J . Phys. Chem. 1990, 94, 4398-4400
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= -17.9 f 0.1 kcal/mol,6 and ArH(02+) = 278.37 f 0.02 kcal/m01,~*~~ we determine AfH298(H02) = 3.8 f 1.2 kcal/mol (where the uncertainty represents our 95% confidence limits and includes the uncertainties of the reference data). This result is in particularly good agreement with the results of Howarda and the average value cited by SB12 (Table I). On the basis of our value for A,H(H02), we also derive Do(H-OOH) = 88.4 f l .2 kcal/mol and D0(H-02) = 48.3 f l .2 kcal/mol. While the former value agrees with Sawyer's determination of 90 kcal/mol, the latter value is considerably lower
than Sawyer's value, D0(H-02) = 59 k ~ a l / m o l . Using ~ ~ the well-known heats of formation A@(H) = 52.103 f 0.001 kcal/mol and ArH(HOOH) = -32.53 f 0.01 kcal/mol,6 one can calculate the sum of D0(H-O2) and Do(H-OOH) as 136.74 f 0.01 kcal/mol. The sum of Sawyer's bond dissociation energies is 149 kcal/mol, clearly inconsistent with the known thermochemistry. Acknowledgment. This work is supported by Air Force Wright Aeronautical Laboratories and by the National Science Foundation, Grant CHE-8917980.
Vibrational Predlssociatlon Spectroscopy of (CH,OD), and (CH,OH) (CH,OD) in the 9.6-pm Region Jeffrey P. LaCosse and James M. Lisy*3+ School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received: March 13, 1990)
Vibrational predissociation spectra of isotopomers of methanol dimer have been recorded in the 9.6-clm region. Using the beam depletion method, we have observed vibrational transitions at 1028.29 and 1053.49 cm-l for (CH30D), with Lorentzian line widths of 2.83 and 1.18 cm-I, respectively. These transitions are blue-shifted from the fully protonated dimer by 1.88 and 1.30 cm-I, respectively. Spectra were also measured for the mixed dimer (CH30H)(CH30D). Analysis shows that there is a preference of the deuterated subunit to occupy the donor position in the mixed dimer. It is estimated that the population of dimer to monomer ratio is 1% under conditions where only the monomer and dimer are present in the molecular beam .
Introduction In the past few years, experimental study of the methanol dimer has been increasing at a steady rate. These studies have been carried out primarily in the 9.6-pm region, where the C-0 stretching transition of methanol is located. Techniques of larger cluster elimination have been developed, in this laboratory] and by others,2 allowing the measurement of the infrared dissociation spectrum without congestion due to larger cluster contamination. Huisken and Stemmler3 observed a doublet structure with bands centered near 1026 and 1052 cm-I, indicating that the C-0 stretch in each subunit experiences a different environment with respect to each other. This is in contrast to the result obtained by Hoffbauer et a1.4 where one band was observed at 1045 cm-l. This discrepancy can be explained by the difference in expansion conditions; Hoffbauer et al. study favored a distribution of larger clusters, which were detected at the same m / e as the dimer in their mass spectrometer. In addition, Buck et al. have shownS that methanol cluster sizes from three to five subunits give a single absorption band near 1045 cm-I. In another study, Buck et a1.2 have shown that extensive fragmentation can occur under electron impact ionization even with low electron energies. In an effort to gain more information on the structure and dynamics of this system, vibrational predissociation (VP) spectroscopy was performed on various isotopomers of methanol dimer, using a technique of larger cluster elimination developed by this laboratory. Experimental Section A discussion of the large cluster elimination technique can be found in Kolenbrander and Lisy.I A detailed description of the experimental apparatus can be found in Michael and LisyS6 In the present studies, a continuous wave (CW) C 0 2 laser is used instead of the optical parametric oscillator in refs 1 and 6. The methanol dimers were generated by a supersonic expansion of first run (FR) (-70% Ne, -30% He; Airco) neon seeded with the appropriate isotopomer(s) of methanol. The CH,OX (X = H, 'Alfred P. Sloan Foundation Fellow 1987-1991.
