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Reactions of indium phosphide cluster cations with ammonia and trimethylamine
International Journal of Mass Spectrometry and Ion Processes 130 C1994) 133- 142
133
Elsevier Science Publishers B.V., Amsterdam
Reactions of indium phosphide cluster cations with ammonia and trimethylamine C.R. Chris Wang, Anthony AT&T Bell Laboratories,
M. DeSantolo,
Mary L. Mandich,
William D. Reents, Jr.*
Murray Hill, NJ 07974, USA
(Received 19 August 1993; accepted 23 September 1993) Abstract Chemistry of indium phosphide clusters is studied using the powerful trapped ion cell techniques of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in conjunction with an external cluster source and ion guide. The external source is capable of generating a wide range of cluster ions which the ion guide loads with high efficiency into the FTICR cell. The differential pumping of the ion guide allows for operation of the FTICR at requisite low pressure conditions while extracting clusters generated in a high pressure environment. Highly selective reactions of indium phosphide clusters are observed with ammonia and trimethylamine. Of all the In,P,? cluster sixes and stoichiometries studied, only the indium dimer ion reacts exothermically with ammonia. Thermalized In: reacts by indium ion transfer to ammonia. Owing to its much higher basicity, trimethylamine is much more reactive. The smaller indium phosphide clusters react by indium ion transfer to trimethylamine. As the clusters become larger, however, the reaction probability decreases to zero. Key words: Indium phosphide; Ammonia; Trimethylamine; Clusters; Fourier transform ion cyclotron resonance
Introduction Reactivity studies of gas phase semiconductor clusters have provided new insights into the chemistry of nanosize semiconductor surfaces [l] and the chemistry of gas phase semiconductor precursor species which arise during semiconductor processing [2-51. The majority of these studies have focussed on silicon clusters where both striking analogies to and contrast with silicon surface chemistry have been found. Much less is known about the chemistry of clusters of compound semiconductors [5-71. Herein, we report on the formation and reactivity of clusters of indium phosphide, a III-V semiconductor finding
* Corresponding
author.
SSDI 0168-I 176(93)03913-7
increasing application as an optoelectronic material [8]. To date, no reactivity studies have been performed on indium phosphide clusters. Clusters of indium phosphide are an attractive III-V system for study since they do not present problems of overlapping isotopomer masses which have plagued studies of gallium arsenide clusters. In accordance with various reagents designed to either grow larger clusters or to remove atoms from the clusters in a prototypical “etching” reaction both of these types of reactions have been seen for small silicon clusters [2,3,5,9]. The first reagent that we have examined is trimethylindium, where we hoped to simulate a cluster growth sequence via addition of indium atoms analogous to those we have seen for silicon cluster cations in reactions with silane. No
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indication of cluster growth is observed for In,P; reactions with In(CHs). Instead, one or two methyl groups are transferred from In(CH& to the cluster. Alternatively, P atoms could be added to the cluster from PHs; however, we did not select this reagent due to its toxicity. Thus, we chose to examine the reactions of indium phosphide clusters with NH,. The ammonia system is of particular interest since a number of different reactions are possible. Depending on conditions, NH3 can be used to etch semiconductor surfaces or to grow semiconductor nitrides. Different reactivity has been seen with NH3 for semiconductor clusters as well. Both small gallium arsenide and silicon clusters containing at least six or nine atoms, respectively, chemisorb NH3 [ 1,6, lo]. Depending on the reaction temperature, the NH3 binds molecularly or dissociatively on these silicon clusters [l]. The reaction probability of gallium arsenide clusters containing 6 or 7 total atoms is reported to be less than for larger Ga,As; clusters. The clusters smaller than six reactivity of Ga,A$ total atoms has not been reported. The reaction of smaller silicon cationic clusters with NH3 are known, however, and exhibit complex behaviour [ll]. The smallest silicon clusters, SiLs, do not simply chemisorb NHs, instead they react to add NH, which is accompanied by loss of silicon atoms for some of the products. And silicon clusters containing 4-8 silicon atoms do not react at all with NH, [lO,l 11.We find that none of the small InxP,t clusters studied react with ammonia with only one exception: In;. We then examine the reactions of small indium phosphide clusters with trimethylamine for comparison as it is expected to be more reactive than ammonia. This is indeed found to be the case; trimethylamine reacts with about half of the small In,P; clusters that we have studied. Experimental
All measurements are performed using the trapped ion cell of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer,
Wang et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 133-142
equipped with 2.96 T superconducting magnet and an Ionspec Omega @Jdata station. In,Pl clusters are formed in an ion source external to the magnet. Previous workers have shown the versatility of using an external ion source to generate a broad range of cluster sizes and that these cluster ions can be transferred through the magnet fringing fields into the ICR cell [12-171. Our external ion source uses fast atom bombardment (FAB) sputtering of a bulk indium phosphide target to generate In,P; clusters. The FAB gun (Phrasor Scientific) produces a continuous primary Xe/Xe+ beam and is operated typically at 10 kV and 25 PA cm-* [ 181. The resulting cluster ion beam is accelerated to 1 kV and then transferred into the ICR cell (2in cubic cell) using an electrostatic ion guide. The ion guide is designed so that it isolates the high pressure (> 10p4Torr) of the source region from the relatively low pressure (< lo-‘Torr) of the ICR cell with three stages of differential pumping. The ion current from the source is 30-50 nA and the ion current incident at the cell is 3-5nA, giving an overall transmission efficiently through the ion guide of M 10%. Details of this external source and electrostatic ion guide will be published elsewhere. Cluster ions from the source are trapped in the ion cell and then specific cluster ions are isolated from the total population for reactivity studies. Figure 1 shows the required pulse sequence for ion loading, trapping, isolation and reaction. Briefly, prior to ion collection in the cell, all ambient ions are quenched from the cell. Since ions are streaming continuously from the source towards the cell, they must be prevented from entering the cell during this quench pulse. This is accomplished by pulsing one of the ion guide deceleration lenses near the ion cell entrance to a large positive voltage (typically + 260 V). During ion collection, the ion blocking pulse is removed which allows ions to enter the cell. A collection time of 20ms is found to be sufficient to “fill” the ion cell. Once the cell is loaded, ions are again blocked from reaching the cell until the next experimental cycle. The ion cell is operated with a
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130 (1994)
Quench
Blocking
Stopping
_I
Isolation
i
Detectlon
j
Fig. 1. Timing sequence required for an experimental cycle. At time t = 0, a blocking pulse stops ions from entering the cell, during which a quench pulse is applied to remove all ambient Ions from the cell. During the ion collection window, the blocking pulse is removed allowing ions to enter the cell and a “stopping” pulse is applied to assist ion trapping. After a sufficient ion population is collected, the blocking pulse is reapplied. Then, an isolation pulse ejects all ions except for a single mass cluster ion. The reaction time begins at the end of this pulse and lasts until the ions are excited by the r.f. detection pulse.
constant + 1.OV trapping voltage on both the near and far trapping plates except for during the ion collection period. While ions are loading into the cell, the far trapping plate is pulsed to a higher voltage (l-10 V) which is found to assist the trapping process. The initial ion population in the cell usually contains a number of different cluster ions. This ion population s typically reduced to an ion population containing a single stoichiometry In,P,f cluster by strongly irradiating the other cluster ions at their cyclotron frequencies. We employ a LeCroy Arbitrary Function Generator to generate an isolation pulse which is the sum of the multiple sine waves corresponding to the frequencies of the ions to be ejected. Individual In,P; cluster sizes and stoichiometries can easily be isolated because P is monoisotopic, In occurs naturally as nearly one isotope (96% mass 115), and the In: P mass ratio is not an integer. The trapped ions are allowed to react with reagent gases for variable reaction time periods ranging from 10 ms to 20 s, after which the mass spectrum of the ion population is recorded.
