J. Phys. Chem. 1994,98, 6514-6521
6514
Reactions of Pulsed-Laser Evaporated Ca, Sr, and Ba Atoms with 0 2 . Infrared Spectra of the Metal Oxides, Oxide Dimers, Dioxides, and Peroxides in Solid Argon Lester Andrews,. Jason T. Yustein, Craig A. Thompson, and Rodney D. Hunt Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received: January 1 1 , 1994'
Pulsed-laser ablated Ca, Sr, and Ba atoms have been reacted with 0 2 in excess argon during condensation at 10 K. Infrared spectra and oxygen isotopic substitution show that the major products are the symmetrical OM0 metal dioxide and the rhombic metal oxide dimer. The metal oxides and cyclic M02 peroxides are minor products. In the case of calcium, activation energy provided by pulsed-laser ablation makes the reaction to give CaO, CaO2, (CaO)2, and OCaO possible. Strontium and barium are more reactive and the dioxides and peroxides increase on annealing the argon matrix to 25 K, which shows that no activation energy is necessary for the Sr and Ba atom reactions with 02.Photolysis decreased the peroxide in favor of the dioxide species. The alkaline earth metal peroxides are characterized by 0-0 stretching absorptions in the 700-cm-1 region and are distinctly different from the alkali metal superoxides, which are characterized by 0-0stretching absorptions in the 1lOO-cm-' region.
Introduction Reactions between group 1 and group 2 metal atoms with 0 2 are strikingly different. The alkali metals are open-shell species with low ionization energies, and they spontaneously undergo charge-transfer reactions to give alkali superoxide M+O2- species.l.2 On the other hand, alkaline earth metal atoms are closedshell species with higher ionizationenergies,and there is a barrier along the entrance channel of the potential energy surface for the reaction with oxygen.3~4Accordingly, the alkaline earth metal atom reactions with 0 2 are much slower than the alkali metal reactions, but stable alkalineearth metal dioxidespecies do exist.s7 One interesting aspect of the alkaline dioxides is that three basic electronic configurations are possible: linear (3Zg-) and cyclic (3A2 and IAl), which qualitatively consist of (O-)(M2+)(O-) dioxide, M+02-superoxide, and M2+022-peroxide configurations, re~pectively.~The most stable form for Be and Mg is the linear dioxide molecule based on recent calculations7and infraredspectra from pulsed-laser evaporated atom reactionsin a condensingargon stream.8~9 However, CaO2 and Sr02 are predicted to have 'AI ground statesU7Product molecules have been observed in nitrogen matrix reactions.5 These molecules, however, were thought to be superoxidesM+02-(3A2),followingthe example of the alkali metal species,l.2-5 but the theoretical model predicts a more peroxidelike M2+022-(1Al) specie^,^ and the role of the nitrogen matrix in stabilizing these products is not clear. The Ca 0 2 reaction has been studied in the gas phase using He and N2 carrier gases4 and in condensing argon and nitrogen streams.5-10 The gas-phase reaction is 3 times faster in a nitrogen carrier gas, and a Ca02 product species was identified in condensing nitrogen but not in condensing argon.495-10 Similar investigations with Sr and Ba and 0 2 gave more intense and varied products for these more reactive metals in nitrogen than in argon matrix reactions.5 Pulsed-laser evaporation has proven to be an effective method of generating metal atoms with excess kinetic energies, which can undergo reactionsthat require substantial activationenergies. Aluminum, gallium, Be, and Mg are cases in point.8.9Jl.12 It was decided to investigate pulsed-laser evaporated Ca, Sr, and Ba atom reactions with 0 2 in a condensing argon stream to explore the electronic configuration of the metal dioxide product species
+
*Abstract published in Advance ACS Absrracrs, June 1, 1994.
0022-3654f 94f 2098-6514304.50f 0
through matrix infrared spectroscopy. In addition, metal monoxide products were also observed in solid argon.
Experimental Section The apparatus for pulsed-laser ablation of metal atoms into a condensing argon stream for matrix infrared spectroscopic studies has been described in previous r e p ~ r t s . ~ l -Metal l ~ targets (Alfa Inorganics) were cut, epoxy-glued to a l/4-in. X 20 nut, polished, attached to a rotatable l/4-in. stainless steel rod, and positioned in the vacuum chamber. A similar target was pressed (10 tons) from CaO powder (Baker, reagent). Target material wasablated by the pulsed Nd:YAG fundamental (20-60 mJ/pulse) into an argonstreamcontainingoxygen(1%,0.5%,0.33%) andcondensed at 1 1 f 1 K for 2-3-h periods. Fourier-transforminfrared spectra were recorded on a Nicolet 750 or 60SXR at 0.5-cm-1 resolution down to 420 cm-I and a Nicolet 5DXB at 2.0-cm-1 resolution down to 300 cm-l. Reported frequencies are from the higher resolutionspectra and are accurate to fO.l cm-I. Matrix samples were annealed from 11 f 1 K to 25,30,35,40, or 45 K, and more spectra were recorded. Selected samples were photolyzed by a 175-W mercury street lamp (Philips H39 KB) with the globe removed. ReSultS
Infrared spectra from the Ca, Sr, Ba, and 02 systems will be presented. Complementaryreactions with N2O are also included. Ca + 0 2 . A series of pulsed-laser ablated Ca experiments were done at three laser energies in the 20-60 mJ/pulse range with 1% and 0.5% 0 2 in argon. Typical spectra are shown in Figure 1 from 1050to450cm-I. Thecodepositedsamplespectrum (a) isdominatedbyasharpnew bandwith peaksat 516.3 (labeled 4) and 515.7 cm-1 (labeled 1). Sharp weak bands at 1039.4, 1033.0 (03), 953.8 (04-), and 804.0 cm-1(03-) arecharacteristic of oxygen ~ystems.15-1~Sharp new bands were also observed at 798.9, 783.3 (labeled 2), 746.7 (labeled 3), 736.2 (labeled 6), 610.7 (labeled 5), and 584.6 cm-1 (labeled 4). Photolysis, spectrum b, almost doubled the 515.7-cm-1 band but decreased the 516.3-cm-1 shoulder and the 584.6-cm-l band, decreased the 746.7- and 736.2-cm-' absorptions, and increased the 798.9-, 783.3-, and 610.7-cm-1 bands. Annealing to 25 K decreased the 5 15.7- and 610.7-cm-I bands, markedly increased the 798.4-cm-I band, and produced a broad 635-cm-1 band, as shown in spectrum 0 1994 American Chemical Society
Reactions of Ca, Sr, and Ba with
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6515
0 2
ln
1'
I
1
m
N
P
a
1050
975
400
825
$50
675
600
w 650
525
WAVENUMBER
Figure 1. Infrared spectra in the 1050-450-~m-~ region for pulsed-laser evaporated Ca atoms reacted with oxygen moleculesand trapped in excess argon at 10 K (a) Ar/O2 = 100/1 sample codeposited with Ca atoms for 3 h, (b) after full arc photolysis for 30 min, and (c) after annealing to 25 K.