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TABLE I:
Lorentzian Fit Parameters"
(CHIOH), (CH;0H)ib (CHPDh (CHpOH)(CH,OD)
1026.41 1026.5 1028.29 1026.81
(4) ., (4) (4)
2.38 4.5 2.83 2.68
(40) .~ (28) (18)
1052.19 (16) . . 1051.6 1053.49 (19) 1052.98 ( 5 )
3.18 (39) . . 4.2 1.18 (67) 1.54 (90)
L ? w o ( x )peak , x position in cm-I. Awo(x), fwhm of peak x in cm". Peak 1 is known as the acceptor band; peak 2 is the donor band (see ref 16). Numbers in parentheses indicate error in last digit (iu). *Results of Huisken and Stemmler, see ref 3.
D, or both) was introduced to the FR neon by passing the rare gas mixture over temperature controlled methanol in a gas bubbler. We have found6,' that FR neon is an excellent carrier gas for forming small clusters with low internal energy. Others have since exploited the benefits of FR neon.* The concentration of CH30H and isotopomers in the expansion mixture was controlled by varying the temperature of the methanol bath to give the desired concentration. Typical concentrations of methanol were on the order of 0.02% mole fraction, in order to limit the extent of condensation to mainly dimer production. The corresponding temperature of methanol inside the bubbler is -50 OC. Results and Discussion In order to observe the VP spectrum of any of the isotopomers of the methanol dimer in the 9.6-pm region, it is necessary to (1) Kolenbrander, K. D.; Lisy, J. M. J . Chem. Phys. 1986,85(10), 6227. (2) Buck, U.; Meyer, H. Phys. Rev. Lett. 1984, 52(2), 109. (3) Huisken, F.; Stemmler, M. Chem. Phys. Lett. 1988, 144(4), 391. (4) Hoffbauer, M. A.; Giese, C. F.; Gentry, W. R. J . Phys. Chem. 1984, 88, 181. (5) Buck, U.; Gu, X.;Lauenstein, C.; Rudolph, A. J. Phys. Chem. 1988, 92, 5561. (6) Michael, D. W.; Lisy, J. M. J . Chem. Phys. 1986,85(5), 2528. (7) Kolenbrander, K. D.; Lisy, J. M. J. Chem. Phys. 1986,85(5), 2463. (8) Klots, T. D.; Chuang, C.; Ruoff, R. S.; Emilsson, T.; Gutowsky, H. S. J . Chem. Phys. 1987, 86(10), 5315.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. I I, 1990 4399
Letters
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Frequency (cm-') Figure 2. Vibrational predissociation spectra of isotopomers of methanol dimer. (A) VP spectrum of (CH,0D)2 in FR neon. Mass spectrometer is set at m / e 35. (B) VP spectrum of (CH,OH)(CH,OD) in FR neon with (CH,OD), contamination removed. Mass spectrometer is set at m / e 34. See text for other experimental conditions.
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Frequency (cm-') Figure 1. Elimination of larger clusters: (A) 0.2% methanol in FR neon (bubbler at -20 "C); (B) 0.07%methanol in FR neon; (bubbler at -35 "C); (C) 0.02%methanol in FR neon (bubbler at -50 "C). All spectra taken at m / e 33. See text for other experimental conditions.
eliminate the spectral congestion due to clusters larger in size than the dimer. To observe the effect on the concentration of methanol in the expansion mixture in the V P spectrum of (CH,OH),, a set of three spectra were recorded with different methanol concentrations in the expansion. Figure 1A shows a V P spectrum taken with 0.2% C H 3 0 H (Fisher, 99.9%) in FR neon, recorded at m / e 33 ((CH,OH)H+). The stagnation pressure was 66 psia. The crosses are the actual data points, and the smooth curve is a Lorentzian fit to the data. The error (Au) in the dissociation cross section for each point is smaller than the size of the respective cross. Comparing this spectrum with the results of Huisken and Stemmler, it is clear that the two outer peaks are due to the photodissociation of (CH,OH),, while the center peak is due to dissociation of methanol clusters larger than the dimer. Figure Ib is the V P spectrum recorded with identical conditions, except that the methanol concentration in the expansion has been reduced to 0.07%. The dimer peaks have maintained their relative size, but the feature near 1044 cm-' has been attenuated. In Figure IC, the concentration of methanol in the expansion is further recuced to 0.02%. The feature at 1044 cm-I has virtually disappeared, resulting in a pure dimer spectrum. Fitting the spectrum with a double Lorentzian line shape allows the determination of line widths and positions. Line centers and widths found from the fits are found in Table I. These values are in very good agreement with the results obtained by Huisken and Stemmler3 for (CH,OH),, using a crossed He beam to spatially isolate the dimer. This method of larger cluster contamination elimination
