J. Phys. Chem. 1992,96, 10182-10189
10182
Reactions of PulsedLaser Evaporated Aluminum Atoms wWh Oxygen. Infrared Spectra of the Reaction Products In Solid Argon Lester Andrews,; Thomas R. Burkbolder, and Jason T. Yustein Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: July 2, 1992; In Final Form: September 1 1 , 1992)
Reactions of pulsed-laser evaporated Al atoms with 0 2 in a condensing argon stream gave cyclic-A102at 496.3 cm-' as the major product. In addition sharp new absorptions at 1211.2,1176.3,and 1129.5 cm-'are identified on the basis of isotopic shifts and multiplets as hear OAlOAlO, AlOAlO, and OAlO,respectively. The identifhtim of linear OAlOAlO and AlOAlO was confirmed by MP2 calculations of isotopic frequencies. The 1129.5-cm-I band exhibited a sharp mixed oxygen isotopic triplet and isotopic ratio 1.0261 in agreement with that expected for a linear molecule. A sha 1092.5-cm-' absorption is identified as OAlOO on the basis of a mixed oxygen isotopic quartet with 1%)2/1%)'80/1802 and %2/1802 reagent mixtures. The only A102isomers observed were the cyclic and linear forms found by theory to be potential minima. The observation of isotopic shifts and mixed isotopic multiplets is absolutely essential for the identification of new transient species.
Introduction The aluminum atom reaction with molecular oxygen, Al + O2 A10 0, has been investigated in the gas phase, in matrix isolation studies, and by theoretical calculations owing in part to the importance of aluminum as a structural material and as a component in solid propellants. Gas-phase studies have focused on the reaction dynamics and the spectroscopy of the AlO product formed,l" and two of these investigations have employed laser ablated aluminum at0ms.4.~Matrix isolation experiments have reported a variety of new infrared absorptions for aluminumoxygen species,'-12 but there is agreement only on the identification of A120 at 992 cm-l, which has also been observed from heated Al + A 1 2 0 3 mixt~res~~9'~ and recently in the gas phase.15 However, fluorescence spectroscopy has made possible the identification of AlO at 975 cm-l in solid argon.'O There is evidence for both cyclic and linear forms of A102in matrix experiments.8JlJ2 In addition theoretical studies have explored the structures of possible A102 species1618and N202molecules.1e22 Finally, maa spectmcopic studies of group 13 oxides have shown that the major aluminum oxide vapor species in equilibrium with solid A1203 at 2200-2300 K are AlO,A120, and A1202 and that A102 is a minor but identifiable vapor specie^.^^-^^ Furthermore, A102 has been proposed as a product of the A10 + O2 reaction at 1400 K.26 These aluminum oxides and their combination products, in particular molecular Alz03, are the subject of the present study. Several interesting questions concerning the Al 0 2 reaction are paramount. Does the reaction proceed through a complex NO2species? What can be deduced about the structure of this A102species? Given that BO2is a linear molecule and no evidence for a cyclic BO2species has been f~und,~'*~* what are the relative stabilities of linear and cyclic M 0 2 and M202 species going down group 13 in the periodic table? Owing to the experimental difficulties of containing liquid aluminum and of having a high temperature source near a cryogenic window, pulsed-laser evap oration of aluminum has been employed as a source of atoms for reactions with O2before product trapping in solid argon. These studies present new observations and assignments for the linear OA10,AIOAlO, and OAlOAlO molecules and provide new light on earlier discrepancies in the literature.
