r m a l e q u i l i b r i u m b e t w e e n
1 - 2 0 e V [17-19]. T h i s h y p e r t h e r m a l k i n e t i c e n e r g y
t h e s u r f a c e a n d t h e species d e s o r b e d f r o m it is
effectively b r i d g e s the d i f f e r e n c e b e t w e e n t h e surface w o r k f u n c t i o n a n d t h e i o n i z a t i o n p o t e n t i a l ,
*Corresponding author.
e v e n in m o l e c u l e s w h e r e this e n e r g y g a p is l a r g e
64
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
and cannot otherwise be bridged by the average thermal energy. Thus, virtually all organic molecules can undergo efficient ionization following a hyperthermal collision with a surface. Five main types of HSI processes were observed: (i) Positive ion HSI (PHSI), where positive molecular ions were produced as a result of molecule-surface electron transfer [8, 10]; (ii) Negative ion HSI (NHSI), where negative molecular ions were produced due to surfacemolecule electron transfer [8-10]; (iii) Hyperthermal surface induced dissociative ionization (HSIDI), where both negative and positive fragment ions were produced [8]; (iv) Chemically induced HSI, where the hyperthermal kinetic energy-surface scattering results in the creation of new species with favorable electronic properties that promote efficient HSI [12-15]; (v) Local hot spot HSI was observed in Hg/ Pt(111) scattering where the atom-surface energy transfer induced a transient local hot spot which facilitated a high efficiency thermal surface ionization [16]. Surface-molecule electron transfer processes and their several possible outcomes were theoretically treated by Gadzuk and Holloway [20-23]. However, the physical mechanism responsible for the obtained hyperthermal surface induced dissociative ionization is not yet fully established. Recently, low energy organic ion-surface scattering was studied and is reviewed by Cooks et al. [24]. These ion-surface scattering processes are similar to the second half of the PHSI process where the neutral molecule transfer an electron to the surface and then scatters from it as a low energy ion [25]. While ion-surface scattering can involve kinetic-vibrational energy transfer which may induce dissociation other processes of surfacemolecule electron transfer may control the outcome of HSI and induce dissociation, as will be discussed in this paper.
In electron impact mass spectrometry the fragmentation pattern is described by statistical theories such as QET and RRKM [26-30]. The fragmentation patterns solely depend on the molecular and fragment ion's vibrational energy which may originate from the electron ionization process added to the molecular thermal vibrational energy. In HSI the initial vibrational energy of the molecules is very low due to the supercooling in the supersonic molecular beams [31]. However, vibrational energy can be acquired through kinetic-vibrational energy transfer mediated by the surface or by the initial surface-molecule electron transfer process itself. The fragmentation pattern, however, may be largely influenced by the surface-molecule, surface-fragments and surface-ion interaction potentials. In this paper we present further experimental results that help to establish a more consistent mechanism for the HSIDI of alkyl halides. We describe the sequence of reactions leading from the organic parent molecule colliding with the surface to the observed negative and positive mass spectral fragmentation pattern. Molecules such as alkyl iodides and bromides were chosen because of their high ionization yield, their simple fragmentation pattern, and the availability of higher homologs and isomers that can be examined and compared with the fragmentation pattern of normal alkanes in electron impact. These molecules can be viewed as an electron acceptor halogen group which can facilitate a surface-molecule electron transfer and an alkyl radical "tail" residue, which after dissociation can transfer an electron back to the surface and be observed in the positive ion mass spectra. A central question addressed by this work is whether the ion fragmentation occurs near the surface and is affected by its properties or it is a unimolecular process far away from the surface. Piperidine was also studied as an example of a molecule which can dissociate after a molecule-surface electron transfer on a surface with a controlled workfunction.
