J. Phys. Chem. 1985,89.4499-4501
proceeds randomly, the first Ti3+species produced after irradiation should correspond to all of the electrons measured for the visible spectrum, with the ratio decreasing as the concentration of electrons increases. The data in Figure 3 do not show any change in slope at lower electron concentrations, but no measurements were made on alkaline colloid solutions containing less than 2 X IO4 mol of electrons dm-3. An alternative explanation would be that the surface trap sites in alkaline colloids are less efficient than those in acidic solution; this explanation supposes that the measured UV-visible spectrum of alkaline colloids contains contributions from both trapped and free electrons. We cannot distinguish between these two possibilities at present. From the EPR observations the following description can be given of the electron trapping process in colloidal Ti02. Band-gap irradiation produces holes and electrons, most of which recombine in the absence of a hole scavenger. In samples irradiated at 4.2 or 77 K a few electrons (ca. 1 per particle for acidic solutions) are trapped at Ti4+ defect sites in the interior of the colloid, producing interstitial Ti3+ ions. On warming to room temperature these are lost through either recombination or reduction of H 2 0 a t the surface. In the presence of a hole scavenger many more electrons are trapped at surface Ti4+sites, producing a distorted octahedral Ti3+species responsible for the blue color and the broad EPR signals. Both surface and interior trapped electrons are stable at room temperature in the presence of a hole scavenger (and the absence of oxygen). Irradiation at 77 K in the presence of a hole scavenger produces first electrons trapped at interstitial sites but surface trapped electrons are also formed more slowly. At 4.2 K, only interstitial Ti3+is detected. PVA, methanol, and I- all function as effective hole scavengers, while acetate appears to be less efficient. Three suggestions have been made in the literature for the origin of the blue color in irradiated colloids, viz. electrons trapped at oxide vacancies,6free electron^,^ and Ti3+centers3 The first of these can now be dimissed; no EPR signals at g = 2.0 characteristic of electrons trapped at oxide vacancies have been observed in the irradiated colloids. In acidic solution, all of the electrons are trapped as Ti3+; in alkaline solution the possibility of free electrons remaining untrapped cannot yet be discounted. Although the trapped electrons are thus accounted for, the fate of the positive holes simultaneously produced remains unclear. In the presence of PVA, holes are thought to be removed either by direct attack on PVA or through initial trapping at surface hydroxide ions followed by .OH attack on PVA? but the a-alcohol radicals so formed were not detected by EPR at 77 or 4.2 K. A similar mechanism can be envisaged for hole scavenging by methanol, although again no paramagnetic products were detected. Flash photolysis studies3J6 have shown that the IC radical anion is the initial product of hole trapping by I-, but this paramagnetic species was not detected by EPR. Only in the case of acetate, ( 1 6 ) Moser, J.; Gratzel, M. Helu. Chim. Acta 1982, 65, 1436
4499
a relatively inefficient hole scavenger, were products of hole trapping detected. Formation of methyl radicals from acetate can be attributed to either h+ + CH3C02or
.OH
+ CH3C02-
-
+
CH3.
CH3.
+ C02
+ C02 + OH-
In the absence of a hole scavenger no EPR signals were detected on irradiation at 4.2 or 77 K which could be attributed to trapped holes although a trapped electron signal (Ti3+) was clearly developed. Similar experiments with hydrated (solid) anatase have shown the formation of a signal assigned to 0- species (holes trapped at lattice oxide ion sites) together with Ti3+on irradiation at 4.2 K,lZbut neither this species nor the .OH radicals expected for hole trapping at OH- sites are seen in the colloids. Either the trapped holes or their reaction products are too short-lived to be detected even at 4.2 K, or possibly holes are trapped at adjacent sites which interact too strongly for a signal to be detected. anion) has Pairwise trapping of holes (as the diamagnetic 022been proposed for MgO by King and Freund.I7 The orthorhombic signal produced on irradiation of colloid solutions containing peroxide at 4.2 or 77 K is identical with a signal observed following oxygen photoadsorption on hydrated anatase surfaces at 77 K and attributed to the 02-radical anion.I2 Peroxide is known to be adsorbed on hydrated anatase surfaces,ls and the yellow color of the colloid solutions containing peroxide can be attributed to the adsorbed species. Adsorbed 0:- can function as both a hole and an electron scavenger, viz. 0:-
H202+ e.OH
- +
+ h+
Oy
.OH
+ 0 2 2 - -.+
02-
OH-
+ OH-
Le., 02-formation may occur as a result of both hole and electron trapping reactions, and the absence of any Ti3+ signals due to trapped electrons for colloids irradiated in the presence of H202 is also accounted for. Acknowledgment. We thank Mr. J. Moser for preparation of the colloid solutions, and Dr. J. van der Klink of the Institute of Experimental Physics for permission to use the EPR spectrometer. This work was supported by the Swiss National Science Foundation and the Gas Research Institute, Chicago, IL (subcontract with the Solar Energy Research Institute, Golden, CO). Registry No. Ti02, 13463-67-7. (17) King, B. V.; Freund, F. Phys. Reo. B 1984, 29, 5814. (18) Boonstra, A. H.; Mutsaers, C. A. H. A. J . Phys. Chem. 1975, 79, 1940.