TABLE 1: Product Absorptions (cm-l) Observed from the Reaction of Pulsed-Laser Ablated Calcium Atoms and Oxygen Molecules Ca + 1 6 0 2 Ca + I 8 0 2 ratio 16/18 798.9 783.3 746.7 736.2 635 610.7 59 1 584.6 576 5 16.3 515.7 509.1 484.4 435 371
P/Aa
identification (2)CaO3 (2)CaO3 (3)CaO (6) CaO2 peroxideb (O,)(CaO) aggregate (5) (CaOCaO) CaxOzy (4) (CaO)z (&h)' Cax02y (4) (Ca0)2 (&h) (1) OCaO dioxided OCaO site
753.9 740.2 716.7 698.8 610 586.5
1.05969 1.05822 1.041 86 1.053 52 1.041 0 1.041 26
+/+ +/ -/ -/ +/+ +/-
561.2
1.041 70
-/
495.8 499.0
1.041 35 1.033 47
-/
42 1 365
1.033 3 1.016 4
/+ (CaO2) aggregate +/+ CaxOyaggregate
+/-
?
a Photolysis/annealing behavior summarized by + (increase) and (decrease). b Nitrogen matrix counterparts 742.1, 555.7 cm-I for 1 6 0 2 and 702.5, 536.8 cm-I for l 8 0 2 . Nitrogen matrix counterparts 559.4 cm-1 for 1 6 0 2 and 536.6 cm-I for 1 8 0 2 . d Nitrogen matrix counterparts 496.9, 484.7 cm-1 for 1 6 0 2 and 481.2, 469.7 cm-I for 1802.
c of Figure 1. Increasing the laser power in different experiments increased the 610.7-, 584.6-, and 5 16.3-cm-I bands relative to the bands labeled 1, 2, 3, and 6. Experiments with I 8 0 2 gave the isotopiccounterparts listed in Table 1. Photolysis and annealing behavior and changing laser power assured the correct band association. Studies were done with l 6 0 2 / 1 8 0 2 mixtures; doublet absorptions with pure isotopic values reported in Table 1 were observed for bands 1, 3,4, and 6, but bands 2 became quartets at 783.3, 749.7, and 740.2 cm-I on photolysis and at 797.5, 789.0, 763.5, and 754.0 cm-I on annealing. Similar experiments with statistical 1602/160180/ 1802 produced a sharp 1:2:1 triplet at 515.7, 508.3, and 499.0 cm-I with 1.6-cm-1 full widths at half maximum. The bands labeled 4 gave 1:2:1 triplets with intermediate components at 573.6 and 503.2 cm-I. The band labeled 6 exhibited a 736.2, 721.0, 698.8 cm-I triplet. Figure 2 illustrates the spectrum of such a sample after deposition and after ultraviolet photolysis. The bands labeled 2 became 1:2:1:1:2:1 sextets at 783.5, 774.0, 765.0, 759.0, 749.8, and 740.0 cm-I on photolysis and 797.6, 789.0, 779.0, 773.0, 763.8, and 754.2 cm-I (partially resolved) on annealing. Ca N20. Pulsed-laser evaporated Ca and 1%N20 in argon were codeposited. The major product was a sharp 746.7,742.3 cm-1 doublet (A = 0.03) and a sharp 516.3, 515.7 cm-1 doublet (A = 0.02). Annealing to 25 K decreased these bands in favor of a broad 813-cm-I band.
+
CaO Pellets. Calcium oxide pellets were employed in two investigations. The first trapped the ablated vapor species in a condensing argon stream, and the spectrum shown in Figure 3a reveals product bands at 747.0, 592.2, 584.5, and 515.5 cm-1 (labeled 3, 4, and 1, respectively) and 371 cm-1. The second experiment trapped the ablated vapor species in a 2% 0 2 argon stream: and the spectrum in Figure 3b shows the same 3,4, and 1 band:; plus a weak 2 band at 797 cm-1. Annealing to 25 K reduced the former bands and markedly increased the 797-cm-1 and 0 3 bands as shown in Figure 3c. Sr + 0 2 . Pulsed-laser experiments were performed with strontium and natural isotopic oxygen, and a representative spectrum is shown in Figure 4a. The spectrum was dominated by strong, sharp new bands at 532.4 cm-I (labeled 1) and at 530.1, 529.1, and 441.3 cm-I (labeled 4). Stepwise annealing from 25 to 45 K in 5 K intervals revealed the gradual appearance of a strong broad 789-cm-l band, gradual decrease of the 652.8cm-I band, increase of the sharp 532.4-cm-l band to 35 K and then decrease, and progressive decrease of the 530.1- and 441.3cm-I bands as summarized in Table 2. In an Ar/O2 = 100/1 experiment the sample was irradiated by the full light of the medium-pressure mercury arc, and the 6 bands were destroyed, the 3 band was decreased, the 1 band was doubled, and the 4 bands were decreased.. A different experiment using Ar/02 = 300/ 1 sample gave slightly weaker product absorptions,as shown in Figure Sa. Annealing to 25 K doubled the 6 absorptions, increased the 1 band by 50%, and produced a strong new 2 band (Figure 5b). A subsequent photolysis decreased 2, destroyed 6, decreased 3, slightly increased 1, and slightly decreased 4 absorptions (Figure 5c). Comparison of these two experiments run at constant low Sr concentration revealed that the 1, 3, and 6 bands were halved at 300/1 whereas the 4 bands reduced by only 20%. Similar experimentswith 1 8 0 2 isotopicsamplesgave the product band measurements compared in Figure 4c and Table 2. An experimentwithAr/l6O2/l8O2 = 150/1/1 showed sharpdoublets for the 1, 3, 4, and 6 bands. A final experiment with an Ar/ 1 6 0 2 / 1 6 0 1 8 0 / 1 8 0 2 = 800/1/2/1 samplegave thespectrumshown in Figure 4b. The important multiplets to note are a triplet at 729.9,710.2,689.3 cm-I for the 6 band, a doublet at 6523,621.5 cm-1 for the 3 band, and a triplet for the 1 band at 532.4, 522.4, 510.4 cm-1, triplets for the major 4 band sites at 530.1, 518.1, 504.8 cm-1 and at 441.3,428.1,419.9 cm-1, and part of a triplet for the lower 6 band at 500 (shoulder) and 488.0 cm-1. The 5 band exhibited a broad 596-589-cm-I intermediate component. After annealing a broad sextet was observed for the 2 band. In order to complement the pulsed-laser experiments with Sr atoms, a thermal Sr atom codepositionswas done with an Ar/02 = 100/ 1 sample. The spectrum was dominated by a strong band at 479.8 cm-I with a weaker (10% as strong) satellite band at 532.8 cm-1 and medium-intensity bands at 459 and 361 cm-1. The experiments reported previouslys also contained a weak 532cm-I absorption and a medium-intensity 370, 362 cm-I band. Sr + N20. An experiment with 1%N2O in argon gave strong site split multiplets at 658.2, 652.8, and 648.3 cm-1 for species 3, 532.4 cm-I (species 1), and stronger 528.0- and 440.4-cm-I (species 4) bands. Annealing to 25 K increased the 1 band and produced a broad 790-cm-I band while the species 3 and 4 bands decreased. Further annealing to 35 K decreased species 1,3, and 4 bands and increased the 790-cm-1 feature. Ba + 0 2 . The data from pulsed-laser evaporated Ba and oxygen experiments are collected in Table 3. Figure 6 compares oxygen isotopic spectra. The l a 0 2 reaction (Figure 6a) gave the 570.2cm-I band (labeled 1) observed previously,s but other bands at 634.2 (labeled 3), 501.0, and 401.4 cm-1 (labeled 4) were much stronger in the pulsed-laser than in the thermal Ba atom experiments. Weak bands were also observed at 754.5 and 468.3 cm-1 (labeled 6) and 460.0 cm-l (the latter was strong in ozone
6516 The Journal of Physical Chemistry, Vol. 98, No. 26, 1994
Andrews et al.