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
2. Experimental
2.1. UHV Mass Spectrometric Experiments The U H V mass spectrometer experimental apparatus was described in detail elsewhere [8]. Here we briefly describe the modifications pertinent to the present experiments. The organic molecules were aerodynamically accelerated in a seeded beam of H2 of He. The gas mixture was expanded through a 8 0 # m diameter ceramic nozzle which could be differentially heated up to 1000°C (for detailed description of the nozzle, see ref. 32). The molecular beam was skimmed and collimated through two differentially pumped chambers into a U H V chamber (base pressure 1 x 10 -9 Torr). The beam was square-wave modulated by a mechanical chopper for lock-in amplification, or pulsed for time of flight kinetic energy measurements. In the UHV chamber the beam collided with a single crystal surface or a filament. The beam spot on the surface was 3.2 × 1.2 mm 2. The surfaces used in these experiments were d i a m o n d ( I l l ) , silicon(Ill) and a technical rhenium filament. The diamond surface was mechanically polished and chemically treated by a hot KNO3/H2SO4 solution before insertion into the vacuum. It was also annealed in vacuum at 875°C and checked by helium diffraction as described in ref. 11. The silicon surface was cleaned and annealed in vacuum at 1200°C. The rhenium filament used was a 0.1 mm diameter wire. It was cleaned by heating to 2000°C in vacuum and by oxygen treatment cycles at 1 x 10-6Torr. Two quadrupole mass spectrometer (QMS) heads (UTI 100C) were used as ion mass analyzers and detectors: one for the direct unscattered beam and the second, mounted 45 ° to the molecular beam axis, for the scattered ion detection. This head was modified for external ion collection and negative ion detection (in ion detection measurements the QMS filament was turned off).
65
2.2. Total expansion experiments Another apparatus served us for direct expansion of an unskimmed beam-free jet experiment [33]. We utilized a single stage, small vacuum chamber which was pumped by a rotary pump alone. The surface was placed about 3 - 5 m m downstream from the nozzle. As a result, all or most of the accelerated molecules collided with the surface. This small chamber was coupled to a capillary gas chromatograph (HP5890A) by installing it above the flame ionization detector (FID), without removing the FID. In order to operate this HSI detector, helium was used as a carrier gas in the gas chromatograph column (5 meter megabore 530#m i.d. DB-1 of J&W). Molecular samples for study, calibration or intra-nozzle chemical reactions were introduced through a special opening. The air flow to the FID was shut down and the hydrogen flow was increased to 150-200 ml min -l to achieve high kinetic energies. The total flow was expanded through the nozzle (the same nozzle as in section 2.1). The detector was a simple Faraday cup collector placed at a right angle to the surface. In this apparatus we directly measured the positive and negative total ion currents from the Faraday cup, where the surface was biased at positive or negative voltage respectively.
3. Results Figure 1 shows the PHSI mass spectra of a homologous series of normal alkyl iodides (iodoethane-iodoheptane), scattered from a d i a m o n d ( I l l ) surface. The mass spectra have been redrawn from analog presentations with pen limited noise level. Since adjacent masses were insufficiently resolved, peaks due to natural abundant 13C were omitted. The mass spectra always exhibit the radical alkyl fragments at mass 29 (C2H~-), 43 (C3H~'), 57 (C4H~-), 71 (CsHfi), 85 (C6H+3) and 99 (C7H~-5) which are produced through the dissociative ionization of n-iodoethane to n-iodoheptane molecules respectively. This dissociative ionization also results in
66
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
CnHzn÷l I/DIAMOND POSITIVE IONS
)
r
I
)" ,tb /
//
MASS
I
/' / //
//
/~3H7!
(u)
Fig. ]. The molecular tail effect on PHS] mass spectra: PHS! mass spectra of normal alkyl iodides (C.H2.+1 !; n = 2-7) scattered from a diamond(l 1l) surface. The surface temperature was 380°C, the nozzle temperature was 480°C and the hydrogen backing pressure was 1000Torr in all the measurements. The mass spectra are normalized and peaks due to t3C were omitted. Note that the mass spectra are mostly dominated by propyl and butyl radical ions.
