W. D. Hewett, J. H.
2640
Newton, and W. WeRner
Absorption Spectra of Molybdenum Oxide Molecules and Molybdenum Atoms in Neon and Argon Matrices at 4 K W. D. Hewett, Jr., J. H. Newton, and W. Weltner, Jr.’ Department of Chemistry, Univers/?yof F/orMa. Gainesville, FlorMa 326 7 1 (Received November 78, 1974; Revised Manuscript Received September 2, 7975) Publication costs assisted by the Air Force Office of Scientific Research and the National Science Foundation
MoO2, Moos, and (Mo03)2-5 have been trapped in neon and argon matrices and studied spectroscopically in the infrared, visible, and ultraviolet regions. No evidence for a MOOmolecule was found and no definite electronic spectra were observed for any of these molecules, in sharp contrast to the spectra of WO and WO2 produced and isolated in the same way. The molybdenum oxide molecules were prepared by vaporization of the solid oxides or by passing l 6 0 2 and 1 8 0 2 over molybdenum metal at 2000 to 2600 K. From Mo isotopic splittings in the ir, Moo2 was found to be bent at an angle of 118 f 4O and to have vibrational frequencies (in neon for 98Mo) ul”(a1) = 948, Y3”(bl) = 899 cm-l. Moo3 was deduced to be pyramidal (C3”) with angle /3 = 61.5 f 2 O and with stretching frequencies vl”(a1) = 976 and v3”(e) = 922 cm-l. Frequencies of 978,858, and 840 cm-’ were assigned to the trimer (Mo03)3of D3h symmetry. Two sets of triplets corresponding to the z7P a7S3 and y7P a7S3transitions of Mo atoms have been observed in solid neon and correlated with the earlier observations in the heavier matrices.
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Introduction The molecular oxides of the refractory metals Nb, Ta, and W have been thoroughly investigated in rare gas matri~ e s , I and - ~ in each case spectra were observed which were assignable to the monoxide and higher oxides. Like tungsten,’ molybdenum forms a large number of molecular oxides at high temperatures, and also, as with the tungsten oxides, there is essentially no reliable analysis of the complex gas-phase spectra. Mass spectrometric studies have identified the molecules Moon, Moos, (Mo03)z, (M00313, (Mo03)4, and (MoO3)s in the vapor over the various solid oxides of molybdenum. The molecules vaporizing from solid Moo3 were investigated by Berkowitz, Inghram, and Chupka4 who found the most abundant species to be (Mo03)3, (Mo03)4, and ( M 0 0 3 ) ~with relative intensities of 1.0:0.33:0.05, respectively, at a temperature of 850 K. The vaporization of solid Moon5has been shown to produce Moo3 as the most abundant gas-phase molecule, followed by (MOO&, MoO2, and (Mo03)3 over a range of temperatures from 1470 to 1785 K. When a stream of molecular oxygen is passed at low pressure over the surface of hot Mo metal, the major gaseous products formed are Moo2 and MOOSwith relative abundances depending on the 0 2 pressure and the temperature of the metaL6 The only reported mass spectrometric observations of the MOO m o l e ~ u l resulted e ~ ~ ~ from experiments in which Mo metal was heated in an aluminum oxide crucible t o temperatures above 2170 K, where the gaseous oxides were formed in the order MoOz > MOO > MOOS.Unfortunately, this system is not suitable for matrix isolation work because the pressures of Al, 0, Al20, and A10 are one to three orders of magnitude greater than those of the Mo oxides under these conditions. Early reports of emission spectra assigned to Moo9-” have been shown by Howard and Conway12 to be largely due to the nitrides of the form Mo,N. According to these authors, a series of near-infrared bands at 865.63, 861.39, 859.90, and 859.51 nm and others at 650.9 and 644.5 nm The Journal of Physical Chemistry, Voi. 79, No. 24, 1975
definitely arise from oxides of Mo, but an assignment to particular vapor species could not be made. The gas-phase infrared spectrum of the vapors over solid MOO3 at high t e m p e r a t u r e ~has ~ ~ been , ~ ~ reported to exhibit absorption bands at 969 and 815 cm-l which have been interpreted as the terminal M e 0 and Mo-0-Mo bridge vibrations of rings of (MOO&. A matrix isolation study’s in solid Ar of the vapors produced when solid Moo3 was heated to 66OOC revealed absorptions at 977,865,856, and 837 cm-’. The 977-cm-’ feature was assigned to the out-ofplane M e 0 vibrations of rings of Moo3 units, and the remaining bands were assigned to the ring vibrations of (M003)5, (Mo03)4, and Mo(03)3, respectively. Previous studies of Mo atoms isolated in rare gas matrices16,17 have shown that the strong atomic transitions which are observed can be correlated with gas-phase transitions from the a7S3 ground state to the z7P and y7P states. Since at the highest temperatures used