J. Phys. Chem. 1994, 98, 11978-11987
11978
Gas-Phase N-Si Ion Clusters in AmmonidSilane Mixtures J.-F. Gal, R. Grover, and P.-C. Maria Laboratoire de Chimie Physique Organique, Groupe FT-ICR, Universitt de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Ctdex 2, France
L. Operti, R. Rabezzana, G.-A. Vaglio,' and P. Volpe Dipartimento di Chimica Generale ed Organica Applicata, Universith di Torino, Corso Massimo d'Azeglio 48, 10125 Torino, Italy Received: July 27, 1994@
Ammonidsilane mixtures have been studied by ion trap mass spectrometry, and variations of ion abundances with reaction time in 1 5 , 1:1, and 5 :1 mixtures have been reported. Mechanisms of ion-molecule reactions have been elucidated by single and multiple isolation steps, and exact mass measurements of isobaric ions have been carried out by Fourier transform mass spectrometry. The SiH,+ (n = 0-3) primary ions give self-condensation processes in which Si,H,+ species are formed. These ions react with NH3 and give Si2NH,+ (n = 3-6) and Si2N2Hn+(n = 4-7) ions in successive steps with elimination of H2. In parallel processes, the SiH,+ (n = 0-3) primary ions react with NH3 to give ions belonging to the SiNH,+ (n = 2-4) and SiN2H,+ (n = 4-7) families, which do not react with Si& furtherly. A number of precursors give SiNH6+ and N&+ through different pathways, the last ion being the most abundant one also after short reaction times in all the mixtures examined. The rate constants of the gas-phase reactions of the most important ions have been determined by ion trap mass spectrometry, compared with calculated collision rate constants, and the efficiencies have been determined. Formation of Si2N2H,+ (n = 4-7) and SigN2Hn+(n = 7, 8) ions, even if with rather low efficiencies, suggests that ionic species in addition to radicals can give a contribution to the deposition of solid silicon nitride from ammonidsilane mixtures by radiolytical methods.
Introduction Interpretation of the mechanisms of ion-molecule reactions in volatile hydrides of elements of groups 14 and 15 has been successfully obtained by chemical ionization mass spectrometry, Fourier transform mass spectrometry, or ion trap mass spectrometry.'-15 These studies are important from a fundamental point of view, as interesting insights on the chemistry of these systems can be obtained in the absence of perturbations, such as the effect of solvent. Moreover, they permit us to investigate the role of charged species in the deposition of solid polymers, containing elements of groups 14 and 15 of the periodic table, from gaseous mixtures by radiolytical techniques.l6-l8 In fact, the preparation of new materials from the gas phase is acquiring increasing imp~rtance,'~ and in particular, the preparation of ceramicsz0 and semiconductors2' at a very high-purity degree by deposition from single compounds or by reactions of mixtures of gaseous components is very interesting. In this paper, we study gas-phase ion-molecule reactions in ammonidsilane mixtures by ion trap mass spectrometry (ITMS) and report the mechanisms of the reactions of the most important ions which lead to the formation of hydrogenated ionic species containing up to five silicon and nitrogen atoms and the kinetic constants of the first nucleation reactions of ionic species. These processes can give a contribution to the deposition from the gas phase of silicon nitride, which is a ceramic material of great interest to advanced engine construction and mechanical engineering for its favorable properties.22 Fourier transform mass spectrometry was previously applied to study the first reaction steps in a 1:3 NH3/Si& mixture, leading to the formation of N-Si bonds.14 Moreover, ionic species containing silicon and @
Abstract published in Advance ACS Abstracts, October 15, 1994.
nitrogen have been extensively studied for their relevance in the gas-phase chemistry of dense interstellar clouds.23
Experimental Section Silane and ammonia were obtained commercially in high purity. Prior to use, each of them was introduced into a separate flask, containing anhydrous sodium sulfate as drier, which was connected to the gas inlet system of the instrument. Helium was supplied at an extrahigh-purity degree and was used without further purification. All experiments were run on a ITMS 70 Finnigan Mat ion trap mass spectrometer. The gas inlet system was modified in order to introduce simultaneously two reagent gases and helium buffer gas into the ion trap through three different lines. A Bayard Alpert ionization gauge was used to measure the pressures. Helium buffer gas was admitted to the vacuum chamber at a pressure of about 3.5 x Torr. The temperature was maintained at 333 K in order to avoid thermal decomposition, and ions were detected in the 14-200 u mass range. Frequent bake-up periods of the manifold and of the lines for the introduction of reactants and helium and addition of the drier have reduced the water background to such a low level that products from the well-known reactions of water with silicon-containing ions'3 were not observed at a significant abundance. The scan modes for ion-molecule reaction experiments both without and with mass-selective storage have been previously described in detaiL5 Formation of ions was obtained by bombardment with an electron beam at ionization times in the range 1-10 ms. In the first kind of experiments, ionization was followed by reaction (times ranging from 0 to 500 ms) and acquisition events. When isolation of ions was performed,
0022-3654/94/2098-11978$04.50/0 0 1994 American Chemical Society
N-Si Ion Clusters in AmmonidSilane Mixtures
J. Phys. Chem., Vol. 98, No. 46, 1994 11979
SCHEME 1
\+ Si3NH;
J
103
Nc
Si
88
2H;
65
63
62
78
a reaction time suitable to maximize the abundance of ions to be stored was applied following the ionization event. Isolation of the selected ions, their reactions with neutrals present in the trap for convenient reaction times, and acquisition were the successive steps. In these systems, ions with different composition can have the same nominal mass, as the nominal mass of the most abundant isotope of silicon (28Si)is twice the nominal mass of the most abundant isotope of nitrogen (14N). To identify the ions, experiments were run at different ratios of partial pressures of reagent gases. However, in some cases, to univocally assign a formula to a mlz value, it has been necessary to identify the precursor of the ion and its formation mechanism by multiple ion isolation steps. As an example, with the aim to determine the reaction mechanism of SiN&+, mlz 60 (Scheme l), it is not possible to simply isolate ions at mlz 60, as they mainly consist of SizH4+ (Scheme 3 and Tables 2 and 3). Therefore, it is necessary to isolate the ionic species at mlz 45, SiNH3+, which react to form among others SiNz&+, which is in turn selectively stored for variable reaction times, allowing the study of its products. The scan function relative to these experiments is reported in Figure 1, where the ionization event (period A) is followed by a first reaction time (period B) during which all ions in the trap react with neutral molecules. After isolation of ions 1 (period C), achieved by applying an rf and a dc voltage, the rf level is lowered again for a suitable delay time (period D) in order to trap all ions formed in ion-molecule reactions of ions 1 and to maximize the abundance of ions 2. Again, isolation of ions 2 is obtained by a combination of the rf and dc voltages (period E), and trapping of the product ions is achieved by decreasing the rf voltage for variable reaction times (period F). Acquisition (period G) and interscan (period J) follow as described previou~ly.~ It must be pointed out that every time an rf or a dc voltage is applied, a settling time is necessary to stabilize the trap.