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The detection parameters used to record the mass spectrum are 1.OV trapping voltage, 140 V r.f. excitation chirp, and 1 kHz r.f. chirp step. The ion/molecule reactions of each InxPy’ cluster are studied by allowing them to react with the reagent gas in the ion cell and monitoring the resulting ions as a function of reaction time. Reagent pressures of (1.9-6.3)x lo-’ Torr are used for trimethylamine and ammonia. These true pressures are determined from measured pressures by correcting for the ion gauge sensitivity towards each reagent gas and instrumental factors as described previously [ 191. Reaction products are verified by standard double resonance techniques [20]. Reaction rates are extracted by fitting the time dependence of the normalized ion intensities to exponential functions. The rate of reactant ion decay and the rate of product ion formation in each case differ by less than 10%. No additional inert gas is employed in the ion cell for cluster ion thermalization in these studies. Also, at the Xe pressure of 10-4-10-5Torr in the source, the cluster ions do not undergo any thermalizing collisions in the cluster source chamber [18,21]. Thus, relaxation of any excess internal and/or kinetic energy contained by the cluster cations will occur in the ion cell by collisions with the reagent gas. Note that we do not observe formation of smaller cluster ions via unimolecular decay from larger metastable cluster ion during the reaction time. As discussed below, the effects of excess energy on the cluster ion reactions can be discerned from the reaction products and kinetics. Results and discussion Ion beam characterization: angular distributions
kinetic energy and
The kinetic energy distribution of the total In,P,f ion bean is studied by applying a variable retarding potential to the cell entrance trapping plate and measuring the ion current on the far cell plate. The results show that the cluster ions have a very
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broad kinetic energy distribution. However, 80% of the cluster ions have kinetic energies less than 20 V. The distribution of the kinetic energies of the remaining 20% of the cluster ions tails out to more than 1OOV. The observed broad kinetic energy distribution for the In,P,f clusters is typical for a sputter ion source [13,22]. As described below, the atomic In+ ion beam can be isolated from the remaining InXPc cluster ions by applying appropriate deflection potentials in the ion guide. The retarding potential analysis of the In+ beam shows that the atomic ions mimic the kinetic energy distribution of the other In,Pl cluster ions. A rough measure of the angular distribution of the cluster ion beam is obtained by tuning the voltage on X-Y deflection plates contained within the ion guide and observing the resulting ion current at the cell (Z refers to the ion beam axis) [23]. When the X-Y deflection plates are optimized for the maximum total ion throughput, only In+ and In: are observed to enter the ICR cell. This optimization requires a X- Y deflection voltage of +4OV relative to the field free voltage. The ion current roughly exhibits a gaussian dependence about this optimal voltage with a width of x 15 V. This indicates that the In+ and In: angular distribution peaks off-axis. The cluster ion current into the ion cell is optimized at a substantially lower X-Y deflection voltage of + 26V relative to the field free voltage. The gaussian lineshape of the cluster ion beam current with respect to the deflection voltage has a width of M 20V. Thus, while the cluster ion angular distribution is broader than the In+ angular distribution, it peaks at lower angles. Retarding potential measurements of the ion current of both In+ and the cluster ions at two different deflection voltages shows no angular dependence of the overall kinetic energy distribution. No attempt is made to correlate the magnitudes of the X-Y deflection voltages to absolute angles relative to the target surface normal. Cluster cation abundance distribution
The relative intensities of In,P,f
cluster ions
Wang et al./lnt. J. Mass Spectrom. Ion Processes 130
(1994)
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generated by the FAB source are shown in Fig. 2. The cluster abundance distribution differs markedly from a previously reported FTICR mass spectrum where the InXPy’ clusters were generated by laser ablating a bulk InP target located at the entrance of the ICR cell [9]. For example, the earlier mass spectrum contains numerous phosphorus/phosphorus rich species such as Pf, Pl, P4f, and InP,f that are totally absent in our cluster ion distribution. All of these phosphorus rich species are expected to have higher ionization potentials than species containing more indium atoms. Our FAB gun source conditions obviously favour generation of ions with lower ionization potentials. We cannot determine from the cluster ion abundances alone whether the two different methods of cluster source differ in their neutral cluster abundances or in the secondary cluster ion formation. In addition to the overall low phosphorus content of the observed In,P,f clusters, several discontinuities appear in the cluster abundance spectrum. For example, InsP: and In7Pi are missing in the series Iir_sPT. This may signal either exceptionally high ionization potentials or high unimolecular dissociation probabilities for In5P,f and In7P,f. The former can be verified by ionization efficiency measurements and the latter can be estimated by measuring the unimolecular dissociation rates. Both require modification of our current configuration and are beyond the scope of this paper. Inl_jP&2 reactions with ammonia: rates and products
Of all the InXPl cluster ions investigated, only In; reacts with NH3. Two reaction channels are observed: In; + NH3 + In+ + (In + NHs)
(1)
In; + NH3 + In(NH3)+ + In
(2)
A plot of normalized ion intensities vs. reaction time is shown in Fig. 3. The fast dissociation
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x 12
481
200
400
600
BOO m/z
,,,,,,,, 1000
1200
Fig. 2. The mass spectrum showing the In,P& cluster abundance as generated from FAB source and trapped voltages have been optimized for cluster cation intensities.
process forming In+, Eq. (l), occurs at a single rate and is completed very early on during the decay of the total In; population. In(NHs)+, Eq. (2) forms at two distinct rates. Table 1 lists the reaction rate constants derived from a kinetic analysis of the data in Fig. 3 [24,25]. The three reaction channels described above for In: occur because the In: population contains kinetically and internally excited ions. In the first reaction channel, Eq. (l), In; forms In+ by a collision induced dissociation (CID) process. The
in the ion cell.Ion guide
relative fraction, 15%) of the In: population which forms this CID product is found to be independent of the neutral collision partner. For example, when In; is reacted with Nz or N(CH&, a comparable fraction of 19% or 18% respectively, of the In; population react to form In+ of w 1 eV [26,27]. Given the insensitivity of this channel to different collision partners, the In; population which undergoes this CID reaction most likely contains excess kinetic energy. The other two reaction channels both form the
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Wang et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 133-142
800
400
00 000
Fig. 3. Normalized
ion intensities
3 00
600 Reaction
rates and product
12 00
15.00
vs. reaction time for the In; + NH3 reaction at P(NHs) = 4.8 x lo-’ Torr. Solid lines represent result of a kinetic fit to the data using the parameters in Table 1.