is simple and easily carried out. In principle, it can be used with any system that will be in a condensed phase in the temperture range of the cooling bath. To exploit this method of pure dimer production, as in the case of methanol, the technique was used with some of the isotopomers of methanol to observe the V P spectrum of the corresponding dimers. The present study concerns on the V P spectra of isotopomers of (CH,OH),, namely (CH,OD), and (CH,OH)(CH,OD). The generation of these dimers is done in exactly the same manner as the fully hydrogenated dimer. The only differences in the experimental conditions are the slightly different mass setting of the quadrupole mass spectrometer, along with a slightly richer expansion mixture. Figure 2A is the (CH,OD), spectrum, recorded at m / e 35, with similar source conditions as the Figure 1C spectrum. The expansion mixture used was 0.05% methanol-d (Aldrich, 99.5%) in FR neon. Lorentzian fitted line positions and widths are found in Table I. There is a significant increase in error in fitting peak 2, due to its sharpness, compared to the Figure 1A spectrum. It is clear that there is a shift to higher frequencies in the two transitions compared to the fully hydrogenated dimer. The shifts are measured to be 1.88 cm-I for peak 1 and 1.30 cm-' for peak 2. These shifts are in the same direction, but much less than seen for the corresponding monomers, of about 6 cm-]. In the case of (CH,OH)(CH,OD) where the spectrum is taken at m / e 34, there is a problem due to the contamination of (CH,OD),, which is also present in the molecular beam, and is detected at m / e 34. The contribution of (CH30D)2to the m / e 34 spectrum was estimated by measurement of the electron impact fragmentation branching ratio of (CH30D)2to m / e 34 and m / e 35. This was accomplished by taking VP spectra of the d, dimer at m / e 34 and 35 simultaneously using CH,OD in the expansion mixture, under dimer-only conditions. The resulting ratio was found to be 1:1, indicating that there is an equal chance of (CH30D), being detected at m / e 34 or m / e 35. With this knowledge of the branching ratio, the contamination due to the d, dimer can be subtracted, leaving the true mixed dimer spectrum. Figure 2B shows the spectrum of (CH,OH)(CH,OD), corrected for (CH,OD), contamination, taken with the mass spectrometer set at m / e 34. The expansion mixture is a 1:l mixture of C H 3 0 H /
J . Phys. Chem. 1990,94,4400-4403
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C H 3 0 D in FR neon, with the total mole fraction of the methanol species being 0.05%. The fitted Lorentzian line parameters are listed in Table I. These results can be used to determine if the deuterated subunit occupies either the donor or acceptor position in the mixed dimer, assuming there is little interaction between the two C - 0 stretches within the complex. Comparison of the transition frequencies of the mixed dimer with frequencies observed for (CH30H)2and (CH30D)2reveals that the mixed dimer has a definite configuration with respect to the subunits. If the donor and acceptor C-0 stretches are assigned to the 1052- and 1026-cm-' bands, respectively, the mixed dimer transitions will enable us to assign where the CH,OH and CH30D reside in the dimer. These dimer assignments were first made by Kabisch and Pollmerg and have been supported by solvated alkali-metal-ion studies carried out in this laboratory.IO Examination of the band near 1027 cm-' in the mixed dimer spectrum shows that it lies nearest the acceptor peak in the (CH30H)2VP spectrum, after subtraction of the contamination from (CH30D)2. This strongly suggests that the acceptor position in the mixed dimer is occupied by CH,OH. Similarly, comparison of the 1053-cm-' mixed dimer band with the donor peaks of (CH,OD), and (CH,0H)2 shows that the donor position is very likely occupied by CH30D. This configuration is expected for the mixed dimer, since it would have a lower zero-point energy compared to the other possible configuration. This was observed in a matrix isolation study of (HF)(DF) by Hunt and Andrews.l' However, matrix effects have been shown to exclude some observed gas-phase structures, as shown by Andrews et al.I2 This preferred structure is possible especially at the expected temperature of the dimer; time of flight measurements show a translational temperature of 6 K. If the expansion conditions were different so that the dimer would be somewhat warmer, the production of the other mixed dimer might be possible, as o b s e r ~ e d ' ~with - ' ~ (H(9) Kabisch, G.; Pollmer, K. J . Mol. Struct. 1982, 81, 35.