A New High-pressure Phase of Solid COP Roland C. Hanson Physics Department,t Arizona State University, Tempe, Arizona 85287, and Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received: June 7, 1985)
Experimental evidence based on Raman scattering data in a diamond anvil cell is presented for a new phase I11 of solid C 0 2 which occurs above 18 GPa at ambient temperature. This phase has a quite different low-frequency Raman mode pattern from the well-known Pa3 phase I. Introduction C 0 2 is one of the most important gaseous components of the earth as well as the other planets in the solar system. There have Current address.
0022-3654/85/2089-4499$01.50/0
recently been several studies of solid C 0 2 at high pressures. Most of these studies have been on the cubic Phase I (space P U P Pa3). The Present Paper will Present evidence for a new high-pressure phase of solid CO, that occurs above 18 GPa ambient temperature. 0 1985 American Chemical Society
4500
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
A review of the recent high-pressure work on CO, will first be given. Powder X-ray diffraction studies at 296 K have been done by 0linger.l These show that CO, remains in the Pa3 phase throughout the range of pressure from freezing to 10 GPa at ambient temperature. Liu2 has reported a phase I1 of solid CO, on the basis of optical and powder X-ray studies. This new phase is reported to be stable in the 0.5-2.3-GPa region at room temperature. Recently, Liu3 has also published powder X-ray diffraction studies up to 50 GPa. This work indicates that C 0 2 remains in the Pa3 structure up to Mbar. However, this conclusion is based on the observation of only the three strongest diffraction lines in the pressure range above 9 GPa. The X-ray studies of Olinger’ and L i d are in substantial agreement in their range of overlap up to 10 GPa. Olinger does not report any evidence for phase I1 in the 0.5-2.3-GPa region. Raman studies of the three librational modes in phase I have been made by Schmidt and Daniels4 up to a few kilobars. Extensive Raman and infrared studies up to 12 GPa have been presented by Hanson and Bachman’ and by Hanson and Jones.6 These studies have focused on understanding the behavior of the internal vibrational modes and especially the pressure tuning of the Fermi resonance. They also reported studies on the pressure dependence of the librational modes. In these Raman studies4d crystalline CO, was solidified from the liquid at room temperature in some cases. The Raman spectrum characteristic of the Pa3 phase I was reported throughout the region where Liu2 reports phase 11. One must conclude that the evidence for phase I1 is contradictory and more investigation is needed. Bertran’ has analyzed the Fermi resonance data for C 0 2 in the dense fluid, liquid, and solid phases. Theoretical density functional studies have been done by LeSar and Gordon.* They studied the structural stability of N2 in a number of phases as well and also studied CO, in the Pa3 phase. Etters and Helmy9 have constructed a model for the internal molecular vibrational modes as a function of pressure that is able to reproduce the experimental frequencies very well. Gibbons and Klein’O and Kobashi and Kihara” have modeled the pressure dependence of the librational modes in the Pa3 structure. Experiment and Results In the present work, Raman scattering studies were made on solid CO, in the pressure range up to approximately 30 GPa. The measurements were all done at room temperature. Liquid CO, was loaded from a cylinder into the region around the gasket using an indium dam in a standard diamond anvil cell. The loading was done at a few bars pressure and a temperature in the liquid-phase range of CO,. The Raman scattering measurements were done with a standard Raman system with argon ion laser illumination and photon counting electronics. Specially selected low-luminescence type I diamonds were used. Observations were taken in the low-frequency region (50-600 cm-I) and of the two components, v+ and v-, of the vl, 2v2 Fermi resonance doublet. Above 12 GPa the lower component, v-, becomes too weak to observe except with very long runs. The upper component, v+, remains a single peak and was observable throughout these experiments. The frequency of v+ continues to increase gradually as an extension of our earlier work.4 The modes in the librational region were the focus of most of these experiments. Above 18 GPa the appearance of the Raman modes in the low-frequency region changed drastically in their appearance. This is shown in Figure 1 where the librational modes ( 1 ) Olinger, B. J . Chem. Phys. 1982, 77, 6255-6258. (2) Liu, L. Nature (London) 1983, 303, 508-509. (3) Liu, L. Earth Planet. Sci. Lett. 1984, 71, 104-110. (4) Schmidt, J. W.; Daniels, W. B. J . Chem. Phys. 1980, 73,4848-4854. ( 5 ) Hanson, R. C.; Bachman, K. Chem. Phys. Lett. 1980, 73, 338-342. (6) Hanson, R. C.; Jones, L. H. J. Chem. Phys. 1981, 75, 1102-1112. (7) Bertran, J. F. Spectrochim. Acta, Part A 1983, 39A, 119-121. (8) LeSar, R.; Gordon, R. G. J . Chem. Phys. 1983, 78, 4991-4996. (9) Etters, R. D.; Helmy, A. Phys. Reu. B. 1983, B27, 6439-6445. (10) Gibbons, T. G.; Klein, M. L. J. Chem. Phys. 1974, 60, 112-126. (11) Kobashi, K.; Kihara, T. J. J. Chem. Phys. 1980, 72. 3216-3220.
