5072
J. Phys. Chem. 1982, 86,5072-5075
Molecular Species in the As-0 System I?.D. Brlttain, K. H. Lau, and D. L. Hlidenbrand’ SRI International, Menlo Park, California 94025 (Received: April 22, 7982; In Final Form: August 16, 1982)
Mass spectrometric analyses of saturated and unsaturated vapors over arsenolite,Asz03(s),and over an Asz03-Ca0 mixture have identified a surprisinglylarge number of thermodynamicallystable A s 4 molecular species. These species,which include As406, As403, Asz03, AsOz, AsO, and perhaps h . 3 0 4 , were identified primarily from threshold appearance potential measurements. Considerable care was required, since some ions are produced both by fragmentation and by simple ionization. There may be several fragmentation pathways yielding & 3 0 4 + , but the evidence regarding this ion is complex and a clear decision cannot yet be made. The possible effects of temperature on the mass spectra of As406and other complex As-0 species are discussed, but the evidence to date appears inconclusive.
Introduction It has been established from vapor density’ and electron diffraction2 studies that the saturated vapor over As203(s) (arsenolite) is made up of As406gaseous molecules. Additionally, the diatomic oxide As0 has been identified from its electronic band s p e c t r ~ m and , ~ recently by mass ~pectrometry,~ but there appears to be little information about the existence of other stable gaseous species in the As-0 system. One might reasonably expect that lying between As406and As0 there would be other stable As-0 species, and that these would prevail in the undersaturated vapor or in the vapor over condensed phases of reduced As203activity. By analogy with the N-0 and P-O5 systems, other gaseous species such as As2O3, Asz04,and Asz05 could play a role in the As-0 system chemistry, but the chemical bonding in the latter might be sufficiently different so that other species would predominate, depending on temperature and oxygen potential. In modeling the equilibrium and nonequilibrium chemistry of the As-0 system in copper smelting processes, it will be necessary to have thermochemical information about the various gaseous species. This could be particularly important in treating the condensation of arsenic oxides in flues and precipitators, since vaporization and condensation will not be simple congruent processes if the flue gases contain a mixture of As-0 molecular species. The exploratory studies described here were carried out to define the As-0 vapor chemistry and to obtain some indication as to the degree of complexity of the system. Several types of effusive sources containing a variety of As-0 phases were utilized, and the effusing vapors were analyzed by mass spectrometry. While this work was in progress, Drowart6 reported on other mass spectrometric studies of the A s 4 system that have an important bearing on the present investigation. Experimental Section Preliminary runs on most of the vaporization sources were made with an Extranuclear Labs quadrupole mass filter system; the effusion oven source for the quadrupole (1) H. Biltz, 2.Phys. Chem. (Leipzig),19, 417 (1896). (2) L. R. Maxwell, S. B. Hendricks, and L. S. Deming, J. Chem. Phys., 5, 628 (1937). (3) K. P. Huber and G. Herzberg, “Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules”, Van Nostrand-Reinhold. New York. 1979. (4) K. H. Lau, R. D. Brittain, and D. L. Hildenbrand, Chem. Phys. Lett., 81, 227 (1981). (5) S. Smoes and J. Drowart, Faraday Symp. Chem. Soc., 8, 139 (1973). ~ - - I
(6) J. Drowart, ‘Thermodynamic Studies at High Temperatures by the Mass Spectrometric Knudsen Cell Method”, 29th Annual Conference on Mass Spectrometry and Allied Topics, May 24-29, 1981.
and its method of operation are quite similar to those of our magnetic-sector mass spectrometer described in the l i t e r a t ~ r e . These ~ quadrupole runs were quite useful in rapidly surveying the vapor chemistry and in selecting the cell configurations and sample compositions for generating the various species. All of the appearance potential (AP) measurements and subsequent characterizations were made with the magnetic sector i n ~ t r u m e n t .The ~ automatic recording of ionization efficiency (IE) curves8 and resultant evaluation of AP’s by the vanishing current method proved to be crucial to the identification of the neutrals. Several distinct threshold energies were observed for some of the ions, as well as significant shifts with temperature, so that it was necessary to have accurate records to make the proper identifications. Alumina effusion cells were used in all of the studies reported here. The cell and sample configurations were as follows: (1)As203(s)in a conventional single-chamber effusion cell; (2) As203(s)in the low-temperature chamber of a double cell in which the vapor could be superheated; and (3) a mixture of CaO(s) and As203(s)as the vapor source in the double cell. The As203(s)(arsenolite) sample was Johnson, Matthey “Specpure” grade material.