I- negative ion formation. The molecular ion peaks are missing through all the series in the obtained mass spectra. Lower alkyl ions, which are produced due to sequential fragmentation of the primary alkyl ions, are also observed. In iodoethane, iodopropane and iodobutane molecules, the undissodated ions are the major peaks, while in the larger molecules the dissociation of the alkyl ion is dominant. The unimolecular dissociation of the primary alkyl ion in the lower enthalpy pathway usually forms ethylene or propylene molecules. As an example, the mass spectrum of iodohexane consists of the hexyl ion at 85u and butyl and propyl ions at 57 and 43 u respectively according to the following dissociation pathways of the hexyl ion: C6H~-3 - - * C 4 H ~- -F C 2 H 4
C6H~-3 --* CaH~- + C3H6 These two dissociation pathways of the hexyl ion are the lowest energy unimolecular dissociation pathways [34]. In addition, the peaks at 55 and 41 u are the consequence of hydrogen elimination which is a well known phenomenon in hydrogen-
rich ions. Normally, (under electron impact ionization) the propyl and butyl ions continue to dissociate to form ethyl and methyl ions, and the electron impact mass spectra of these molecules consist of peaks from methyl up to the molecular ion. In the HSIDI, the ethyl ion signal is very weak (except in iodoethane where the ethyl ion is the molecular residue and is the only peak observed), and the methyl ion signal is missing throughout all the series (we could not observe any CH~- ions even from iodomethane). Another notable difference in comparison with similar electron impact ionization spectra is the lack of single hydrogen atom consecutive dissociation peaks and the relative weakness of the molecular hydrogen elimination peaks. This is clearly the outcome of molecular hydrogen iodide elimination in electron impact ionization while in HSI, as will be discussed, the initial dissociation is that of the iodine negative ion and atomic hydrogen consecutive dissociation is unfavorable. In order to compare the relative positive and negative ionization yield of these molecules, their mixture solution should be separated in time, and time integrated using gas chromatography. Figure 2 presents the PHSI and NHSI chromatograms of a mixture of normal iodoethane, iodopropane, iodobutane, iodopentane and iodohexane [33]. The mixture contains 1% of each molecule in ethanol solution. The positive ion chromatogram (upper spectrum) includes all the possible ions emerging from the molecules as shown in Fig. 1. The negative ion mass spectra of these molecules consist of iodine ions alone, meaning that the negative ion peaks are due to the iodine current. Note also that the ethanol solvent peak is unobserved in both traces due to the extreme ionization selectivity which can be encountered in HSI. Table 1 shows the integrated peak areas of the positive and negative chromatographic peaks of Fig. 2. As can be seen from Table 1, the negative ion formation yield of this series is practically constant or decreases slowly with the alkyl chain length (small steric effect). However, the positive ion formation yield of the iodoethane is very low (0.8%)
A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
and the lack of the ethyl and methyl ions in the positive ion mass spectra can be rationalized in terms of ion reneutralization probabilities in their exit trajectories as will be discussed in section 4. Cyclic alkyliodides show similar behavior to the linear alkyliodides, except that they exhibit a relatively more intense cyclic alkyl ion peak than that of the linear molecules. In Fig. 3 the PHSI mass spectra of cyclohexyliodide and 1-iodohexane are compared. It is clearly observed that the relative undissociated alkyl ion (hexyl- ion) is more intense in the cyclic molecule than in the linear one. Moreover, the relative total yield is two orders of magnitude higher in cyclohexyliodide (not shown in the figure). In order to induce fragmentation of the cyclic C6H+I ion, two C - C bonds must be broken and much more energy is
HSI/DIAMONO
C4H9I
CsHnI
POSITIVE IONS C6HI31
C3H71
A C3HTI
NEGATIVE IONS C4HgX CsHIII
C6HI3I
A
I
I
2
0
3
67
I
I
PHSI
TIME(MIN)
I-Iodohexone
Fig. 2. The molecular tail effect on PHSI and NHSI total ionization yields: Positive ion (upper) and negative ion (lower) hyperthermal surface ionization-gas chromatographic detection of a mixture of 1% normal alkyl iodides (C, Hz~+tI; n = 2-6) in ethanol. The mixture was injected into a gas chromatograph with a 5 m long megabore capillary at 50°C. A diamond(Ill) single crystal at 450°C was used as a surface. Note the constant size molecular independent peak area in the negative ion trace, while the positive ion chromatogram exhibits a large peak area increase with the "tail" size.