3 n
1
om
-
4
I
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4
6
-1
1
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B50
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I
700
'
,
an -
Wavmumbn ("1)
I 560
'
'
'
1
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7
8
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450
-
Figure 2. Infrared spectra in the 820480-cm-I region for Ca atoms reacted with 1% 1 6 0 2 / 1 6 J E 0 2 / 1 8 0 2 in argon: (a) after sample deposition for 2 h, (b) after h > 290 nm photolysis for 30 min, and (c) after full arc photolysis for 30 min. n
a
-I
I
11OD
1000
-0
.OO
700
I*YLNW.L".
000
-0
400
800
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Figure 3. Infrared spectra in the 1100-300-cm-~region for vapor species from pulsed-laser evaporated CaO pellet: (a) CaO evaporated into argon for 2 h, (b) CaO evaporated into Ar/Oz = 50/1 for 2.5 h, and (c) after annealing to 25 K. experiments).lO Photolysis doubled the strong 1 band, destroyed the 6 bands, halved the 3 band, and decreased the 4 bands. In different experiments, the samples were annealed to 1 5 2 0 , and 25 K; the sharp 1 and 6 bands increased. In the series of Ba experiments, the initial depositedsamplesshowed constant relative intensities for the 1,3, and 6 bands and more or less of the 4 bands depending on Ba concentration. An experiment with 1 8 0 2 revealed isotopic shifts for all product bands as depicted in Figure 6c; again, photolysis doubled the 1 band and annealing produced a broad 748-cm-1 band. A similar experiment with scrambled 1602, 160180,and l a 0 2 shown in Figure 6bshowsa triplet at 754.5,733.8,and712.2cm-l for the6 bands, a doublet a t 634.2 and 602.0 cm-1 for the 3 bands, a sharp triplet at 570.2, 557.8, and 544.3 cm-1 for the 1 bands (fwhm = 2.0 cm-I), triplets for the 4 bands with central components at 488.7 and 390.0 cm-1, and triplet a t 468.3, 459.2, and 446.7 cm-1 for the lower 6 bands. The 1118-cm-I band became a weak sextet, which was destroyed on photolysis that doubled the 1 triplet.18
Figure 4. Infrared spectra in the 820-420-cm-I region for pulsed-laser evaporated Sr atoms reacted with oxygen molecules in excess argon: (a) Ar/l602 = 200/1 sample deposited with Sr atoms for 2 h, (b) Ar/ 1 6 * L E 0 2= 200/1 sample deposited with Sr atoms for 3 h, and (c) Ar/ I E 0 2 = 200/1 codeposited with Sr atoms for 3 h. This band is seen in other oxygen systems and is due to Oq+ in solid argon.18 An additional experiment with scrambled isotopic oxygen and 0.5-cm-1 resolution gave sharper spectra than in Figure 6b. This sample wassubjected to470-1000-nmfiltered photolysis, and the 1 and 6 triplets increased 5%. Irradiation at 290-1000 nm further increased the 1 triplet by 20% and decreased the 6 triplets by 75%. A full-arc irradiation destroyed the 6 triplets and decreased the 1 triplet by 5%.
Discussion The new product species will be identified, and comparisons to thermal experiments will be made for insight into the reactivity ofCa,Sr,andBaatomswith02. Structuresofthedioxideproduct species, reaction mechanisms, and bonding trends will be considered. O M 0 and MO2. The strong sharp bands labeled 1 in Ca, Sr, and Ba experiments a t 515.7, 532.4, and 570.2 cm-1 dominate each experiment and grow approximately 2-fold on photolysis. The Sr and Ba products increased on annealing to 25 K, but the
Reactions of Ca, Sr, and Ba with 0
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6517
2
TABLE 2 Product Absorptions (em-') Observed from the Reaction of Pulsed-Laser Evaporated Strontium Atoms and Oxygen Molecules Sr + 1602 Sr + 1 8 0 2 ratio 16/18 P/Aa identification 789 729.9 658.6 652.8 605.1 602.5 532.4 530.1 529.1 509.2 441.3 370
745 689.3 627.1 621.5 574.0 571.7 510.4 504.8 503.9 488.0 419.9 352
1.059 1 1.058 90 1.050 23 1.050 36 1.054 18 1.053 87 1.043 10 1.050 19 1.050 00 1.043 44 1.050 96 1.051 1
-/+ -/+ -1-1-
I+
/+ +/+
-1-1-
-/+ -1/+
(2) sa3 (6) SrO2 peroxide (3) SrO site (3) SrO (5) Sr,Oy aggregate (5) site (1) OSrO dioxide (4) (SrO)z (h) (4) (site) (6) SrO2 peroxide (4) (Sr0)2 ( D u )
Sr,O, aggregate 0 Photolysis/annealing behavior summarized by + (increase) and (decrease).