-
+
+
Experimental Section
The technique for pulsed-laser evaporation of aluminum is identical to that employed in recent boron ~ t u d i e s . ~Targets *~~~ (AI, Aesar, 99.998% Puratronic and A1203,'Lucalox", General Electric) were mounted on a rod rotating at 1 rpm. A Nd:YAG laser was operated at 1064 nm and 6 Hz with 10-ns pulse width, and laser power of 15-80 mJ/pulse was focused on the target sample with a 20-cm-focal length quartz lens. Microscopic examination of the target revealed laser tracks approximately 0.1 0022-3654/92/2096-10182$03.00/0
mm wide. This agrees with the spot size predicted from focal length (20 cm) X laser beam divergence (0.5 X rad). Aluminum atoms were adeposited with Ar/02samples at 2 mmol/h for l-5-h periods, and infrared spectra were recorded on a Nicolet 60SXR at 0.5- and 0.2-cm-' resolution; wavenumber accuracy is f0.1 cm-'. Matrix samples were annealed from 11 1 K to 20,25, or 30 K, and more spectra were recorded. Photolysis of selected samples employed a 175-W mercury street lamp (Philips H39KB) with the globe removed. Results Infrared spectra of aluminum atom reaction products with oxygen molecules over a wide range of concentrations and of laser evaporation products from solid alumina will be presented. Al + 0 2 . The infrared spectrum from 1250 to 450 cm-I for a typical experiment using average conditions, Ar/02 = loo/ 1 and 40 mJ/pulse at the aluminum target, is shown in Figure la. The spectrum is dominated by the very strong 496.3-cm-' band (measured accurately with dilute reagents) assigned by the Moscow group to A102.11 Another obvious product band at 992.8 cm-'is due to A120,13-15 Also noted are O3at 1039.5 and 1033.2 cm-l 30 and a weak AlO band at 975 cm-l.Io Important product bands at 1211.2, 1176.3, 1167.5, 1144.8, 1129.5, 1092.5, 939.6-945.4, 897.7, 843.2, 804.6, and 529.6 cm-'are numbered 1-10 in Figure 1 and Table I. Many of these bands have been reported by earlier worker^,^^^^^' but only the 940-cm-' band has been correctly assigned to linear A 1 2 0 2 by the Moscow group."J' Sample annealing to 25 K caused several noteworthy changes in the spectrum, as shown in Figure 1b. The 2-6, AlzO, AlO, and 7 bands decreased by varying degrees, the 10 band was destroyed, and the 496.3-cm-' N O 2band was decreased by 35%. The sharp 1 band at 1211.2 cm-' increased 3-fold. Strong new bands a p peared at 1126.7 cm-I (labeled ll), 852.6, 849.5, 837.9 cm-' (A103) and at 686.5, 609.2, and 542.7 cm-'. Annealing more dilute samples also produced weak bands at 964, 911, and 778 cm-l, a strong new 886.5-cm-' band, and substantial growth of the A102 band at 496.3 cm-'. Although trace H20 absorptions were present in these experiments, there was no evidence for the HAlOH insertion product.32 The effect of 254-nm photolysis for 30 min is shown in Figure IC. The O3band was reduced and the 852.6- and 849.5-cm-' bands were markedly reduced, as expected for metal ozonide species.33 The strong 496.3-cm-' band was decreased by 70% on photolysis, the Sll.S-cm-' band was regenerated, and the 1129.5and 1126.7-cm-l bands increased slightly. The laser power was varied from threshold 15 mJ/pulse to 50 mJ/pulse at the sample in a series of experiments with A r / 0 2 = 200/1 samples; the infrared spectrum from 1250 to 750 cm-I is contrasted for the lowest power 15 mJ/pulse (Figure 2a) with Q 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10183
Pulsed-Laser Evaporated A1 Atoms with 0
TABU I: lntrwd Abmrptha (cm-I) Observed for Ahl" AtolbOxygea Mokuk Reaction Fkdwta Trapped in Solid Argon at 11 f 1 K
'602
mixed
160-1800
?
1214.4 1211.2 1176.3 1169.2 1167.5 1144.8 1140 1129.48 1126.7 1092.5 lO5Obr 1025.6 1009 992.8 975.2 964.2 953.8 945.5 942.9 939.6 915.4 910.7 897.7 886.5 861.3 852.6 849.5 837.9 843.2 808.1 804.6 800.9 804.6 778 686.2 681.3 640 609.2 542.7 529.6 511.5 496.3 490.0
1206.1, 1200.0, 1187.4, 1181.3 1160.8 1153.8 1152.4 ? ? 1115.95 1120.6, 1095.6 1089.0, 1068.0 ? ? ? C
940.5, 928.4, 925.1, 914.4 928, 914 922, 910 broad broad 891, 886.0, 881.0, 879.0 840, 832,830, 819 broad doublet 807 br, 794 br, 781,790 br 790 br 795, 786, 778,768 679, 673 br 602 br 534 br 520.9 ? 488.1 ?
