J. Phys. Chern. 1982, 86, 617-621
macroion. This is possible in a cluster which has been collisionally excited even though a cluster with a peripheral H30+may be less stable then one with a centrally located H30+. More complex mechanisms of H30+solvated or H30+ losss can be imagined.
-
H30+(H20),
-
H30+(H20)n-20H-+ H30+ (H2O)n + H30+
In this case a low threshold process would require the availability of the energy of neutralization of H30+and OH- in the system prior to detachment of the H30+. Otherwise a threshold energy of roughly 11eV12would be needed. The processes considered show a range of threshold energies within the limits of the observed values. Charge transfer reaction may indeed take place at higher center-of-mass energies. Conclusions A TOF mass spectrometer capable of analyzing massive ion clusters produced by a dc ion source has been employed (12) J. L. Beauchamp, 'Reaction Mechanisms of Organic and Inorganic Ions in the Gas Phase", in "Interaction Between Ions and Molecules", P. Ausloos, Ed., Plenum Press, New York, 1975.
617
in a study of the interaction of water cluster ions with neutral gases, at low center-of-mass energies (0.5-300 eV). Results on mass shift and attenuation studies with cluster ions show energy transfer processes which initially lead to the loss of individual water molecules from the large cluster. At a critical collision energy which is insensitive to the properties of the neutral collision process there appears to be an efficient process of cluster ion attenuation which is interpreted as a loss of charge with the probable formation of relatively low molecular weight charged species. No evidence was observed for either the loss of several water molecules per collision or the formation of multicharged clusters in ion neutral collisions. The observed attenuation cross sections at high center-of-mass energies are equal to those obtained by assuming a spherical cluster at liquid density colliding with a target gas molecule. This indicates that the clusters do form in a closely packed structure.
Acknowledgment. The assistance of A. P. Irsa and J. Yarmoff in the course of this work is gratefully acknowledged. The authors also acknowledge stimulating discussions with Professor Richard Porter of Cornel1 University. This research was carried out at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences.
Electron Attachment to Volatile Uranyl Molecules A. Yokozekl, E. L. Qultevls, and D. R. Herschbach" Department of Chemlstty, Haward Unkerslty, Cambrklge, Massachusetts 02 128 (Received: August 24, 198 1; In Flnal Form: September 23, 198 1)
Negative ion production by endoergic charge transfer from fast alkali atoms to a crossed molecular beam of uranyl complexes has been studied for three systems, U02L2,UO2L2-THF,and U02LyTMP,where L denotes hexafluoroacetylacetonate, THF tetrahydrofuran, and TMP trimethyl phosphate. Near the lowest threshold, only negative ions of the parent molecules appear. The corresponding nominal electron affinities are 1.9 f 0.3, 1.6 f 0.2, and 1.5 f 0.3 eV, respectively. At higher collision energies, smaller negative ions appear, all containing the L- anion; the variation with collision energy and source temperature indicates these come from dissociative electron attachment to the parent molecules rather than attachment to products of thermal unimolecular decomposition. Qualitative electronic structure arguments suggest the attaching electron first enters a uranium 5f orbital and then migrates to the lowest vacant ?r* orbital of the L ligand.
Introduction A large class of volatile uranyl complexes has recently been synthesized and found to exhibit remarkable phot~chemistry.'-~ The most extensively studied systems have the form U02L2-B,where L denotes a hexafluoroacetylacetonate ligand, (CF,CO),CH, and B a base such as THF, tetrahydrofuran. These molecules are of particular interest for IR-laser separation of uranium isotopes, by virtue of the high volatility of the complexes (informally called "teflon-coated uranyl") and the feasibility of inducing highly selective and efficient photodissociation by pumping the asymmetric stretching vibration of the U02 (1) Kramer, G. M.; Dines, M. B.; Hall, R. B Kaldor, A.; Jacobson, A. J.; Scanlon, J. C.Inorg. Chem. 1980, 19, 1340. (2) Ekstron, A.; Randall, C.H. J. Phys. Chem. 1978,82, 2180. (3) Levy, J. H.; Waugh, A. B. J. Chem. Soc., Dalton Trans. 1977,17, 1678. 0022-3654/82/2086-0617$01.25/0
group with an ordinary C02 A practical isotope separation scheme requires some means of scavenging the laser-selected species, to prevent secondary scrambling reactions. One approach would be to employ an endoergic charge-transfer reaction, as illustrated schematically in Figure 1. Here X represents the labeled fragment (e.g., U02L2)produced by isotopically selective IR-laser-induced dissociation of the parent molecule Y (e.g., U02L2-THF),and D represents a suitable reactant which can donate an electron to either X or Y. (4) Cox,D. M.; Hall, R. B.; Horsley, J. A.; Kramer, G. M.; Rabinowitz, P.; Kaldor, A. Science 1979,205, 390. (5) Kaldor, A.; Hall, R. B.; Cox,D. M.; Horsley, J. A.; Rabinowitz, P.; Kramer, G. M. J. Am. Chem. SOC.1979,101,4465. (6) Cox,D.M.; Horsley, J. A. J. Chem. Phys. 1980, 72, 864. (7) Cox,D.M.; Levy,M. R.; Horsley, J. A.; Hall, R. B.; Bray, R. G.; Kaldor, A. J. Phys. Kramer, G. M.; Priestley, E. B.; Brickman, R. 0.; Chem. To be published.
0 1982 American Chemical Society
618
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
Yokozeki et ai.