(no PHSI yield in CH3I), and increases dramatically in iodopropane and further in iodobutane and then it slowly saturates. The peak between the iodopropane and iodobutane in the PHSI chromatogram was identified as a tert-butyliodide impurity which possesses a much higher positive ionization yield than that of normal alkyliodides (lower ionization potential of the tertiary ion). This trend in the positive ion formation yield,
Cyclohexyl
I0 Table 1 Integrated peak areas (%) of the PHSI and NHSI peaks of Fig. 2 C2H5I PHSI (integrated) 0.8 NHSI (integrated) 88
C3H7I C4H9I C5HIuI C6HI3I 31.5 100
69 95
1
4
83 91
100 80
I
30
I
iodide
i
50
70
MASS
(U)
I
90
I10
Fig. 3. Molecular structural effects on PHSI mass spectra: PHSI mass spectra of 1-iodohexane (upper) and iodocyclohexane (lower). The nozzle temperature was 450°C, resulting in about 8.5 eV incident molecular kinetic energies for both molecules. The diamond surface temperature was 400°C in both spectra. Note the higher degree of fragmentation of the normal 1-iodohexane.
A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
68
required. In addition, the secondary cyclic ion is more stable than the primary alkyl ion; thus, its relative ionization yield is much higher due to both the higher C6H+I ion kinetic energy and its lower ionization potential as a secondary cation. However, once the ring is broken, the residual ion behaves like ions formed from the linear molecules, giving bunches of ions 14 u apart, due to ethylene or propylene expulsion. The positive and negative ion hyperthermal surface ionization yield and the obtained fragmentation pattern depend on the scattered molecules, their initial kinetic energy, the surface used, its work function and temperature. Figure 4 shows the PHSI mass spectra of piperidine scattered M-!
Piperidine/Re PHSI TN =435"C
POz =1=10" 6 torr TN=435" C
/
M
POz = I. I0 -6 l o r r TN = 8 0 ° C
= I0
i MASS
(U)
I10
Fig. 4. The role of the surface on PHSI mass spectra: PHSI mass spectra of piperidine scattered from Re and ReO surfaces as indicated. The PHSI mass spectra is much richer from the ReO surface which possesses a very high surface work function. Thus, even lower mass fragments are not reneutralized on the surface.