Hanson
Phase I 14.5 GPa
t
I
400
300
100
200 FREQUENCY (cm-’)
Figure 1. Raman scattering spectra of solid C 0 2 in phase I (Pa3) at 14.9 GPa and in phase 111 a t 18 GPa a t ambient temperature.
II
PHASE I Pa 3
300
r
I
L
1
$200-
3
2.
.
E
PHASE Dl
... .. .. 8
1001
0
4
8
12
16
20
24
PRESSURE (GPa )
Figure 2. Pressure dependence of the low-frequency Raman modes in solid C 0 2 a t ambient temperature. The data below 12 GPa are taken from ref 4. The three modes in phase I are the well-known librational modes. The pressure dependence of the Raman modes in phase 111 is poorly defined because of large pressure gradients. at 14.9 GPa in phase I and the new mode structure at 18.1 GPa are shown. In the new phase there are three modes at 245, 270, and 320 cm-’ as well as a suggestion df some overdamped modes in the low-frequency region. In a run at 24.9 GPa, there also was a suggestion of another mode at about 550 cm-I which is not shown here. This spectrum is very different from that of the three well-defined librational modes which are shown for phase I at 14.9 GPa. The appearance of a new mode structure in the librational mode region is taken as evidence for a new phase which will be called phase 111. Several other changes occurred at this phase change. The sample appeared to become birefringent. This suggests that the new phase I11 may be of lower than cubic symmetry. However, the birefringence may also be due to the large pressure gradients which appear in phase 111. The sample showed very large pressure gradients as the pressure was raised further. The ruby fluorescence lines broadened and rubies in different parts of the sample showed significantly different pressures. This prevented obtaining satisfactory Raman spectra at higher pressures except in a few cases. The pressure differences within the sample soon became of the order of 5 GPa or larger. The pressure differences in the cubic Pa3 phase are much smaller (of the order of 0.5 GPa). Figure 2 shows the data that we were able to obtain on the pressure dependences of the low-frequency modes. The data below 12 GPa are taken from our previous work.4 Discussion These Raman data are taken as strong evidence for the existence of a new high-pressure phase I11 of CO,. The X-ray diffraction data of L i d which are interpreted in terms of the Pa3 structure should not be taken as being in disagreement with this new phase since his X-ray data gave only three lines in the pressure range where phase I11 is stable.
J . Phys. Chem. 1985, 89, 4501-4517 What may be the nature of the crystal structure of phase III? The structure of C 0 2 phase I is largely determined by its large quadrupole moment as shown by Kihara.I2 Presumably the new phase of C 0 2has a structure which is also dominated by this large quadrupole moment in some as yet to be determined way. It should be a phase closely related to the Pa3 structure such that the three strong diffraction lines observed by L i d would also be strong lines for phase 111. The phases of solid N 2 might be a place to look for a suggestion of what is happening here. The Pa3 structure of solid N 2 is stable up to 0.4 GPa at which pressure N 2 transforms to the tetragonal P42/mnm phase. The Raman scattering pattern of this phaseI3 is very different from our observations and is thus ruled out. Other higher pressure structures (12) Kihara, T.; Koide, A. Adu. Chem. Phys. 1975, 33, 51-72.
4501
of N 2 have been reported recently, but these are disordered structures and are unlikely for C 0 2because of its large quadrupole moment. Detailed structural information on the new phase 111 of solid C 0 2 will await a more detailed high-pressure powder X-ray diffraction study. Improving on Liu's3 study will be difficult because of the weak X-ray scattering of first-row elements coupled with the small sample volume in a diamond anvil cell. Acknowledgment. Acknowledgments are made for help in the experiments and discussions with R. L. Mills, D. Schiferl, and B. Olinger. Registry No. C 0 2 , 124-38-9. (13) Medina, F. D.; Daniels, W. B. J . Chem. Phys. 1976, 64, 150-161.