Results The ions observed in the mass spectrum of the saturated vapor over arsenolite at 370 K were As406+, As405+,h 3 0 4 + , and Asz03+ with threshold AP’s of 10.3, 15.5,13.0, and 18.0 eV, respectively. In the use of the term “appearance potential” we make no formal distinction between threshold energies for ionization processes leading to the formation of parent or of fragment ions; for the former, the measured AP’s should however lie close to the corresponding ionization potentials (IP). No other ions were observed at ionizing energies up to 25 eV. From the magnitudes of the threshold AP’s it is clear that As406+ is a parent ion, and that the other ions are formed by fragmentation processes. Our observed AP for As406+ agrees well with values of the first ionization potential of As406determined by photoelectron spectroscopy, ranging from 10.01 to 10.05 eV.+” Thus there is no evidence for species other than As406 in saturated arsenolite vapor. When measured at ionizing energies 3 eV above the respective thresholds, the ion intensities As406+/As405+/ (7) D. L. Hildenbrand, J . Chem. Phys., 48, 3657 (1968); 52, 5751 (1970). (8) D. L. Hildenbrand, Int. J.Mass Spectom. Ion Phys., 4, 75 (1970); 7, 255 (1971). (9) P. H. Cannington and H. J. Whitfield, J . Electron Spectrosc. Relat. Phenom., 10, 35 (1977). (IO) E. Diemann, Inorg. Chem. Acta, 24, L 27 (1977). (11)R. G. Edgell, M. H. Palmer, and R. H. Findlay, Inorg. Chem., 19, 1314 (1980).
0022-3654/82/2086-5072$01.25/00 1982 American Chemical Society
The Journal of Physical Chemistry, Voi. 86, No. 26, 1982 5073
Molecular Species in the As-0 System
TABLE I: Threshold Appearance Potentials of As-0 Species under Various Conditions ~
T
=
1050 K
~~~~
condi-
ion As,06+ As405+ As403+ As404+ As304+
As303+ As302+ As203+
As202+ AsO,' AsO'
AP, eV
*
10.3 15.5 2 9.0 f 8.8 i 8.9 13.0 i 11.0 10.5 < 9.0 10.8 i 10.6 +. 18.0 f 10.4 t 14.5 i 10.8 i 9.0 t
*
*
0.3 0.5 0.3 0.3 0.3 0.3 0.3 0.3
0.3 0.3 1.0 0.3 0.5 0.3
0.3
tionn
neutral
A A
AS406
B D
As405
C A
B
As403 As404
AS406
C
D D D
(As304)
A
B C
D B
AsO, As0
a A, saturated vapor over As203(s),370 K; B, unsaturated vapor over As,O,(s), 1050 K ; C, unsaturated vapor over As,O,(s), 1160 K ; D, unsaturated vapor over As203(s)-
CaO(s), 1400 K .
As304+/As203+were in the approximate ratio 100/0.5/ 3610.5. The temperature dependence of As406+,as measured at 13-eV ionizing energy, was determined at four points over the range 317-372 K. Least-squares analysis of log [(I(AS&+)T)] vs. 1 / T yielded the second law slope heat AH", = 27.7 f 0.5 kcallmol, in close agreement with the corresponding value of 27.8 kcallmol determined by Behrens and Rosenblatt12 from mass effusion rates over the range 367-429 K. A pressure calibration based on gold and tin vaporization, together with the observed ion intensities, indicated an As406vapor pressure of 4.4 X lo-' atm at 372.4 K, compared to the mass effusion value of 4.1 X atm.12 When arsenolite was vaporized from the double cell and the unsaturated vapor superheated to 1050 K, the threshold AP's of As405+,As304+,and Asz03+ decreased to 9.0,11.0, and 10.4 eV, and the new ion AsO' appeared at 9.0 eV. These and other AP's measured under the various experimental conditions are summarized in Table I. The recorded ionization efficiency curves of As406+, AS405+, As304+,A s 2 0 3 + , and AsO' at 1050 K are shown in Figure 1; the sharp break in the As405+curve at 15 eV and the low-energy "foot" extending below 10 eV constitute clear evidence for at least two distinct ionization processes. The threshold energies of As405+, As2O3+, and AsO+ are in accord with the expected ionization potentials of the corresponding neutrals, since these should be similar for b o 3and As406,while the IP's of the valence-unsaturated species AS405 and A s 0 are expected to be somewhat lower and to lie below 10 eV. By the same reasoning, As304+at 11.0 eV must result from a dissociative ionization process, possibly involving a new neutral precursor such as AS405. In any event, the mass spectra unambiguously identify As405,As203, and A s 0 as new components of the neutral vapor under these conditions. As406+continued to be the major ion in the mass spectrum, and its threshold AP remained unchanged. Relative ion intensities As406+/ As405+/As304+/Asz03+/AsO+ measured at AP + 3 eV were in the ratio 100/0.3/ 101 113. On increasing the upper cell temperature further to 1160 K, some additional changes were evident. Although the lowest threshold AP's of As406+,As405+, As203+,and AsO+ (12)R. G.Behrens and G.M. Rosenblatt, J. Chem. Thermodyn., 4, 175 (1972).