same product, In(NH3)+: Eq. (2). This product is formed at two different reaction rates which differ by nearly two orders of magnitude. Thus it is formed from two distinctly different In; populations. The reaction probability of the slower Table 1 Reaction
9 00 time (sets)
ions for the ion/molecule
reaction
reaction is only 0.3% of the ion/molecule collision rate. Ion internal energy relaxation occurs at roughly a rate of 10% of the ion/molecule collision rate for polyatomic molecules [28]. This gives the ion population involved in the slower reaction
of In: with NH,
Reactant ion
Product ions
Product fraction
Reaction rate (x lo-‘s)a
Reaction probabilityb
In;
In(NH$
0.29 0.55
3.7 f 0.1 0.064 f 0.002
0.20 f 0.01 0.0036 f 0.0001
In+
0.15
7.6 f 0.2
0.42 & 0.01
a cm3 molecule-’ SC’. bReaction probabilities are the ratio of the measured dipole orientation (ADO) theory [24]:
reaction
the
rate to the ion/molecule
collision
where q is the charge, p is the reduced mass, c is the dipole locking constant, (Yis the polarizability dipole moment of ammonia (1.47 D) [25], and T is the temperature of the reaction (300 K).
rate which is calculated
of ammonia
using average
(2.81 A3) [25], po is the
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Wang et al./Int. J. Mass Spectrom.
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130 (1994)
sufficient time to be thermalized so we ascribe this reaction to the ground state In; population. The low probability of this reaction for ground state thermalized Inzf means that the microscopic reaction pathway involves at least one nearly thermoneutral intermediate. Such reactions are particularly sensitive to the presence of additional energy in the reagents. This is consistent with the observation of a much faster reaction rate by another portion of the In2f population. The fast reaction is due to “hot” In2f which must be distinct from the “hot” In; fraction that undergoes CID. The excess energy is present as electronic and/or vibrational excitation. We rule out the possibility that this second “hot” In; fraction has excess kinetic energy which is insufficient to cause CID but large enough to change the reaction efficiency. Excess kinetic energy would be expected to slow down this exothermic reaction; this is not observed. Furthermore, the “hot” I$ reaction efficiency is only 20% of the ion/molecule collision rate which allows for excess kinetic energy to be relaxed to the neutral reagent gas and/or converted into internal energy. Further experiments will be needed to elucidate the form of excess energy contained in this “hot” In: which reacts according to Eq. (2). It is striking that none of the other In,P; clusters react with NH3 either by a similar CID process or by indium ion transfer. The best explanation is that clusters larger than the I$ are more strongly bound such that the energy released by forming a new In-N bond is not sufficiently large to overcome the energy required to remove an indium ion from the cluster [29]. A rather weak In-N bond would also explain why incorporation of NH, into small In,P,f clusters is not observed as it is for the smallest silicon cluster ions [l 11. Additional energy must be required in the reaction for the NH3 to disrupt the intramolecular bonds of the larger In,Pl clusters. Finally, we do not observe any (In,P,f)-NH3 association complex products as have been seen for Ga,Asc(x + y 2 6) and Si,’ (x 2 9) clusters [ 1,6, lo]. An acid-base bond
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between the lone pair on the ammonia and the charged cluster undoubtedly forms. But, for these small clusters, the chemisorption complex must be too weakly bound to survive long enough for collisional stabilization by a third body at the pressures of our experiments. Inl_5P&2 reactions with trimethylamine:
rates and
products
The reactions of small indium phosphide cluster cations with trimethylamine follow the general pathway: In,P&
+ N(CHs)j - In(N(CHj)s)’ + In,- 1b2
X<5
(3)
All of these reactions can be described as a displacement reaction where the nitrogen lone pair on the N(CH& attaches to the cluster ion and displaces a In,-&2 neutral cluster. Several exceptions occur. In3P+, In,Pc2 and InsPi do not react with trimethylamine. Also, as shown in Fig. 4, 18% of In,f population reacts with N(CH& to form In+. As described in the previous section, this latter reaction occurs for a kinetically excited portion of the In; population. Table 2 summarizes the reaction rates and product fractions for the ion/molecule reactions of the various InXPy’ clusters with N(CH& [24,251. The reaction probabilities of small indium phosphide cluster cations with trimethylamine are dependent on size and composition. In general, the reaction probabilities decrease with an increasing indium content for a given phosphorus content. An exception is the association reaction between In+ and N(CH& which is second order in the N(CH3), pressure. The greater reactivity of trimethylamine versus ammonia with In,P; clusters can be explained by their relative gas phase basicities [30]. The proton binding energy of N(CH3)3 is x 1 eV stronger than that of NH3. The stronger basicity of N(CH3)3 accounts for why the ion/molecule association
140
C.R.C. Wang et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 133-142
800
600
0.00
080
040
1.20
1.60
Reaction time (sets) Fig. 4. Normalized ion intensities vs. reaction time for the In:+ N(CH& reaction at PF[(CH&]= 1.9 x lo-‘Torr. represent the results of a kinetic fit to the experimental data using the parameters in Table 2.
product for In+ is observed with N(CHs)s and not with NH,. Thus, formation of the (CH&N(In,PT) acid-base complex that undoubtedly occurs during the microscopic reaction releases much more energy to the reaction complex as compared to formation of the H3N-(In,Pl) complex. Based on their relative gas phase basicities, the observed In(N(CHs),)+ product is predicted to be much more strongly bound than the In(NHs)+ product which makes reactions with trimethylamine more exothermic. Both effects are responsible for the increased reactivity of indium phosphide clusters with N(CHs)s relative to NHs. However, as the In,P,f clusters become larger, the energy released by binding the amine is insufficient to compete successfully with the stronger intramolecular cluster bonds. This rationalizes the decreasing reaction probability with increasing cluster size. Only when the In,Pc clusters become sufficiently large that the lifetime of the association complexes compete successfully with unimolecular
Solid lines
dissociation will we observe reaction with either amine. Based on the ammonia chemisorption studies with silicon and gallium arsenide clusters, this will require somewhat larger clusters than we reported on here [1,6,10]. Conclusions Indium phosphide clusters are generated in an external FAB cluster ion source and transferred into the trapped ion cell of an FTICR mass spectrometer. This has enabled us to generate a wide variety of In,P; clusters and measure the rates and products of their reactions with NH3 and N(CHs)s. The FAB source produces both thermal and nonthermal cluster ions as evidenced by some of the observed reactions. Of all of the In3,Pc2 (1
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Wang et al.lInt. J. Mass Spectrom.
Table 2 Reaction
rates and product
Ion Processes
ions for the ion/molecule
130 (1994)
reaction
141
133-142
of InXPl with N(CH,)3
Reactant ion
Product ion?
