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The long-time limiting value for AP(C,H,+) = 11.95 f 0.2 eV is in agreement with the photoionization and PIPECO v a l ~ e s . ' ~ JIf~ we take into account the thermal energy distribution of pyridine at 423 K (Figure 12), we can estimate the threshold at 0 K to be -0.2 eV higher. It seems therefore that our apearance potential is in better agreement with the more recent value of 12.15 f 0.02 eV for the 0 K threshold, obtained from variable-time PIPECO.18 It is interesting to notice that reactions such as (l), which are characterized by large kinetic shifts and whose thermochemical onsets occur in Franck-Condon gaps, cannot be easily studied by ordinary photoionization methods. Even the very careful two s t u d i e ~ ' ~give J ~ an uncertainity in the threshold energy of 0.35 eV. What seems to be needed is trapped photoion mass spectrometry (TPIMS).
This could combine the excellent energy resolution of photoionization with the wide time range available to TIMS. We are currently constructing a TPIMS device which includes a cylindrical ion trap (CIT),29*30 a pulsed vacuum-UV light source and monochromator, and a quadrupole mass spectrometer. This instrument will be able to trap photoions for up to milliseconds and will enable us to measure kinetic shifts at constant detection sensitivity with excellent energy resolution. One of the first molecules to be studied will be pyridine. (29)R. F. Bonner, J. E. Fulford, R. E. March, and G . F. Hamilton, Int. J. Mass Spectrom. Ion Phys., 24,255(1977);J . E.Fulford, R. E. March, R. E. Mather, J. F. J. Todd, and R. M. Waldren, Can. J . Spectrosc., 25, 85 (1980). (30)We thank Dr. R. E. March for very helpful advice in the design of the cylindrical ion trap.
Interaction of Massive Water Cluster Ions with Neutral Gases H. Udseth, H. Zmora,+ R. J. Beuhier, and L. Friedman' Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 (Received: June 26, 1981; In Final Form: August 24, 1981)
The attenuation and disintegration of water cluster ion beams, interacting with neutral gases, were studied for a range of cluster energies and sizes. Scatteringcross sections and rates of water molecule loss were measured. For these experiments, a time-of-flightmass spectrometer capable of analyzing dc ion sources has been developed and is described. The observed cross sections for attenuation of water cluster ions are consistent with the assumption of structures having densities comparable with that of liquid water.
Introduction Prior to the development of high molecular weight mass spectrometric techniques, interest in water cluster ions has generally been limited to solvated protons or hydroxyl ions containing less than 10 or 15 water molecules. Such clusters can be prepared in mass spectrometer ion sources operating at a few torr or less and have been the subject of a variety of thermodynamic and kinetic studies.'-5 Larger water cluster ions have been generated in our laboratory for studies of energy transfer in ion impact processes and as model compounds in the development of ion source techniques devoted to the synthesis of cluster ions of high molecular eight.^,^ A particularly interesting application of high molecular weight cluster ions is in the exploitation of their low charge-to-mass ratio for injection of fuel atoms into thermonuclear devices? An important question that arises in connection with this application deals with the interaction of relatively large cluster ions with residual gases in a vacuum system and is concerned with mechanisms of cluster ion beam attenuation as the result of cluster ionneutral molecule collisions. Large cluster ions produced by condensation of molecules at very low temperatures may have very low densities and consequently larger cross sections for a variety of gas-phase collisional interactions. Croas sections for the degradation of the respective clusters into lower molecular weight ionic species or of endothermic dissociativeprocesses were expected to provide information on cluster ion densities as well as the applied problem of 'On leave from Soreq Nuclear Research Center, Yavneh, Israel.
beam transport in vacuum systems. In the present work we have studied the interaction of water clusters with neutral gases at various energies. In order to be able to analyze high mass clusters (>loo00 amu), a specially built time-of-flight mass spectrometer has been employed. Since the water source in its present configuration could not be pulsed, a novel, three-grid beam-chopping system was incorporated into the TOF spectrometer.