ELECTRON E N E R G Y , eV
Flgure 1. Ionization efficiency curves of species in unsaturated As,O, vapor at 1050 K.
remained unchanged, that of As304+,decreased further to about 10.5 eV, and the new ions AS404+ and As202+appeared at 8.9 and 14.5 eV, respectively. For reasons noted earlier, As404+ clearly is a parent ion, while h 3 0 4 + , in spite of the still lower threshold AP, is believed to be yet a fragment ion; the new source of As304+could be AS404, but the latter could not be verified. AszOz+is also a fragment ion of unknown origin. When the activity of As203(s) was reduced by mixing with CaO(s) and the vapor was superheated in the double cell to 1400 K, the additional ions AS403+,As303+,As302+, and As02+appeared with threshold AP's of 8.8, 10.8, 10.6, and 10.8 eV, respectively. Yet a further change was evident in the As304+IE curve, as a weak tail extending to 9 eV or below was observed, although the intense second threshold continued to be observed at 10.3 eV, now essentially unchanged from the value at 1160 K. It would seem that both As403+and AsOz+ are parent ions, formed by simple ionization of the corresponding neutrals. The assignment for As403+is straightforward, while an IP of 10.8 eV is within expectations for the dioxide molecule AsOz with substantial oxygen character in its ionizing orbital. For As303' and As302+ the situation is less clear, but fragmentation processes are the most likely source. The weak, low-energy tail for AS304' suggests a simple ionization process, but it was not possible to increase the relative contribution from this tail portion by changing experimental conditions. When the sample was heated to 1670 K, the major onset of the As304+IE curve remained at 10.3 eV, with the same weak tail indicating a first threshold below 9 eV; since it was not possible to enhance the low-energy tail of the curve, the assignment to neutral As304+ remains in doubt. No change was noted in the threshold energies of AS404' or As02+ which also were observed at the highest temperature. As it became apparent that the threshold AP of AS304+ decreased significantly with increasing effusion cell temperature, a more detailed examination was made in order to determine the nature of the temperature dependence. In this sense, the As304+ threshold behavior differed markedly from that of the other As-0 ionic species, where discrete threshold energies of parent and fragment ions, independent of temperature, were observed. Since AS304+ was one of the more abundant ions in the mass spectra under each of the conditions summarized in Table I, it was possible to determine AP(As304+)at a number of cell
5074
The Journal of Physical Chemistry, W. 86,NO. 26, 1982
1 3 0 1
0
I
0 0
100
~.
t
k 400
u 800
1200
1600
CELL TEMPERATURE, K
Flgure 2. Appearance potential of As30,+ as a function of cell temperature.
temperatures over the range 400-1700 K; the results are shown graphically in Figure 2. The data do not include the weak threshold process below 10.5 eV that was observed at temperatures above 1400 K. The relative magnitudes of the two processes at 1400 K can be judged from the fact that the total As304+signal decreased by 99% as the ionizing energy was lowered from 12.5 to 10.5 eV. Figure 2 shows that AP(As304+)decreases in seemingly continuous fashion from 13.0 eV at 400 K to 10.5 eV at 1160 K, but then remains constant at 10.4 f 0.2 eV over the temperature range 1200-1700 K. A complete mass spectrum with IE curves was not obtained for each individual temperature point in Figure 2, so that the vapor composition at each point is not available. However, neutral As405,as evidenced by AP(As405+)at 9.0 eV, was detected at 845 K along with As406,indicating at least two potential neutral precursors for As304+at that temperature. One possible conclusion is that As304+ is formed by fragmentation of lighter oxides such as As405and AS404 as well as from As406, and that the shifting vapor composition with increasing temperature leads to the observed temperature trend in AP(AS304'). On this basis, one would conclude that there is only one major neutral source of As304+above 1200 K. Other explanations are possible, as discussed in the next section.
