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(15)J. R. Lacher and H. A. Skinner, J. Chem. SOC.A , 1034 (1968). (16) B deB. Darwent, Natl. Stand. Ref. Data Ser., Natl. Bur. Sfand., No. 31 (1970). (17) R. J. Cvetanovib, Adv. Photochem., 1, 155 (1963). (18) P. B. Ayscough, A. J. Cocker, F. S. Dainton, and S. Hirst, Trans. Faraday Soc., 58, 318 (1962). (19) E. Sanhueza and J. Heicklen, J. Photochem., 4, 17 (1975). (20) E. Sanhueza and J. Heicklen, Int. J . Chem. Kinet., 6, 553 (1974). (21) E. Sanhueza and J. Heicklen, Int. J . Chem. Kinef., 7, 399 (1975).
Butler et al.
(22) P. Cadman, A. W. Kirk, and A. F. Trotman-Dickenson, J. Chem. Soc., Faraday Trans. 1, 72, 1027 (1976). (23) C. Cillien, P. Goktfinger, G. Huybrechts, and G. Martens, Trans. Fara&y SOC.,63, 1631 (1967). (24) T. N. Bell, T. Yokota, and A. G. Sherwood, Can. J. Chem., 54,2359 (1976). (25) M. I. Christie, Proc. R. SOC.London, Ser. A , 246, 248 (1959). (26) J. R. Majer and J. P. Simons, Adv. Photochem., 2, 137 (1964). (27) H. Hippler and J. Troe, Int. J . Chem. Klnet., 8,501 (1976).
Identification of HgO, Species by Matrix Isolation Spectroscopy R. Butler, S. Katr, A. Snelson," IIT Research Institute, Chicago, Illinois 606 16
and J. B. Stephens NASA George Marshall Space Flight Center, Huntsville, Alabama 35812 (Received May 16, 1979) Publication costs assisted by the National Aeronautics and Space Administration
The condensation of Hg atoms in an ozone-doped argon matrix gas with irradiation from a medium-pressure mercury arc lamp at 10 K resulted in the formation of several trapped HgO, species. Tentative identification of HgO has been made from absorption bands appearing in the visible and IR regions of the spectrum, based on 1602 and laOz isotope studies. The following spectroscopic constants were obtained: urn = 14634 cm-l, v1 = 548 cm-l, and ul1 = 676 cm-l. Some qualitative evidence was obtained for the existence of HgO,.
Introduction Mercury oxide, HgO, is a well-known solid but the gas phase species, despite attempts at characterization, has not been identified.l Gaseous HgO has been suggested as being formed in the decomposition of O3 by Hg atoms., The production of ozone in the mercury-photosensitized reaction with oxygen has been suggested to take place through an excited HgOz s p e ~ i e s . More ~ recently, the existence of HgO, has been questioned as important in the 02-photosensitized reaction, though the data were not sufficiently unequivocal to completely rule out the pos~ibility.~ In this paper we report the results of some matrix isolation studies which indicate the existence of at least two mercury oxides. Experimental Section A matrix isolation cryostat, based on an Air Products Displex 202 closed cycle refrigerator, and molecular beam furnace of conventional design were used in the study. Linde ultra-high-purity argon was usually used as the matrix gas. Oxygen used to dope the matrix or prepare ozone was Linde research grade. l8OZ,99% isotopic purity, was obtained from Prochem. Isotopes. Ozone, prepared in a low-pressure electric discharge between platinum electrodes, was purified by trap-to-trap distillation at liquid nitrogen temperature and stored on silica gel. Mercury was contained in a resistively heated stainless steel Knudsen cell and vaporized at temperatures in the range of 80-160 O C . Deposition times of 0.5-4 h were used. A Hanovia medium-pressure mercury arc lamp was used for photolysis studies. Spectral measurements in the visible region were recorded on a Jarrell Ash 1-m Czerny-Turner scanning spectrometer, using a tungsten ribbon light for a source, and a Hamamatsu S-20 photomultiplier for detection in conjunction with conventional electrometer signal amplification and strip-chart recording. The spectrometer calibration was checked against a mercury 0022-3654/79/2083-2578$01 .OO/O
discharge lamp and found to be good to f O . l nm. A Perkin-Elmer 283 spectrophotometer was used to record infrared spectra in the 4000-200-~m-~ range with an estimated accuracy of f 2 cm-l in the region of interest in this study.