The TOF Mass Spectrometer ConstructionDetails. A 2 m length time-of-flight (TOF) mass spectrometer capable of analyzing in dc ion signals was built and incorporated in interaction studies of water clusters with neutral gases. The spectrometer consists of a dc ion source coupled to a three-grid beam-chopping assembly followed by an acceleration region, a drift tube, and a secondary electron detector. (1)M. DePaz, J. J. Leventhal, and L. Friedman, J . Chem. Phys., 51, 3748 (1969). (2)P. Kebarle, "Higher Order Rea$ion-Ion Clusters and Ion Solvations", in "Ion-Molecule Reactions , J. L. Franklin, Ed., Plenum Press, 1972. (3)J. J. Solomon, N. Meot-Ner, and F. H. Field, J . Am. Chem. SOC., 96,3727 (1974). (4)I. N. Tang and A. W. Castleman, J. Chem. Phys., 60,3981(1974). ( 5 ) J. Q. Searcy and J. B. Fenn, J. Chem. Phys., 61,5282 (1974). (6)R.J. Beuhler and L. Friedman, Nucl. Inst. Meth., 170,309(1980). (7)R. J. Beuhler and L. Friedman, to be published. (8) W. Henkes, V. Hoffman, and F. Mikosch, Reo. Sei. Instrum., 48, 675 (1977).
0 1982 American Chemical Society
Interaction of Water Cluster with Neutral Gases SOURCE
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Flgure 1. Schematics of the TOF spectrometer with the water cluster source: (1) dlscharge wlre, (2) noulaskimmer, (3) extracting lens, (4) deflection plates, (5) scattering chamber, (6) aperture, (7) pulsed grlds, (8) stopper, (9) accelerating region, ( I O ) secondary electron emmer, (1 1) detector.
A three-grid system and a high power, fast rise time pulser were used to chop the ion beam for either a thermal emission source or a water cluster generator. The grid system was mounted perpendicular to the beam direction and is shown as elements 7 and 8 in Figure 1,which is a schematic representation of the apparatus. Grid number 8 (the “stopper”) is biased at a positive potential, sufficient to prevent any ions from passing through it when the high-voltage pulse is off. The pair of grids numbered 7 are the pulsed grids and are driven by a Cober Model 605 high-power pulser. The grid farthest from the stopper is held at ground potential and serves as a return path for the pulsed current. The grid nearest the stopper is the “pulsed grid” and is pulsed positively to a voltage sufficient to drive the ions which have accumulated in the space between it and the stopper into the acceleration region (number 9 in Figure 1). At the same time, this pulse prevents any ions on the source side of the pulsed grid from entering the acceleration region. The acceleration region is composed of four, equally spaced electrodes on a four-step voltage divider between the drift tube and the stopper to ensure a uniform change of field. The drift tube is a stainless steel tube 10 cm in diameter and 2 m long with a 2-mm entrance aperture and a 7-mm exit aperture. The source end of the drift tube is mounted in a plexiglass flange with an inside O-ring seal which provides electrical insulation (the drift tube is operated at a negative high voltage), and serves to separate the spectrometer into two separately pumped regions. The detector and drift tube region are pumped by a 4-in. oil diffusion pump with a liquid nitrogen trap. The source end is pumped by a 6-in. oil diffusion pump with a LN trap. This pumping arrangement makes it possible to work with either a low- or a high-pressure source. A secondary electron emission detector is used for ion detection. The ion beam impacts on a copper electrode mounted off the beam axis at a potential of -20 kV and secondary electrons are detected with a Bendix Model M308 magnetic multiplier. This arrangement is sensitive to ions up to a mass of approximately 15000 amu with a high efficiency.6 The timing and data acquisition of the experiment are controlled by a clock-interface system. The interface provides a fast rise 15 V start pulse with a variable period from 150 ps to 10 ms. This pulse is used to trigger the grid pulser and initiates an internal clock in the interface. After a delay, which is adjustable from zero to the full period, data acquisition begins. The interface accepts pulses from the detector, which have gone through a current-sensitive preamplifier and an Ortec Model 474 timing filter amplifier, and provides address and increment information
5, 1982 613
