9091
J . Phys. Chem. 1991, 95,9091-9094 in which the wavepacket can move or spread also increases. Any motion or spread of the wavepacket in a direction away from the C-I bond coordinate would decrease the overlap with excited C-I stretch levels of the electronic ground state and therefore decrease the emission to stretch overtone or bend-stretch combination bands. Conclusion
relative to the C-I stretch overtone progression, suggesting that the photodissociation has significant multidimensional character. The gas-phase resonance Raman spectra are overall similar to resonance Raman spectra of these alkyl iodides dissociating in solution, indicating that the short-time dynamics are similar for the isolated and solvated molecules. However, there are significant differences in detail between the gas and solution Raman spectra, indicating that solvent effects are important.
Resonance Raman spectra of ethyl, isopropyl, and tert-butyl iodides provide mode-specific information about the early time dynamics of alkyl iodide photodissociation. As the alkyl group becomes heavier and more branched the Raman spectra show increased intensity in bendstretch combination band progressions
Acknowledgment. This work is supported by the National Science Foundation through Grant CHE-88- 10557. Registry No. Ethyl iodide, 75-03-6: isopropyl iodide, 75-30-9: f e r f butyl iodide, 558-17-8.
Determination of the Ionization Potentials of Aluminum Oxides via Charge Transfer Stephan B. H. Bacht and Stephen W. McElvany* Chemistry DivisionlCode 61 I O , Naval Research Laboratory, Washington, D.C. 20375-5000 (Received: April 24, 1991: In Final Form: May 30, 1991)
The ionization potentials of aluminum oxides (AI,Oy) with x = 2, y = 1-4; x = 3, y = 2-4; and x = 4, y = 4 were determined to within 0.2 eV by charge-transfer bracketing measurements in a Fourier transform ion cyclotron resonance (ICR) mass spectrometer (FTMS). The method of determining the ionization potentials (IPS) of molecules by charge-transfer bracketing is extended from species produced by direct laser vaporization to species formed by reactions within the ICR cell. The IPS of aluminum oxides containing two aluminum atoms (A120y)increase from 8.35 to 8.9 eV for y = 1-3. Upon addition of a fourth oxygen the IP decreases by approximately 2 eV, indicating a possible structural change from linear to cyclic. Aluminum oxides containing three aluminum atoms (A130y)have IPSof 6.85 eV for y = 2 and 3. A dramatic increase in IP of 1.5 eV is observed upon addition of a fourth oxygen atom. The changes in IP are related to the oxidation state of the aluminum in the oxide species.
Introduction
There has been recent interest in the reactions of the group 111 metal oxides because of their importance to the semiconductor industry. Aluminum oxides are insulators and exist in two anhydrous f0rms.l The CY form, called corundum, is a hexagonal close-packed array where the aluminum ions are distributed symmetrically among the octahedral interstices. The y form can be regarded as having a “defect” spinel structure derived from a face-centered-cubic array of oxygen anions. The oxidation and fragmentation properties of pure and oxidized aluminum cluster ions have recently been an area of intense research.2” This study was initiated to determine the ionization potentials of aluminum oxides (A1,O ) formed by direct laser vaporization and aluminum oxides produced by subsequent ion/molecule reactions with NO2 or N 2 0 . An important aspect of this study is the extension of the charge-transfer bracketing technique from species formed solely by direct laser vaporization (DLV) to species formed by reactions within the ICR cell. This would enable the determination of the ionization potentials of a variety of transient species (e.g., reaction intermediates), which may not be possible when more standard techniques are used. The charge-transfer bracketing technique has been used to determine the ionization potentials (IPS) of phosphorus, and arseniclo clusters which were produced by DLV. This investigation determines whether the IPS of the oxides converge to some common value in a smooth manner or whether there are large differences between the ionization potentials of the oxides as their size increases. It might be possible to determine changes in the structures of these oxides upon the addition of either an oxygen or aluminum atom if large Alps were observed between neighboring oxides in a series (either the same number of alu-
’NRC/NRL Postdoctoral Research Associate.