8 8
1
6
9
(C)
d '
I
.
4
o J : WZS
700
s7s
WAVLWH(ILRS
4SO
3ZS
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Figure 6. Infrared spectra in the 825-325-cm-1 region for pulsed-laser
evaporated Ba atoms reacted with oxygen moleculesand trapped in excess argon at 10 K: (a) Arlo2 = 100/1 sample deposited with Ba atoms for 2 h, (b) Ar/1691802= 100/1 sample codeposited with Ba atoms for 3 h, and (c) Ar/I802 = 100/1 sample codeposited with Ba atoms for 3 h.
w."bn
Isml)
Figure 5. Infrared spectra in the 900-300-~m-~ region for pulsed-laser
evaporated Sr atoms reacted with oxygen moleculesin excess argon: (a) Arlo2 = 30011 sample deposited with Sr atoms for 2 h, (b) after annealing to 25 K, and (c) after broad-band photolysis for 30 min.
TABLE 3: Product Absorptions (cm-I) Observed from the Reaction of Pulsed-Laser Ablated Barium Atoms and Oxygen Molecules
Ba + 1 6 0 2
Ba + 1 8 0 2
ratio 16/18
P/AO
1118 803.9 792 754.5 634.2 570.2 501.0 468.3 460.0 422 401.4
1054 759.1 748 712.2 602.0 544.3 475.8 446.7 437.5 40 1 38 1.2
1.060 7 1.059 02 1.059 1.059 39 1.053 49 1.047 58 1.052 96 1.048 35 1.051 43 1.052 4 1.052 99
-/ -/ /+ -/+ -/ +/+ -1 -/+ -1 +/ -/
identification (04+)
BaO3 BaO3 Ba02 peroxide BaO OBaO dioxide (BaO)z ( D u ) Ba02 peroxide BaOBaO Ba,O, aggregate (4) (BaO)2 (&h) 0 Photolysis/annealing behavior summarized by + (increase) and (decrease). (2) (2) (6) (3) (1) (4) (6)
Ca product decreased. These bands form sharp 1:2:1 triplets with 1602/16J802/"J02 reagent, which indicates a product species with two equivalent oxygen atoms. The strong bands labeled 1 are clearly due to the major primary M + 0 2 reaction product. The 570-cm-1 band has been assigned by previous workers to the v2 (symmetric M-O stretching) mode of Ba02 with cyclic superoxide configuration in solid a r g ~ n . ~However, J~ no associated superoxide stretching mode was observed here, which raises doubt about the superoxide character of BaO2. The strong 570.2cm-1 band doubled on photolysis, and no other absorptions followed this behavior. The new 532.4- and 5 15.7-cm-1 bands are assigned to analogous Sr02 and Ca02 species.
These assignments to CaO2 and SrO2 differ from previous nitrogen matrix reaction products,S which probably are CaOz and Sr02 strongly interacting with N2, hence the need for the noble gas environment. The strong argon matrix band a t 48 1 cm-1 assigned to SrO2 in retrospect must be due to a Sr,OZ,, cluster species. Note that a small yield of Sr02 (532.4 cm-1) was formed with thermal S r atoms. The 593- and 582-cm-1 bands tentatively assigned to CaO2 from the Ca O3reaction in argonlo are probably due to a Ca,OZy cluster species. Ab initio CCSD(T) calculations predict the ground-state C a 0 2 molecule to have the 1Al cyclic peroxide configuration with a 45.7O 0-Ca-0 apex angle and strong infrared absorptions at 720 cm-' (al, 121 km/mol), 592 cm-l (al, 88 km/mol), and 512 (bl, 202 km/mol).' The 515.7/499.0 = 1.033 47 isotopic ratio is substantially lower than the harmonic Ca-0 diatomic ratio (1.042 29), but not as low as the harmonic ratio expected for linear 0-Ca-O (1.032 43). This band could in principle be due to the v2 (al) mode of cyclic CaO2. However, the predicted stronger V I (al) and v3 (bl) bands were not observed in these experiments, and the calculated harmonic 16/18 ratio for v2 of cyclic Ca02 is predicted a t 1.034 87, which is slightly higher than the observed ratio. Theoxygen isotopic ratios observed for theSrO2 (532.4/510.4 = 1.043 10) and BaOz (570.2/544.3 = 1.047 58) molecules are even smaller than harmonic ratios predicted for the linear molecules (1.043 45 for 0 S r - O and 1.048 45 for 0-Ba-O) which have the minimum 16/18 ratio for a harmonic M-O vibration in a metal oxide molecule. The v2 (a,) modes of cyclic SrOl and Ba02 are predicted to have even higher 16/18 ratios (1.045 15 for Sr02 with the calculated' 42.6O apex angle and 1.049 45 for BaOZ with an estimated 3 6 . 9 O apex angle) and to be higher still for ~2interactingwithvl(al). Theobserved 16/18 ratiosquestion cyclic structures for CaOz, SrOz, and BaOz and open the possibility for bent-linear open structures. On a different track, if CaO2 is presumed to be an obtuse bent molecule and the strong 5 15.7-cm-1band is v3, the antisymmetric Ca-0 stretching mode, the harmonic 16/18 ratio is calculated to be 1.033 47 for a 150' O-Ca-O angle. Agreement with the observed 16/ 18 ratio provides strong support for a bent molecule with an angle near 150°. Valence angle calculations for C,
+
6518 The Journal of Physical Chemistry, Vol. 98, No. 26, 1994
molecules from isotopic frequencies using terminal substitution predict an upper limit to the bond angle,20-21 so the 150' value from matrix infrared data is an upper limit and the true valence angle is slightlysmaller than 150° dependingupon anharmonicity. Electronic structure calculations at the CASSCF level have been performed for bent triplet O-Ca-0.22 Three open bent states were found with similar energies and similar frequencies. The lowest bent triplet state (3B2, 0-Ca-0 angle 140') has calculated frequencies 514.5 cm-1 (bl, 229 km/mol), 444.1 cm-l (al, 39 km/mol), and 80.5 cm-I (al, 92 km/mol). Clearly these preliminary CASSCF calculations on 0-Ca-0 are in excellent agreement with the argon matrix spectrum of M a - 0 . Since this bent molecule has only a small dipole moment, matrix interaction should be minimal and the gas-phase fundamental is expected at 520 f 10 cm-I. The structural relationship between OBeO, OMgO, and OCaO parallels the structures of the difluorides. The BeF2 and MgF2 molecules are linear whereas CaF2 is bent with a 140 f 5' angle,23-27 which has been explained by the involvement of d orbitals on the metal in the bonding to fluorine. On this basis the OSrO and OBaO molecules may be bent followingthe example of SrF2 and BaF2. The observed anharmonic 16/ 18 ratios for the major product OSrO and OBaO dioxide molecules are slightly lower than 16/18 ratios calculated