to the digital input module of a Tracor Northern TN-1710 multichannel analyzer. The interface provides a dwell per channel, variable by factors of two, from 50 ns to 1.6 ps starting with address 0. The memory content increments at that address for each detector pulse received and the memory address increments at the end of each dwell period until either the section of memory chosen is used or a new start pulse is received. The output of the multichannel analyzer is thus a display of intensity vs. time starting at some fixed time after the start pulse (the selected delay) and proceeding in uniform steps in time (the dwell time per channel). Operation and Performance. The two main factors affecting the resolution of a TOF mass spectrometer are the initial spatial and velocity distributions of the ions in the source. Wiley and McLareng have analyzed these factors for their TOF spectrometer and this analysis is applicable to the pulsed grid, stopper, and acceleration region in our apparatus. It was shown that by adjusting the accelerating voltage V D(the voltage on the drift tube) and the pulse amplitude Vc (E,in Wiley and McLaren) so as to reach a compromise between best space and energy focusing conditions, one can achieve a mass resolution of better than 100. In the present configuration, there is an additional, voltage-dependent source of spread which arises from the fact that ions are being accumulated in the space between the grids during the off period of the pulser. These ions come to a stop in front of the stopper, at a distance determined by their initial energy, E, a qVl and the stopper bias V,, and are then accelerated back toward the skimmer and are lost. When the voltage Vc is applied, ions with the same kinetic energy, traveling in the opposite directions, will arrive at the detector at different times, leading to widened mass peaks and to “tails” on the high mass side. Wiley and McLaren have compensated for a similar effect by introducing a time lag between the ion formation and the accelerating pulse. Such compensation is not possible in the present case because of the nature of the cw water cluster source used in these experiments. The effect of this velocity distribution can be estimated as follows: During the off period of the pulser, ions with an initial energy, qVl, enter the TOF analyzing region and will reach a maximum distance x , ( x , = DIVl/ V,) from the pulsed grid, due to the retarding potential V, on the stopper. D1 is the separation between the pulsed grid and the stopper. The velocity of an ion at any point x , 0 < x < x,, is given by
where a = x / x , . When the pulse V, is applied, all ions within the region between grids 7 and 8 will be accelerated toward the drift tube, and those to the left of the pulsed grid (see Figure 1) are lost. The time difference associated with ions with velocities fv, is given by AT = 2v,/a, where a = (q/m)(V,- V,)/Dl or
To a first approximation, the total transit time is given by the time T spent by the ion in the drift tube, T [ ‘ f 2 (m/q)L2/VD]lI2, and (9) W. C. Wiley and I. H. McLaren, Rev. Sci. Instrun., 26,1150 (1955).
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[viVD(1 - a)]'" L (V,- V,)
AT - D1 _ 4T
In order to estimate the effect of this time spread on the resolution, one has to calculate the average A T / T , weighted by the number density n(a)of the ions in the space 0 < a < 1. n(a) is proportional to l/ua, and
Best resolution is achieved by setting D1(the spacing between the pulsed grid and the stopper) and V, to their minimum possible values. In practice, V, is set just above the threshold value, below which ions leak into the acceleration region during the off period. The spectrometer was first tested and optimized with a thermal emission source. A rhenium ribbon, coated with solutions of sodium, rubidium, and cesium salts, was mounted immediately in front of the first grid (number 7 in Figure 1). The filament was biased a t +4.4 V. Optimum performance with a thermal emission source was achieved with the following configuration: The spacing between the first and the pulsed grids was 3 mm, 1.25 mm between the pulsed grid and the stopper. A 500 lines/in. mesh was used for all three grids (in those instances where a high resolution is not an important fador, the grids were changed to 100 lines/in. in order to increase the counting rate). The pulsed grid was driven with a 20-11s rise, 3-ps long, 80-V square