1,nm
A
B
C
D
E
F
0
J
Figure 1. Scan function for successive isolation of two ions (timing events: A = ionization, B = reaction of all ions, C = isolation of ions 1, D = reactions of ions 1, E = isolation of ions 2, F = reactions of ions 2, G = acquisition, J = interscan).
Therefore, even if the reaction time is set to zero, some delay has elapsed from the ionization event, and it cannot be ruled out that ion-molecule reactions have already occurred to some extent. Some complementary experiments were performed by using the FT-ICR spectrometer built at the University of Nice-Sophia A n t i p o l i ~ . In ~ ~particular, ion composition was examined by exact mass measurements to confirm the results of the previous procedures and to identify ions with the same nominal mass and different formula, formed from a unique precursor, which cannot be identified in any ion trap mass spectrometry experiment. These experiments were conducted in the broad-band mode in the mass range from 14 to 200 u at pressures of about 2 x lo-’ Torr. In these conditions, the resolution is about 1500 at mlz 60 and 3000 at mlz 30. Exact masses were obtained by calibrating the instrument with known ions (accuracy: 1-3 ppm). Then the ion composition is checked from its mass by
11980 J. Phys. Chem., Vol. 98, No. 46, 1994
Gal et al.
using the Bruker CMS 47 software. The difference between the calculated and the measured mass is in the range of 1-30 ppm. This accuracy allows us to easily differentiate ions of the same nominal mass containing either 28Si(27.97693 u) or 14N2(28.00615 u) and to determine that addition of 16 mass units is always due to 14NH2(16.0187 u) rather than to l6O (15.9949 u). When experiments were run to determine the rate constants by ion trap mass spectrometry, accurate pressure measurements were necessary. In fact, the read pressure, even if corrected for the relative sensitivity of the ion gauge,25does not reflect the real pressure in the ion trap. Therefore, its calibration was performed by determining the rate constant of the well-known ion-molecule reaction CH;
+ CH, -CH,' + CH3
(1)
The ratio between the measured rate constant and that reported in the literature,26k = 1.2 x cm3 molecule-' s-l, leads to a correction factor of 0.58 f 0.03 used to calibrate the ion gauge. The rate constants were determined for reactions of primary and secondary ions of silane with both ammonia and silane as neutral molecules, with the aim to determine the rate constants of formation of species containing new N-Si bonds. It was checked that the pressure of helium buffer gas at different pressures does not affect these measurements. Every value reported is the average of at least three different experiments, and uncertainties of the measurements fall within 20%. In these experiments, the same kind of scan file was used as for mechanism determinations: a dc voltage was applied to mass selectively store ions of a defined mlz. Then, they were reacted for variable delay times with the neutral reactants present in the ion trap. The reaction time was raised by dynamically programmed scanningz7 which works within the acquisition software environment. It operates by continuously increasing the reaction time from 0 to 50 ms by 0.2-ms steps after the acquisition of each mass spectrum. In the determination of the rate constants, it has been taken into account that during the acquisition event, ions are ejected from the trap at the rate of 5500 uls. Hence, the total reaction time includes the variable time at the fixed rf value plus the time during the analytical ramp until the reactant ions are ejected. In kinetic calculations, ionic abundances, corrected for silicon isotopic distribution, were replaced by their percentage normalized with respect to the sum of the abundances of the parent and product ions which are formed at abundances higher than 2% in the range of reaction times considered. In the systems studied here, the disappearance of the reactant ions follows pseudo-first-order kinetics, as expected, and from the linear regression of the natural logarithm of their normalized percentages vs reaction time in seconds, the absolute value of the slope is calculated. This is the observed constant, ki. in s-l. With regard to the appearance of the secondary ions, in the hypothesis that reactions of primary ions with the two neutral molecules (NH3 and Si&) are independent, then, they are formed in parallel pseudo-first-order reactions. Their rate constants are obtained from plots in which the abundance of the secondary ion is reported against (1 - e-kd'3 corresponding to the following equation, in the formation of Si2&+ from SiH*+:
k'
[Si2H4+]= '[SiH,+],( kd
1 - e-kd'r)
(2)
where ka) is the rate constant of appearance of the Si2&+ ions in s-l, kd' is the rate constant of disappearance of the SiH2+
ions in s-l, and [SiH2+]o is the percentage abundance of the SiH2+ ions at the initial reaction time.28 Any K , pseudo-firstorder rate constant in s-l, is transformed in k, expressed in cm3 molecule-' s-l, by the following equation:
k=
k' (2.9 x 10l6)p
(3)
where p is the corrected pressure of reagent gases in Torr and 2.9 x 10l6 molecule Torr-' cm-3 is the density factor at 333 K. For a generic reacting ion X+, eq 4 can be written as (4) where kal and ka2 are the rate constants of appearance of secondary ions from X+ by reactions with Si& and NH3, respectively; k d is the rate constant of disappearance of the X+ ions; p1 and p2 are the partial pressures of Si& and NH3, respectively; and P is the total pressure of the reacting gases. Linearity of the logarithmic plots is considered indicative of the thermalization of reactant ions8 in gaseous systems. All the kinetic experiments reported here show this kind of behavior, and this is ascribed to collisional cooling which removes the majority of the excitation energy in a short time for the high pressure (3.5 x Torr) in the trap. Experiments in which argon was added to a large excess with respect to SiH4 were also performed, as argon is much more efficient than helium at removing the internal energy of the reactant ions. The very similar self-condensation rate constants of SiH+ to Si2H3+ and of SiH2+ to SiH3+ and Si&+ obtained for a Si& (3.0 x lo-' Torr)/He (3.5 x Torr) mixture and for a Si& (3.0 x lo-' Torr)/Ar (7.0 x Torr)/He (3.4 x Torr) mixture have also confirmed the thermalized ground state of ion populations. Further proof of the ion thermalization is the absence of silicon cluster ions formed in self-condensation reactions with loss of two hydrogen molecules, which are typical nonthermal hot reactions."