Product fraction
Reaction rate (x lO@)b
Reaction probabilityC
In+ In; In; In; In:
In(N(CHs)s)+ In(N(CHs)s)+ In+ In(N(CHs)X In(N(CH&)+ NR
1.0 0.82 0.18 0.65 I.0
0.0085 3.36 f ll.Of0.3 0.22 f 0.20 f < 0.04
0.00070 f 0.00002 0.31 * 0.01 1.00 f 0.03 0.021 f 0.05 0.019 f 0.001 < 0.004
In,P+ In,P+ In,P+ InsP+
In(N(CH&)+ NR NR NR
1.0
0.59 f 0.01 < 0.01 < 0.03 < 0.07
0.055 f 0.001 < 0.001 < 0.003 < 0.007
In2Pi InsPz In4Pi InsP;
In(N(CH)s)$ In(N(CH&)+ NR NR
1.0 1.0
4.6 f 0.2 2.8 f 0.2 < 0.01 < 0.08
0.43 f 0.02 0.27 f 0.02 < 0.001 < 0.008
f 0.0002d 0.04 0.05 0.01
a “NR” indicates that no reaction products are observed on the time scale of these experiments. b cm3 molecule-’ s-‘. ’ Reaction probabilities are the ratio of the measured reaction rate to the ion/molecule collision dipole orientation (ADO) theory as given by the equation in Table 1, footnote b [24]; a[N(CHs)s] ]251. d Reaction
rate evaluated
at P[N(CH,),]
rate which is calculated using average = 8.15.A’ [25], P~[N(CH~)~] = 1.47 D
= 5.5 x lo-‘Torr.
Kinetically excited I$ ions form In+ by CID. In(NH3)+ is formed at a fast rate by non-thermal In2f and at a much slower rate by ground state thermal In; ions. No (In,P;)-(NHs) association products are seen at the pressures and time scales of our experiments. Trimethylamine, however, reacts with most of the small indium phosphide cluster cations to form In(N(CH,)$ via a displacement reaction. A small fraction of the In: population also forms In+ by the same CID process seen in the reaction with NH3. Unlike the reaction with NH3, the presence of excess energy in a second “hot” portion of In; does not affect the formation rate of In(N(CH,),)+. An association product is observed to form between In+ and N(CH&. A large gas phase basicity explains the greater reactivity of trimethylamine versus ammonia with indium phosphide clusters. However, this basicity is not sufficiently large to compete with the increased internal binding energies of the larger clusters. Thus, the
reaction probabilities cluster size.
decrease
with increasing
References
9
10
M.F. Jarrold, Science, 252 (1991) 1085. W.D. Reents, Jr., M.L. Mandich and C.R.C. Wang, I. Chem. Phys., 97 (1992) 7226. M.L. Mandich, W.D. Reents, Jr. and K.D.J. Kolenbrander, Pure Appl. Chem., 62 (1990) 1653. M.L. Mandich and W.D. Reents, Jr., J. Chem. Phys., 96 (1992) 4233. W.D. Reents, Jr., J. Chem. Phys., 90 (1989) 4258. L. Wang, L.P.F. Chibante, F.K. Tittel, R.F. Curl and R.E. Smalley, Chem. Phys. Lett., 172 (1990) 335. L. Wang, L.P.F. Chibante, F.K. Tittel, R.F. Curl and R.E. Smalley, Chem. Phys. Lett., 194 (1992) 217. For example: (a) Proc. 4th International Conference on InP and Related Materials, April 21-24, 1992. (b) Properties of Indium Phosphide; EMIS Data Reviews Ser. No. 6, INSPEC, New York, 1991. M.L. Mandich, W.D. Reents, Jr. and V.E. Bondybey, in E.R. Bernstein (Ed.), Atomic and Molecular Clusters, Elsevier, Amsterdam, 1990, pp. 69-357. J.M. Alford, R.T. Laaksonnen and R.E. Smalley, J. Chem. Phys., 94 (1991) 2618.
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W.R. Creasy, A. G’Keefe and J.R. McDonald, J. Phys. Chem., 91 (1987) 2848. J.M. Alford, P.E. Williams, D.J. Trevor and R.E. Smalley, Int. J. Mass Spectrom. Ion Processes, 72 (1986) 33. M.P. Irion, A. Selinger and R. Wendel, Int. J. Mass Spectrom. Ion Processes, 96 (1990) 27. R.T. McIver, R.L. Hunter and W.D. Bowers, Int. J. Mass Spectrom. Ion Processes, 64 (1985)67. P. Kofel, M. Allemann, H. Kellerhals and K.P. Wanczek, Int. J. Mass Spectrom. Ion Processes 65 (1985) 97. P. Kofel and T.B. McMahon, Int. J. Mass Spectrom. Ion Processes, 98 (1990) 1. P. Sharpe and C.J. Cassady, Chem. Phys. Lett., 191 (1992) 111. J.F. Mahoney, J. Perel and A.T. Forrester, Appl. Phys. Lett., 38 (1981) 320. W.D. Reents, Jr., J. Chem. Phys., 90 (1989) 4258 (and references cited therein). J.L. Beauchamp, Ann. Rev. Phys. Chem., 22 (1971) 527. Although the neutral Xe pressure directly at the nozzle exit of the FAB gun is quite high, typically 50-IOOTorr, the Xe pressure falls off quickly with distance away from the nozzle. For our FAB gun, the neutral Xe pressure reaches the chamber pressure well before the target surface. See: J. Perel and J.F. Mahoney, Proc. 30th ASMS Conference on Mass Spectrometry and Allied Topics, June 6- 11, 1982. K. Wittmaack, Phys. Lett. A, 69 (1979) 322.
Wang et al./Int. J. Mass Spectrom. Ion Processes I30 (1994) 133-142
23 The term “angular distribution” employed herein represents the ion spread after the ions have left the source and have travelled half-way through the ion guide. It does not necessarily depict the image of ion angle distribution coming right out of the bombarding sample surface, although they might correlate with each other. 24 T. Su, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Vol. 1, Academic Press, New York, 1979, pp. 83-118. 25 D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 71st edn., CRC Press, Boca Raton, FL, 19901991. 26 K. Balasubramanian and J. Li, J. Chem. Phys., 88 (1988) 4979. 27 D.R. Lide (Ed.), J. Phys. Chem. Ref. Data, 17 (suppl. 1) (1988). 28 R.C. Dunbar, in T.A. Miller and V.E. Bondybey (Ed.), Molecular Ions: Spectroscopy, Structure and Chemistry, North-Holland, New York, 1983. 29 Another possibility is that the In: is formed with higher nascent internal energies than the other In”Py’ ions. This latter explanation does not explain the reaction of thermal In: and is inconsistent with the observed In,P,’ reactivities with trimethylamine as described in the In,_sP& reactions with trimethylamine: rates and products” section. 30 D.H. Sue and M.T. Bowers, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Vol. 2, Academic Press, New York, 1979, pp. 2-51.