voltage pulses. The stopper was set just above the threshold voltage needed to prevent the beam from reaching the acceleration region during the off period, typically 5 V. The drift tube was usually set at -3 kV. With this arrangement, a mass resolution (fwhm) of 150 was achieved. The experiments discussed below were carried out with a high-pressure water cluster source.6 Ions are initially produced in a discharge in an atmosphere of a suitable carrier gas, usually He or Ar, saturated with water vapor. The discharge is drawn between a 175-pm diameter platinum wire and a 150-pm diameter gold nozzle placed about 1 mm apart. The discharge current was 0.1-0.6 mA. The gas was expanded into a chamber pumped by a 41-cfm mechanical pump (Hereaus Type E 75), maintaining a pressure of 50-200 mtorr. The beam then passed through a skimmer with a 250-pm aperture, closely followed by an extracting lens, and on into the spectrometer grid system. The nozzle and skimmer were biased at ground potential and the extractor at -20 to -200 V. Details of the source design and performance will be published el~ewhere.~ The typical cluster size could be varied from 200 to about 20 000 amu, with currents of a few nanoamps being detected immediately after the extractor. At the detector end, count rates as high as 6 X lo5 s-' could be obtained, but typical working count rates were in the range of lo5 s-l. It is expected that this source would yield an inferior TOF mass spectrum to that achieved with a thermal emission source ( m / A m of 150) due to the higher initial ion energy and energy spread. The best mass resolution obtained with the water source was about 50, which was found to be adequate for the purpose of the experiments reported in this paper. Figure 2 shows a water cluster spectrum typical of the low mass clusters range. At masses greater than 1000, resolutions of individual water peaks was not achieved. Interaction Studies with Neutral Gases Experimental Details. The purpose of the experiment was to examine the cluster ion beam attenuation upon
-55
58
J
61 64 67 T I M E OF F L I G H T ( p s e c )
Figure 2. Water cluster timeof-flight spectrum. The mass designation and number of water molecules of each peak is shown. I
AVERAGE CLUSTER S I Z E TARGET GAS CROSS-SECTION C M ENERGY
2
4
5
Plmtorr)
6
l50Oan~ A,
295%'
1
50eV
1
\
8
9
Flgure 3. Attenuation of the ion beam as a function of pressure.
passage through a neutral scattering gas. For these experiments, a collision cell and two pairs of x-y deflection plates were added to the apparatus, between the cluster source and the grid assembly. The deflecting plates have a gap of 1.6 mm, are separated by 1.25 mm from each other, and are located at a distance of 4 mm from the collision cell. These plates serve to focus the beam on the 250-pm diameter entrance aperture of the collision cell. The cell is 8 mm long and has a circular exit aperture of 1 mm. It is followed by a 10-mm long drift region terminated by another aperture 350 pm in diameter, which determines the acceptance angle of the drift tube. The cell is heated to a temperature of 100 O C . Gas is introduced into the collision chamber through a variable leak and the pressure is measured on a separate line connected to a MKS Baratron (Type 170 M)capacitance manometer. He, Ar, C6F6, and C3F8were used as target gases in the scattering chamber. The laboratory energy of the water cluster beam was varied from 10 to 2000 eV. Two types of experiments were carried out to determine the average loss of mass from clusters in collisions at a particular energy of interaction: (a) the mass spectrum of the fraction of the beam passing through the chamber was recorded as a function of pressure; (b) the total intensity of the transmitted beam (mass integrated) was recorded as a function of pressure to measure the total loss of ion beam intensity from all causes. In these mass-integrated experiments the triple grid network was held at the same potential as the collision cell, effectively extending the drift region to the beginning of the acceleration
Interaction of Water Cluster with Neutral Gases
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IO
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Flgure 4. Summary of cross section measurements. r , and rt are cluster and target gas radii, respectively. E ,, is the center-of-mass energy.