minum atoms or the same number of oxygen atoms). The average aluminum oxidation state in the metal oxide molecule may also influence the ionization potential of the species. Large differences in ionization potentials therefore might be indicative of a change to a more favorable oxidation state (low IP) or to an unfavorable oxidation state (high IP) following ionization. The aluminum oxides in this work were produced either directly by laser vaporization, as is the case for A120+,AI3O2+,and A1404+, or by the reaction of these species with N 2 0 or NO2. Typical reactions” for the addition of oxygen to an aluminum oxide are Al,Oy+
+ NO2
--
+
Al,Oy+l+ NO NO+ + (Al,O,,+J
AI+ + (AlrlOy+l)
+ NO
This type of oxidation of aluminum oxide cations is possible up to y = 4 for clusters with two aluminum atoms and y = 6 for clusters with three aluminum atoms. ( I ) Cotton, F. A,; Wilkinson, G.Advanced Inorganic Chemistry, 4th ed.; Wiley and Sons: New York, 1980. (2) Jarrold, M. F.; Bower, J. E.; Kraus, J. S. J . Chem. Phys. 1987, 86, 3876. (3) Jarrold, M. F.; Bower, J. E. J . Chem. Phys. 1987, 87, 5728. (4) Ruatta, S. A.; Anderson, S.L. J . Chem. Phys. 1988, 89, 273. (5) Jarrold, M . F.; Bower, J. E. J.Chem. Phys. 1987, 87, 1610. (6) Jarrold, M. F.; Bower, J. E. Chem. Phys. Lerr. 1988, 144, 311. (7) McElvany, S. W. J . Chem. Phys. 1988,89, 2063. (8) Bach, S.B. H.; Eyler, J. R. J . Chem. Phys. 1990, 92, 358. (9) Zimmerman, J. A.; Eyler, J. R.; Bach, S. B. H.; McElvany, S. W. J . Chem. Phys. 1991, 94, 3556. (10) Zimmerman, J . A.; Bach, S. B. H.; Watson, C. H.; Eyler, J. R. J . Phys. Chem. 1991, 95, 98. (11) King, F. L.; Dunlap, B. 1.; Parent, D. C . J . Chem. Phys. 1991, 94, 2518.
This article not subject to US.Copyright. Published 1991 by the American Chemical Society
9092
Bach and McElvany
The Journal of Physical Chemistry, Vol. 95, No. 23# 1991
TABLE I: Results of Charge-Transfer Bracketing Experiments IP,22eV A120 Al2O2 nickelocene 6.28 Y 6.71 Y ferroccnc Y 7.00 N,N-diethylaniline 7.50 Y m-toluidine Y Y 8.04 durenc Y 8.29 m-cresol N Y 8.44 p-xylene Y 8.82 toluene N 9.04 I ,2,4-trichlorobenzene N 9.25 benzene
AI2@
Y Y Y Y
-
+ CTA, Al,Oy+ + CTA2
AI,O,
-
+ CTAl+
no reaction
IP(AI,O,) IP(AI,O,)
A1303
Al,04
AI@,
Y N
Y
Y N
Y Y Y Y Y N N
Y
N N
Y
N
N N
+ Argon/Oxidizer Profile
Ejection Sweeps
> IP(CTA), C
A1302 Y
N
Charge-transfer bracketing consists of allowing a series of compounds (charge-transfer agents (CTA)) with known ionization potentials to interact with the cation of a species of unknown IP. The compounds between which charge transfer is observed and charge transfer is not observed are said to "bracket" the IP of the species of unknown IP: A1,OY+
Al20,
Transmit Receive
IP(CTA)2
The IP of the species of interest is then bracketed between the two CTAs, and the ionization potential is defined as the average of the reported ionization potentials of the two bracketing compounds. The error in this value is designated as half the difference between the ionization potentials of the two bracketing compounds or 0.1 eV, whichever is greater. The only reported experimental IPS of aluminum oxides are for AI20 and A1202 as determined by appearance potential measurements yielding values of 7.7 f O.5I2-l5to 8.5 f 1 1 6 and 9.9 f 0.5 eV,13*15 respectively. A more accurate determination of the IPS of these and other aluminum oxide species will lead to a better understanding of the reactions observed by King, Dunlap, and Parent," where the relative IPS of the aluminum oxides and the other species ( N O and AI) produced by the oxidation reaction would influence the products observed and their branching ratios. So, to examine the role, if any, that charge transfer plays in these systems, it is necessary to determine to within a few tenths of an electronvolt the ionization potentials of the aluminum oxides involved. This can be accomplished readily by the charge-transfer bracketing technique.