1802
1176.7 1172.9 1142.2 1135.4 1133.8 1114.2 1106 1100.75 1089.5 1064.1 1020br ? 966? 950.2
v ( ~ ~ O ) / V ( ' * O ) annealb 1.0320 1.0326 1.0299 1.0298 1.0297 1.0274 1.0305 1.02610 1.0341 1.0267 1.029
++ + ++
++
-
1.0445 1.04483
-
++
934.4 901.6 899.9 899.9 892.4 881.2 876.0 874.6 ? 818.1 807.5 804.6 791.7 818.2 780.2 777.1 773.3 758.8 749 670.1 665.0
1.0319 1.0579 1.0507 1.0507 1.0529 1.0388 1.0396 1.0264
--
1.0528 1.0559 1.558 1.0584 1.0306 1.0358 1.0357 1.0357 1.0598 1.0389 1.0240 1.0245
++ ++ ++ 0 -+
594.8 532.0 513.2 496.0 480.6 474.1
1.0242 1.0201 1.0320 1.0312 1.0327 1.0339
+ ++
-
++ ++ ++
-
0
-
+ ++ ++ ++ ++ ++ -+l+/+l-
identification 1. site 1; OAlOAlO 2, AloAlo 3, AlOAlO site 3, AloAlo 4, (OAlO)(X) aggregate 5 , OAlO 11, (OAl)(OOo) 6, OAlOO high 02/Al high A1/02 (A120)(02)? AlOAl A10
c,
OAl02
isolated C0,2, AloAlo 3, AlOAlO site 3, AloAlo aggregate aggregate site 7, AlX(O2)2 ? A103 A103 A103
AI03 8, ? 9, Alx(02)2 99 AlX(O212 9, AlX(O212 isolated C3? aggregate aggregate site aggregate aggregate aggregate 10, (Al02)(Al) perturbed N O 2 cyclic A102 perturbed A102
'New absorptions in scrambled isotopic oxygen experiment: 1602/16J802/L802 = 1/2/1. Some of these bands were observed with 1 6 0 2 / 1 8 0 2 = 1/1 samples. bAnnealing behavior: appears; grows; 0, no change; -, decreases; --, destroyed. 'Isotopic sextet in N2 matrices?
++,
+,
- Ilb
" I
WRVENilMBER
"1. Infrared spectra in the 450-1250-cm-' region for aluminum atoms (40mJ/pulse) with Ar/02 = 100/1 sample: (a) with codeposition for 2.5 h; (b) after annealing to 25 i 1 K; (c) after full arc photolysis for 30 min.
spectra for higher laser powers, namely, 30 mJ/pulse (Figure 2b) and 50 mJ/pulse (Figure 2c). The band absorbance yield of product species followed the laser power and deposition period, but the relative yields of different products varied depending upon the number of aluminum atoms in the product species. The strong NO2band at 496.3 cm-I is taken as a monoaluminum reference and the A Z Oband at 992.8 cm-I as a dialuminum reference. The sharp 1129.5-cm-' band labeled 5 exhibited constant relative
Bso Fiopn 2. Infrared spectra in the 750-1250-cm-I region for aluminum a t o m and Ar/02= 200/1 samplts comparing different laser powers at the target: (a) 15 mJ/pulse; (b) 30 mJ/pulse; and (c) 50 mJ/pulse.
absorbance with the strong 496.3-cm-' band (25%) at the two lower laser powen employed but increased (to 50%) at the highcat laser power, although the 496.3-cm-' band yield was somewhat less than expected, p i b l y owing to photolysis by radiation from the target. As can be seen in Figure 2, the A120band absorbance increased relative to the 1129.5-cm-' band with increasing laser power. It is therefore concluded that the 496.3- and 1129.5-cm-'
Andrews et al.
10184 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 0
Nl
I
Y O
",is0
Figure 3. Infrared spectra in the 450-1250-cm-' region for aluminum atoms (30 d/pulse) deposited for 4 h with low oxygen concentration samples: (a) argon with trace O2impurity; (b) Ar/02 = 800/1.