Results Preliminary experimentation showed that the only conditions under which HgO, formation could be induced was by irradiation with a mercury arc during the deposition of Hg atoms with a matrix gas containing ozone. If oxygen was used instead of ozone, or if the matrix was first deposited with either O2 or O3 and subsequently irradiated no new compound formation was observed. Experimentation was therefore concentrated on the ozone-containing systems, and efforts were made to maximize the intensities of the new absorption features, which, in the initial experiments, were very weak. The presence of ozone at moderate levels in oxygen and argon matrices markedly reduced the optical transmission and hence the amount of material that could be deposited. The best compromise appeared to be between 0.5 and 5% 03, in argon matrices with mercury vaporization temperatures at 70-100 O C . Up to 800 cm3 NTP of this gas mixture could be deposited in a 4-h period and absorbance values for the more intense new features of -0.3 (in decadic form) obtained, though many bands were much weaker than this. The matrix was usually pale green, though occasionally light orange areas could be seen. The latter coloration showed only when the concentration of Hg in the matrix was high and seemed to correlate with a broad absorption feature lying a t -500 cm-' in the infrared spectrum. A similar broad absorption feature was also found in the spectrum of pure solid HgO. Most of the study was made with 1603.Some 15 new infrared absorption bands appeared in the spectra which could be assigned reasonably to mercury-oxygen species. 0 1979 American
Chemical Society
The Journal of Physical Chemistty, Vol. 83, No. 20, 1979 2579
Study of HgO, by Matrix Isolation Spectroscopy
TABLE I: Infrared Bands of Hg/Oxygen Compounds in an O,/Ar Matrix
OIZO I I -
ratio
U ( ~ ~ Ocm-' ,),
u( l8O,), cm-'
0 10-
u( ' 6 0 ) / u (
1365' 1341'
008-
1265 1217
1.060 M
1202' 1198' 1081' 730
1025 (weak) 1020 689 679
690 676 645 607 591 485 447 429 a
i\
009-
1362 1355
f 1.060 1.060
642
1.053
578 558 458 424 407
1.050 1.059 1.059 1.054 1.054
I I
006t 6583
n
O 04 0 o5t
I
002c
6200
6300
6400
6500
ii
I
/
6600 6700 WavelenQlh I A 1 ~
I
6800
+
bands were observed in the infrared which indicated a compound or compounds containing more than one oxygen atom. Relative intensity measurements made on these features and the above electronic absorption bands showed that they were unrelated. As noted before some 15 absorption bands appeared in the IR spectrum which could probably be assigned to HgO, species. Intensity measurements were made on all absorption bands lying below 800 cm-l, a reasonable upper limit for diatomic Hg160. Relative intensity ratios were determined for the bands referenced to the most intense transition in the electronic spectrum. Only one IR absorption band appeared to correlate with the electronic transition, a weak feature at 676 cm-'. Measurements from six different spectra for this band had a standard deviation of 12%, which, in view of the generally low intensity of the band, may be taken as a fairly good indication that the two features had a common precursor. Similar relative intensity measurements were not made on the le02spectra since only two spectra were obtained in which both the electronic and IR spectra were recorded simultaneously. spectra appeared The corresponding feature in the lS02 at 642 cm-l. The frequency shift ratio for these two bands is 1.053. For the remaining absorption bands appearing in the infrared spectrum of the mercury oxide species, definitive assignments are not possible. It is perhaps worth noting that the 1081- and 606-cm-l bands in the l6OZspectra, which were relatively strong in relation to other absorption features, were reduced in intensity by about 40% after being exposed to radiation from the IR Nernst glower in
Most of these absorption bands had counterparts in the lS03 spectra. A list of these frequencies is presented in Table I together with the observed frequency shift ratios. For comparison, the expected frequency shift ratio Y(Hg160)/v(Hg180)is calculated at 1.056. In those cases where corresponding l60and lS0bands are not reported, overlap by an O3 band almost certainly prevented identification. One experiment was tried with ozone containing scrambled l60(50%) and lSO (50%) isotopes. Several of the new absorption features showed structure indicative of a molecule containing more than one oxygen atom, but, in many cases, the absorption bands of the unreacted scrambled O3prevented definitive conclusions from being made for many bands. Unfortunately, in many cases band intensities were low and insufficient time was