Results Ion-Molecule Reactions in NHs/SW Miutures. Schemes 1, 2, 3, and 4 report the reaction pathways of the Si& primary ions Si+, SiH+, SiH2+, and SiH3+, respectively. The schemes were built by isolating and storing the ions for reaction times up to 500 ms. Every identified product ion was again isolated and stored for variable reaction times. This procedure was repeated until loss of instrumental sensitivity prevented reproducible results. Experiments were performed in mixtures with different NH3/Si& ratios (5:1, 1:1, 1:5), and this, together with the long reaction time, is the reason why not all the ions reported in the schemes are present in Tables 2-4. Table 1 reports the meaning of the labels of the main reaction pathways of Schemes 1-4 in order to facilitate their reading. Moreover, in Schemes 2, 3, and 4, the ion-molecule reactions initiated by SiNH3+ (mlz 43,SiN&+ (mlz 46), and SiNZ&+ (mlz 60) are not reported, as they have been already shown in Scheme 1. When the neutral reagent is Si&, addition of a SiH2 group is generally observed with elimination of a hydrogen molecule, path Sa. This reaction occurs for both ions containing only silicon atoms and ions in which a nitrogen atom is bonded to silicon. In the first case, self-condensation processes take place to form Si2H,+ (n = 2-5), Si3H,+ (n = 4-7), and Si&+ (n = 6-9) in successive steps. In the latter, the ionic species Si2NH,+ (n = 3-6) react, leading to the formation of the Si3-
N-Si Ion Clusters in AmmonidSilane Mixtures
J. Phys. Chem., Vol. 98, No. 46, 1994 11981
SCHEME 2
I
Si3NH6
Si4H7+ IYc
4
119
\
lo4
SiN,Hgf 65
\Na
\
Si3N2H; Si4NH8+
119
134
SCHEME 3
NH,+ (n = 5-8) ionic family. The SiH2+ ion also reacts with neutral silane by hydride abstraction with formation of the SiH3+ ion and the SiH3 radical species, path S.,
When the neutral reagent is ammonia, the main reaction pathway leads to the addition of a NH group to the ionic species and elimination of a hydrogen molecule, path N,. This reaction
11982 J. Phys. Chem., Vol. 98, No. 46, 1994
Gal et al.
65
Si4NHld 136
TABLE 1: Labels on the Main Reaction Pathways As Reported in Schemes 1-4 label reacting molecule or type of process neutral loss H hydrogen transfer P protonation N, NH3 Hz NH3 NH3 NH3 NH3 NH3
H
NC Nd Ne Nr Sa
SiH4
S,
SiH4
Hz SiH3
Nb
Si& SiH3 SiZH5
pathway is followed by all ions, except NH,+ ( n = 2, 3) and leads to the formation of the most abundant ions containing nitrogen and silicon bonded together. In particular, the following reactions take place by the N, path: the SiNH,+ (n = 2-4) ionic family is formed from SiH,+ (n = 1-3), and SiNH,+ (n = 3, 4) give SiNzH,+ (n = 4, 5) ions; Si*NH,+ ( n = 3-6) ionic species are formed from SizHnC(n = 2-5) and give the SiZN*H,+ (n = 4-7) ions; the Si3NH,+ (n = 5-8) ions are originated from Si3Hn+(n = 4-7) species, and when n = 6,7, they form Si3N2Hn+(n = 7, 8); and S i a H , + ( n = 8-10) ions are given by the Si4H,+ ( n = 7-9) ionic species. Alternative reaction patterns of neutral ammonia lead to elimination of a hydrogen radical, path Nb, or to addition of the whole NH3 molecule without any neutral loss, path N,. The Nb pathway is displayed by Si+ and SiNH,+ ( n = 2-4) ions in Scheme 1 forming SiNHz+ and SiN2H,+ (n = 4-6) ionic species, respectively. Again, reaction path N, is shown in Scheme 1 by the SiNH,+ ( n = 4,6) and SiN*H,+ (n = 4,5) ionic families which give the SiNzH,+ ( n = 7, 9) and SiNsH,+ (n = 7, 8) ions, respectively. Addition of an ammonia molecule followed by loss of a silane molecule, path Nd, is displayed by the SizH,+ (n = 3-5) and Si3H,+ (n = 4-7) ionic families to form SiNH,+ ( n = 2-4) and Si2NH,+ (n = 3-6) ions, respectively. The
reverse reaction, Le., addition of a S i b molecule and elimination of NH3, has never been observed. This behavior has required high-resolution experiments by FT-ICR to be confirmed. In fact, Si*NH,+ (n = 3-6) ionic species, at masses 73-76 u, react in the NH3/Sib mixture to form ions having masses 8891 u, which were previously identified as Si3H,+ (n = 4-7). However, exact mass measurements have determined that the ionic products at masses 88-91 u coming from Si*NH,+ (n = 3-6) were Si2N2Hnf (n = 4-7) ions formed in a N, reaction path. The same reaction path was also verified for the formation of ions at masses 60 and 61 u from SiNH3+ and SiNH4+, respectively, which were identified as SiNzH,+ (n = 4,5) rather than Si2Hn+ (n = 4, 5). Elimination of a SiH3 radical after addition of a NH3 molecule, path Ne, is observed less frequently, and only the SiNH,+ (n = 2, 3) ionic species are originated from Si2Hnf ( n = 2, 3) ions following this pathway. Finally, N&+, which is shown in all the schemes, is originated from a variety of ions by protonation of an ammonia molecule. Also, the SiNH6+ ion is displayed in all the schemes and is formed from many different ions following various reaction pathways which are not related to those described above. Therefore, these paths are not indicated in the schemes by a particular label. The SiNH6+ ion further reacts at very long reaction times to give SiNzHg+ through pathway N,. In Table 2 , the relative abundances of the significant ions in the ion trap mass spectra of the NH3 (3.5 x Torr)/Si& (3.5 x Torr) mixture are reported, at reaction times from 0 to 200 ms. All abundances refer to the most abundant ion at any time studied here, which is N&+ after a reaction time of 200 ms. Ions showing relative abundances lower than 0.5% up to 200-ms reaction time are not reported in Table 2 even if they are shown in Schemes 1-4. At zero reaction time, the most abundant species are the primary ions, NH,+ (n = 2, 3) and SiH,+ ( n = 0-3), which transport 20.3% and 71.0% of the total ion current, respectively.