in this research these atomic absorptions formed a background spectrum, this information was extremely helpful in the analysis of the ultraviolet spectra observed here. Also, because of the use of neon matrices in this research, these absorptions were also identified and assigned in that matrix gas. This paper describes an attempt to learn something about this complex system and to compare the spectra with those of the tungsten oxides. Some definitive results are only obtained for the simpler molybdenum oxides, and the results indicate that further progress on the larger species will require considerably more spectroscopic work. Experimental Section The Dewar used in this work h y been described previously.18The matrices were formed on calcium fluoride or cesium iodide windows cooled to about 4 K with liquid helium. The background pressure in the Dewar was typically 2 Torr after X Torr before filling with He and 4 X filling. Research grade argon was purchased from Matheson, and ultrapure neon was obtained from Airco. The two
Spectra of Molybdenum Oxide Molecules and Molybdenum Atoms rare gases were used without further purification other than passage through a copper coil immersed in liquid nitrogen to freeze out any condensable impurities. The molybdenum metal (99.9+%) was purchased from General Electric. High-purity oxygen (99.998%) was purchased from Airco, and 91% enriched l 8 0 2 was purchased from Miles Yeda Laboratories to be used for isotopic substitution. The molybdenum oxides were produced in the apparatus shown in Figure 1. The molybdenum cells were made from l/s-in. 0.d. tubing with a wall thickness of 0.020 in. and were cut to a nominal length of 1 in. on a Pistorius wheel with a carborundum disk. The ends of the cells were ground to a slight taper to assure a tight fit into the holes in the Mo rods which served as the endcaps and electrical contacts for the cells. These rods were shrink fitted into holes in the copper blocks to give good thermal and electrical connections and to prevent 0 2 leakage. One of the Mo supports was drilled out and used as the 0 2 inlet to the hot cell. The 0 2 gas was leaked into one end of the Mo cell from a 30-ft3 cylinder at 2000 psig. An oil free regulator was used to step the pressure down to about 2 psig, and a needle valve was used to control the flow rate of gas through the cell. Under operating conditions; the cells were typically heated to temperatures between 1700 and 23OO0C, and the 0 2 flow was controlled so that the pressure inside the furnace chamber could be varied from 2 X to 5 X lov5 Torr with a typical background pressure being 2 X lom6Torr with the cell at 22OOOC. The cells were resistively heated by passing currents of several hundred amperes through them, and the surface temperature was monitored with a Leeds and Northrup optical pyrometer. All temperatures reported in this paper have been corrected for the nonideality of molybdenum as a blackbody emitter using the data of Worthinglg as reported in the book by EspeZ0and are probably within 25' of the true temperature of the cell. Experiments were also performed in which solid MOOS (Spex Industries, 99.99+%) was vaporized from a tantalum cell a t temperatures ranging from 600 to 800'C with the products trapped in Ar and Ne matrices. The cell temperature was monitored with a chrome1 vs. alumel thermocouple which was strapped to the hot cell with tantalum wire. The thermocouple output was measured using a Leeds and Northrup volt potentiometer. These experiments were done to confirm the Ar matrix results of Mal'tsev et al.15 and to obtain higher resolution infrared spectra of the matrices than those reported in that work. Absorption spectra were recorded from 1200 to 199 nm using a Jarrell-Ash 0.5-m Ebert mount monochromator. Gratings blazed a t 300,500,800, and 1000 nm were used to span the spectral regions of interest. A tungsten ribbon lamp was used as the light source from 1200 to 350 nm, and a Sylvania DE350 D2 lamp was the source in the ultraviolet region. The detectors for this work were a RCA 7102 photomultiplier tube for the near-infrared region and a RCA 7200 PMT for the visible and ultraviolet regions. Their outputs were fed into a Jarrell-Ash electrometer amplifier and then recorded on a Bristol Model 570 strip chart recorder. The visible and ultraviolet spectra were calibrated using a low-pressure mercury arc lamp and measured to the nearest 0.5 nm unless otherwise noted. Absorption spectra in the infrared were taken using a Perkin-Elmer 621 grating spectrophotometer and were routinely scanned from 4000 to 200 cm-l. The double-beam spectra were recorded vs. a nitrogen-purged cell with two
2841
..