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Elsevier Science Publishers B.V., Amsterdam
Reactions of indium phosphide cluster cations with ammonia and trimethylamine C.R. Chris Wang, Anthony AT&T Bell Laboratories,
M. DeSantolo,
Mary L. Mandich,
William D. Reents, Jr.*
Murray Hill, NJ 07974, USA
(Received 19 August 1993; accepted 23 September 1993) Abstract Chemistry of indium phosphide clusters is studied using the powerful trapped ion cell techniques of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in conjunction with an external cluster source and ion guide. The external source is capable of generating a wide range of cluster ions which the ion guide loads with high efficiency into the FTICR cell. The differential pumping of the ion guide allows for operation of the FTICR at requisite low pressure conditions while extracting clusters generated in a high pressure environment. Highly selective reactions of indium phosphide clusters are observed with ammonia and trimethylamine. Of all the In,P,? cluster sixes and stoichiometries studied, only the indium dimer ion reacts exothermically with ammonia. Thermalized In: reacts by indium ion transfer to ammonia. Owing to its much higher basicity, trimethylamine is much more reactive. The smaller indium phosphide clusters react by indium ion transfer to trimethylamine. As the clusters become larger, however, the reaction probability decreases to zero. Key words: Indium phosphide; Ammonia; Trimethylamine; Clusters; Fourier transform ion cyclotron resonance
Introduction Reactivity studies of gas phase semiconductor clusters have provided new insights into the chemistry of nanosize semiconductor surfaces [l] and the chemistry of gas phase semiconductor precursor species which arise during semiconductor processing [2-51. The majority of these studies have focussed on silicon clusters where both striking analogies to and contrast with silicon surface chemistry have been found. Much less is known about the chemistry of clusters of compound semiconductors [5-71. Herein, we report on the formation and reactivity of clusters of indium phosphide, a III-V semiconductor finding
* Corresponding
author.
SSDI 0168-I 176(93)03913-7
increasing application as an optoelectronic material [8]. To date, no reactivity studies have been performed on indium phosphide clusters. Clusters of indium phosphide are an attractive III-V system for study since they do not present problems of overlapping isotopomer masses which have plagued studies of gallium arsenide clusters. In accordance with various reagents designed to either grow larger clusters or to remove atoms from the clusters in a prototypical “etching” reaction both of these types of reactions have been seen for small silicon clusters [2,3,5,9]. The first reagent that we have examined is trimethylindium, where we hoped to simulate a cluster growth sequence via addition of indium atoms analogous to those we have seen for silicon cluster cations in reactions with silane. No
134
C.R.C.
indication of cluster growth is observed for In,P; reactions with In(CHs). Instead, one or two methyl groups are transferred from In(CH& to the cluster. Alternatively, P atoms could be added to the cluster from PHs; however, we did not select this reagent due to its toxicity. Thus, we chose to examine the reactions of indium phosphide clusters with NH,. The ammonia system is of particular interest since a number of different reactions are possible. Depending on conditions, NH3 can be used to etch semiconductor surfaces or to grow semiconductor nitrides. Different reactivity has been seen with NH3 for semiconductor clusters as well. Both small gallium arsenide and silicon clusters containing at least six or nine atoms, respectively, chemisorb NH3 [ 1,6, lo]. Depending on the reaction temperature, the NH3 binds molecularly or dissociatively on these silicon clusters [l]. The reaction probability of gallium arsenide clusters containing 6 or 7 total atoms is reported to be less than for larger Ga,As; clusters. The clusters smaller than six reactivity of Ga,A$ total atoms has not been reported. The reaction of smaller silicon cationic clusters with NH3 are known, however, and exhibit complex behaviour [ll]. The smallest silicon clusters, SiLs, do not simply chemisorb NHs, instead they react to add NH, which is accompanied by loss of silicon atoms for some of the products. And silicon clusters containing 4-8 silicon atoms do not react at all with NH, [lO,l 11.We find that none of the small InxP,t clusters studied react with ammonia with only one exception: In;. We then examine the reactions of small indium phosphide clusters with trimethylamine for comparison as it is expected to be more reactive than ammonia. This is indeed found to be the case; trimethylamine reacts with about half of the small In,P; clusters that we have studied. Experimental
All measurements are performed using the trapped ion cell of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer,
Wang et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 133-142
equipped with 2.96 T superconducting magnet and an Ionspec Omega @Jdata station. In,Pl clusters are formed in an ion source external to the magnet. Previous workers have shown the versatility of using an external ion source to generate a broad range of cluster sizes and that these cluster ions can be transferred through the magnet fringing fields into the ICR cell [12-171. Our external ion source uses fast atom bombardment (FAB) sputtering of a bulk indium phosphide target to generate In,P; clusters. The FAB gun (Phrasor Scientific) produces a continuous primary Xe/Xe+ beam and is operated typically at 10 kV and 25 PA cm-* [ 181. The resulting cluster ion beam is accelerated to 1 kV and then transferred into the ICR cell (2in cubic cell) using an electrostatic ion guide. The ion guide is designed so that it isolates the high pressure (> 10p4Torr) of the source region from the relatively low pressure (< lo-‘Torr) of the ICR cell with three stages of differential pumping. The ion current from the source is 30-50 nA and the ion current incident at the cell is 3-5nA, giving an overall transmission efficiently through the ion guide of M 10%. Details of this external source and electrostatic ion guide will be published elsewhere. Cluster ions from the source are trapped in the ion cell and then specific cluster ions are isolated from the total population for reactivity studies. Figure 1 shows the required pulse sequence for ion loading, trapping, isolation and reaction. Briefly, prior to ion collection in the cell, all ambient ions are quenched from the cell. Since ions are streaming continuously from the source towards the cell, they must be prevented from entering the cell during this quench pulse. This is accomplished by pulsing one of the ion guide deceleration lenses near the ion cell entrance to a large positive voltage (typically + 260 V). During ion collection, the ion blocking pulse is removed which allows ions to enter the cell. A collection time of 20ms is found to be sufficient to “fill” the ion cell. Once the cell is loaded, ions are again blocked from reaching the cell until the next experimental cycle. The ion cell is operated with a
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Wang et aLlInt. J. Mass Spectrom.
Ion Processes
130 (1994)
Quench
Blocking
Stopping
_I
Isolation
i
Detectlon
j
Fig. 1. Timing sequence required for an experimental cycle. At time t = 0, a blocking pulse stops ions from entering the cell, during which a quench pulse is applied to remove all ambient Ions from the cell. During the ion collection window, the blocking pulse is removed allowing ions to enter the cell and a “stopping” pulse is applied to assist ion trapping. After a sufficient ion population is collected, the blocking pulse is reapplied. Then, an isolation pulse ejects all ions except for a single mass cluster ion. The reaction time begins at the end of this pulse and lasts until the ions are excited by the r.f. detection pulse.
constant + 1.OV trapping voltage on both the near and far trapping plates except for during the ion collection period. While ions are loading into the cell, the far trapping plate is pulsed to a higher voltage (l-10 V) which is found to assist the trapping process. The initial ion population in the cell usually contains a number of different cluster ions. This ion population s typically reduced to an ion population containing a single stoichiometry In,P,f cluster by strongly irradiating the other cluster ions at their cyclotron frequencies. We employ a LeCroy Arbitrary Function Generator to generate an isolation pulse which is the sum of the multiple sine waves corresponding to the frequencies of the ions to be ejected. Individual In,P; cluster sizes and stoichiometries can easily be isolated because P is monoisotopic, In occurs naturally as nearly one isotope (96% mass 115), and the In: P mass ratio is not an integer. The trapped ions are allowed to react with reagent gases for variable reaction time periods ranging from 10 ms to 20 s, after which the mass spectrum of the ion population is recorded.