INTERNUCLEAR DISTANCE
Figure 1. Schematic potential curves illustrating endoergic charge transfer from a donor species D to two acceptors X and Y such that X has the higher electron affinity. R = e 2 / A indicates the nominal radius for crossing of ionic and covalent curves, and A is the difference between the donor ionization potential and the acceptor electron affinity.
The thresholds for ion-pair formation A, = IP - EA, and A, = IP - EA, are chosen so that A, > A,. The avoided curve crossings occur at the "electron-jump" radii, R, and Ry If the collision energy for D X and D + Y is less than the energy barrier due to the avoided crossing at R,, but greater than that due to the avoided crossing at R,, ion-pair formation will occur only for the isotopically selected species X and not for the parent molecule Y. The product containing X can then be chemically or physically extracted. Such a scavenging scheme requires that the labeled photodissociation product X have an appreciably higher electron affinity than the parent Y molecule. This is plausible for X = U02L2and Y = UO2L2.B,by analogy with simpler molecules like the halogens; e.g., EA = 3.6 eV for the C1 atom whereas EA = 2.5 eV for the C12 molecule.8 The experiments reported here were undertaken to replace this distant analogy with data for electron attachment to some prototype uranyl complexes. We use a crossed molecular beam method to study the production of negative ions by endoergic charge transfer from fast alkali atoms to the target uranyl complexes. This method has been applied to determine electron affinities for many simpler moleculesgand recently to study electron attachment to weakly bound molecular clusters.1° It is a relatively gentle method, and indeed we find the uranyl complexes yield quite simple negative ion spectra despite their elaborate structure.
+
Experimental Section The apparatus and procedures used here were developed and described in previous studies of electron attachment to molecular clusters.1° A beam of fast Rb atoms, seeded in a supersonic jet of diluent He or Hz, crossed at 90" the target beam containing the uranyl compound. By scanning the Rb nozzle temperature for various diluent compositions, with the target beam conditions held constant, the relative kinetic energy of the collision partners could be varied up to -7 eV. The supersonic target beams of uranyl compounds were formed by passing argon carrier gas ~~
~
(8) Massey, H. S. W. "Negative Ions", Cambridge University Press:
London, 1976. (9) For reviews, see (a) Wexler, S.; Parks, E. K. Annu. Rev. Phys. Chem. 1979,30, 179. (b) Loss, J.; Kleyn, A. W. In "The Alkali Halide Vapors",Davidovita, P.; McFadden, D.; Ed.;Academic Prees: New York, 1978. (c) Baede, A. P. M. Adv. Chem. Phys. 1975,30,463. (d) Janev, R. K. Adv. atom. Mol. Phys. 1976,12, 1. (10)Bowen, K. H.; Liesegang, G. W.; Sanders, R. S.; Herschbach, D. R. J. Phys. Chem. Submitted for publication.
Flgwe 2. Molecular structures of uranyl complexes. L denotes the hexafluoroacetylacetonate ligand; the B, denote base adducts: (1) no adducts; (2) tetrahydrofuan; (3) trimethyl phosphate. Elsewhere in this paper these complexes are denoted by UO,L,, UO,L,-THF, and UO,L,.TMP, respectively.
at -130 torr through a reservoir containing the solid sample and mounted 30 cm upstream from a 0.008-cm diameter nozzle. A stainless steel in-line filter (Nupro "F" Series) was used as the sample reservoir." The stainless steel sintered element in the filter was filled with typically 1 g of the sample and packed with glass wool. The reservoir, the supersonic nozzle, and the connecting tube were each resistively heated by nichrome wires insulated with fiberglass tubing and controlled by Variacs. The temperature of each section was monitored by chromel-alumel thermocouples, referenced to an ice-water bath. For the nozzle, the tube, and the reservoir the operating temperatures were typically -110, 80, and 50 "C, respectively. At this reservoir temperature, the uranyl compounds sublimate with vapor pressures of 0.1 torr or Negative and positive ions emerging from the collision zone were observed with a quadrupole mass filter (Extranuclear, Model 4-162-8). The effective resolving power of the detection system was m/Am lo3 (fwhm) and the mass range up to 1600 amu. Initially,the negative ion mass spectra showed prominent peaks due to impurities in the target beam, chiefly water and hydrated species. Prior to quantitative measurements, these were eliminated by heating the reservoir and nozzle assembly -30 "C above the eventual operating temperatures and increasing the Ar flow for -1 h.
-
-
Results Near the threshold for endoergic charge transfer, the only significant channel observed produced just the negative ion of the parent uranyl complex
Rb
+ U02L2.B
+
Rb+ + U02LZ.B-
The threshold energies for this process were determined for each of the three complexes pictured in Figure 2 with B = nothing, THF, or TMP. At collision energies above threshold, the U02L2-and U02L-ions appeared in addition to the U02L2-B-ion. Figure 3 shows typical negative ion mass spectra obtained for the three systems, each at a collision energy 1.5eV above the threshold. At first, we suspected that the presence of U02L2-and U02L- in the spectra produced from UO2L2-THFand UO,L.TMP was due to contamination of the samples by U02L2 or to thermal decomposition of the samples. However, for the three systems the UOzLz- and U02L- ions appeared at different threshold energies and exhibited markedly different variation with collision energy and source temper-
-
(11) We indebted to Dr. Will Lee for developing this source.
The Journal of Physical Chemistv, Vol. 86, No. 5, 7982 619
Electron Attachment to Uranyl Molecules
ELECTRON IMPACT MASS SPECTRUM
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