for the harmonic linear molecules. This small discrepancy can only be accounted for by anharmonicity in the u3 vibration. Unfortunately,this discrepancy cannot be precisely apportioned between anharmonicity and apex angle dependence so it is not possible to determine the apex angles for OSrO and OBaO from the infrared spectra. It is concluded that OSrO and OBaO are (triplet) dioxide molecules with linear or nearly linear structures. The Ca, Sr, and Ba experiments reveal two product species with the MO2 stoichiometry,which give rise to the 1 and 6 bands. Both species exhibit triplet mixed isotopic absorptions for two equivalent oxygen atoms. The 1 and 6 bands track with the 3 bands (to be assigned to MO) and not the 4 bands (to be assigned to (MO)2) on changes in metal concentration; hence, species 1 and 6 contain a single metal atom. The 1 and 6 bands show opposite photolysis behavior-the 1 bands increase, and the 6 bands are ultimately destroyed by photolysis. The spectra of species 6 are strikingly similar to the spectrum calculated' for thecyclic 'A1 CaOzperoxidestate that isdescribed above. Theupperbandshowsalarge 16/18ratio(l.O53to 1.059), which characterizes an almost pure 0-0 stretching mode, and appears in the peroxide anion stretching region.28 The lower 6 bands for Sr and Ba at 509.2 and 468.3 cm-I, respectively, are about 5-fold stronger than the upper 6 bands and exhibit 16/18 ratios that are comparable to the strong 1 bands, which indicate substantial metal participation in the M-02 stretching mode, and are appropriate for symmetric M-02 stretching modes; however, no bands were observed for an antisymmetric M-02 motion. Thespecies6 band (752.1 cm-1) wasenhanced in thermal Ba reactions in nitrogen relative to species 1 (540.6 cm-I), and species 6 bands (730.5 and 474.0 cm-I, present reassignment) were strong in Sr O2/N2 experiments,s which are expected to stabilize polar product molecules. No lower frequency band was observed for species 6 with Ca in argon probably due to masking by the very strong 1 band. However, the present nitrogen matrix experiments associated 742.1- and 555.7-cm-l bands on photolysis and triplet mixed isotopic multiplets, and these bands can be assigned to species 6 in solid nitrogen. Recall the enhanced reactivity of Ca in nitrogen carrier gas.4 Species 6 bands are therefore identified as the cyclic peroxide ('AI) CaO2, SrO2, and BaO2 molecules. Note that for all three metals photolysis decreases the peroxide in favor of the dioxide. MO. The alkaline earth monoxides have been of considerable interest in part owing to the identificationof the ground electronic state.29 In the case of SrO and BaO, molecular beam electric
+
Andrews et al.
TABLE 4 Vibrational Fuadameotals (cm-1) of Alkaline Earth Metal Oxides in the Gas Phase and Solid Argon 16/18
molecule Be0 MgO CaO SrO BaO
gad 1463.7 174.7 722.4 645.5 665.8
argon 1526.lC 825Sd 746.7 652.8 634.2
shiftb 62.4 50.8 24.3 7.3 -31.6
obs 1.01896 1.041 86 1.05036 1.05349
harmonic 1.02069 1.035 17 1.042 29 1.05070 1.05392
a Reference 33. Shift = argon - gas. Reference 9. Reference 8, observed from NzO reaction and N2180 not available.
resonance experiments established the ground state as X1Z+.30.31 In the case of CaO, matrix experiments5 helped to identify the ground state as 1 9 by matching matrix fundamentals to states characterized from gas-phase emission spectra. Recent diode laser investigations gave 722.40 cm-I for the infrared spectrum of gaseous CaO in the ground state, the only infrared spectrum for a gaseous alkaline earth monoxide.32 The CaO, SrO, and BaO molecules have been studied by electronic spectroscopy in the gas phase33and infrared spectroscopy in solid nitrogen," which supported the thermal metal atomozone reaction. Barium is the most reactive of the alkaline earth metal atoms, and the Ba-03 reaction proceeded in argon carrier gas to give a 634.3-cm-1 fundamental for BaO in solid argon, but no evidence for SrO or CaO was found in the analogous reactions.IO The sharp 746.7-cm-l band (labeled 3) was observed in Ca reactions with 0 2 and N20 and from the ablation of a solid CaO pellet. The observation of a sharp doublet absorption with scrambled isotopicoxygen shows that one oxygen atom is involved inthisvibration. Theobserved 16/18ratio(l.041 86) isO.O41% less than the harmonic diatomic value, which is appropriate for cubic contributions to anharmonicity. If the harmonic and anharmonic constants33for Ca160 are corrected for CaI80, the calculated anharmonic 16/18 ratio is 1.041 67, very near the observed matrix value. This agreement confirms assignment of the 746.7-cm-1 band to CaO in solid argon. Note that solid argon blue shifts the CaO fundamental by 24.3 cm-I. The sharp 652.8-cm-1 band was observed from the Sr reaction with 0 2 and N2O. The doublet character with statistical "5.1*02 and 16/18ratio 1.050 36justO.O32%belowtheharmonicdiatomic ratio verify assignment to SrO in solid argon. Note that solid argon blue shifts the SrO fundamental by 7.3 cm-I. The sharp 634.2-cm-1 band has already been assigned to BaO in solid argon,IOJ5 and the present results stand in agreement. The observed 16/ 18 ratio 1.053 49 is 0.043% below the harmonic diatomicvalue. Theargon matrixredshifts the BaOfundamental 3 1.6 cm-1. This is thedirection of shift expected for ionic molecules like alkali metal fluorides. The gas-to-solid argon shifts for alkaline earth metal oxides are summarized in Table 4. The large blue shift for Be0 has been rationalized by formation of the ArBeO c ~ m p l e x .Note ~ the observed 16/18 ratio is 0.169% below the harmonic value, which is much more than can be expected for anharmonicity. This 16/18 ratio is indicative of Be vibrating with another atom, i.e., Ar in the ArBeO complex. Why then does the CaO molecule give a blue matrix shift when the isoelectronic KF molecule36 experiences a red shift (429 to 397 cm-I)? The CaO molecule is smaller (r = 1.82 A) than KF (r = 2.17 A),33 and CaO fits into a single argon atom vacancy (d = 3.76 A). The SrO molecule (r = 1.92 A) likewise can fit tightly into a substitutional site in the argon lattice. Although the BaO molecule is only a little larger (r = 1.94 A), the increase in polarizability of Ba cation as compared to that of Sr cation and reflected in the relative ionic character of SrO and BaO30.31 leads to an attractive interaction with argon rather than repulsive. Furthermore, the BaO molecule may occupy a site replacing two argon atoms in the lattice. MOJ. The bands labeled 2 at 798.9 cm-l for Ca, 789.4 cm-1 for Sr, and 792 cm-l for Ba are near bands assigned previously