135
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The detection parameters used to record the mass spectrum are 1.OV trapping voltage, 140 V r.f. excitation chirp, and 1 kHz r.f. chirp step. The ion/molecule reactions of each InxPy’ cluster are studied by allowing them to react with the reagent gas in the ion cell and monitoring the resulting ions as a function of reaction time. Reagent pressures of (1.9-6.3)x lo-’ Torr are used for trimethylamine and ammonia. These true pressures are determined from measured pressures by correcting for the ion gauge sensitivity towards each reagent gas and instrumental factors as described previously [ 191. Reaction products are verified by standard double resonance techniques [20]. Reaction rates are extracted by fitting the time dependence of the normalized ion intensities to exponential functions. The rate of reactant ion decay and the rate of product ion formation in each case differ by less than 10%. No additional inert gas is employed in the ion cell for cluster ion thermalization in these studies. Also, at the Xe pressure of 10-4-10-5Torr in the source, the cluster ions do not undergo any thermalizing collisions in the cluster source chamber [18,21]. Thus, relaxation of any excess internal and/or kinetic energy contained by the cluster cations will occur in the ion cell by collisions with the reagent gas. Note that we do not observe formation of smaller cluster ions via unimolecular decay from larger metastable cluster ion during the reaction time. As discussed below, the effects of excess energy on the cluster ion reactions can be discerned from the reaction products and kinetics. Results and discussion Ion beam characterization: angular distributions
kinetic energy and
The kinetic energy distribution of the total In,P,f ion bean is studied by applying a variable retarding potential to the cell entrance trapping plate and measuring the ion current on the far cell plate. The results show that the cluster ions have a very
136
C.R.C.
broad kinetic energy distribution. However, 80% of the cluster ions have kinetic energies less than 20 V. The distribution of the kinetic energies of the remaining 20% of the cluster ions tails out to more than 1OOV. The observed broad kinetic energy distribution for the In,P,f clusters is typical for a sputter ion source [13,22]. As described below, the atomic In+ ion beam can be isolated from the remaining InXPc cluster ions by applying appropriate deflection potentials in the ion guide. The retarding potential analysis of the In+ beam shows that the atomic ions mimic the kinetic energy distribution of the other In,Pl cluster ions. A rough measure of the angular distribution of the cluster ion beam is obtained by tuning the voltage on X-Y deflection plates contained within the ion guide and observing the resulting ion current at the cell (Z refers to the ion beam axis) [23]. When the X-Y deflection plates are optimized for the maximum total ion throughput, only In+ and In: are observed to enter the ICR cell. This optimization requires a X- Y deflection voltage of +4OV relative to the field free voltage. The ion current roughly exhibits a gaussian dependence about this optimal voltage with a width of x 15 V. This indicates that the In+ and In: angular distribution peaks off-axis. The cluster ion current into the ion cell is optimized at a substantially lower X-Y deflection voltage of + 26V relative to the field free voltage. The gaussian lineshape of the cluster ion beam current with respect to the deflection voltage has a width of M 20V. Thus, while the cluster ion angular distribution is broader than the In+ angular distribution, it peaks at lower angles. Retarding potential measurements of the ion current of both In+ and the cluster ions at two different deflection voltages shows no angular dependence of the overall kinetic energy distribution. No attempt is made to correlate the magnitudes of the X-Y deflection voltages to absolute angles relative to the target surface normal. Cluster cation abundance distribution
The relative intensities of In,P,f
cluster ions
Wang et al./lnt. J. Mass Spectrom. Ion Processes 130
(1994)
133-142
generated by the FAB source are shown in Fig. 2. The cluster abundance distribution differs markedly from a previously reported FTICR mass spectrum where the InXPy’ clusters were generated by laser ablating a bulk InP target located at the entrance of the ICR cell [9]. For example, the earlier mass spectrum contains numerous phosphorus/phosphorus rich species such as Pf, Pl, P4f, and InP,f that are totally absent in our cluster ion distribution. All of these phosphorus rich species are expected to have higher ionization potentials than species containing more indium atoms. Our FAB gun source conditions obviously favour generation of ions with lower ionization potentials. We cannot determine from the cluster ion abundances alone whether the two different methods of cluster source differ in their neutral cluster abundances or in the secondary cluster ion formation. In addition to the overall low phosphorus content of the observed In,P,f clusters, several discontinuities appear in the cluster abundance spectrum. For example, InsP: and In7Pi are missing in the series Iir_sPT. This may signal either exceptionally high ionization potentials or high unimolecular dissociation probabilities for In5P,f and In7P,f. The former can be verified by ionization efficiency measurements and the latter can be estimated by measuring the unimolecular dissociation rates. Both require modification of our current configuration and are beyond the scope of this paper. Inl_jP&2 reactions with ammonia: rates and products
Of all the InXPl cluster ions investigated, only In; reacts with NH3. Two reaction channels are observed: In; + NH3 + In+ + (In + NHs)
(1)
In; + NH3 + In(NH3)+ + In
(2)
A plot of normalized ion intensities vs. reaction time is shown in Fig. 3. The fast dissociation
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Wang et aLlInt. J. Mass Spectrom.
Ion Processes
130 (1994)
137
133-142
x 12
481
200
400
600
BOO m/z
,,,,,,,, 1000
1200
Fig. 2. The mass spectrum showing the In,P& cluster abundance as generated from FAB source and trapped voltages have been optimized for cluster cation intensities.
process forming In+, Eq. (l), occurs at a single rate and is completed very early on during the decay of the total In; population. In(NHs)+, Eq. (2) forms at two distinct rates. Table 1 lists the reaction rate constants derived from a kinetic analysis of the data in Fig. 3 [24,25]. The three reaction channels described above for In: occur because the In: population contains kinetically and internally excited ions. In the first reaction channel, Eq. (l), In; forms In+ by a collision induced dissociation (CID) process. The
in the ion cell.Ion guide
relative fraction, 15%) of the In: population which forms this CID product is found to be independent of the neutral collision partner. For example, when In; is reacted with Nz or N(CH&, a comparable fraction of 19% or 18% respectively, of the In; population react to form In+ of w 1 eV [26,27]. Given the insensitivity of this channel to different collision partners, the In; population which undergoes this CID reaction most likely contains excess kinetic energy. The other two reaction channels both form the
C.R.C.