Branching Fractions for Penning Ionization in Quenching of He(2%), Ar('P,,,), Ne(3P2,,) Atoms'
and
Michael T. Jones, T. D. Dreiling, D. W. Setser,* and Richard N. McDonald Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: October 17, 1984)
A cold-cathode discharge was used to produce metastable He(2%), Ne(3P2,0),and Ar(3P2,0)atoms in a flowing-afterglow apparatus with He as the carrier gas, and a mass spectrometer was used to detect the relative intensities of the positive ion signals from the reactions of various added reagents. The branching fractions for Penning ionization of 15 compounds with He(2%), Ar(3P2,0),and Ne(3Pzo) have been measured relative to standard reference reactions, which were CO for He(2%) and Ne(3P2,0)and NO for Ar(fP2,0).For each reaction the primary distribution of ions from the Penning ionization event was recorded. For both He(2%) and Ne('P2,,), Penning ionization appears to be the dominant quenching channel: however, Penning ionization branching fractions of less than unity were found in most cases for Ar(3Pz,o)atoms. In addition to measurements for the ionization channels, branching fractions for neutral channels that give emission are reported in an appendix for some Ar(3Pz,o)and Kr(3P2)reactions with bromine and iodine containing reagents. The ionization and neutral product branching fractions can be combined to obtain a general viewpoint for quenching of metastable rare gas atoms for several reagents. Penning ionization by Ar(3P2,0)is a useful way for generating parent ions with a minimum of fragmentation for subsequent studies of ion-molecule reactions in a flowing-afterglow apparatus.
I. Introduction Penning ionization (PI) reactions of electronically excited He(2%; 19.8 eV), Ne(3P2,0;16.6 and 16.7 eV), and Ar(3P2,0;11.5 and 1 1.7 eV) atoms have been actively studied in recent The branching fractions for ionization with He(%) have been studied for several small molecules; but, less is known regarding exit channel distributions for quenching of Ar(3P2,0)and Ne(3P2,0) atoms. While our work was in progress, some PI branching fractions were reported for Ar(3P2,0)with several small molecules using an indirect technique."' Branching fractions do not appear (1) This work taken in part from the M.S. thesis of M.T.J., Kansas State University, 1983. (2) Hotop, H.; Neihaus, A. Int. J . Mass Spectrom. Ion Phys. 1970,5,415. ( 3 ) Cermak, V. J. Electron. Spectrosc. Relat. Phenom. 1976, 9, 419. (4) Hotop, H. Electron. At. Collisions, Proc. Int. ConJ, l l t h , 1979, 1980, 271. ( 5 ) Niehaus, A. Adu. Chem. Phys. 1981, 45, part XI, 399. (6) Kolts, J. M.; Setser, D. W. In "Reactive Intermediates in the Gas Phase"; Setser, D. W., Ed.; Academic Press: New York, 1979; Chapter 3. (7) Golde, M. F. "Gas Kinetics and Energy Transfer"; The Chemical Society: London, 1977; Vol. 2. (8) King, D.; Setser, D. W. Annu. Reu. Phys. Chem. 1976, 27, 407. (9) Golde, M. F.; Ho, Y.-S.; Ogura, H. J . Chem. Phys. 1982, 76, 3535. (10) Balamuta, J.; Golde, M. F. J. Chem. Phys. 1982, 76, 2430. (1 1) Balamuta, J.; Golde, M. F.; Ho, Y . 4 . J . Chem. Phys. 1983, 79, 2822.
0022-3654/85/2089-4501$01.50/0
to have been reported for Ne(3P2,0)atom reactions, although N, and CO probably quench mainly by PI.'2-'4 In this work, the branching fractions for ionization, rQ+= kQ+/kQ,have been measured at room temperature by using a Rg*
+Q
- + kQ+
Rg kQ
Q'
+ e-
kQ = kQt
other products
+ kQ*
(la) (1b)
flowing-afterglow apparatus, which employed a quadruple mass spectrometer to monitor the product ions. A cold-cathode discharge located near the entrance to the flow reactor generated the metastable rare gas atoms (Rg*).6 The rQ+ were measured by observing the ion signals for a given reagent molecule relative to that of a reference reaction with a known branching fraction for a common metastable atom concentration. By changing the reagent concentration the primary and secondary ion channels could be identified and the primary ions from Rg* quenching are results, some electronically excited reported. In addition to the rQ+ (12) West, W. P.; Cook, T. B.; Dunning, F. B.; Rundel, R. D.; Stebbings, R. F. J. Chem. Phys. 1975, 63, 1237. (1 3) Brom, J. M.; Kolts, J. H.; Setser, D. W. Chem. Phys. Lett. 1978, 55. 44. (14) Bruno, J. B.; Krenos, J. J . Chem. Phys. 1983, 78, 2800.
0 1985 American Chemical Society