available to investigate many of the features in depth. For this reason attention here is largely restricted to the assignment of absorption bands to HgO with only a brief consideration of some possible features attributable to Hg02. Details of the visible spectra will be presented first. In Figure 1 the matrix spectrum of an HgO, species containing leOz is shown. An analogous spectrum, with small wavelength shifts, was obtained by using 1602From the spectral intensities in Figure 1 it is concluded that there are two band systems, a more intense progression at 6829, 6583, and 6355 A and a much weaker one at 6690 and 6456 A. The spectral data for both the l60and l80species are presented in Table I1 together with the observed isotope frequency shift ratios. As noted later, some absorption TABLE 11: Wavelength of the Visible Absorption Bands of HgO,
--
With
l6O,
14634 i 14948 ?: 15191 i 15489 i 15736 ?:
4 cm-' 1 2 cm-' 13cm-' 1 2 cm-'12 ern"------ 545 i 17 cm-' 12 cm-'
6827 -c 2 A = 14648 f 6688 i. 5 A = 14952 i 6598 i 2 A = 15156 f 6468 i: 5 A = 15461 ?: 6389 * 5 A = 15652 f
With lSO, 4 cm" 12 cm-' _508-*_6_cm~ 4 cm-' . 12 ern-"----- 496 * 13 cm-' 1 2 cm-'
= = = = =
7000
Figure 1. Visible matrix isolation spectrum of HgO, species formed argon matrix with by the interaction of Hg atoms with a 5 % irradiation during deposition.
Low precision measurement.
6829 * 2 A 6690 ?: 5 A 6583 f 5 A 6456 i. 5 A 6355 i. 5 A
6900
--- - --
--- - --
_ _ 541 -- -- -- -_- ----- _- --_ -- - - - - ---_ 509
Isotope Shift Ratios a/a' = 1.08 f 0.03 b/b' = 1.10 i. 0.05 c/c' = 1.06 ?: 0.05
a i
17 cm-'
b C
a' f
17 cm-
b' C'
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The Journal of Physical Chemistry, Vol. 83, No. 20, 7979
the spectrometer for a period of some 18 h. This behavior suggests a common precursor for these bands. It was not observed if the corresponding bands in the l8OZspectra exhibited the same type of photobleaching.
Discussion The observed and calculated isotope frequency shift ratios shown in Table I1 are consistent, within the precision of the experimental measurements, with an assignment of the electronic transitions to diatomic HgO. From the present data it is not possible to determine if the two observed progressions are due to two different electronic transitions or if they are the result of matrix site effects perturbing a single energy state. HgO is expected to have a ground state by analogy with the alkaline earth diatomic oxides. Data for these species5 indicate the existence of AIX and lA’II states which are readily accessible to the ground state with energies in the range 3500-22000 cm-l. It is conceivable that the above electronic transitions could arise from species such as HgO, or Hg03. Unfortunately, the limited spectral resolution available did not allow a definitive conclusion to be made on this point from experiments with scrambled isotopically labeled ozone. Matrix studies on metal ozonides have shown electronic transitions in the 5100-3700-A region6 This region was examined in the present study, but no absorption features were evident. As noted some absorption bands were observed in the infrared which indicated a compound or compounds containing more than one oxygen atom. Relative intensity measurements made on these features and the above electronic absorption bands showed that they were unrelated. On the basis of the good correlation in the relative intensity measurement made on the electronic absorption spectra), we band and the infrared band at 676 cm-l (1603 concluded that the latter may be assigned reasonably to the fundamental vibration frequency of ground state Hg160;the corresponding frequency of Hg180 is 642 cm-l. The frequency shift ratio observed at 1.053 agrees within experimental precision with the calculated value at 1.056. Because of absorption band overlap, the isotopically scrambled ozone experiment did not provide positive evidence for the presence of only one oxygen atom in the species responsible for these two absorption bands. Indirect evidence in support of the present assignment of these frequencies to HgO is their similarity with the analogous group IIA oxides of BaO (670 cm-l) and SrO (645 cm-l). It is tempting to assign the two absorption bands lying at 1081 and 606 cm-l (l60experiments), which showed the photobleaching behavior, to an ionic HgOz species analogous to that assigned to other metal superoxide^.^-^ The superoxide ion in these compounds exhibits vibration frequencies in the 1100-1000-cm-’ region, very close to that of an isolated 02-ion, and shows v ( l 6 0 ) / v ( l 8 0 ) isotopic frequency shifts close to that of the free ion at 1.061. The
Butler et al.