Discussion A summary of the ions and their threshold energies observed under the various experimental conditions is given in Table I; neutral precursors are assigned where the evidence is clear. Only the initial conditions are listed, but some of the species were observed over a wide range of temperatures, e.g., As406up to 1360 K, As404and As405 up to 1670 K. The results indicate clearly that As406 is the sole constituent of the saturated vapor over arsenolite, but that the additional thermodynamically stable neutral species As405, As404, As4Q3,Asz03,AsOz, AsO,and possibly As304appear as the undersaturated vapor is progressively heated. Drowart6 reported identification of AsO,As02, and As203, and of the more highly oxygenated species As407, As408, As409,and As4OI0in mass spectrometric studies of the As-0 system conducted respectively at higher and lower oxygen potentials than those prevailing here. It is evident that the vapor chemistry of the As-0 system is much more complex than one might have expected from the known behavior of other group 5 oxides. However, there is strong evidence from recent mass spectrometric studies of Sidorov and colleague^'^ that the Bi-0 system (13)L.N.Sidorov, I. I. Minayeva, E. Z. Zasorin, I. D. Sorokin, and A. Ya. Borschevskiy, High Temp. Sci., 12, 175 (1980).
Brittain et al.
exhibits similar complexity. Ions of the same type were observed in the saturated and unsaturated vapor over Bi203(s),and the presence of the neutral species Bi406, Bi30,, Bi203,Bi202,Bi20,and BiO was inferred. It is significant that some of the ions such as A S 4 0 4 + , As403+,As303+, and As302+were not observed in the mass spectrum of the saturated vapor over arsenolite up to 30-eV ionizing energy. Their appearance in the undersaturated vapor at elevated temperature thus provides conclusive evidence, aside from the AP data, that thermal dissociation yields a new set of thermodynamicallystable As-0 species. However, the AP data alone are sufficiently conclusive, since the lowest thresholds of As405+,As404+,AS&+, and AsO+ are below 10 eV, as one would expect for the IP's of the parent neutrals. Likewise, the near equality of the and As02+is in accord with lowest AP's O f A%&+, the anticipated IP's of those neutral species. The temperature dependence of the As304+threshold AP, shown in Figure 2, is quite interesting and deserves further comment. As noted earlier, one possible explanation for this behavior is that thermal dissociation of the vapor effects a progressively larger proportion of lighter oxide species (As405,A S 4 0 4 ) with lower threshold AP's for these individual threshold curves fragmentation to As304+; could then fold in together to yield a composite curve that appears to shift with temperature, as we have observed for other gas mixtures yielding a common ion. Based upon independent measurements on the As-0 system, Drowart6 suggested an alternate explanation of the As3F4+threshold behavior. He reports not only that AP(As304 ) varies from 12.7 eV at 400 K to 11.5 eV at 1080 K, in fair agreement with our results in Figure 2, but also that the ion ratio As4o6+/As3o4+when measured with 20-eV ionizing electrons decreases from 3.9 at 400 K to 0.9 at 1200 K. On the assumption that As406(g) is the sole source of AS304+, Drowart6 interprets this behavior in terms of the theory of unimolecular reactions, assuming that the thermal vibrational energy present in the neutral molecule is quantitatively transferred during the ionization process, and that the total energy so imparted to the dissociating molecular ion is randomized and made available for bond rupture. The observed AP for the fragment ion thus would be lowered, while the parent/fragment ion ratio would decrease with temperature as a result of the increased As406+ ion decomposition rate at the higher temperatures. This latter interpretation is entirely reasonable for a complex molecule such as As406,having a large number of internal modes. The underlying basis for temperature-dependent mass spectra has been described by Vestall4 in terms of the quasi-equilibrium theory. Chupka15has discussed the effect of thermal energy on fragment ionization thresholds and has presented evidence that thermal energy is fully effective in the dissociation of parent ions. This is in accord with the quasi-equilibrium theory of mass spectra, which indicates that all of the thermal energy in the original molecule is available for bond fission in the dissociative ionization process. If this were so in the dissociative ionization of As406,then one could calculate the maximum effect on the AS@,+ fragment ion threshold AP from the thermodynamic functions of As406given by Behrens and Rosenblatt.12 From these functions,12 we calculate that the total increase in the average internal thermal energy of As4O6(g)between 400 and 1200 K is 35.4 kcal/mol, or 1.53 eV/molecule. In the same temperature range, the data in Figure 2 show that AP(As304+)decreases by 2.6 f 0.4 eV, so that one cannot (14)M.L. Vestal, J. Chem. Phys., 43, 1356 (1965). (15)W. A. Chupka, J. Chem. Phys., 30, 191 (1959);54, 1936 (1971).