absorptions arise from monoaluminum species. The sharp 4 and 6 bands also behave appropriately for single A1 atom species. However, the 1-3 and 7-10 bands show higher AI dependencies, namely, the same as A120, and hence are due to dialuminum species. The sharp 953.8-cm-' band prominent at lower laser power is due to the isolated 0, anion.34 The oxygen concentration was also varied from 2% to trace levels. The spectrum from an experiment with argon and trace oxygen and 30 mJ/pulse laser power is illustrated in Figure 3a. Note that the 992.8-cm-' A120 band is almost as strong as the 496.3-cm-' A102 absorption! Annealing had the same effect as described above, except that the 496.3-cm-' band absorbance doubled, and new aluminum rich aggregate species appeared at 1115 and 1140 cm-I; the 1129.5-cm-' band was reduced by half. Two satellites appeared near the 496.3-cm-' band at 5 11.5 and 490.0 cm-I. The spectrum from a similar Ar/02 = 800/1 experiment is contrasted in Figure 3b; in this experiment annealing produced the 5 11.5- and 490.0-cm-' satellites, increased the 496.3-cm-' band by 30%, and decreased the 1129.5-cm-' band by 50%. Higher laser power gave a lower yield of 04-due to photolysis by the laser plume. At the other extreme, 2% 0 2 gave very strong familiar product bands, new broad bands at 1071, 1056, 913, 686, and 609 cm-I, which increased markedly on annealing. Two experiments were done using a Nicolet SDXB FT-IR, and spectra were recorded down to 250 cm-I. Although very strong product bands were observed, as described above, no product absorptions were observed below 480 cm-I. Photolysis for 30 min with a Pyrex filter gave a 25% reduction in the 496-cm-' band, a 20% reduction in the 529-cm-' band, a 5% growth in the 1129-cm-' band, a 30% growth in the 1211-cm-' band, and no change in the 1176-, 1169-, 1092-, 945, and 940-cm-I bands. Continued photolysis for 30 min with the unfiitered mercury lamp decreased the 496cm-' band by 40% and the 529-cm-' band by lo%, increased the 1129- and 1211-cm-l bands by lo%, and left the species 2, 3, and 6 bands unchanged. Finally, two AI experiments were done with nitrogen. A nitrogen matrix with l % O2gave a weak product multiplet at 1091 cm-', A120 at 991 cm-I, and ozone and nitrogen oxide bands. Another study was done with an Ar/02/N2 = 200/ 1/ 1 sample; product features were identical to those described for Ar/02 = 200/1 samples. A120y Polycrystalline alumina was ablated by the laser, and the products were trapped in a condensing argon stream on the 11 f 1 K window; the spectrum is illustrated in Figure 4a. As in the oxygen deficient experiment of Figure 3a, the A120 and A102 bands are of comparable intensity. Note the increased absorbance of the 1 band and A120 and A10 bands relative to Figures 1 and 3. Annealing the ablated A1203sample to 25 K caused a marked decrease in A120 and AlO absorptions, as shown in Figure 4b. In addition the 10 band was destroyed while the 496.3-cm-' band increased by 1096, the 1 band increased by 40096, and the 1126.7cm-' band (labeled 11) appeared out of a shoulder
il
I
2
iiso
so
ibso
so
+so
650
550'
WAVENUMBER
'
lis0
Figure 4. Infrared spectra in the 450-1250-m-' region for A1203laser evaporated (60dlpulse) into an argon stream condensed at 12 1 K (a) for sample deposited for 3 h; (b) after annealing to 25 i 2 K. 0
11 9 I
W UW -
a
a $N
m-
ii
5
E l
.IT50
li5C
1050
450
as0
is0
650
+so
650
WAVENUMBER
Figure 5. Infrared spectra in the 450-12SO-cm-' region for aluminum atoms deposited with '*02in exam argon at 12 1 K: (a) from 3-h = 200/1; (b) after annealing deposition at 30 mJ/pulse with Ar/1802 to 25 i 2 K (c) from 2-h codeposition at 50 mJ/pulse with Ar/'*02 = 100/1.
*
on the 1129.5-cm-' band, which decreased 30%. The 2, 3, and 9 bands decreased as before. Finally, alumina powder (from Lucalox) p r d into a KBr disk gave a strong broad absorption centered at 620 cm-I. Abhtioa. A pressed disk of LAO2was ablated by low laser power into a condensing argon stream. The spectrum was dominated by a strong broad 825-cm-' band and weaker bands at 670,570,450,380, and 340 cm-I. Finally, LAOz powder was mixed with KBr (1/20) and pressed into a pellet; the infrared spectrum showed a strong broad 820-cm-' band and weaker bands at 670,570, and 440 cm-'.The argon matrix spectrum of ablated LW02and the KBr/LW02 pellet spectrum were quite similar. AI l%Mhwkkd&. Three experiments were done with I8O2, and representative spectra are shown in Figure 5. All product bands shifted as is shown in an Ar/1802 = 200/1 experiment in Figure