observed lower frequency would reasonably correspond to the HgO stretching mode. A third frequency expected of an HgOz species was not identified. Some further evidence in favor of such an assignment was that the intensity of the high-frequencyO2 stretching mode was only about 1/5 of that of the lower frequency stretching mode, a similar intensity pattern as found in other superoxides. If indeed the above assignment is correct, the observed photobleaching behavior of these bands could perhaps be justified in terms of the expected lower stability of Hg+02-, compared to alkali and alkaline earth metal superoxides by virtue of the high ionization potential of Hg at E 10.4 eV compared to that of the alkali and alkaline earth metals which fall in the range 3.9-6.1 eV. Finally, we make a few comments with respect to HgO, formation in the present study. The prerequisite for the formation of HgO, species appeared to be irradiation during matrix deposition in the presence of Hg atoms and ozone. This result strongly suggests that excited mercury atoms Hg(3P)are required to obtain reaction with O3under our experimental conditions. The lack of reaction when O2 was used under irradiation during deposition, instead of ozone, implies that the reactive oxygen source was either 03,singlet 02,or 0 atoms, the latter probably in the 3P ground state. Since reaction between Hg(3P) and these oxygen species will be highly exothermic it is likely that production of “stable” HgO, species occurs during condensation at the matrix surface where the exothermicity could be effectively dissipated.
Conclusion Medium-pressure mercury arc lamp irradiation of the matrix during the simultaneous deposition of mercury atoms and ozone doped argon gas resulted in the formation of mercury-oxygen compounds. From the visible and IR absorption spectra a tentative assignment for diatomic HgO has been made. The following spectroscopic constants from the matrix data were derived: Y’ = 548 f 13 cm-l, J’ = 676 f 1cm-l, voo = 14634 f 4 cm-l. Lack of data prevented definite assignment for most of the observed infrared absorption bands, though some qualitative evidence in favor of the formation of Hg02 is presented. Acknowledgment. The authors are grateful to Jim Lazar, NASA Headquarters, for supporting this study. References and Notes Herzberg, G. “Spectra of Diatomlc Molecules”, D. Van Nostrand: New York, 1959; p 539. Calvert, J. G.; Pitts, Jr., J. N. “Photochemistry”; Wiley: New York, 1967; pp 111, 112. Volman, D. H. “Advances in Photochemistry”, Noyes, W. A,; Hammond, 0. S.; Pitts, J. N.; Ed., Interscience: New York, 1963; p 52. Hlppier, H., Wendt, H. R.; Hunziker, H. E. J. Chem. fhys. 1978, 68, 5103. Field, R. W. J. Chem. fhys. 1974, 60, 2400. Jacox, M. E.; Milligan, D. E. J . Mol. Spectrosc. 1972, 43, 148. Auk, 6. S.;Andrews, L. J. Chem. fhys. 1975, 62, 2312. Andrews, L. J . Chem. Phys. 1969, 50, 4288. Smardzewski, R. R.; Andrews, L. J. Chem. fhys. 1972, 57, 1327.