N-Si Ion Clusters in AmmonidSilane Mixtures
J. Phys. Chem., Vol. 98, No. 46, 1994 11983
TABLE 2: Relative Abundances of Significant Ions in the IT Mass Spectra of the NHJSiH4 1:l Mixture as a Function of the Reaction Time
TABLE 3: Relative Abundances of Significant Ions in the IT Mass Spectra of the NHJSib 1 5 Mixture as a Function of the Reaction Time
reaction time, ms mlzb
ion
0
10
20
30
40
50
75
100
200
m/zb
ion
0
10
20
reaction time, ms 30 40 50 100 200
500 ~~
16 17 18 28
NH2+ NH3' N&+ Si+
7.7 10.5 4.0 19.1 29SiH+ 6.1 30 SiH2+ 19.6 31 SiH3+ 18.9 44 SiNH2+ 0.5 45 SiNH3+ 0.6 46 SiN&+ 0.9 48 58 SiZH2+ 0.7 59 Si2H3+ 0.3 60 SiZ&+ 0.8 61 Si2H5+ 73 Si2NH3+ 74 Si2N&+ 75 Si2NH5+ 76 Si2NH6+ 88 Si3&+ 89 Si3H5+ 104 Si3NH6+ 106 Si3NHg+ 119 SLH7+
6.0 11.7 12.6 17.5 6.1 15.2 20.1 1.1 1.3 2.4 0.3 1.6 0.9 2.0
3.9 11.3 21.0 14.1 5.6 11.2 19.8 1.5 2.0 3.9 0.5 2.2 1.3 2.7 0.2
2.7 1.8 1.4 9.8 8.7 7.6 31.4 37.6 44.9 11.3 9.7 7.9 5.1 4.3 3.8 8.4 6.3 5.0 18.6 18.2 17.1 2.1 2.5 2.6 2.3 2.6 2.7 5.2 6.3 7.6 0.6 0.8 1.0 2.7 3.3 3.6 1.5 1.9 1.9 3.3 3.3 3.7 0.4 0.5 0.5 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.5 0.3 0.3 0.2 0.3 0.2 0.3 0.4 0.2
0.6 4.9 3.3 59.1 72.9 100.0 5.3 3.3 0.5 2.5 1.8 0.3 2.5 1.2 14.7 12.4 5.6 2.8 2.8 2.0 2.9 3.1 1.7 8.9 10.6 13.7 1.5 1.9 3.5 4.0 4.1 3.4 2.1 2.2 1.7 3.2 2.7 1.3 0.5 0.7 0.9 0.3 0.4 0.5 0.3 0.5 0.7 0.6 0.8 0.9 0.5 0.6 1.0 0.3 0.4 0.5 0.4 0.5 0.7 0.2 0.3 0.5 0.3 0.5 0.3 0.5
The pressure of both NH3 and Si& is 3.5 x Torr, the total pressure is 3.5 x Torr, and the temperature is 333 K. Masses are calculated on 'H, 14N,and %i.
At longer reaction times, the abundances of the primary ions decrease, and NH,+ ( n = 2, 3) ions disappear after 100 ms of reaction, while the fraction of the ionic current transported by SiH,+ (n = 0, 1, 3) at 200-ms reaction time is 4.6%. Actually, the ionic abundance of SiH3+ decreases only slightly up to 100 ms, as it is also formed from SiHz+ reacting with Si& (Scheme 3). The N&+ ionic species displays a unique behavior, as its abundance sharply increases in the reaction time considered here, and at 200 ms, it transports 71.2% of the total ion current. This trend can be correlated to the reaction mechanisms reported in Schemes 1-4, in which N H 4 + is formed from a great number of both primary and secondary ions. It is worth noting that all the ions containing nitrogen and silicon bonded together transport a fraction of the total current which is 17.8% after 200-ms reaction time, while secondary ions which do not contain nitrogen transport only 6.4% of the total ion current after the same reaction time. As from the reaction mechanisms it is shown that ionic species containing both nitrogen and silicon are formed in reactions between silicon-containing ions and ammonia molecules, a NH3 (2.3 x Torr)/Si& (1.1 x loe6 Torr) mixture has been studied with a higher silane partial pressure in order to have more silicon reacting ions (Table 3). From Table 3, it is evident that a higher number of ionic species which contain nitrogen and silicon together are present in this system as compared to the NH3/Si& 1:1 mixture (Table 2). In fact, the SiNzH,+ (n = 5-7), SizNzH,+ (n = 4, 5 , 7), and Si4NH,+ ( n = 8-10) ions are observed in appreciable abundances at 500 ms of reaction. Opposite behavior is observed for the NH3 (1.O x Torr)/ Si& (2.0 x lov7 Torr) mixture (Table 4). In fact, in this system, very few ionic species are displayed at any reaction time considered, and SiNH,+ (n = 2-4,6), and SiNZH,+ (n = 5, 7) ions are the only products of the ion-molecule reactions.