Sa and given in Table 1. A new band labeled 11 at 1089.5 cm-' grows on annealing, as does the 1173.0-cm-' band labeled 1 (Figure 5b). The band at 901.6 cm-' masks the 2 band at 899.9 cm-I, which remains on annealing. The '80counterparts behaved on annealing as did the '60-isotopic bands. Figure 5c shows the spectrum from an Ar/'802= 100/1 sample using higher laser power; note the larger initial 1 and 11 bands and the absence of Two different 1602 I8O2samples were reacted with Al atoms. Many bands showed no new mixed isotopic unnpcmcntq exceptions were a very weak 1116.0-cm-' central component between the 1129.5- and 1100.78-~m-~ pure isotopic components of species 5, a triplet with a 886.0-cm-' central component for species 7, a quartet at 1211.2, 1206.2, 1181.5, and 1173.0 an-'for species 1, a quartet at 1092.5, 1089.0, 1068.0, and 1064.1 cm-'for s@es 6, and a quartet for ozone absorptions. Wotolysis for 30 min gave a 20% reduction in the 496.3- and 480.5-cm-' bands and 10%
+
+
The Journal of Physical Chemistry, Vol. 96, No. 25, I992 10185
Pulsed-Laser Evaporated A1 Atoms with 0
11 i 11
I
I
11
5
2 1 3
S
I
m 0
0
oijzs
itoo
ii7s
iiso
iizs
WRVENUMBER
iioo
ib75
ibso
Figure 6. Infrared spectra in the 1050-1225-~m-~region for aluminum = 1/2/1 in a t o m (60 mJ/pulse) deposited with 1602/160180/'802 argon at Ar/02= 100/1: (a) for sample deposited for 4 h at 12 1 K; (b) after annealing to 25 2 K. Arrows denote the isotopic sextet for species 1 .
growth in the 1129.5- and 1100.7-cm-' bands. Annealing substantially decreased these bands and produced a quartet of mixed isotopic ozonide bands. Experiments were done with Ar/'69'802 = 200/1 samples at two different laser powers; expanded scale spectra at 0.2 cm-' resolution are shown in Figure 6. The sharpest feature in the spectnun is the 1129.48-, 1115.95, and 1100.75-cm-' 1/2/1 triplet for species 5 (fullwidth at half-maximum = 0.5 cm-I). The species 2 and 3 bands produced a complicated multiplet due to site splittings for species 3 and band overlap. Annealing reversed site absorptions for species 3, decreased species 2, and suggested that these bands are mixed isotopic triplets; species 2 exhibited absorptions at 1176.3, 1160.8, and 1142.2 cm-'. Annealing also increased species 1 and produced a 1/2/1/1/2/1 isotopic sextet reminiscent of ozone, but with relatively more spacing between the central components than that found for Species 11 increased a factor of 3 on annealing and revealed a quartet at 1126.7, 1120.6, 1095.6, and 1089.5 an-'. In the lower region (not shown) multiplets were observed for other product absorptions; the strong A102 band exhibited a 1/2/1 triplet as did the 529.6-cm-' satellite. The 939.7-945.4-cm-' band exhibited intermediate components at 922-928 and at 910-914 cm-'. The sharp 897.7-m-' band became a sextet, and the split band at 804.6 an-'became a broad 1/2/ 1/ triplet. In addition the ozonide band at 800-850 cm-'increased on annealing and gave a clear sextet with overlapping central components. Finally, the 686.2-cm-' band broadened and revealed at least two intermediate components.
Discussion The major product absorptions will be assigned based on isotopic shifts and multiplets and ab initio calculations of structure and spectra. M2. Cyclic and h e a r . The major band at 496.3 cm-' was but its appearance at low aluminum first as~igned'~ to AJ202, concentration and the isotopic shift led the Moscow group to reassign this band to cyclic AJ02.'l Subsequent theoretical calculationsI6I8 and the present work are in agreement with the cyclic-AI02assignment. In the present threshold laser evaporation experiment, the 496.3-cm-' band was strong (A = 0.21) whereas the 992.8-an-' A120band was weak ( A = 0.01) and the species 5 band at 1129.5 cm-' was of medium intensity (A = 0.05). Increasing laser power slightly increased the absorbance of the 1129.5- and 496.3-cm-' bands, but markedly increased the absorbance of the 992.8-cm-' band relative to the 1129.5- and 496.3-cm-' bands. These data support the conclusion that the 496.3-cm-I band is due to cyclic-A102 and foster a similar conclusion that the 1129.5-cm-' band is also due to a monoaluminum species. The symmetrical 1/2/1 triplets observed for the latter bands in 16J802experiments require the presence of two equivalent oxygen atoms for each species.36