138
Wang et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 133-142
800
400
00 000
Fig. 3. Normalized
ion intensities
3 00
600 Reaction
rates and product
12 00
15.00
vs. reaction time for the In; + NH3 reaction at P(NHs) = 4.8 x lo-’ Torr. Solid lines represent result of a kinetic fit to the data using the parameters in Table 1.
same product, In(NH3)+: Eq. (2). This product is formed at two different reaction rates which differ by nearly two orders of magnitude. Thus it is formed from two distinctly different In; populations. The reaction probability of the slower Table 1 Reaction
9 00 time (sets)
ions for the ion/molecule
reaction
reaction is only 0.3% of the ion/molecule collision rate. Ion internal energy relaxation occurs at roughly a rate of 10% of the ion/molecule collision rate for polyatomic molecules [28]. This gives the ion population involved in the slower reaction
of In: with NH,
Reactant ion
Product ions
Product fraction
Reaction rate (x lo-‘s)a
Reaction probabilityb
In;
In(NH$
0.29 0.55
3.7 f 0.1 0.064 f 0.002
0.20 f 0.01 0.0036 f 0.0001
In+
0.15
7.6 f 0.2
0.42 & 0.01
a cm3 molecule-’ SC’. bReaction probabilities are the ratio of the measured dipole orientation (ADO) theory [24]:
reaction
the
rate to the ion/molecule
collision
where q is the charge, p is the reduced mass, c is the dipole locking constant, (Yis the polarizability dipole moment of ammonia (1.47 D) [25], and T is the temperature of the reaction (300 K).
rate which is calculated
of ammonia
using average
(2.81 A3) [25], po is the
C.R.C.
Wang et al./Int. J. Mass Spectrom.
Ion Processes
130 (1994)
sufficient time to be thermalized so we ascribe this reaction to the ground state In; population. The low probability of this reaction for ground state thermalized Inzf means that the microscopic reaction pathway involves at least one nearly thermoneutral intermediate. Such reactions are particularly sensitive to the presence of additional energy in the reagents. This is consistent with the observation of a much faster reaction rate by another portion of the In2f population. The fast reaction is due to “hot” In2f which must be distinct from the “hot” In; fraction that undergoes CID. The excess energy is present as electronic and/or vibrational excitation. We rule out the possibility that this second “hot” In; fraction has excess kinetic energy which is insufficient to cause CID but large enough to change the reaction efficiency. Excess kinetic energy would be expected to slow down this exothermic reaction; this is not observed. Furthermore, the “hot” I$ reaction efficiency is only 20% of the ion/molecule collision rate which allows for excess kinetic energy to be relaxed to the neutral reagent gas and/or converted into internal energy. Further experiments will be needed to elucidate the form of excess energy contained in this “hot” In: which reacts according to Eq. (2). It is striking that none of the other In,P; clusters react with NH3 either by a similar CID process or by indium ion transfer. The best explanation is that clusters larger than the I$ are more strongly bound such that the energy released by forming a new In-N bond is not sufficiently large to overcome the energy required to remove an indium ion from the cluster [29]. A rather weak In-N bond would also explain why incorporation of NH, into small In,P,f clusters is not observed as it is for the smallest silicon cluster ions [l 11. Additional energy must be required in the reaction for the NH3 to disrupt the intramolecular bonds of the larger In,Pl clusters. Finally, we do not observe any (In,P,f)-NH3 association complex products as have been seen for Ga,Asc(x + y 2 6) and Si,’ (x 2 9) clusters [ 1,6, lo]. An acid-base bond
139
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between the lone pair on the ammonia and the charged cluster undoubtedly forms. But, for these small clusters, the chemisorption complex must be too weakly bound to survive long enough for collisional stabilization by a third body at the pressures of our experiments. Inl_5P&2 reactions with trimethylamine:
rates and
products
The reactions of small indium phosphide cluster cations with trimethylamine follow the general pathway: In,P&
+ N(CHs)j - In(N(CHj)s)’ + In,- 1b2
X<5
(3)
All of these reactions can be described as a displacement reaction where the nitrogen lone pair on the N(CH& attaches to the cluster ion and displaces a In,-&2 neutral cluster. Several exceptions occur. In3P+, In,Pc2 and InsPi do not react with trimethylamine. Also, as shown in Fig. 4, 18% of In,f population reacts with N(CH& to form In+. As described in the previous section, this latter reaction occurs for a kinetically excited portion of the In; population. Table 2 summarizes the reaction rates and product fractions for the ion/molecule reactions of the various InXPy’ clusters with N(CH& [24,251. The reaction probabilities of small indium phosphide cluster cations with trimethylamine are dependent on size and composition. In general, the reaction probabilities decrease with an increasing indium content for a given phosphorus content. An exception is the association reaction between In+ and N(CH& which is second order in the N(CH3), pressure. The greater reactivity of trimethylamine versus ammonia with In,P; clusters can be explained by their relative gas phase basicities [30]. The proton binding energy of N(CH3)3 is x 1 eV stronger than that of NH3. The stronger basicity of N(CH3)3 accounts for why the ion/molecule association
140
C.R.C. Wang et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 133-142
800
600
0.00
080
040
1.20
1.60
Reaction time (sets) Fig. 4. Normalized ion intensities vs. reaction time for the In:+ N(CH& reaction at PF[(CH&]= 1.9 x lo-‘Torr. represent the results of a kinetic fit to the experimental data using the parameters in Table 2.
product for In+ is observed with N(CHs)s and not with NH,. Thus, formation of the (CH&N(In,PT) acid-base complex that undoubtedly occurs during the microscopic reaction releases much more energy to the reaction complex as compared to formation of the H3N-(In,Pl) complex. Based on their relative gas phase basicities, the observed In(N(CHs),)+ product is predicted to be much more strongly bound than the In(NHs)+ product which makes reactions with trimethylamine more exothermic. Both effects are responsible for the increased reactivity of indium phosphide clusters with N(CHs)s relative to NHs. However, as the In,P,f clusters become larger, the energy released by binding the amine is insufficient to compete successfully with the stronger intramolecular cluster bonds. This rationalizes the decreasing reaction probability with increasing cluster size. Only when the In,Pc clusters become sufficiently large that the lifetime of the association complexes compete successfully with unimolecular
Solid lines
dissociation will we observe reaction with either amine. Based on the ammonia chemisorption studies with silicon and gallium arsenide clusters, this will require somewhat larger clusters than we reported on here [1,6,10]. Conclusions Indium phosphide clusters are generated in an external FAB cluster ion source and transferred into the trapped ion cell of an FTICR mass spectrometer. This has enabled us to generate a wide variety of In,P; clusters and measure the rates and products of their reactions with NH3 and N(CHs)s. The FAB source produces both thermal and nonthermal cluster ions as evidenced by some of the observed reactions. Of all of the In3,Pc2 (1
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Wang et al.lInt. J. Mass Spectrom.
Table 2 Reaction
rates and product
Ion Processes
ions for the ion/molecule
130 (1994)
reaction
141
133-142
of InXPl with N(CH,)3
Reactant ion
Product ion?
