21] Rossini, F. D., Selected values of physical and thermodynamic properties of hydrocarbons, Am. Petroleum Inst. Proj. 44, (Carnegie Press, Pittsburgh, Penna., 1953). 22] See for example, Tester, H. E., METHANE, in Thermodynamic Functions of Gases, Vol. 3, Din, F., Editor (Butterworths, London, 1961). 23] McDowell, R. S., and Kruse, F. H., Thermodynamic functions of methane, J. Chem. Eng. Data 8, 547 (1963). This was kindly pointed out by Lester Haar of NBS. 24] Barieau, Robert E., Analytical expressions for the zero pressure thermodynamic properties of nitrogen gas including corrections for the latest values of the atomic constants and the new carbon-12 atomic weight scale, J. Phys. Chem. 69, 495 (1965). [25] Stewart, R. B., The thermodynamic properties of oxygen, (Dissertation, Dept. of Mech. Engrn'g., Univ. of Iowa, June, 1966.) [26] Mechtly, E. A., The international system of units (NASA SP-7012, Sept. 1966, U.S. Gov't. Printing Office, Washington, D.C. 20402.) [27] Harrison, Roland H., private communication (U.S. Bureau of Mines Petroleum Research Center, Bartlesville, Oklahoma 74003, August, 1970.) (Paper 75A1-645) JOURNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry The Infrared Spectrum of Matrix Isolated BɑO2* S. Abramowitz and N. Acquista Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (September 21, 1970) The infrared spectra of matrix isolated BaO2 have been observed and assigned. Ba atoms were allowed to react with an argon oxygen mixture and condensed on a liquid hydrogen cooled surface. Spectra observed using 18O2 and 160180 confirm this assignment. Key words: Barium: barium oxides; high temperature: infrared; matrix isolations. 1. Introduction The reaction of O2 with barium in the gas phase has been studied by several workers [1, 2]. This reaction is thought to go through an intermediate BaO2 (1), even though mass spectrometric evidence for this species is not available. Because of the importance of such an intermediate species, particularly in connec tion with the recent use of barium releases in the atmosphere to form BaO and Ba clouds, it was decided to study the products of this reaction using matrix isolation techniques. 2. Experimental Procedure A beam of barium atoms was evaporated from either a stainless steel Knudsen cell or a heated stainless steel small-diameter tube and allowed to codeposit with O2/Ar onto a liquid hydrogen cooled CsI window. The electron beam furnace used for heating the Knudsen cell, the Air Products Cryotip and the Perkin Elmer 301 spectrophotometer 2 have been described previously [3]. In other experiments a Beckmann [R-7 spectrophotometer with a CsI interchange and a conventional cryostat were used. Higher resolution spectra were obtained using a Perkin-Elmer 99G monochromator equipped with interference filters and suitable diffraction gratings. A chromel-alumel thermocouple inserted into a 0.030" diameter hole drilled halfway up the wall of the Knudsen cell, was used to measure the temperature of the effusing gas. The recent vapor pressure data of Hinnov and Ohlendorf for barium were utilized [4]. A vapor pressure of barium of about 10-3 torr was allowed to effuse through "This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force under AFOSR-ISSA-69-001. Figures in brackets indicate the literature references at the end of this paper. *Certain commercial instruments are identified in this paper to specify completely the experimental procedure. In no case does such identification imply a recommendation or endorsement by the National Bureau of Standards. ABSORPTION oxygen to argon. (Scan (a) is for a 1/300 O2/Ar concentration while scan (b) and (c) are for concentrations of 1/100 and 1/50 respectively.) This indicates that these multiplet structures are not due to isotopic effects (barium has five naturally occurring isotopes with abundances greater than 2%). They could possibly be due to molecular complexes with oxygen neighbors, since the oxygen concentration is about 10 times that of the barium in the matrix. Alternatively they could be ascribed to a matrix effect which is concentration dependent. In any event the feature at 570 cm-1 certainly becomes more prominent as the concentration of Ar/O2 and Ba/O2 increases. The reaction of Ba+1802 was then studied. The spectrum shown in figure 2(b) is for about an 180/Ar In spectrum (c) Ba has been codeposited with 1602 +160180+1802. The ratio of the 1602/18O2 in the sample, which was then discharged electrically with a Tesla coil to scramble the oxygen sample, was about 2/1. The observed spectra contains all the features which are found in spectrum (d). In addition to these features there are lines in between which correspond on a one to one basis with those due to 1602 and 1*02. These lines are about midway between the oxygen 16 and 18 lines. It was then decided to try to obtain spectra of Ba+O with about equal concentrations of both in the matrix. Figure 3 shows such spectra. The barium was heated resistively in