Two recent ab initio calculations predict similar structures for cyclic-AI02 ( & A I 4 angle = 39.3O or 41.8O; A 1 4 = 1.932 A; 0-0 = 1.298 or 1.377 The symmetric A 1 a 2 stretching mode for a 4O0 0 - A I 4 angle should exhibit a 16/18 ratio of 1.0284 assuming no interaction with the symmetric 0-0 stretching mode. The observed isotopic ratio for cyclic-A102 is 1.0327, somewhat higher than the expected value, which is consistent with a small amount of 0-0 stretching character in the 496.3-cm-' mode. We have no evidence for the weak 1097-cm-' band originally assigned to v ( M ) of cycli~AlO~? This p i b l e band is important for the bonding picture; if the original assignment were correct, the implication of superoxide character for A 1 0 2 is apparent. However, NO2 cannot be as ionic as Li02, which is clearly a superoxide speck3' Furthermore, the weak 1097-cm-' band assigned originally to c y c l i ~ A I 0has ~ not been confirmed by isotopic substitution. We believe that the 1092.5-cm-' species 6 band observed here is probably the same band previously assigned to cyclic-A102 and the same band used to identify bent asymmetric AIOO? howeuer, this 1092.5-cm-I band is better reassigned to OAIOO, os will be discussed below. The 0-0 fundamental of cyclic-A102is expected to be very weak, and it may be masked by O3 or other product absorptions in this region. Very recent CASSCF multiconfiguration calculations for AlO2predict u1 at 1081 cm-'and u2 at 591 cm"." If the u1 value is scaled by the same 0.84 factor required to fit the observed 4964"' value for 4,then u1 is predicted near 900 cm-I. The 16/18 ratio for the u2 band is therefore explained by mixing of a small amount of symmetric 0-0 stretching character with the symmetric A142 stretching mode. Finally, cycIic-AlO2 is not sufficiently ionic" to be considered a superoxide, particularly in the ionic lithium superoxide sense." The sharp band labeled 5 at 1129.5 cm-l is assigned to u3 of the linear 0 - A I 4 molecule based on the sharp mixed isotopic triplet and agreement between the observed isotopic ratio, 1.026 10, and the ratio calculated for a linear molecule, 1.02648. In contrast to OBO, which showed evidence of quartic anharmonicity in the u3 fundamental, OAlO appears to be normal with a small amount of cubic anharmonicity in the u3 mode. The broader 918-cm-' band assigned to linear OAlO by the Moscow group was not observed here. However with 2% 02,a 913-an-' band was observed which increased on annealing. It must be concluded that the 9 18- and 9 13-cm-' bands are due to an aggregate species since the Moscow experiments also employed 2 4 % oxygen.36 Theoretical calculations, which sup rt a u3 fundamental for linear OAlO in the 900-cm-' region,'cust be reevaluated. Other UHF calculations predict a u3 fundamental that is too high (1764 cm-').18Clearly calculating the vibrational spectrum of linear (21'1) 0-A1-0 is a formidable theoretical problem, which merits further attention. Finally, no evidence was found here for any A 1 4 molecular species other than the linear and cyclicforms absorbing at 1129.5 and 496.3 cm-I, which have been shown by calculations to be stable structures.I6I8 In samples with excess trapped A1 atoms, cyclic-A102increased on annealing to 25 K, but linear-AI02 decreased. However, similar boron atom experiments showed an increase in linear-B02 on annealing to 25 K. Clearly, the addition of AJ to O2to give cyclieA102 requires no activation energy, but insertion to give linear-AI02 requires activation energy. In contrast, cold B atoms insert to give l i ~ ~ e a r - B O The ~ . decrease ~~ of linear-A102 (and cyclic-AI02 in the absence of e x a s AI) on annealing shows that these molecules undergo further association reactions. Perturbed Species. The sharp 1144.8-m-' band labeled 4 appears to be due to a species with one aluminum atom, and the 16/18 isotopic ratio is very near that for the antisymmetric stretching mode of linear-OAlO. The 1144.8-cm-' band is attributed to OAlO perturbed by another molecule owing to the small (15-cm-I) blue shift from isolated OAJO. The perturbing molecule could be 0 2 , and AI is the most likely site for this weak interaction based on the observed isotopic ratio being that for an antisymmetric 0 - A 1 4 stretching mode rather than a terminal (diatomic 1.0369) OAl mode.
a
10186 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
Andrews et al.
TABLE Ik observed rad Cdcuhted Freqoeocies (cm-') for the Two Highest Boad Stretching Modes of Linear-AH)-Ato" @)
v 4 m
isotope 27-16-27-16 27-18-27-16 27-16-27-18 27-18-27-18
ObS
946 914 928 900
RHF/DZP 1025 987 1008 972
MP2/DZP 939 909 921 893
ObS
1176.3 1160.8 1160.8 1142.2
RHF/DZP 1295 1282 1274 1258
MP2/DZP 1173 1157 1158 1138
"Reference 39: similar results in ref 38.