from a rhenium filament with different experimental conditions. Piperidine is a molecule which is efficiently ionized even under thermal surface ionization [35]. Its HSI on rhenium oxide is under current investigation by Kishi and Fujii [36]. The upper mass spectrum is of piperidine at 435°C nozzle temperature (high kinetic energy) scattered from a technical Re filament (about 1100 K filament temperature). It shows only an ( M - 1) peak which is due to molecule-surface H - transfer. This H - transfer is believed to be due to molecule-surface hydrogen atom transfer followed by a radical-surface electron transfer [14]. The addition of 1 × 10-6 Torr oxygen in the vacuum chamber changes the surface properties and turns the Re surface to ReO (unknown stochiometry) which possesses a very high work function of 6.4eV [37]. The middle spectrum shows the obtained PHSI mass spectrum from this ReO surface, at the same beam conditions as those used to obtain the upper mass spectrum. The (M - 1) mass peak intensity is increased by about two orders of magnitude in comparison with its intensity obtained with the technical Re surface. Moreover, extensive fragmentation is now observed and the molecular ion appears as well (in the upper spectrum the peak at the molecular mass is solely due to the 13C contribution of the (M - 1) peak). By reducing the initial kinetic energy, the molecular ion relative intensity increases and the fragmentation to low mass ions decreases as seen in the lower mass spectrum. We note that in scattering from a ReO surface the molecular ion is a major peak (71%), combined with lower mass ions, while in scattering from the Re surface it is not detected at all, and (M - 1) + is the dominant peak. HSI is driven by the incident kinetic energy. In l-iodopropane/diamond scattering the positive and negative ion signal increased by five orders of magnitude between near threshold and 10eV incident kinetic energies [11]. In molecules with rich fragmentation patterns the incident kinetic energy can also largely change the relative height ratio among the various peaks [25]. In Fig. 5 the results of 1-iodopentane scattered
69
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
I - B r o m o p e n l o n e / S ~ (111)
CsHIIIIOIAMOND PHSI
Ts = ~ 3 0 0 ° C
A PHSI
TM,280"C
B
Ts=~800°C
j I
20
I
4o
80 MASS
10
50
50 70 90 MASS (U)
II0
Fig. 5. The incident molecular kinetic energy effect on PHSI mass spectral fragmentation pattern: PHSI mass spectra of 1-iodopentane at three incident kinetic energies. (A) Nozzle temperature, 280°C (low kinetic energy); (B) nozzle temperature, 375°C; (C) nozzle temperature, 455°C. The surface was a diamond surface at 400°C in all the three measurements. Note the tail increased dissociation with the increase of the incident kinetic energy. from diamond(l 11) surface at three different initial kinetic energies are shown. At relatively low incident molecular kinetic energy of 6 eV (Fig. 5(A)) the pentyl (m = 71 u) and propyl ions practically dominate the spectrum and their height ratio is 1 : 1. As the incident molecular kinetic energy increases to ~ 7.2eV (Fig. 5(B)) and 8.1eV (Fig. 5(C)), the dissociation of the pentyl to propyl ion gradually increases and in addition butylene ( m = 55u) and ethyl peaks appear. Similar behavior was found in most of the molecules we have worked with and with all surfaces. Figure 6 shows the fragmentation pattern dependence on the surface temperature in the 1-bromo-
,60
(u)
Fig. 6. The surface temperature effect on PHSI mass spectra: PHSI mass spectra of 1-bromopentane scattered from Si(111) surface at the indicated surface temperatures. The tail dissociation may be affectedby the temperature of the surface,probably due to a change in its cleanliness. pentane/Si(111) system. The bromopentane P H S I mass spectra consist mostly of pentyl and propyl ions. As seen from the spectra, the ratio between these two peaks depends on the surface temperature. The surface temperature affects the surface cleanliness during the scattering. It can also change the surface reactivity, its work function, the adsorption probabilities, the energy transfer to the molecules and the reneutralization probabilities of the different fragments. The Si surface at 300°C is covered by atomic hydrogen which can emerge from the carrier hydrogen gas or from the scattered molecules. Thus, atomic hydrogen or other contamination coverage on the surface can change the ionization probabilities and the fragmentation pattern. In contrast with the Si surface, on a diamond surface we could not observe such a surface temperature dependence of the fragmentation pattern. The ionization yield also depends on the molecular weight and exhibits an isotope effect. Figure 7 shows the N H S I mass spectra of 1,2-dibromopen-
A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 53-?4
70
I, 2-DIBROMOPROPANE/DIAMOND
\. Ek:6e
I
77
I
79 MASS
I
81 (U)
the natural isotope abundance ratio. This means that at kinetic energies where the ionization yield dependence on the molecular kinetic energy is stiff, a considerable (up to 1.24) isotope effect is observed which is largely reduced at higher kinetic energies. We note that the results shown in Fig. 7 are with pen limited noise due to extensive mass spectra averaging with Le-Croy 9400 signal averager so that the isotope effect can be accurately extracted. At lower incident kinetic energies the isotope effect is presumably higher but the ion statistical noise prohibits its precise measurement. As the kinetic energy is increased the isotope effect is gradually reduced. A similar effect of heavy ion enrichment at intermediate kinetic energies was observed with chlorine isotopes and with the positive fragment ions.