Experimental Section The experiments were performed with a Fourier transform ion cyclotron resonance mass spectrometer (FTMS). The system consists of a Nicolet FTMS-1000 data system," a 3-T Nicolet superconducting magnet, and a custom vacuum system with a 1 in. x 1 in. x 2 in. (z-axis) cell made of stainless steel plates, except for the trapping plates which are composed of 90% transparent nickel mesh. This apparatus has been described in detail.I8 The stored waveform inverse Fourier transform (SWIFT)'9s20 ion excitation/ejection technique has been incorporated into this instrument. The sample, an aluminum oxide (Al,03) pellet formed from the corresponding powder (Aldrich), was introduced into the instrument via a solids insertion probe. The 532-nm output of a Quanta-Ray DCR-2 Nd3+:YAG laser2I ( 0 . 1 4 5 mJ/pulse) (12)Thompson, K. R. High Temp. Sci. 1973, 5, 62. (13) Drowart, J.; DeMaria, G.;Burns, R. P.; Inghram, M. G.J . Chem. Phys. 1960, 32, 1366. (14) Burns, R. P. J . Chem. Phys. 1966,44, 3307. (15) Fu, C. M.; Burns, R. P. High Temp. Sci. 1976,8, 353. ( 1 6 ) Farber, M.;Srivastava, R. D.; Uy. 0. M . J . Chem. Soc., Faraday Trans. I 1972,68, 249. (17) Presently sold by Extrel, P.O. Box 4508, Madison, WI 5371 1. ( I 8) Parent, D. C.; McElvany, S. W . J . Am. Chem. SOC.1989, I I I , 2393. (19)T. L.Ricca Associates, Columbus, OH 43212. (20) Marshall, A. G.;Wang, T.-C. L.; Chen, L.; Ricca, T. L. Fourier Transform Mass Spectrometry: Evolution, Innovation, and Applications, Buchanan, M. V., Ed.; ACS Symposium Series 359; American Chemical Society: Washington, DC, 1987;pp 21-33. (21)Now Spectra Physics, Laser Products Division, P.O. Box 7013, Mountain View, CA 94039-7013.
I
Formation I.P. Deterrninakn (oxidationreaction) (chargetransfer reaction) Laser Triggered Pulse Valves Triggered Eject AIf Figure 1. FTMS pulse sequence used for these experiments. The laser is triggered, the pulsed valve is opened, and AI+ is ejected during the beam event. The argon/oxidizer profile shows that most of the gas is pumped away before the excitation/ejection sweeps. The 1P determination period is a variable time (0.003 to 3 s) for the charge-transfer reaction. was collimated by using several irises and focused through the cell onto the sample by using a 1 m focal length lens. This procedure was used to produce AI20+, A1302+,and A1404' by direct laser vaporization (DLV). These cations were then oxygenated by allowing them to react with NO2 or N20, which entered the vacuum chamber through a pulsed valve (General Valve Corp.). The oxide ion of interest was then allowed to react with a particular charge-transfer agent which was maintained at a static pressure of 1-2 X Torr above background (ca. 1 X Torr). The charge-transfer reactions were studied to near completion (>90%) and exhibited only single-exponential firstorder kinetic behavior. This indicates that isomeric oxide ions (with different neutral IPS) are not generated by this method. The ion production/oxidation and IP determination procedure are illustrated under Results. The CTAs used in these experiments are listed in Table I and were obtained from commercial sources. The CTAs underwent several freeze-pumpthaw cycles prior to their introduction into the vacuum chamber, and their purities were verified by their electron ionization mass spectra in the FTMS.
Results The experimental parameters had to be adjusted carefully and monitored so that the charge-transfer bracketing technique could be applied successfully to species produced via direct laser vaporization (DLV) and subsequent oxidative reactions within the ICR cell. A general schematic of the experimental pulse sequence used is shown in Figure 1. At the time the laser was fired, AI+ was continually ejected from the ICR cell to reduce space-charge interferences caused by the abundant monatomic ion produced by DLV. The AlzO+,AI3O2+.and AI4O4+ions generated by DLV can possess large amounts of internal and translational energy. Argon was used as the thermalizing gas in the present experiments ( 103-104 collisions during a 1-4-s reaction period), although previous charge-transfer studies8-l0 have shown that the IPS of the clusters determined by this method remain unchanged upon removal of the argon (or even SF6) thermalizing step of the
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9093
Ionization Potentials of Aluminum Oxides
i
0
TABLE 11: Ionization Potentials of Aluminum Oxides ionization potentials, eV AI oxidation state a'
~
species
A'zo'
A120
this work 8.35 f 0.2
previous work 7.7 f 0.5' 7.9 & 0.3b 8.20 f 0.15c 8.5 I d 9.9 & 0.Y l o & Id
neutral
cation
+1.0
+0.5
A1202
8.9 f 0.2
+2.0
+IS
A1203 AI204
8.9 f 0.2 6.85 f 0.2
+3.0
+2.5
+4.0
+3.5
A1102 A1303 A1304
6.85 f 0.2 6.85 i 0.2 8.35 f 0.2
+1.3
+1.0
+2.0 +2.7
+1.7
A1404
6.85 f 0.2
+2.0
+1.75
=Zo mh
60
m/z
+2.3
References 12-1 5. Reference 23. Reference 24. Reference 16. CReference~ 13 and 15.