3 j C u Block
1
OAG-3
Inlet
Water Cooled Cu Electrode
Figure 1. Apparatus for preparatlon of molybdenum oxide mole-
cules.
CsI windows which were positioned in the reference beam. This cell was equipped with an adjustable iris which allowed the operator to attenuate the reference beam to compensate for light scattered from the sample beam by the thicker matrices. The spectra were calibrated with the absorption spectrum of a thin film of polystyrene and are accurately reported within a range of f0.5 cm-l. The relative positions of bands could be measured and experimentally reproduced with an accuracy of f0.05 cm-' when the higher scale expansions were used. Results I. Ultraviolet and Visible Absorption Spectra. a. Neon Matrices; Mo Atom Spectra. The absorption spectra of the neon matrices were recorded over the range 800-245 nm with the exception of one experiment in which the CsI target and windows were replaced with CaF2 optics where the ultraviolet range was extended to 190 nm. When samples were deposited with the Mo cell a t temperatures above 20OO0C, the strongest absorptions were Mo atomic transitions. Figure 2 shows the ultraviolet spectrum of a matrix deposited from a Mo cell at 2125'C. The prominent features are the strong triplet a t about 305 nm and the single band a t 359.3 nm. The features at 303.0, 305.5, and 307.4 nm are attributed to the transitions from the atomic ground state (Kr) 4d55s 7S3to the excited levels y7P4, y7P3, and y7Pz, respectively.21 The lone band at 359.3 nm is the third member of the triplet shown in Figure 3. These absorptions occur at 371.5, 366.5, and 359.2 nm and are assigned as transitions to the z7P2, z7P3, and z7P4 atomic states, in that order. Neon matrices having infrared absorptions in the Mo-0 stretching region (1000-800 cm-l) also exhibited a few very weak absorption features in the uv such as those at 284.5 and 344.0 nm in Figure 2. These bands have been assigned in Table I to molybdenum oxides, but this must be considered as tentative in view of their low intensity and lack of any observable progressions. The only strong oxide bands occurred in the far-uv at 212.0 and 229.7 nm with a shoulder a t 243.7 nm. Unfortunately, it was not possible to observe these bands in the same experiments as those in which the infrared spectra were recorded because of the infrared cutoff frequency of the CaFz optics, but in similar The Journal of Physical Chemistry, Vol. 79, No. 24, 1975
2642
W. D. Hewett, J. H. Newton, and W. Weitner Hs 36pO
i
CSI Backgrcund
Z
v
A
TABLE I: Ultraviolet Absorptions in Neon Matrices
I
/
W M
3440
VNe
ANe,
3593
371.5 366.5 359.2 344.0 332.9 307.4 305.5 303.0 284.5 282.6 271.5 243.7 229.7 212.0
t- 3074
a
I
I
L305.5
303.0
280
290
300
310
320
340
330
350
360
vNe, cm" vgas,cm-ia
nm
370
Wavelength, nrn Flgure 2. Ultraviolef absorption spectrum of a neon matrix at 4'K
containing molybdenum atom and weak molybdenum oxide bands.
a
26 910 27 277 27 831 29 061 30030 32 521 32 723 32 994 35 139 35 375 36 822 41 022 43 522 47 154
25 614 25 872 26 321
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vras
1296 1405 1510
Assignment z IPZ
z7P3 ZTP4 Oxide Oxide
31 300 31 533 31 913
1221 1190 1081
33 955
1420
Y IP2 YIP3 YIP4 Oxide
Y5P3 Oxide Oxide (sh) Oxide Oxide
.
Reference 21.
I
I
i
1
MO atoms
-\
\
-Mootoms
1
I 350
I 360
,
I
1
370
Wavelength, nm Flgure 3. Absorption spectrum of the y7P in a neon matrix at 4'K.
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r n 320 330 340 350
I
380
4
Pa 'S3
triplet of Mo atoms
experiments the most abundant oxides which were formed were Moo3 and its polymers. A summary of the atomic and oxide absorption bands in neon matrices is given in Table I. In that table, the feature a t 282.6 nm was assigned as the atomic transition to the fP3 state on the basis of its gas to matrix frequency shift and its constant intensity relative to the other Mo atom absorptions in different experiments. (This absorption was obscured in the Ar matrices by the CsI background absorption.) The band listed in Table I a t 332.9 nm does not appear in Figure 2 but was observed in matrices formed from lower temperature deposits (T