1
83
Fig. 7. Isotope effectin HSI: NHSI of 1,2-dibromopropanenear the bromineisotopemasses at two differentkineticenergies.The two spectra were taken at the same surface conditions. Each mass spectrum shown was averaged over 500 times with a Le Croy 9400signalaverager.A substantial isotopeeffectis demonstrated with increasedheavy isotope ionization at intermediate kinetic energies. tane scattered from diamond, near the two bromine isotope masses, at two different incident molecular kinetic energies. The upper mass spectrum was obtained at 9 eV incident kinetic energy of the bromopentane molecules. At this kinetic energy the height ratio of the two bromine isotopes is close to their natural isotope abundance (50.5% :49.5% to 79Br and SIBr respectively) as has been also measured in electron impact ionization at the same QMS head and experimental conditions. However, when the kinetic energy is lowered to the range of 5-6 eV, (near the practical threshold) it is clearly observed (also in PHSI) that the low mass bromine isotope relative height decreases (lower spectrum), while the high mass bromine isotope height increases. At this incident kinetic energy the height ratio between the two isotopes is 45.2% :54.8%o, while in the electron impact ionization mass spectrum the ratio is still
4. Discussion The available experimental results shown here allow us to present an improved qualitative picture of the HSIDI mechanism. Accordingly, in all those molecules which contain an atom or a group with a high electron affinity, the first step in the HSIDI process is an electron transfer from the surface to the molecule to form a negative molecular ion [8,9]. In the alkyliodide (or bromide) molecules, in which the difference between the electron affinity of the iodine atom and that of the molecule is larger than the carbon-iodine bond strength in the negative ion, a consecutive spontaneous dissociation occurs. As a result, the initial outcome of the surface-alkyliodide electron transfer is a dissociative ionization into I - and an alkyl radical residue. This process can be further enhanced by the possible radical-diamond (or another) surface reactions, which can also lower the threshold energy. Both diamond and silicon are semiconductors so that the electron transfer from the valence band forms a single hole near the negative ion. The existence of only a single positive "image" charge near the negative ion reduces its reneutralization and substantially enhances the negative ion forma-
A. Danon and A. Amirav/ lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
tion which was observed from semiconductor surfaces in contrast with metal surfaces [8]. Above a certain kinetic energy the radical can also scatter from the surface. As radicals usually possess a low ionization potential, they can effectively transfer an electron back to the surface (to the single hole formed by the initial surface-molecule electron transfer) and scatter as positive ions. Thus, the mass spectra obtained will consist of negative iodine (or bromine) ions and positive ions of the alkyl residue. Therefore, the kinetic energy dependence and the threshold energy and ionization yield of the negative iodine and of the positive alkyl ions will be different. The fragmentation of the alkyl ions into lower mass ions, in the positive mass spectra, can take place by two possible mechanisms. One possibility is that the primary alkyl radical dissociates near the surface to form a lower mass homologous radical and a neutral alkene molecule. Then the lower alkyl radical transfers an electron to the surface and scatters as a positive ion. The other possibility is that the dissociation occurs in the primary alkyl ion after the scattering from the surface, unimolecularly during the flight time to the detector as was observed with large polyatomic molecules such as cholesterol [25]. In this case the dissociation will be to a lower mass positive alkyl ion and a residue according to statistical dissociation rates. Here we will try to determine which of the two mechanisms is more appropriate for the alkyl iodide molecules. The total measured ionization yield depends on the molecule-surface electron transfer yield and the reneutralization probabilities of each radical and ion. The reneutralization probabilities depend on the surface, the molecular properties and the kinetic energy of the ions which scatter from the surface. When an ion dissociates, its kinetic energy is distributed among the resulting fragments approximately according to their mass (momentum conservation). In C3H7I , for example, 75% of the available kinetic energy goes to the iodine atom upon dissociation while only 25% goes to the propyl radical [8]. Since ions near the surface are attracted to it due to its image potential, ions with