I
i 63
70
61)
" 0
l0i
I
/I/ I IL
12b
14"
mlz
Figure 2. (a) Mass spectrum obtained following DLV of an aluminum oxide pellet and mass selection of AI20+( m / z 70). (b) Mass spectrum following oxidation of A120+with N20(see text for details). (c) Mass selection of A1203+( m / r 102) from (b). (d) Charge-exchangereaction of Al2O3+with durene ( m / z 134).
experiment. The thermalizing time and the argon pressures used in these experiments were designed to allow for multiple collisions between the thermalizing gas and the oxide ions. It was possible for the clusters to cool radiatively and/or rearrange into their lowest energy (most stable) structure during the thermalization period. Adjustment of the experimental parameters was critical to arrest the oxidation of the aluminum oxide at the desired stoichiometry and to control the competition between the oxidizing reaction and the charge-transfer reaction during oxidation. Otherwise, an insufficient amount of the oxide of interest would remain to undergo charge transfer in the subsequent charge-transfer reaction period. One must also recognize and address the possibility of competitive charge transfer from intermediate oxides and the production of hydrated aluminum oxides from reactions with background species (e.g., HzO).The extent of oxidation was controlled by using a fixed reaction (oxidation) time of 1 s and varying the amount of oxidizing agent (N20or NO2) allowed into the cell by adjusting the length of time that the pulsed valve remained open. Pressures of the CTAs were carefully adjusted to reduce the amount of charge transfer occurring during the oxidation reaction so that a sufficient quantity of the oxide cation of interest remained for the final charge-transfer phase of the experiment. An example of the entire experimental sequence is shown in Figure 2, which uses A1203to illustrate the steps necessary to form these oxides and then bracket their ionization potentials. The AI2Ot ( m / z 70) cation was generated by DLV and mass selected as shown in Figure 2a. Argon and N 2 0 were added via pulsed valves to thermalize and oxidize AI20+ as shown in Figure 2b; note the presence of AIZ0,+ with y = 1-4 along with minor
hydrogenated oxide cations and the durene molecular ion (CloHI4+; m / z 132) which results from charge transfer that occurred during the thermalizing/oxidizing period. The A1203+ion was then mass selected prior to the charge-transfer reaction period as shown in Figure 2c. Figure 2d was obtained after a 1-s reaction, indicating that charge transfer occurs from A1203+to durene and thus IP(Alz03) > IP(durene) = 8.04 eV. The ionization potentials of aluminum oxides (Al,O,) with x = 2, y = 1-4; x = 3, y = 2-4; and x = 4, y = 4 were determined by the charge-transfer bracketing technique described above. Table I lists the charge-transfer experiments performed and the results. The ionization potentials of the aluminum oxides are between the IPSof the compounds for which charge transfer was ( Y ) and was not (N) observed. Table I1 lists the IPS of the aluminum oxides and their uncertainties which were derived from the charge-transfer bracketing experiments. The IP of only a single cluster containing four aluminum atoms (A1404) was determined because of the small amount of AI4O4+produced by direct laser vaporization. Discussion The IPS of AIZ0 and A120z determined by charge-transfer bracketing are compared to previous mass spectrometric appearance potential measurements in Table 11. The IP of A120 (8.35 f 0.2 eV) is in reasonable agreement with all previous values but in particular with the highest precision measurement of 8.20 f 0.15 eV.24 The origin of the disparity in the A$Oz values (8.9 f 0.2 vs 9.9 f 0.5 eV1391s)is not known a t this time. One possibility is structural differences (e.g., linear and cyclic) of the AI2O2 ions or neutrals in the two experiments. Large alternations were observed in the IPS of a series of oxides that contain equivalent numbers of aluminum atoms but different numbers of oxygen atoms as shown in Table 11. The IPS of the aluminum oxides containing two aluminum atoms decrease by over 2 eV from 8.9 eV for those that contain two and three oxygen atoms to 6.85 eV for A1204,indicating a structural change between Alz03and A1204. This structural change has been suggested by King et aI.'l as attributable to a change in structure from linear to cyclic:
0 =AI - 0- A I = 0
/
0-AI,
0\
,AI-0 0
For the series of oxides containing three aluminum atoms the IPS are 6.85 f 0.2 eV for oxides with two and three oxygen atoms, but upon addition of the fourth oxygen, the IP increases by 1.5 (22) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.;Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref.Data 1988, Suppl. No. I . (23) DeMaria, G.; Gingerich. K. A.; Piacente, V. J . Chem. Phys. 1968, 49, 4705. (24) Hildenbrand, D. L. Chem. Phys. Leu. 1973, 20. 127.