Product fraction
Reaction rate (x lO@)b
Reaction probabilityC
In+ In; In; In; In:
In(N(CHs)s)+ In(N(CHs)s)+ In+ In(N(CHs)X In(N(CH&)+ NR
1.0 0.82 0.18 0.65 I.0
0.0085 3.36 f ll.Of0.3 0.22 f 0.20 f < 0.04
0.00070 f 0.00002 0.31 * 0.01 1.00 f 0.03 0.021 f 0.05 0.019 f 0.001 < 0.004
In,P+ In,P+ In,P+ InsP+
In(N(CH&)+ NR NR NR
1.0
0.59 f 0.01 < 0.01 < 0.03 < 0.07
0.055 f 0.001 < 0.001 < 0.003 < 0.007
In2Pi InsPz In4Pi InsP;
In(N(CH)s)$ In(N(CH&)+ NR NR
1.0 1.0
4.6 f 0.2 2.8 f 0.2 < 0.01 < 0.08
0.43 f 0.02 0.27 f 0.02 < 0.001 < 0.008
f 0.0002d 0.04 0.05 0.01
a “NR” indicates that no reaction products are observed on the time scale of these experiments. b cm3 molecule-’ s-‘. ’ Reaction probabilities are the ratio of the measured reaction rate to the ion/molecule collision dipole orientation (ADO) theory as given by the equation in Table 1, footnote b [24]; a[N(CHs)s] ]251. d Reaction
rate evaluated
at P[N(CH,),]
rate which is calculated using average = 8.15.A’ [25], P~[N(CH~)~] = 1.47 D
= 5.5 x lo-‘Torr.
Kinetically excited I$ ions form In+ by CID. In(NH3)+ is formed at a fast rate by non-thermal In2f and at a much slower rate by ground state thermal In; ions. No (In,P;)-(NHs) association products are seen at the pressures and time scales of our experiments. Trimethylamine, however, reacts with most of the small indium phosphide cluster cations to form In(N(CH,)$ via a displacement reaction. A small fraction of the In: population also forms In+ by the same CID process seen in the reaction with NH3. Unlike the reaction with NH3, the presence of excess energy in a second “hot” portion of In; does not affect the formation rate of In(N(CH,),)+. An association product is observed to form between In+ and N(CH&. A large gas phase basicity explains the greater reactivity of trimethylamine versus ammonia with indium phosphide clusters. However, this basicity is not sufficiently large to compete with the increased internal binding energies of the larger clusters. Thus, the
reaction probabilities cluster size.
decrease
with increasing
References
9
10
M.F. Jarrold, Science, 252 (1991) 1085. W.D. Reents, Jr., M.L. Mandich and C.R.C. Wang, I. Chem. Phys., 97 (1992) 7226. M.L. Mandich, W.D. Reents, Jr. and K.D.J. Kolenbrander, Pure Appl. Chem., 62 (1990) 1653. M.L. Mandich and W.D. Reents, Jr., J. Chem. Phys., 96 (1992) 4233. W.D. Reents, Jr., J. Chem. Phys., 90 (1989) 4258. L. Wang, L.P.F. Chibante, F.K. Tittel, R.F. Curl and R.E. Smalley, Chem. Phys. Lett., 172 (1990) 335. L. Wang, L.P.F. Chibante, F.K. Tittel, R.F. Curl and R.E. Smalley, Chem. Phys. Lett., 194 (1992) 217. For example: (a) Proc. 4th International Conference on InP and Related Materials, April 21-24, 1992. (b) Properties of Indium Phosphide; EMIS Data Reviews Ser. No. 6, INSPEC, New York, 1991. M.L. Mandich, W.D. Reents, Jr. and V.E. Bondybey, in E.R. Bernstein (Ed.), Atomic and Molecular Clusters, Elsevier, Amsterdam, 1990, pp. 69-357. J.M. Alford, R.T. Laaksonnen and R.E. Smalley, J. Chem. Phys., 94 (1991) 2618.
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W.R. Creasy, A. G’Keefe and J.R. McDonald, J. Phys. Chem., 91 (1987) 2848. J.M. Alford, P.E. Williams, D.J. Trevor and R.E. Smalley, Int. J. Mass Spectrom. Ion Processes, 72 (1986) 33. M.P. Irion, A. Selinger and R. Wendel, Int. J. Mass Spectrom. Ion Processes, 96 (1990) 27. R.T. McIver, R.L. Hunter and W.D. Bowers, Int. J. Mass Spectrom. Ion Processes, 64 (1985)67. P. Kofel, M. Allemann, H. Kellerhals and K.P. Wanczek, Int. J. Mass Spectrom. Ion Processes 65 (1985) 97. P. Kofel and T.B. McMahon, Int. J. Mass Spectrom. Ion Processes, 98 (1990) 1. P. Sharpe and C.J. Cassady, Chem. Phys. Lett., 191 (1992) 111. J.F. Mahoney, J. Perel and A.T. Forrester, Appl. Phys. Lett., 38 (1981) 320. W.D. Reents, Jr., J. Chem. Phys., 90 (1989) 4258 (and references cited therein). J.L. Beauchamp, Ann. Rev. Phys. Chem., 22 (1971) 527. Although the neutral Xe pressure directly at the nozzle exit of the FAB gun is quite high, typically 50-IOOTorr, the Xe pressure falls off quickly with distance away from the nozzle. For our FAB gun, the neutral Xe pressure reaches the chamber pressure well before the target surface. See: J. Perel and J.F. Mahoney, Proc. 30th ASMS Conference on Mass Spectrometry and Allied Topics, June 6- 11, 1982. K. Wittmaack, Phys. Lett. A, 69 (1979) 322.
Wang et al./Int. J. Mass Spectrom. Ion Processes I30 (1994) 133-142
23 The term “angular distribution” employed herein represents the ion spread after the ions have left the source and have travelled half-way through the ion guide. It does not necessarily depict the image of ion angle distribution coming right out of the bombarding sample surface, although they might correlate with each other. 24 T. Su, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Vol. 1, Academic Press, New York, 1979, pp. 83-118. 25 D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 71st edn., CRC Press, Boca Raton, FL, 19901991. 26 K. Balasubramanian and J. Li, J. Chem. Phys., 88 (1988) 4979. 27 D.R. Lide (Ed.), J. Phys. Chem. Ref. Data, 17 (suppl. 1) (1988). 28 R.C. Dunbar, in T.A. Miller and V.E. Bondybey (Ed.), Molecular Ions: Spectroscopy, Structure and Chemistry, North-Holland, New York, 1983. 29 Another possibility is that the In: is formed with higher nascent internal energies than the other In”Py’ ions. This latter explanation does not explain the reaction of thermal In: and is inconsistent with the observed In,P,’ reactivities with trimethylamine as described in the In,_sP& reactions with trimethylamine: rates and products” section. 30 D.H. Sue and M.T. Bowers, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Vol. 2, Academic Press, New York, 1979, pp. 2-51.