7l
low kinetic energy will be trapped near the surface and will be reneutralized on it. Accordingly, if the dissociation of the primary positive alkyl ion occurs on the surface, the low mass ions (methyl and ethyl), which carry a low portion of the molecular ion kinetic energy, will not be able to escape from the image potential of the surface and will be reneutralized. Additionally the ionization potentials of the radicals CH3, C2H3 and Call 7 are 9.83, 8.4 and 8.1 eV respectively [38]. Thus, ethyl and especially methyl ions are expected to be effectively neutralized on the surface even if radicalsurface electron transfer Occurred and only propyl radical exhibits efficient ion formation. However, if the dissociation occurs far away from the surface, then the ratio between the different alkyl ions' height will be given by the intramolecular dissociation theories alone, and the resulting fragmentation pattern will be similar to that obtained in electron, impact ionization. In Fig. 1 we show that the fragmentation pattern of the alkyl halide molecules consists mostly of high mass alkyl ions while the low mass methyl and ethyl ions are missing. This observation suggests that the primary alkyl ion dissociation occurs near the surface and therefore the methyl and ethyl ions which are effectively neutralized on the surface are not detected. The same arguments explain the PHSI and NHSI yields of the gas chromatograms shown in Fig. 2. In the negative ion chromatogram the measured current was from negative iodine ions alone through all the series. Since the iodine atom is heavier than the alkyl chain, it will carry most of the kinetic energy which is weakly dependent on the alkyl chain length. In addition, because all components of the mixture have the same velocity in the beam, the kinetic energy in the beam increases linearly with the molecular mass. In the experiments presented in Fig. 1, the molecular kinetic energy of iodopropane was 7.2 eV and the kinetic energies of the other alkyl iodides were linearly dependent on the molecular weight. Iodohexane for example, contains 30% more initial kinetic energy than iodoethane as the same nozzle
72
A. Danon and A. Amirav/lnt. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
conditions. While the incident kinetic energy increases as the molecular alkyl chain increases, the heavier radical residue obtains the excess energy and the iodine velocity and kinetic energy remains unchanged. Therefore according to the proposed mechanism, the kinetic energy of the iodine ions from all the series in the gas chromatogram will be similar and one can expect the same NHSI yield to all the series (see Table 1). Only minor differences are expected due to steric effects on the electron transfer probabilities. However, the opposite is true for positive ion formation. In iodomethane the ionization yield is very low and we could not detect any positive ions. The positive ionization yield of iodoethane is also low because the mass spectrum contains only the low mass ethyl ion. The ionization yield increases in iodopropane and iodobutane and saturates in the longer alkyl iodide molecules. The increase in iodopropane and iodobutane molecules is because the positive ion mass spectra of these molecules consist of higher mass propyl and butyl ions (also lower ionization potentials). With larger molecules the molecular alkyl radical dissociates to form mostly propyl and butyl ions and thus the positive ionization yield of these molecules is expected to saturate. Again, we assume here that the molecular alkyl radical dissociates near the surface, then the lower mass radicals transfer back an electron to the surface and scatter as ions. The mechanism discussed is also consistent with the observed differences between the normal 1-iodohexane and cyclohexyliodide mass spectra. Since the mass spectrum of cyclohexyliodide is dominated by the heavier hexyl ions which also possess a lower ionization potential (secondary cation), its neutralization probability is low and the ionization yield is much higher. It is important to note that the mechanism of the molecular ion dissociation near the surface is appropriate for small molecules in which the vibrational energy redistribution rate is very fast. However, in large molecules such as cholesterol, the dissociation time scale is expected to be much longer and the fragmentation will be unimolecular