9094
eV to 8.35 f 0.2 eV for Al3O4. The average oxidation state of the aluminum atoms in the oxide species may also have a part in determining the first ionization energies. These changes in the average oxidation state of the aluminum atoms may contribute to large changes in IP of the oxides upon addition of another aluminum or oxygen atom. The primary oxidation state for aluminum is +3. An oxidation state of 1 can be a stable oxidation state for some aluminum compounds through the inert pair effect.' The inert pair effects results from a s2-p electron configuration of the metal atoms in the molecule. This translates into a resistance of a pair of s electrons to participate in bond formation. There are also certain group I l l compounds in which the metals are in a +2 oxidation state.' These generally are thought to contain metal-metal bonds. The average aluminum oxidation states of the neutral and cationic oxides are also included in Table 11. Most of the oxides studied in this work that have low IPS (6.85 eV) are also oxides, where upon ionization the average oxidation state of aluminum in the oxide is brought closer to either +3, which is its preferred oxidation state, or + I , where the inert pair effect gives the oxide added stability. In the oxides that have large IPS (8-9 eV) the average oxidation state of aluminum moves away from +3 or +I upon ionization. An example of this is AI20,, which has a large IP because the average AI oxidation state moves away from the stable +3 state upon ionization. In contrast, AI2O4has a low IP because the average oxidation state of aluminum moves closer to +3 upon ionization. Similar trends are found for the other oxides studied in these experiments. This though is not the case for A1202,even though the aluminum in the oxide cation has an average oxidation state which is lower than the +2 found in the neutral (Le., getting closer to the + I oxidation state of Al). In this case, the neutral may contain a metal-metal bond, thus producing a stable +2 oxidation state. This suggests that the structure of A1202is one with a AI-A1 core, rather than a structure with alternating aluminum and oxygen atoms. The oxidation reactions of aluminum oxide cations have recently been investigated by King, Dunlap, and Parent." They reported the formation of AlxOy+,+,NO+ (only with NO2), and AI+ in reactions of the oxide ion with NO2 or N20. It was reported that -20% of the total NO2 pressure added was actually NO resulting from decomposition processes. Thus, it is possible that the NO+ production was due to charge transfer between neutral NO (IP = 9.26 eV)22 and aluminum oxide cations, rather than being produced directly by the oxidation of the aluminum oxide. They
+
Bach and McElvany
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
were unable to rule out charge transfer with N O because the ionization potentials of most of the oxides at the time of their study were unknown. From our work in establishing the IPS of AIX0, molecules, the NO+ observed in the oxidation of AIX0,+ by NO2 cannot originate from a charge-transfer reaction because the IPS of all of the oxides are lower than the IP of NO. Thus, the NO+ is produced directly from the oxidation of the aluminum oxides. The fact that NO+ is formed, which has a relatively high ionization potential, indicates the large exothermicity of these oxidizing reactions and the relative stability of neutral AIxOy species. Conclusion
The charge-transfer bracketing technique was successfully applied to determine the IPS of aluminum oxides formed by direct laser vaporization and by subsequent oxidation reactions within the ICR cell. The ionization potentials of aluminum oxides, AlxO with x = 2-4 and y = 1-4, were bracketed to within 0.2 eV. is not clear from the present results whether the oxidation state of the aluminum in the oxide or the structure of the aluminum oxide is the most important factor in determining the ionization potential of these oxides. Large changes were observed in the IPS of the aluminum oxides with x = 2 and 3 upon the addition of a fourth oxygen atom. The large variation in the ionization potentials of these aluminum oxides is indirect evidence for structural changes that occur as the species become more highly oxidized. The 2-eV decrease in IP between A1203 and A1204is consistent with the proposed structural change from linear to cyclic from previous ion/molecule reactivity studies. The average oxidation state of the aluminum in the aluminum oxide can be used to derive structural information about the oxide. This was the case for AI2O2that was found to have a relatively high IP (8.9 eV), which indicates that the aluminum prefers the +2 oxidation state and suggests the presence of a metal-metal bond in the molecule. The ionization potentials determined in this work rule out the possibility that the NO+ observed by King et aLii was produced from charge transfer between the oxide cation and neutral NO.
fi
Acknowledgment. We thank the Office of Naval Research for support of this research. We also thank Denise Parent for helpful discussions and results prior to publication. Registry NO.A120, 12004-36-3; A1202, 12252-63-0; A1203, 1344-28-1; A1204 136238-08-9; AI302, 51 198-46-0; A1303, 14457-64-8; AIJO,, 12253-16-6.