a stainless steel tube with about a 5 mm orifice yielding roughly 6 times the barium (at the same pressure) as the Knudsen cell. Also the orifice was closer to the CsI low temperature surface. In these experiments only one prominent feature is found for Ba+ 1602 and Ba+1802 at 570 and 545 cm respectively (an 16O180 impurity is present in the 180). An experiment with the scrambled isotopic constitution shows features at 570 and 545 cm ̄1 and another at 559 cm-1 due to Ba16018O. This result verifies our initial results indicating that the species responsible for this absorption has two oxygen atoms or one O2 unit per barium atom. While definitive spectroscopic proof of one barium atom per molecule has not been obtained in this study, any other conclusion doesn't seem warranted. Mass spectral, Knudsen, and optical data indicate that barium evaporates as an atom. Also the only group II diatomic molecule spectroscopically found to date is Mg2 [5] which has a dissociation energy of about 400 cm-'. Reports in the Russian literature [6] purporting to show the existence of Ca are most probably misinterpreted. More likely emitters for the species are thought to be either CaC or CaN because of the we, Be and extrapolated D. values reported. Rather long extensive runs of 36–40 h duration were necessary to produce the spectra shown in figure 4. This is the region of 0-0 stretching mode for a proposed BaO molecule. This mode is found at 1066 cm-1 -1 for the Ba16O2 and at 1004 cm -1 for Ba18O2. These spectra as those shown in figure 3 were obtained with the Perkin-Elmer 99G monochromator. Unfortunately it was not possible to obtain spectra of the mixed isotope in this region (because of the large amount of sample needed). Attempts at longer runs were unsuc cessful because of loss of thermal contact between the liquid hydrogen surface and the cryostat's low temperature window, or increased scattering of the argon/O/Ba film. The results and assignment for the BaO2 species are summarized in table 1. of 1/200. The same number of features were observed as in the 160/Ar experiments. The relative intensities. of these features are comparable to similar concentrations of 160/Ar. It is perhaps interesting that the ratio of the frequencies of the corresponding features for 160 and 180 are the same within the experimental error for all four of the observed features for each species. Curve (a) is for 160/Ar of 1/300 and is shown for comparison purposes. Curve (c) shows a Ba+1602 +180, experiment. In this experiment the 1602 and 1802 were at a concentration of 1/100 yielding a total O2/Ar of 1/50. It is seen from this experiment that the observed spectrum (d) is a superposition of spectra (a) and (b) neglecting relative intensities of TABLE 1. Observed fundamental modes for BaO2 (cm-1) the various features within each of the two multiplets. There is no absorption between or midway between corresponding pairs of bands indicating all these features are due to one oxygen molecule per species. ABSORPTION σ FIGURE 4. FIGURE 3. The infrared spectra of Ba+O2/Ar with Ba/O2 about 1:1. The intensity of the 0-0 stretching mode (at 1066 cm-1) is very weak compared to the Ba-O stretching mode found at 570 cm-1 for Ba16O2. This low intensity coupled with a frequency which is similar to the Oz fundamental frequency reported on by several workers found in solids such as Oz doped alkali halide single crystals [8, 9] tends to support a charge transfer type structure consisting of Ba++ Oz. A completely ionic model would yield an O-O bond which would indeed not be infrared active. Crude attempts by us to observe fluorescence spectra of the Oz by using Hg 2537 A line were not successful possibly because of the low fluorescence yield or scattering matrix. Unfortunately the other BaO stretching mode has not been observed as yet. It is not at all clear why the intensity of this mode is weak compared to the other BaO stretching mode. Weak features have been found in the Ba16O2 spectra; however, comparable bands in the Ba18O2 have not been found. Without these data it is not possible to assign the 570 and 545 cm-1 in The infrared spectra of Ba+O2/Ar with Ba/O2 about 1:1. Ba16O2 and Ba18O2 to the symmetric or asymmetric 1004 1066 CM-1 (The break in curve b is due to a slit change). (a), O/Ar= 1/100 (b), O/Ar= 1/100. stretching frequencies of BaO2. 5. References [1] Sakurai, K., Johnson, S. E., and Broida, H. P., J. Chem. Phys. 52, 1625 (1970). [2] Newbury, R. S., Balton, G. W., Jr., and Searcy, A. W., J. Chem. [3] Mann, D. E., Calder, G. V., Seshadri, K. S., White, D., and [4] Hinnov, E., and Ohlendorf, W., J. Chem. Phys. 50, 3005 (1969). [5] Balfour, W. J., and Douglas, A. E., Can. J. Phys. 48, 901 (1970). [6] Kovalenok, G. V., Sokolov, V. A., Izvestia Vysshikh, Uchebnykh, Zavedenii Fizika, 3, 27 (1968). [7] Linevsky, M. J., private communication. [8] Holzer, W., Murphy, W. F., and Bernstein, A. J., J. Mol. Spec- [9] Rolfe, J., Holzer, W., Murphy, W. F., and Bernstein, H. T., J. |