m+,
16 17 18 28 29 30 31 44 45 46 48 58 59 60 61 61 62 73 74 75 76 88 89 90 88 89 91 103 104 106 118 119 134 135
NHz+ NH3+ N&+ Si+ SiH+ SiH2+ SiH3+ SiNHz' SiNH3+ SiNH4+ SiNH6+ Si2H2' Si2H3+ Si2&+ Si2Hsf SiN2H5+ Si&&+ Si2NH3+ Si2N&+ Si2NH5+ SizNH6+ Si3&+ Si3H5+ Si3Htjf Si2N2&+ Si2N2H5+ SizNZH?+ Si3NH5+ SisNH6+ Si3NHs+ Si&,+
2.1 1.3 0.6 0.4 3.3 2.4 1.9 1.5 2.7 6.0 10.5 13.8 18.1 14.0 10.8 8.1 11.9 8.5 6.4 4.8 35.2 22.4 13.0 8.9 49.0 51.7 54.8 54.4 0.4 0.8 1.1 1.2 0.6 1.2 1.8 2.0 0.6 1.8 3.0 3.8 0.5 0.8 3.0 6.3 8.4 9.6 1.6 3.4 4.7 4.7 4.2 10.2 12.6 14.3 0.3 0.7 1.1 1.5
0.2 0.2
0.3 0.7
0.2 0.1 0.4 0.2 0.5 1.3 0.2
0.3
Sa,'
1.3 17.1 6.7 3.5 5.3 56.9 1.5 2.2 4.7 1.1 10.6 4.7 15.2 2.1
0.9 21.2 4.9 2.4 3.5 53.9 1.6 2.4 5.4 1.3 10.9 4.3 14.8 2.3
0.3 0.2 0.5 0.3 0.6 1.8 0.3
0.3 0.2 0.6 0.6 0.7 2.1 0.4
0.4 0.4 0.6 0.8 0.7 2.2 0.5
0.2
0.3
0.3
0.5
0.3 0.7
0.7 1.0
Si4NHs+ SWH9+
0.4 40.4 64.0 100.0 1.7 0.5 0.6 0.7 41.8 25.2 5.7 2.0 2.1 1.7 2.9 3.3 3.7 9.7 14.1 18.8 3.8 5.5 10.6 10.5 8.0 3.4 3.4 2.6 1.5 12.1 5.7 3.0 3.4 4.4 1.0 0.2 0.3 2.5 0.3 1.2 0.6 0.6 0.5 0.6 0.8 0.7 1.1 2.0 2.4 1.8 4.1 6.7 1.0 0.7 1.8 0.6 0.5 0.4 0.2 0.5 0.2 0.2 0.2 0.6 0.2 0.4 1.6 0.4 0.6 0.6 0.7 0.8 1.7 0.3 0.5 1.1 1.1 2.0 1.9 0.4 0.4 0.8 2.0 0.3 0.6 0.4
Torr and of Si& is 1.1 x The pressure of NH3 is 2.3 x Torr, the total pressure is 3.5 x Torr, and the temperature is 333 K. Masses are calculated on 'H, 14N, and 28Si.
TABLE 4: Relative Abundances of Significant Ions in the IT Mass Spectra of the NHJSiH4 5:l Mixture" as a Function of the Reaction Time reaction time. ms
mlp 16 17 18 28 29 30 31 44 45 46 48 61 63
ion
0
10
20
30
40
NH2+ 16.0 7.8 3.5 1.6 0.8 NH3' 26.2 18.4 10.9 6.3 3.5 N&+ 11.4 38.3 56.4 69.6 77.8 Si+ 2.6 1.9 1.3 1.1 0.8 SiH+ 1.7 1.5 1.2 1.0 0.8 SiH2+ 4.5 2.7 1.6 0.9 0.5 SiH3+ 4.3 3.6 2.6 1.9 1.4 SiNH2+ 0.4 0.9 1.2 1.2 1.3 SiNH3+ 0.3 0.6 0.6 0.5 0.4 SiN&+ 0.9 2.2 3.0 3.5 4.0 SiNH6+ 0.2 0.2 0.3 0.3 SiNZHs+ SiNzH,+
50
100 200
500
0.4 2.3 82.8 94.0 95.4 100.0 0.6 0.2 0.7 0.2 0.3 1.0 0.2 1.1 0.5 0.3 4.2 4.5 4.3 3.5 0.4 0.5 0.6 0.6 0.4 0.5 0.3 0.7
The pressure of NH3 is 1.0 x Torr and of Si& is 2.0 x Torr, the total pressure is 3.5 x Torr, and the temperature is 333 K. Masses are calculated on 'H, I4N, and 28Si.
Considering all ions containing N-Si bonds, not only are they formed in a greater variety of species in the NH3/Si& 1 5 mixture, but they are also more abundant, and after 200-ms reaction time, they transport a fraction of the total ion current which is 24.3% compared with 17.8% of the 1:l mixture and 4.8% of the 5:l mixture. Moreover, at 500-ms reaction time, the abundance of these ions containing both nitrogen and silicon increases as they transport 32.8% of the total ion current in the NH3/SiH4 1:5 mixture against 5.3% in the 5:l system.
Gal et al.
11984 J. Phys. Chem., Vol. 98, No. 46, 1994
TABLE 5: Rate Constants for Reactions of NH,+ and Si&+ Ions in a NHJ/SiI&Mixturep ion neutral ionic products (formation rate consts) kXP kL or k m b efficiencyc 16 23.0 0.70 NH2+ NH3 NH3" ( 16) 10.8 14.9 0.72 Si& NH3+ (7.8), SiH3+ (1.5), SiN&+ (1.5) 22 22.7 0.97 NH3+ NH3 N&+ (22) 3.5 14.6 0.24 Si& N&+ (3.0), SiH,+ (0.2), SiNH6+ (0.3) 7.8 20.3 0.38 Si+ NH3 SiNH2" (7.8) 5.2 12.6 0.41 Si& Si2H2+ (5.2,7.5,d 6.9,' 7.29 6.5 20.2 0.32 SiH+ NH3 N&+ (1.5), SiNH2+ (5.0) 7.5 12.5 0.60 Si& Si2H3+ (7.5,6.6,d 5.6') SiH2+ NH3 N&+ (3.8), SiNH3+ (7.3) 11.1 20.1 0.55 Si& SiH3+(5.5,5.2,d 11.3,' 5.09, Si2&+ (8.2,7.5,d 8.2,'7.29 13.7 12.4 1.10 SiH3+ NH3 N&+ (0.9). S i m + (3.6) 4.5 19.9 0.23 5.2 12.3 0.42 Si& *SiH3+ (4.6, 8.5e),Si2H5' (0.6,0.4,d 0.6,'O.q a Rate constants are expressed as cm3molecule-' s-l; experiments were run at 333 K; uncertainty is within 20%; the asterisk indicates the product of isotopic exchange of silicon. Rate constants have been calculated according to the Langevin theory for reactions of S i h and to the ADO theory for reactions of NH3, taking polarizabilities from ref 35 and 36, respectively, and the dipole moment of NH3 from ref 37. Efficiency has been calculated as the ratio k,,dkL or kexdkmo. Rate constant of the same reaction in Si& from ref 27. e Rate constant of the same reaction in SiD4 from refs 8, 10, and 12. fRate constant of the same reaction in a GeH4/Si& mixture from ref 27. TABLE 6: Rate Constants for Reactions of SiNH,+, Si&,+, SiZN&+, and Si3Hs+ Ions in a NHJ/SiI&Mixture ion neutral ionic products (formation rate consts) kem kL or kAmb efficiency' SiNH2+ NH3 N&+ (6.6), SiN2&+ (0.3) 6.9 18.9 NH3 or Si& SiNH3+ (0.7) 0.7 Si& 11.3 8.8 18.8 SiNH3+ NH3 N&+ (8.1), SiN2&+ (0.4), SiN2H5+(0.3) SiN&+ (0.9) NH3 or Si& 0.9 Si& 11.2 SiN&+ NH3 N&+ ( 1.5) 1.5 18.8 0.080 Si& 11.2 N&+ (OS), SiNH2+ (0.8), Si2NH3+(0.4) 1.7 18.2 0.093 Si2H2+ NH3 Si& Si3&+ (0.5,0.4,d 0.4e) 0.047 10.7 0.5 18.2 2.5 N&+ (l.O), SiNH2+ (0.3), Si2N&+ (1.2) 0.14 SizH3+ NH3 Si& Si3H5+ (3.0, 2.7d) 10.7 3.0 0.28 18.2 10.3 0.57 N&+ (0.3), SiNH3+ (5.4), SiN&+ (2.3), Si2NH5+(2.3) Si2&+ NH3 Si& Si&+ (0.2d) 10.6 N&+ (1.4), SiN&+ (2.4), SiN&+ (1.3), SizNH6+(2.5) 18.1 7.6 0.42 Si2H5+ NH3 10.6 0.5 Si& Si3H7' (0.5,0.5,d0.27 0.047 17.8 3.1 N&" (2. l), SizNzHs+(1.O) Si2N&+ NH3 NH3 or Si& SiNH6" (0.8) 0.8 Si& 0.7 Si3NHs+ (0.7) 10.3 13.5 17.5 N&+ (2.6), siN&+(1.8), SiZN&+ (2.6), Si3NH6+ (6.5) 0.77 Si3Hj+ NH3 Si& 4.0 0.40 10.0 Si4H7+ (4.0,4.79 a Rate constants are expressed as cm3 molecule-' s-I; experiments were run at 333 K; uncertainty is within 20%. Rate constants have been calculated according to the Langevin theory for reactions of Si& and to the ADO theory for reactions of NH3, taking polarizabilities from refs 35 and 36, respectively