In contrast annealing produced a new 1126.7-a-' band, species 11, which exhibited a mixed isotopic quartet and an oxygen isotopic ratio (1.0341) appropriate for a terminal OAl stretching mode. The isotopic quartet characterizes the vibration of a single 0 atom interacting with a second inequivalent 0 atom. Species 11 is probably an 0A10--02 pair interacting strongly enough to break the OAIO symmetry and to give an OAlOOO species. Support for this possible idenflication is found in a preliminary pulsed-laser Al O3experiment, which gave a stronger 1126.7-cm-' band than the 1129.5-cm-' primary product band on sample deposition. In the low frequency region, the 529.6-cm-' band exhibited a mixed isotopic 1/2/ 1 triplet like the 496.3-cm-' band, but the former band required a higher order aluminum dependence than the latter band. The isotopic ratios of the two bands are nearly identical, and the 529.6-cm-' band disappeared on annealing. These observations are in accord with identification of the 529.6-cm-' band as A102 perturbed by another Al atom which reacts on annealing. The (A102)(Al)species is not to be confused with cyclic-A1202;the harmonic isotopic ratio is 1.0369 for the b2" and b3uvibrations of the rhombus shaped A 1 2 0 2 species, which has not been characterized. Other satellite features at 511.5 and 490.0 cm-'on the strong 496.3-cm-' A102fundamental also show similar isotopic ratios and are probably due to cyclic-A102perturbed by an adjacent molecule. Again the isotopic ratios are not appropriate for cyclic-A1202. OAIOO. The reaction of aluminum and oxygen in condensing nitrogengand the possible formation of unsymmetrical AlOO must be considered. The present pulsed-laser experiments with O2in N2 gave substantial nitrogen oxide absortions, which complicated the system. Nevertheless, a weak band was observed at 1091 cm-' corresponding to the strongest absorption assigned earlier to A100.9 The present argon matrix band at 1092.5 cm-' shows a similar quartet splitting with mixed isotopic oxygen and is probably due to the same species; however, no product bands were observed above 1225 cm-'in the present argon matrix experiments. The quartet observed here for the 1092.5Cm' band was observed from both 1602/'802 and 1602/160'80/'802 isotopic mixtures. This means that two inequivalent oxygen atoms are involved in the vibration and that two oxygen molecules supply those oxygen atoms. A preliminary experiment with A1 and O3 markedly enhanced the 1092.5-cm-' band absorbance relative to the other product bands in this region. Accordingly, the 1092.5-cm-' band is reassigned to the OAlOO species. We believe that the evidence offered earlier for AlOO in solid nitrogeng is better attributed instead to the very similar molecule OA100. AlOAIO. The 1176.3- and 1167.5-cm-' band absorbances followed the 945.4- and 9 3 9 . h - ' band abmrbamxs on deposition at all A1 and O2concentrations and in the AlZOp ablation experiments. The 945.4-1 176.3-cm-' pair d c c r d more than the 939.6-1 167.5-cm-' pair on annealing, and intensity shifted from 1167.5 cm-' to a sitesplit absorption at 1169.2 cm-I. Both band systems showed mixed isotopic oxygen components; the upper bands appeared as triplets and the lower bands as quartets. The laser power studies shown in Figure 2 indicate that the species 2 and 3 bands require more aluminum than OAlO, species 5. The 16/ 18 isotopic ratios, 1.0299 and 1.0506, respectively, are higher than values calculated for linear 0-1-0and Al-0-Al, 1.02648 and 1.045 92, respectively. Ab initio calculationspredict two strong bands38v39for linear AlOAlO at 1295 and 1025 cm-' (RHF/ DZP)39with 2 to 1 relative intensity. The calculated bands are in excellent agreement with the observed bands allowing for a scale factor of 0.9 typical for SCF calculations. The 16/18 isotopic
+