Molecular Species in the As-0 System
ascribe the entire temperature effect to thermal energy transfer durng fragmentation; there would have to be an alternate process contributing to AS304' formation at low ionizing energy. If thermal energy transfer during ionization were the primary process responsible for the temperature dependence of AP(As304+), one would not expect this parameter to level off at 10.4 eV and remain essentially constant over a range of 500 K, as shown in Figure 2. However, if the temperature shift of AP(As304+)were due largely to a change in the equilibrium composition of molecular As-0 species, then such a leveling off would be expected as the neutral precursor responsible for the lowest threshold energy begins to predominate in the vapor. Another pertinent point here relates to the temperature variation of the As4o6+/As3O4+ ion ratio. As noted earlier, Drowart6 reported this ratio to be strongly temperature dependent and likewise attributed it to thermal energy effects, in the spirit of the quasi-equilibrium theory. Our ion intensity data, although evaluated at lower ionizing energies, also show such a trend, with the ratio I(As406+, 13 eV)/I(As304+,16 eV) varying from 3.2 at 400 K to 1.2 at 1000 K. In this discussion of the As4o6+/As3O4+ ratio, it should be kept in mind that the sample compositions used by Drowart6 and the prevailing oxygen potentials differed from those used in this work, so that the distribution of gaseous As-0 species in the two studies were not necessarily the same a t a given temperature. However, there is sufficient evidence presented earlier that other molecular precursors appearing with increasing temperature (As405,As404,As304) could also contribute to the AS304+ signal and thereby affect the As4o6+/As3o4+ ratio as observed in our work; the extent to which these same precursors could affect the ion ratio observed by Drowart6 is not yet clear. It is worth noting that the analogous parent/fragment ratio I(As406+,13 eV)/I(As405+,18 eV), after correction for the parent ionization of As405+at the higher temper-
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5075
atures, was essentially constant at 180 f 1when evaluated at 380,850, and lo00 K. Without correction for low-energy parent ionization of As405+,the As4O6+/As4O5+ ratio had values of 181,140,and 128 at the three noted temperatures, so that the correction is significant. As seen in Figure 1 and Table I, the energy separation between the first and second thresholds of AS&+ is relatively large and there are no competing processes yielding As405+,making the correction relatively straightforward. The interpretation of the h 4 0 6 + / h 3 0 4 + ratio might be more complex because of multiple neutral contributors to As304+formation and because of the small energy increment (2.6 eV) between highest and lowest thresholds. In summary, it appears that the vapor phase chemistry of the A s 4 system is highly complex, and that special care is required in interpreting the mass spectrometric evidence. Although there is ample reason to suspect that thermal energy effects may have a major influence on the mass spectrum of h406 and perhaps other large A s 4 molecular species, the evidence to date is not conclusive, and an alternate explanation for the observed temperature effects involving changes in gas-phase composition is at least equally consistent with the results. In order to eliminate possible effects due to temperature-dependent fragmentation in subsequent thermodynamic studies, however, it may be prudent to limit parent ion intensity measurements to the region within a few electrovolts of ionization threshold, where fragmentation is not energetically allowed. Use of the latter approach, of course, demands that parent and fragment ion onset energies be known accurately.
Acknowledgment. The authors are greatly indebted to Drs. J. Drowart and S. Smoes for valuable discussions of this work, and for many helpful suggestions regarding the manuscript. This research was supported by the National Science Foundation, Directorate for Engineering, under Grant DAR 79-11296.