and the dipole moment of NH3 from ref 37. Efficiency has been calculated as the ratio kxdkL or k,,dkmO. Rate constant of the same reaction in Si&. e Rate constant of the same reaction in SiD4 from refs 8, 10, and 12. Table 5 reports the rate constants for reactions of NH,+ (n = 2, 3), and SiH,+ (n = 0-3) and Table 6 those for reactions of SiNH,+ (n = 2-4), SizH,+ (n = 2-5), SizNb+, and Si3H5+ ions in a NH3lSib mixture expressed as cm3 molecule-' s-l. The NI&+ ion is not shown, as it is unreactive in the reaction time examined here. In these tables, the rate constants calculated according to the Langevin theory for reactions with SiH426and to the ADO theory for reactions with NH326are also shown together with the efficiencies of reaction. For some reactions in Table 6, no efficiency has been calculated, as the neutral reactant is not identifiable. The rate constant of the reaction of NH2+ to form NH3+ has been determined to be 23.8 x cm3 molecule-' s-' in the NH3lSiI-b mixture and 16 x cm3 molecule-' s-' in NH3 alone. Therefore, 7.8 x cm3 molecule-' s-' is the rate constant for the formation of NH3+ from SiH4. The same procedure has been used for NH3+, which reacts in the mixture to form N&+ with a rate constant of 25 x cm3 molecule-' s-', while the rate constant of the reaction of NH3+ with NH3 alone to give N&+ is 22 x cm3 molecule-' s-'. It follows that 3.0 x cm3 molecule-' s-' is the ca!culated rate constant of formation of N b + from NH3+ and S i b .
The systems described in Schemes 1-4 are very complex because, besides parallel reactions, consecutive reactions are also involved in these processes. Moreover, some ionic products are common to both parallel and consecutive reactions, and this is especially the case of N&+, which is originated from most of the ionic species reported in the schemes. As an example, the Si2H5+ ion at mlz 61 gives five products, N&+ (mlz 18), SiN&+ (mlz 46), SiNH6+ (mlz 48), SizNH6+ (mlz 76), and Si3H7+ (mlz 91), as reported in Scheme 4. Three of these ionic species, at mlz 46, 76, and 91, in turn, give by reaction with ammonia; two of them, at mlz 76 and 91, form SiNH6+ (m/z 48); and Si3H7+ (mlz 91) also gives Si2NH6+ (mlz 76). Moreover, by isolating and storing the Si*H5+ reactant ion for reaction times up to 50 ms, the ionic species Si3NHg+ (mlz 106) is observed in appreciable abundance, which is a tertiary product of Si2H5+, coming from both Si3H7+ and SizNHs+. Therefore, the determination of such rate constants requires quite complex calculations. In a first step, the rate constant of the disappearance of Si2H5+ and the rate constants of the appearance of the products (mlz 18,46,48,76, and 91) are calculated as described in the Experimental Section, without considering that they are also involved in other processes as reactant or product ions. If
m+
N-Si Ion Clusters in AmmonidSilane Mixtures
J. Phys. Chem., Vol. 98, No. 46, I994 11985
eq 5 is considered,
1
SiNH4+
and the formation of N&+ from SiN&+ (reaction 3) is not taken into account, then the calculated rate constant of the appearance of SiN&+ (reaction 1) is lower than the real one and the calculated rate constant of the appearance of N&+ (reaction 2) is higher than the real one. In fact, a fraction of the abundance of the ion at mlz 46 has reacted, yielding the corresponding amount of the ion at mlz 18, and this quantity of ion at mlz 18 has not been given by SizH5+ in a direct process. A different experiment was necessary to isolate and store the ionic species S i N b + (mlz 46) in order to determine the percentage abundance of the N&+ ion with respect to SiN&+ at all acquisition times of the experiment on Si2H5+ (from 0 to 50 ms by 0.2 ms). These percentage abundances of N&+ originated from SiN&+ have been added to the abundances of S i m + and subtracted from the abundances of N&+ originated from SiZHs+ at the corresponding reaction times. Afterwards, the real rate constants of reactions 1 and 2 could be directly calculated. The rate constants reported in Tables 5 and 6 show that primary ions of both ammonia (NHz+, and NH3+) and silane (Si+, SiH+, SiH2+, and SiH3+) generally react faster than the secondary ions (SiNH2+, SiNH3+, SiN&+, Si2H2+, SizH3+, SiZ&+, and Si2Hsf) considered here. The NH,+ ( n = 2, 3) ionic species mainly react to form the corresponding NHn+l+ ions, displaying constant rates in the lop9cm3 molecule-' sF1 order of magnitude. The primary ions of silane, SiH,+ (n = 0-3), react at comparable rates with ammonia or silane, mainly leading to the formation of SiNH,+ ( n = 2-4) ions in the first case. In the reactions with silane, SiH,+ (n = 0-2) ions give SizH,+ (n = 2-4) ions as the main products, while the fastest reaction of SiH3+ is the isotopic exchange of silicon. The most rapid reactions of the SiNH,+ ( n = 2-4) secondary ions take place with ammonia, leading to the ammonium ion N&+, while reactions of Si2H,+ (n = 3-5) ions give the formation of N-Si bonds with rather high rate constants. The rate constants of the reactions of the tertiary ions SiZN&+ and Si3H5+ have also been determined to see how clustering proceeds. Addition of a NH group to Si*NH4+is observed to give Si2N2Hsf at a rate of 1.0 x cm3 molecule-' SKI, while the same reaction path of Si3Hsf to form Si3NH6+ occurs at a higher rate, as its constant is 6.5 x cm3 molecule-' s-'. For comparative purposes, the rate constants of some processes taking place in different systems (silane alone, deuterated silane alone, germanelsilane mixture) are also reported in Tables 5 and 6.