(IO) Draves, J. A.; Luthey-Schulten, Z.; Liu, W.-L.; Lisy, J. M. Manuscript in preparation. (11) Hunt, R. D.; Andrews, L. J . Chem. Phys. 1985, 82(10), 4442. (12) Andrews. L.;Bondybey, V. E.; English, J. H. J . Chem. Phys. 1984, 8 1 ( 8 ) , 3452. (13) Lafferty, W. J.; Suenram, R. D.; Lovas, F. J.J. Mol. Specfrosc. 1987, 123, 434. (14) Gutowsky, H. S.; Chuang, C.; Keen, J. D.; Klots, T. D.; Emilsson, T. J . Chem. Phys. 1985, 83(5),2070.
F)(DF) and (DF)(HF). It is also possible to estimate the relative amounts of CH,OH and (CH30H)2in the molecular beam by measurement of the signals at m / e ratios respective of the species of interest. With the presence of only the monomeric and dimeric species in the molecular beam, the identification of the contributions of various compounds appearing at a given mass can be made quite easily. In the case of (CH,OH)*, it was shown3that this species fragments under electron impact ionization to m / e 33. It is also known that the trimer can appear at this mass, causing the spectral contamination mentioned earlier. With the trimer and larger clusters absent in the molecular beam, the species contributing signals to m / e 33 are (CH30H), and the carbon-13 isotopomer of CH30H. The contribution of the (2-13 isotopomer can be estimated by measuring the signal at m / e 32, which is due to the carbon-I2 species of C H 3 0 H . After subtraction of this component of the signal at m / e 33, the result is divided by 2 times the signal at m / e 32 to give the relative amount of (CH30H), to the monomer in the molecular beam. The factor of 2 arises from an assumption that the electron impact ionization cross section for the dimer is twice as large as for the monomeric species. With this method, a lower limit of (CH30H)2relative to the monomer at the dimer/monomer only conditions is estimated to be 1%. It is also observed from the VP spectra of Figures IC and 2A,B that the band line widths are not similar between isotopomers. A band contour analysis is presently under way to try to determine if the differences in the line shapes for the different isotopomers are due to VP lifetime differences, or significant rotational constant changes upon isotopic substitution. A measurement of the absolute photodissociation cross sections for the various isotopomers of (CH,OH), is in progress to measure the change of the transition dipole moment upon complexation.
-
Acknowledgment. This work has been supported in part by the National Science Foundation (Grants CHE-8506698, CHE8714735). Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support for this research. (15) Pine, A. S.; Lafferty, W. J.; Howard, B. J. J . Chem. Phys. 1984, 81(7), 2939. (16) Crooks, J.; Stace, A. J.; Whitaker, B. J. J. Phys. Chem. 1988, 92, 3554.
Elucidation of the Initial Stages of the Oxidation of Si( 111) Using Scanning Tunneling Mlcroscopy and Spectroscopy In-Whan Lyo, Ph. Avouris,* IBM Research Division, T . J . Watson Research Center, Yorktown Heights, New York 10598
B. Schubert, and R. Hoffmann Department of Chemistry and Materials Science Center, Cornell University, Ithaca, New York I4853 (Received: March 27, 1990) We use scanning tunneling microscopy, atom-resolvedtunneling spectroscopy,and electronic structure calculations to determine the nature of the adsorption state of oxygen in the initial stages of the oxidation of Si(11 1). We are able to directly image two states of adsorbed oxygen. One of them is identified as a Si adatom site with one oxygen atom inserted in one of the back bonds, while the other involves an oxygen atom tying up the adatom dangling bond with, most likely, another oxygen inserted in one of the back bonds. As the coverage is increased toward the monolayer, the latter site becomes the dominant one.
Introduction The oxidation of silicon is perhaps the most important chemical reaction in the field of microelectronic materials, so it is not Author to whom correspondence should be addressed.
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surprising that it has been the focus of much attention. In particular, the nature of the bonding site of oxygen in the early stages of oxidation has been the subject of extensive study for the past 30 years. Many different configurations have been proposed including configurations where an 0 atom is inserted in Si-Si 0 1990 American Chemical Society