region, with no TOF mass analysis. Results. Attenuation Measurements. Figure 3 shows the results of a typical transmission experiment, in which the mass-integrated intensity, N , is recorded as a function of pressure. Attenuation cross sections were calculated from the linear part of each log N vs. pressure curve. Errors in the absolute values of the cross sections might arise from the determination of the effective interaction pathlength, lestdue to gas diffusion through the apertures. The size of the correction factor depends on the details assumed, ranging from leff = 1 to leff = 1 + 2d, where 1 is the geometric pathlength and d is the diameter of the entrance and exit apertures.1° We have assumed left = 1 in calculating the cross-section values, with a possible error of about 25%. The resulta for the various target gases used are summarized in Figure 4. In order to compare data obtained with different size clusters and different target gases, reduced cross sections are plotted as a function of center-of-mass energy. The reduced cross sections are the measured values divided by r(rc + rJ2 where r, and rt are the cluster and target gas radii, respectively. In calculating the radii, we have assumed that the clusters are spherical, with component water molecules occupying molecular volumes equal to that of liquid water. The radii of C3F8 and CZ6 were estimated from their respective liquid-phase densities." (No correction was made for the free volume in the liquid phase.) Gas viscosity radii were used for He and Ar. As can be seen, the results for all target gases follow the same qualitative behavior. At low center-ofmass energy the cross sections have relatively low values and show little dependence on energy. As the energy increases above a threshold, the reduced cross sections increase about five fold and level off again at high energies to a value close to unity, thus indicating the onset of an additional reaction mechanism which removes clusters from the beam. Figure 5 shows a comparison between attenuation measurements with low (1000 amu) and high (15 OOO m u ) mass clusters. The target gas used in this case was C3F@ In both cases, the same pattern prevails. One should note, however, that, for the heavier clusters, the cross sections change from a lower asymptotic value below threshold to a higher value above threshold as compared with the smaller clusters. The experimental system in its present (10) H.Klinger, A. Muller, and E. Salzborn, J. Phys. Rev. E,8,230 (1975). (11) The liquid density of C6F,is 1.62 (R.E. Banks,"Fluorocarbons and Their Derivatives",Oldbourne Press, 1964, p 136). The liquid density of CsFBis taken a~ 1.513 g/cm3. This value is obtained from the known densities of C3H6(0.585) and the ratio of fluoropentane to pentane densities (1.62 and 0.626, respectively, ibid., p 14).
io0
1000
E,,(eV)
Figure 5. Attenuation cross section measurement of mass 1000 (0) and 15 000 (A)water cluster ions with C3F6target gas. Measurements were made with laboratory energies between 10 and 2000 eV. MASS (ornu) I500 2000 1 I 1860
IO00 1 -
IL
2500 I
1
I
TIME
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Figure 6. Normalizedtimeof-flight spectra as a function of target gas pressure. The average mass of each distribution and, as given above, the respective target gas pressures (in mtorr)are indicated to the right.
form is limited to about 2000 V on the collision chamber. Experiments at higher center-of-mass energies to determine whether the reduced cross sections for the high mass clusters will level off to the same values as for the smaller clusters were therefore not carried out. Mass Shifts. The average cluster size of the beam emerging from the scattering chamber was monitored as a function of the target gas pressure for center-of-mass energies between 0.5 to 200 eV. A typical run is shown in Figure 6, which shows time-of-flight mass spectra of water clusters with 6.4-eV center-of-massenergy, interacting with Ar gas. The spectra were normalized to the same peak intensity. The peak shifts were found to vary linearly with pressure (the higher the pressure, the smaller the cluster size). For each set of runs in the mass range 1000-3000 amu, an average mass degradation figure of about 40 f 15 amulmtorr was found, independent of the center-of-mass
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energy or the type of gas used.
Discussion In the experiments described above, only the “surviving” ions were detected, thus the nature of those ions which were deflected from the beam with an angle greater than the acceptance angle of the system (half cone angle of 0.7’) can be determined only indirectly. The mass-integrated attenuation measurements also do not account for events in which the cluster has been degraded in mass but remains within the acceptance angle of the system, or for small angle elastic scattering. The measured cross sections are therefore smaller than the “total” scattering cross section of the clusters. The cross section for loss of neutral water molecules from cluster ions can be estimated by measurement of the attenuation of the intensity of the most probable mass bin originally observed with no gas added to the collision chamber or by determination of the pressure dependence of the shift in mass of the most probable mass bin, to lower mass values. Water loss cross sections obtained from intensity attenuation measurements are clouded by the shift in mass of the whole cluster ion distribution as shown in Figure 6. Contributions to the most probable mass bin originally observed with no gas added to the collision chamber, from ions formed by water loss from higher molecular weight