Discussion In the NH3/Si& system, ion-molecule reactions involving silane molecules as neutrals generally result in the addition of a SiH2 group and elimination of hydrogen according to the following reaction: SipYH,+
+ SiH, - Six+lNyHn+2f+ H,
(6)
Reactions have been observed for y = 0 and x = 1-3 and for y = 1 and x = 2. When y = 0, the reacting ion does not contain nitrogen, and this Sa path is a self-condensation process which leads to ionic species with an increasing number of silicon
atoms up to the SiJ-I,+ (n = 6-9) ions, as has been observed p r e v i ~ u s l y .When ~ y = 1, only the SizNH,+ (n = 3-6) ions react to form species with three silicon atoms and a nitrogen atom as the highest mass ions. The occurrence of this Sa pathway, typical of all systems in which silane is a reagent molecule, seems to indicate that it is thermodynamically favored. The heats of formation of Si& and H2 in the gas phase are known to be 8 and 0 k c a l / m ~ lrespectively. ,~~ Therefore, it can be deduced that this reaction pathway is exothermic when the enthalpy of formation of the reacting ions is at least 8 kcal/mol lower than that of the product ion. The heats of formation of the ionic species SiH,+ and Si2Hn+2+( n = 0, 1) are reported in the l i t e r a t ~ r e ~and ~ ~permit ,~ calculations on the energetics of the reactions SiH,'
+ SiH, - Si2Hn+; + H,
(7)
for n = 0, AH: = -17 f 9 kcal/mol or 5 -35 ic 3 kcaVmo1 with AHfo(Si2H2+)= 286 f 9 kcaVmol or 1268 f 3 kcal/mol, respectively, and for n = 1, AH," = -35 f 23 kcal/mol or = -14 f 2 kcaVmol with AHfo(Si2H3+)= 245 f 23 or 266 f 2 kcal/mol, respectively. Silane also reacts through pathway S, in the following reaction: SiH2+
+ SiH,
+
SiH,'
+ SiH,
(8)
which is about thermoneutral, as it has been determined to have an enthalpy of reaction slightly positive, AH," = 1.5 f 5.5 k c d mol, but with the uncertainty higher than the value. Calculations at a high level of the energy profiles for the interaction of silicon-containing ions with Si& have been reported and confirm the exothermicity of such proce~ses.~' The presence of different structures has been checked in experiments on Si2Hn+(n = 2-5) ions at long reaction times. Observation of unreactive species has been c o n f i i e d for all the ions in pure silane, as has been already reported.8.10-12However, in the NH3/ SiH4 mixture, the SizH,+ (n = 2-5) ions react completely in the reaction time considered. Silicon isotopic exchange has been detected only for %iH3+, and its rate constant is reported in Table 5. In previous studies, the same reaction was observed in the self-condensation of SiD4 for all the 29SiD,+ (n = 0-3) ions and for some heavier ions,8,10-12SiD3+ reacting much faster than the SiD,+ (n = 0-2) ionic species. The high 28Si/29Siisotopic abundances ratio can prevent us from clearly detecting the isotopic exchange products in our experiments starting with species containing 28Si. However, on the basis of the reported rate constants for the SiD,+ (n = 0-2) ions,8J1J2it has been evaluated that the effect of isotopic exchange processes on the decrease of ionic abundances of the reacting ions is negligible or does not affect the kinetic calculations to an extent higher than the average uncertainty. When the reacting molecule is ammonia, various reaction pathways have been observed which take place with elimination of different neutrals, H2 (N,), H (Nb). Si& (Nd), and SiH3 (Ne), or without any neutral loss (Nc). However, path N,, in which a NH group is added to the reacting ion, according to the reaction SipyH,+
+ NH, -SiJVY+,H,+,+ + H,
(9)
is very frequent. Such behavior suggests that these reactions are very favored, but due to the lack of thermochemical data of the ionic species containing silicon and nitrogen atoms, it has been possible to calculate only the enthalpy of reaction of SiH+
Gal et al.