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6519
Reactions of Ca, Sr, and Ba with 02
to the alkaline earth metal ozonide species M+O3-.I0 The mixed oxygen isotopic sextets confirm the M 0 3 assignment. The 16/ 18 isotopic ratios range from 1.0597 to 1.0582, which verifies the mostly 0-0 character of the vibrational mode. This vibration is a perturbed v3 mode for the ozonide anion. The proximity to alkali metal 0zonides3~also supports this characterization. (M0)2. The bands labeled 4 are all characterized by 16/18 ratios near that for the MO diatomic molecule. This observation suggests (MO)2 species, particularly the rhombic ring. In the Ca experiments, increasing the laser power increased the 4 bands relative to the CaO and OCaO bands, which suggests a molecule containing two or more Ca atoms. Furthermore, Ca gave two sharp bands at 584.6 and 516.3 cm-1, which decreased together on photolysis. The broader 591- and 576-cm-1 satellite bands were observed in earlier Ca/03 and Ca/O2 experiments, and CaO pellet ablation gave a 592-cm-1absorption: these bands are probably due to the Ca,02y cluster species. The 635-cm-l band observed in theca + 0 3 reaction10and assigned to (CaO)2 appears to exhibit a higher multiplet in 1 6 J 8 0 2 experimentsand is probably due to a (02)(Ca0) aggregate. Such a species was observed in Mg O3reactions.6 The isotopic ratios for the sharp 584.7- and 516.3-cm-1 bands (Table 1) are appropriate for assignment to the rhombic (CaO), dimer. In the Be/O2 system both rhombic and linear dimers have been characterized, but in the Mg/O2 work only the linear dimer was observed.8.9 The rhombic dimer is characterized by strong v5(bzu)and vs(bau)motions.38 If the 584.6-cm-l band were due to us and the 516.3-cm-1 band to v6, the Ca-O-Ca angle can be determined as 83O (and the 0-Ca-O angle as 97O) using the ratio 584.6/516.3 = 1.1323 and the previously described relationship.38 The best present explanation for the (CaO)2 species is to suggest assignment of the 610.7-cm-l band as linear CaOCaO dimer since this band grows at the expense of rhombic (CaO), absorptionson photolysis. The similar AlOAlO species also gave a mixed isotopic triplet as inequivalence in the oxygen atoms was not resolved.ll The strontium4xygen system is more reactive than the calcium system, and two strong bands labeled 4 were observed at 530.1 and 441.3 cm-1. Figure4 clearlyshows that the 532.4-cm-1 species 1 and 530.1-cm-1 bands exhibit different oxygen-18 shifts. The 530.1- and 441.3-cm-1 bands track together on annealing in 0 2 , and N 2 0experiments and are assigned to rhombic (SrO)2. The 16/18 isotopic ratios (Table 2) are near that of SrO itself as expected. The ratio 530.1/441.3 = 1.2012 gives a measure of one of the included angles depending upon the mode de~cription.3~ If the 530.1-cm-1 band is assigned to v5(bzu) and defined as 0 moving perpendicular to the S r S r axis and the 441.3-cm-1 band assigned to vs(ba) and defined as 0 moving parallel to the S r S r axis, then the S r - O S r angle is determined as 80° (and the 0 S r - O angle as looo). The barium-oxygen system is also reactive, and the strong 501.2- and 401.4-cm-1 bands observed here are the same as previously assigned to (Ba0)2.5J0935For analogous assignment as described above for (SO),, the vs(b2,,) band at 501.2 cm-1 and the v6(b3J band at 401.4 cm-I, the Ba-O-Ba angle is determined as 78O (and the 0-Ba-0 angle as 1029. The 460.0-cm-1 band was observed previously and assigned to BaO dimer.sJO.35 The previous mixed oxygen isotopic data are consistent with a linear BaOBaO dimer, which is less stable on photolysis than therhombic (Ba0)z species in contrast to the CaOCaO species. Reaction Mechanisms. The central theme here is that pulsedlaser ablation imparts excess kinetic energy to the evaporated atoms39 and that this excess kinetic energy activates reactions of these atoms. The primary products of the Ca + 02 reaction are CaO and OCaO, reactions 1 and 2a. Reaction 1 is endothermic
+
Ca
+ 0,
+
CaO
+0
(1)
Ca + 0, Ca
-
+ 0,
OCaO (dioxide)
(24
CaO, (peroxide)
(2b)
by 23 kcal/mol using experimental dissociation energies33smand requires an activation energy of at least this amount, and the pulsed-laser ablation method is more than equal to the task.39 Reaction 2 also has an activation energy barrier: which is consistent with the failure to observe reaction 2 on annealing or with thermal Ca atoms and argon carrier gas.10 Broad-band photolysis does activate reaction 2 presumably by electronic excitation of calcium atoms. Reaction 2 is expected to be exothermic and the calculated 40 kcal/mol value for 2a7 is reasonable. The dimerization reactions 3 and 4 are exothermic, and these reactions proceed readily. Reaction 3 is observed on annealing of the matrix samples. The secondary reaction 5 is also expected to be exothermic and to proceed under matrix deposition conditions. CaO
+ 0,
CaO + CaO Ca
+ OCaO
-
CaO,
(3)
(CaO),
(4)
(CaO),
(5)
In the case of strontium, the primary reaction 6 is still endothermic (8 f 4 kcal/nlol)," and pulsed-laser ablation provides the activation energy. Reaction 7a proceeds to a small degree on reaction with thermal atoms and reactions 7a and 7b form products during annealing the matrix to 25 K hence, nearly zero activation energy is required for reactions 7.