ratios for the calculated fundamentals are 1,0294 and 1,0545, which are in good agreement with the observed values; however, the upper band is predicted to be a mixed oxygen isotopic quartet and a triplet was observed. Fortunately, MF2/DZP calculations better describe the vibrational modes and give isotopic ratios of 1.0308 and 1.OS 15, respectively, in excellent agreement with the observed ratios, and predict a common frequency (hl c m - I ) for the upper band of the 27-16-27-18 and 27-18-27-16 mixed isotopic molecules, which is in accord with the observed triplet. Table I1 summarizes the calculated and observed frequencies for linear A l U A l - 0 . Note that the MP2/DZP frequencies are a few wavenumbers below the observed values. The 940-cm-l band has been assigned to AlOAlO by the Moscow group; presumably the lack of isotopic data for the 1176.3- and 1167.5-cmd' bands precluded their like assignment. The 1176.3- and 945.4-cm-' bands are assigned to AlOAlO,and the 1167.5- and 939.6-cm-' bands are assigned to the same molecule in a slightly different structure or matrix packing arrangement; the bands have slightly different isotopic ratios.40 Both are presumed to be essentially collinear molecules, but small deviations from overall linearity cannot be ruled out. The decrease of AlOAlO absorptions on annealing with growth of species 1 is probably due to oxygen atom attachment to form OAlOAlO, to be discussed below. We must reconaider the case for assignments to rhombic-A1202. The strong 496.3-cm-' band is not due to A1202as originally assigned.35 Other bands at 690,640, and 545 cm-' have been assigned by different authors to rhombic-A1202. The previous 690-cm-l (or 687-cni') band c m q m d s to the prsent 686.2an' band;the isotopic shift is not appropriate for rhombic-A1202. This band increased markedly on annealing and is due to a higher aggregate than A1202 (five or more atoms). The 64O-cm-' band was only observed on annealing in the most (2%) concentrated samples employed here, and the 542.7-cm-' band appeared here only on anmaling; thee bands are clearly due to higher aggregate species (five or more atoms). The 609.2-cm-' band was also observed after annealing, and it, likewise, is due to an aggregate species. The 16/18 isotopic ratios for the 686- and 609-cm-'band8 are appropriate for aluminum atom vibrations in UAl-0networks. It is noted that a KBr pellet containing A1203powder exhibited a strong, broad absorption centered at 620 c m - I . Where, then, does rhombic-A1202absorb? Ab initio (UHF/ DZP) predict two strong bands at 819 and 476 an-' with the latter 4X stronger; these frequencies must be scaled by 0.9, which results in predictions of 740 and 430 c m - I . No bands were observed in the 250-480-cm-' region. If, however, the strongest rhombic-A1202band falls in the 480-520-cm-' region, it could not be observed here owing to the strong AIOz band. On the other hand, higher level calculations predict a strong band in the 70O-cm-l regi011,"~which was not observed here. We must conclude that there is no solid evidence for rhombic-A1202 in this or any of the earlier matrix isolation studies. The lack of a dipole moment to augment stabilization by the matrix may be noteworthy. OAKIAK). The sharp 1211.2-cm-I species 1 band increased substantially on annealing, which is expected for A 1 2 0 3 on the basis of the obsmed growth of bo3on annealing in similar boron expdments." M i d isotopic oxygen gave a 1/2/1/1/2/l sextet, which indicates a species with two equivalent and one inequivalent oxygen atom Like ozone. The V-shapcd &O, species gave a mixed isotopic quartet, indicating lack of interaction between terminal atoms in the V-shaped structure. Ab initio calculations (SCF/ DZP)3gpredicted a very strong band in the stretching region at
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10187
Pulsed-Laser Evaporated AI Atoms with 0
RHF/ isotope 16-27-16-27-16 16-27-16-27-18 18-27-16-27-18 16-27-18-27-16 16-27-18-27-18 18-27-18-27-18
observed 1211.2 1206.1 1200.0 1187.4 1181.3 1172.9
DZP 1342 1335 1326 1319 1312 1300
scaledb
1211 1205 1197 1190 1184 1173
MP2/ DZP 1232 1228 1222 1207 1201 1193
scaledc 1211 1207 1201 1187 1181 1173
'Reference 39; similar results in ref 38. bScale factor 0.9025. 'Scale factor 0.9831. 1342 cm-' for linear OAlOAlO; scaling by 0.9 predicts a 1210-
cm-'fundamental in excellent agreement with the observed value. The observed 16/18 ratio is 1.0326; the ratio of ab initio values, 1.0323, is in very good agreement. The MP2/DZP level calculations gave a slightly better isotopic ratio, 1.0327, and agreement with the observed isotopic frequencies using scale factors to fit the natural isotopic frequency exactly (Table 111). The fit of six isotopic frequencies at the MP2/DZP level is better than 1 cm-I, which confirms the identification of linear OAIOAIO. The next strongest band (