11986 J. Phys. Chem., Vol. 98, No. 46, 1994 giving SiNHZ+, AH," = -47 kcal/mol, the heat of formation of this last ion being reported in the literature, AHf" = 214.2 kcal/m01.~~ Addition of a NHz group with the elimination of a hydrogen atom is also observed, path Nb: SiPyH;
+ NH, - SiJ'4,+,Hn+,+ + H
(10)
but it occurs for a few ions, the formation of H requiring 52 kcal/mol more than the formation of a hydrogen molecule. Again, it has been possible to determine only the enthalpy of the reaction leading to the formation of SiNH2+ from Si+, AH," = -18 kcal/mol. The formation of adduct ions, path N,, in which a NH3 molecule is added to an ion without any neutral loss, takes place for ions formed in at least two successive steps. These association processes have already been observed in mass spectrometric experiments run at pressures higher than 7.0 x or at pressures of about 2.0 x Torr at long reaction times8 It has been suggested that these reactions take place when the only possible exothermic pathway is the formation of an association intermediate which is stabilized by unreactive collisions with a third body instead of by elimination of a neutral fragment.8 Alternative ion-molecule reactions lead to substitution of a silicon with a nitrogen atom by addition of a NH3 molecule and elimination of a SiH4 molecule, path Nd, or, in one case, a SiH3 group, path Ne. It is evident that the second type of reaction is less favored, the heat of formation of SiH3 being higher than that of SiH4 by about 40 kcal/mol. Again, calculation of the enthalpy of reaction is possible only for processes giving the SiNHz+ ionic species, Si,H,+ Si,H,+
+ NH, - SiNH; + SiH,
+ NH, - SiNH; + SiH,
(11)
(12)
and it has been found to be AH," = -12 f 23 or -33 f 2 kcal/mol for reaction 11 and AH," = -12 f 9 or 25 f 5 for reaction 12, depending on the different values of the heats of formation of Si2H3+ and SizH2+, which are used for the calculations. In Schemes 1-4, some reaction pathways have been indicated by referring to the kind of process (hydrogen transfer, H; and protonation, P) or without any letter at all. In the first case, when a hydrogen atom is added to the reacting ion, the neutral reactant is unidentifiable, as it can be silane or ammonia. However, considering eqs 13 and 14, SiNH,' SiNH;
+ SiH, - SiNH,' + SiH,
+ NH, - SiNH3+ + NH,
(13) (14)
their heats of reaction differ only for the part concerning the neutrals, as the ionic species are identical. As a result, reaction 13 is more favored than reaction 14 by about 16 kcal/mol, thus suggesting that silane is the hydrogenating species. The letter P has been used to indicate protonation of neutral ammonia to form the ammonium ion NI&+ from any reacting ion. It has been possible to calculate the heats of this protonation reaction for the SiH,+ ( n = 1-3), SiNH2+, and SizH,+ ( n = 2, 3) ions, and they are generally negative, the only exception being Si*Hz+. The occurrence of the protonation of NH3 by so many reactant ions is due to the high proton affinity of ammonia which yields the very stable ammonium ion. A similar behavior is displayed by the SiNH6+ ion, which
is formed by quite a number of different ions in different reaction pathways. Therefore, it can be suggested that also the SiNH6+ ionic species has a low heat of formation and a high stability. This hypothesis can be supported considering that the structure of the two N&+ and SiNH6+ ions is very similar, the second one having a silyl group substituting a hydrogen atom. The rate constants displayed in Tables 5 and 6 are in good agreement with the variations of the ionic abundances of the ions shown in Tables 2-4. In particular, N-Si clustering reactions proceed faster from ions containing only silicon and hydrogen atoms than from ions already containing a nitrogen atom. In fact, the SiNH,+ ( n = 2-4) ions are formed very rapidly from SiH,+ ( n = 0-3), but they also react quickly to protonate ammonia and give NH4+, which is useless for our purposes. On the contrary, the SiZH,+ (n = 3-5) ions, which are generally formed at rates comparable with the corresponding nitrogen-containing ions, react with ammonia by addition of a NH group to give SizNH,' ( n = 4-6) with rate constants ranging from 1.2 x to 2.5 x cm3 molecule-' s-'. These ions, containing N-Si bonds, react further with ammonia through an N, reaction path, leading to the formation of ions containing two silicon and two nitrogen atoms, at rates comparable to those of the first reaction with NH3. However, also in this case, the fastest reaction is the formation of the stable ammonium ion, N&+. Cluster ions containing three silicon atoms react with both neutrals faster than the corresponding two-silicon ions which form them and give new nitrogensilicon bonds with a high efficiency of reaction. It has been previously reported that SiNH,+ ( n = 2-4) ions do not add to silane,', and therefore, the chain propagation can proceed only through the Si,H,+ ( n = 2,3) ionic species, which react with both ammonia and silane. This behavior is in agreement with our results, but we have observed that other mixed ions, such as Si2NH,+ (n = 3-6) and Si3NHnf ( n = 6, 7), formed in a successive step, are able to react with NH3, thus resulting in the formation of Si2N2Hn+ ( n = 4-7) and Si3N2Hn+ ( n = 7, 8) ions. A simulation model of the gas-phase processes in remoteplasma-activated chemical vapor deposition indicates that radical species are involved in the deposition of silicon nitride from rare gas/ammonia/silane mixtures.34 This is in agreement with the observation that chain propagation is not possible from SiNH,+ ( n = 2-4) ions.', However, the presence of SizN2H,+ ( n = 4-7) and Si3NzHn+(n = 7, 8) suggests that ionic species can also play a role in the formation of charged cluster precursors of the solid silicon nitride. Moreover, considering that the ionic species of interest in the solid deposition are always formed from ionic precursors containing only hydrogen and silicon atoms which add nitrogen in successive steps, the NH3/SiI& mixtures with an excess of silane are the most suitable ones for the ionic nucleation in the deposition of silicon nitride by radiolytical methods.
Acknowledgment. We thank Italian C.N.R. (Consiglio Nazionale delle Ricerche) and the UniversitC de Nice-Sophia Antipolis for financial support. R. G. thanks the European Community for the Erasmus grant. References and Notes (1) Benzi, P.; Operti, L.; Vaglio, G. A,; Volpe, P.; Speranza, M.; Gabrielli, R. J . Organomet. Chem. 1989, 373, 289. (2) Benzi, P.; Operti, L.; Vaglio, G. A,; Volpe, P.; Speranza, M.; Gabrielli, R. Int. J . Mass Spectrom. Ion Processes 1990, 100, 647. (3) Operti, L.; Splendore, M.; Vaglio, G. A.; Volpe, P.; Speranza, M.; Occhiucci, G. J . Organomet. Chem. 1992, 433, 35. (4)Operti, L.; Splendore, M.; Vaglio, G. A,; Volpe, P. Spectrochim. Acta 1993, 49A, 1213.
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