far away from the surface and will not be affected by it [25]. The high positive ionization yield of the ReO surface and the different fragmentation pattern on it can be explained by its high work function and by assuming that the ion fragmentation occurs near or on the surface. The reduced ionization yield on the bare unoxidized rhenium filament originates from the lower work function of rhenium (5.1 eV) in comparison with that of rhenium oxide (~ 6.4eV). This means that on a Re surface the molecule-surface electron transfer probability is smaller. Again those ions which possess low kinetic energy will have lower survival probability on climbing the image potential barrier. Thus on technical rhenium, only ions with relatively high mass and low ionization potential will be observed, while on ReO the degree of selection on ion's survival probabilities due to their reneutralization is considerably smaller because of the much higher surface work function, and the obtained mass spectrum is much richer. The results o f Fig. 4 serve as a strong support for the role of the surface on the observed mass spectral fragmentation pattern. These results are interpreted in terms of radical dissociation near the surface even though i o n surface interaction can also influence the final outcome. Note that complete dissociation only to ( M - 1 ) + can not be achieved at any kinetic energy on ReO. This result also strongly indicates that the observed differences in ion dissociation pathways are not due to differences in vibrational excitation but rather due to differences in fragment ionization and or ion reneutralization near the surface. Thus the observed mass spectra are not affected by unimolecular dissociation and are determined near or on the surface. Actually these results are in contrast with models based on unimolecular dissociation and thus provide the strongest support for the role of the surface on the ion fragmentation in small polyatomic molecules. Up to now the ReO surface seems to be the best surface for PHSI. The ionization yield of molecules with low ionization potential reaches ~ 10% on
A. Danon and A. Amirav/Int. J. Mass Spectrom. Ion Processes 125 (1993) 63-74
this surface. In addition, simple organic molecules with high ionization yield such as normal alkanes are also chemically ionized with l0 -3-10 -6 ionization yield on this surface [31]. Furthermore, a major advantage of this surface for practical applications is its on-line cleaning by the continuous flow of the oxygen, which successfully survives under the hydrogen carrier gas scattering. However, it should be mentioned that unlike with other surfaces a slight thermal vacuum background is observed from rhenium oxide. Thus, with rhenium oxide, HSI can serve as a universal ion source for organic mass spectrometry with only a few exceptions such as CO, CO2 and CH 4. The ionization efficiency can be high and uniform for molecules with low ionization potential such as polycyclic aromatic hydrocarbons or amines while for aliphatic molecules the ionization efficiency strongly depends on the incident molecular kinetic energy which can be controlled and facilitate a "tunable" HSI selectivity. The demonstrated isotope effect in the bromine NHSI mass spectra (Fig. 7) is also in accord with the proposed mechanism. The increased yield of the heavier bromine isotope at low incident kinetic energies is due to its lower reneutralization probability. Since molecules in the beam contain the same velocity, the incident kinetic energy of molecules containing the st Br isotope is higher than that of those containing the 79Br isotope. In addition, in the dissociation process the bromine negative ion retains kinetic energy proportional to its mass. Thus, at near threshold incident kinetic energies, the SlBr ion will carry 2.5% more kinetic energy than the 79Br ion and its survival probability upon escaping the surface image potential will be exponentially increased. At higher incident kinetic energies, since the produced negative bromine ions contain much more kinetic energy, the isotope effect is reduced. This heavy isotope enrichment is anticipated to be a universal characteristic of HSIDI at near threshold and intermediate kinetic energies.
73
Acknowledgements This work was supported by a grant from the United States Army through its European Research office and by the Wolfson Research Awards administered by the Israel Academy of Sciences and Humanities.
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