Sr + 0,- SrO
+ 0, Sr + 0, Sr
-
+0
(6)
OSrO (dioxide)
(7a)
SrO, (peroxide)
(7b)
The primary reactions with Ba and 02 proceed without activation energy, as BaO is observed in the gas phase40941 and in the matrix reaction with thermal atoms.s Furthermore, OBaO and Ba02 are observed with thermal atomssJ9 and increase on annealing matrix samples. Reaction 8 is exothermic by 15 kcal/ mol?7 and reactions 9 must be substantially more exothermic. It is perhaps surprising that reactions 7 and 9 proceed without activation energy. However, the analogous U atom insertion reaction also proceeds in a cold matrix on annealing.42 Ba
+ 0,
-
+ 0, Ba + 0, Ba
+
BaO
+0
(8)
OBaO (dioxide)
(94
BaO, (peroxide)
(9b)
The photolysis behavior also merits comment. Full arc (A > 240 nm) photolysis doubled the OMOdioxide bands for all metals if performed on the freshly deposited sample; however, after annealing to 25 K, photolysis produced no growth in OCaO and only slight growth of OSrO and OBaO. Furthermore, photolysis A > 380 nm for Ca and A > 470 nm for Ba into the strong atomic resonance lines43344 had little effect. Irradiation of isolated metal atoms in the deposited sample probably leads to diffusion, excitation, and subsequent reaction with an approached 02 molecule. However, annealing clusters the free atoms rendering them unavailable for subsequent reactions. Finally, what role do charged species play in these experiments? Electrons are clearly produced in the ablation process as 04- is observed with all metals. It can reasonably be expected that M+
Andrews et al.
6520 The Journal of Physical Chemistry, Vol. 98, No. 26, 1994
cations are also produced in the ablation process. The recombination of cations and electrons produces excited atoms, which could react directly with 0 2 and contribute to the product yield. Furthermore, a very recent chemiionizationstudy has shown that the formationof (M0)2+is spontaneousand particularly favorable for Ba reactions.45 It is possible that minor bands observed here could be due to metal oxide cation species. Bond* Trends. Two interestingbonding trends in the alkaline earth metal-02 molecule system merit comment. The first is the structure of the dioxide molecules0-M-0 as a function of metal. The OBeO and OMgO dioxide (triplet) moleculesare linear based on isotopic vibrational spectra and electronic structure calcul a t i o n ~ . ~ -However, ~ the OCaO dioxide (triplet) is probably slightly bent, analogousto CaF2, and the OSrO and OBaO dioxides are probably slightly bent as well, analogous to SrF2 and BaF2. We note that OCeO and OThO are nonlinear molecules.46 The second is thecompetitivestabilityof the M02(lA1)cyclicperoxide and OM0 (triplet) dioxide forms as a function of metal. Theoretical calculations predict increasing relative stability of the cyclic peroxide form with increasing metal atomic weight but do not predict the greater stability of the bent dioxide forms for the heavier metals.7 The photochemical increase of O M 0 at the expense of MO2 suggests that the dioxide is more stable. A comparisonbetween cyclic Sr02,Ba02(IA1) and RbO2, Cs02(2A2) is also of interest. The symmetric stretching fundamentals of Rb02 (1 110,255cm-1) and Cs02(1 114,236 cm-l) as compared to those of Sr02 (730,509 cm-I) and BaO2 (754,468 cm-I) reveal theionic bonding in these cyclicspecies. First, the 0-0stretching fundamentals of the former alkali metal species are in the superoxide (02-) stretchingregion and of the latter alkaline species are in the peroxide (022-) stretching region.192 Calculations show that the net charge transfer is in the 0.7e range for the alkali and 1.2erange for the alkalinespecies.7 Accordingly,the electrostatic binding is higher for the alkaline peroxide species, which results in larger dissociation energies and higher symmetric M-02 .stretching frequencies. A similar comparison in the OCaO, OSrO, and OBaO dioxide series is also revealing. Normally, frequencies decrease with increasing atomic weight in a homologous series of molecules. The u3 fundamentals for the above dioxides increase from 5 15.7 to 532.4 to 570.2 cm-I, respectively. This points to increasing bond strength from Ca to Sr to Ba as is also found for the simple monoxides.33 Since these molecules are substantially ionic ObM*Z+-Ob, it is straightforward to propose increasing ionic character with decreasing ionization energy in the series OCaO to OSrO to OBaO. What determines the relative populationsof open OBaO dioxide and cyclic BaO2 peroxide? The 570.2/468.3-cm-I band intensity ratio is 5/1 in the deposited samples (and approximately the same in thermal Ba experiments),s but photolysis increases the 570.2-cm-1 band and decreases the 468.3-cm-1 band. Annealing (without photolysis) increases the 570.2-cm-I band by 50% and the 468.3-cm-I band by 10%. The yield of the cyclic BaO2 species relative to the open OBaO species is enhanced in the N2 matrix, presumably owing to stabilization of the more polar cyclic species (note the much stronger 752.1-cm-1 band in Figure 2b of ref 5). The open OBaO and cyclic Ba02 species probably have similar energies, and the dynamics of the relaxation process affects the relative yield of the barium dioxide and peroxide species. The same can be said for CaO2 and OCaO. In a nitrogen matrix the former dominates, but photolysis destroys CaO2 and increases OCaO. There are differences in the reactivities of Ba, Sr, and Ca. Thermal experimentsSJ9with Ba and 0 2 gave a very strong 570cm-I absorption for OBaO and a weak 754-cm-1 band for BaO2, and these bands increased on annealing to 25 K, however, thermal experiments with Sr gave a weak 532-cm-l band and a strong broader 481-cm-1 band, and thermal experiments with Ca gave
only Ca,Ozy species and no OCaO. The Sr reaction with 0 2 may require a more precise geometric factor (approach geometry) which results in a slower reaction. This allows Sr to cluster during the competitive reaction. Accordingly, the thermal Sr product not observed here is probably due to a Sr,O2, species. Calcium is less reactive still, and clustering proceeds in the absence of direct reaction until the Ca, cluster size is large enough to attach 02.In the bulk metal limit, a fresh calcium metal surface reacts spontaneously with oxygen gas.
Conclusion# Pulsed-laser ablated Ca atoms react with 0 2 to give CaO, (CaO),, CaO2, and OCaO, reactions which require activation energy that is available in these hyperthermal atoms. The oxygen 16/ 18 isotopic ratio for the strong OCaO fundamental indicates an obtuse bent dioxide molecule. The reactivity of Ca, Sr, and Ba atoms increases with atomic number. The O M 0 dioxide species are major products, and the M02 peroxide species are minor products. The MO2 peroxide species corresponds to the 'Al state calculated for CaO2, and the O M 0 dioxide species are triplet obtuse bent or nearly linear molecules. The Sr and Ba reactions to give O M 0 and MO2 proceed on annealing the samples, which indicates that little or no activation energy is required for Sr and Ba reactions with 0 2 as has been found for alkali metals reacting with 0 2 . The alkali metal-oxygen reactions give superoxide products in contrast to the dioxide and peroxide products formed by alkaline earth metal atoms.
Note Added in hoof.Matrix ESR experiments performed by Knight on similarly prepared Ca/O2 and Sr/O2 samples have verified a triplet metal dioxide product but show no evidence of nonlinearity. It appears clear that triplet O M 0 dioxide molecules are stable, but more precise determination of the valence angles must be made. Acknowledgment. We appreciate support from NSF Grant 91-22566, communication of CASSCF calculations on OCaO by C. W. Bauschlicher, and help with experiments by G. V. Chertihin. J.T.Y. acknowledges support from the Howard Hughes Medical Institute through the Undergraduate Biological Sciences Educational Program. References and Notes (1) Andrews, L. J. Chem. Phys. 1%9,50, 4288.
(2) Andrews, L.; Smardzcwski, R. R. J. Chem. Phys. 1973,58,2258. (3) Plane, J. M.C. In Gas-Phase Metal Reactions, Fontijn, A,, Ed.; Elsevier: Amsterdam, 1992. (4) Nein, C.-F.; Rajasekhar, B.; Plane, J. M. C. J . Phys. Chem. 1993, 97,6449. (5) Auk, B. S.; Andrews. L. J . Chem. Phys. 1975,62,2312.The thermal Ba + 0 2 reaction in argon also gave weak absorptions at 792,754,634and 468 cm-l, and the 1802 reaction gave weak counterparts. (6) Andrews, L.; Prochaska, E. S.; Ault, B. S. J. Chem. Phys. 1978,69, 556. (7) Bauschlicher, Jr., C.W.; Partridge, H.; Sodupe, M.;Langhoff, S. R. J. Phys. Chem. 1992,96,9259. ( 8 ) Andrews. L.: Yustein. J. T. J. Phvs. Chem. 1993. 97. 12700. (9j Thompson, C.A.; Andrews, L. J.-Am. Chem. S k . 1!%34,116,423; J. Chem. Phys., to be published.
(IO) Thomas, D. M.;Andrews, L. J. Mol. Spectrosc. 1974,50, 220. (1 1) Andrews, L.; Burkholder, T. R.; Yustein, J. T. J. Phys. Chem. 1992, 96,10182. (12) Burkholder, T.R.;Yustein, J. T.; Andrews, L. J . Phys. Chem. 1992, 96,10189. (13) Burkholder, T.R.;Andrews, L. J . Chem. Phys. 1991, 95, 8679. (14) Hassenzadeh, P.;Andrews, L. J . Phys. Chem. 1992,96,9177. (15) Andrews, L.; Spiker, Jr., R. C. J. Phys. Chem. 1972,76,3208. (16) Spiker, Jr., R. C.; Andrews, L. J . Chem. Phys. 1973,59, 1851. Andrews, L.;Auk, B. S.; Grzybowski, J. M.;Allen, R. 0. J . Chem. Phys. 1975,62,2461. (17) (a) Andrews, L. J. Chem. Phys. 1971,544935,(b) Thompson, W. E.; Jacox, M . E. J . Chem. Phys. 1989,91,3826.(c) Hacaloglu, J.; Andrews, L., to be published. (18) The weak 1118-cm-1 band is due to 0 4 + ; see ref 17b. (19) Abramowitz, S.;Aquista, N. J. Res. Narl. Bur. Srand. 1971,A75,
23.
Reactions of Ca, Sr,and Ba with 02 (20) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, D. E. J. Chem. Phys. 1%9,50,3399. (21) Brabson. G. D.; Mielke, Z.; Andrews, L. J. Phys. Chem. 1991, 95, IO
I I .
(22) Bauschlicher, Jr., C. W.; Partridge, H., personal communication of preliminary results, 1994. (23) Snelson, A. J. Phys. Chem. 1966, 70, 3208. (24) Calder, V.; Mann, D. E.; Seshadri, K. S.;Allavena, M.; White, D. J. Chem. Phys. 1%9,51, 2093. (25) Hastie, J. W.; Hauge, R. H.; Margrave, J. L.Annu. Rev. Phys. Chem. 1970, 21, 475. 126) Haune. R. H.: Margrave. . J. L.:. Kanaan. A. S.J. ChemSoc.. Faradav Trans.-2 1978,.71, 1082. (27) Ogden, J. S . In Cryochemisfry;Moskovits, M., Ozin, G. A., Eds.; John Wiley: New York, 1976; Chapter 7. (28) Evans, J. C. J. Chem. Soc., Chem. Commun. 1969,682. (29) Field, R. W. J . Chem. Phys. 1974, 60, 2400. (30) Wharton, L.; Kaufman, M.; Klemperer, W. J. Chem.Phys. 1962.37, 621. (31) Kaufman, M.; Wharton, L.;Klemperer, W. J. Chem.Phys. 1965,43, 943. (32) Bloom, C. E.; Hedderich, H. G. Chem. Phys. Lett. 1988,145, 143.
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6521 (33) Huber, K. P.; Herzberg, G . Molecular Spectra and Molecular Structures; Van Nostrand Reinhold: New York, 1979. (34) Ault, B. S.;Andrews, L. J . Chem. Phys. 1975, 62, 2320. (35) Lmevsky,M. J. TechnicalReportRADC-TR-70-212, General Electric Co., 1970. (36) Howard, Jr., W.F.; Andrews, L. Inorg. Chem. 1975, 14,409. (37) Spiker, Jr., R. C.; Andrews, L. J. Chem. Phys. 1973,59, 1851. (38) White, D.; Scshadri, K.S.;Dever, D. F.; Mann, D. E.; Linevsky, M. J. J. Chem. Phys. 1963, 39,2463. (39) Wang, H.;Salzberg, A. P.; Weiner, B. R. Appl. Phys. Left. 1961.59, 935. (40) Irvin, J. A,; Dagdigian, P.J. J. Chem. Phys. 1980, 73, 176. (41) Field, R. W.; Bradford, R. S.;Broida, H. P.; Harris, D. 0.J. Chem. Phys. 1972,57, 2209. (42) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1993, 98, 3690. (43) Miller, J. C.; Auk, B. S.;Andrews, L. J . Chem. Phys. 1977,67,2478. (44) Andrews, L.;Duley, W. W.; Brewer, L. J. Mol. Spectrosc. 1978,70, 41. (45) Shaw, A. M.; Dyke, J. M.; Zengin, V.; S u m , S . Chem. Phys. 1994, 179. 455. (46) Gablenick, S. D.; Reedy, G. T.; Chasanov, M. G. J . Chem. Phys. 1974, 60, 1167.