clusters, give rise to errors in the determination of the extent of attenuation of the cluster beam as a result of water loss. Nevertheless a lower limit estimate of water loss cross section, with no correction for such errors, can be made. A value of 55 f 10 A2 is obtained. The calculated “hard-sphere” cross section for a water cluster of 100 water molecules with a density approximating that of liquid water is about 335 A2. Thus the lower limit cross section from intensity attenuation measurements of 55 10 A2 for water molecules loss suggests a rate of water loss of one molecule per five or six colliiions. A more accurate estimate of rate of water molecule loss of one per collision is obtained from observation of the shift in mass of the most probable mass bin as a function of collision chamber pressure. No energy dependence was observed for these processes in the range investigated. This result is consistent with a relatively low-energy threshold for neutral water loss from the cluster ions. The qualitative behavior of the low-energy end of the cross section data (Figures 4 and 5) is consistent with elastic scattering. One would expect that the large clusters will predominantly scatter off a small target gas in the forward direction. The lower the reduced mass is, the more forward the scattering will be (leading to a smaller attenuation). The scattering cross sections of cluster ions by He and Ar are smder than those with c3F8and cp6 below threshold (see Figure 4). Also, the heavier clusters are scattered less than the smaller ones by the same neutral gas (see Figure 5). Note that the cross section data in Figure 5 were not reduced according to the respective areas of the reactants. The reduced data would show a much larger difference between the larger (15000 amu) and smaller (1000 amu) clusters. The measured collision cross sections level off at high energies at the corresponding “hard-sphere” cross section values (i.e., values obtained for a closely packed spherical cluster colliding with a target gas molecule). It is therefore concluded that the clusters do form in a closely packed configuration without a “snow flake” kind of structure. One might expect that the induced dipole term of the interaction would be less significant as the size of the interacting particles gets bigger. Indeed, for a cluster of 1000 amu scattered by a He target, one calculates a “hard
*
Udseth et al.
sphere” radius of 8.5 A, compared with a critical radius for orbiting (based on Langevin’s model) of 2.5 A at a center-of-mass energy of 0.1 eV. The cluster ion loss mechanisms possible in these experiments are charge exchange which results in neutralization of the cluster, elastic scattering, inelastic scattering with or without “evaporation”of water molecules from the cluster (or a complete disintegration of the cluster), and endothermic loss of small ionic species leaving relatively large neutral clusters. The geometry of the experiment was such that a direct observation of the charge transfer products was not possible. If charge transfer neutralization plays a major role in attenuating the beam, the threshold for the onset of this mechanism should be directly related to the ionization potential of the target gas, leading to a higher energy threshold for He and Ar than for the fluorinated hydrocarbons. Such a trend is not observed in the experiment (on the contrary, He and Ar tend to exhibit a slightly lower threshold energy than C3F8and C6F6-see Figure 4). We therefore come to the conclusion that charge transfer can represent only a minor contribution to the observed cross sections at center-of-mass energies below -20 eV. The experimental detection system is sensitive only to charged particles. If the charge remains on the larger fragment when evaporation occurs, it will be detected. Not so if the charge is on the smaller fragment (because it scatters into a larger angle out from the beam). The binding energy of the individual molecules to clusters containing many water molecules should be close to the energy of vaporization of bulk water, less than 0.5 eV. If ion-dipole interactions are considered this energy would be increased to -0.8 eV. The mean free path of the clusters as derived from the maximum measured cross sections is of the order of the collision chamber length (at a pressure of 1 mtorr). It was found that the cluster evaporates with a loss of one or two water molecules per mtorr of target gas, independent of center-of-mass energy (within the range of the experiment, 0.5-300 eV). This is therefore equivalent to a loss of one or two water molecules per collision. Nevertheless, one cannot rule out the possibility that some of the clusters will break up into two or more large fragments as a result of a collision. In such an event, the fragments will most probably scatter into large laboratory angles and will not be detected by the spectrometer. This form of disintegration is not likely to exhibit a threshold behavior and is not considered responsible for the observed results. Furthermore, the mass-shift data do not show any change in the evaporation rate as the energy goes up above threshold. The attenuation of cluster ion beams with center-of-mass energies higher than a few electronvolts can be explained by the assumption of charge loss or charge transfer reactions. The observed thresholds which are insensitive to ionization potentials of collision partners and are at lower energies than required for the rare gases, He and Ar,favor a charge loss rather than charge transfer mechanism. The reaction has a relatively low threshold energy. If solvated H30+ species are produced leaving a relatively large neutral water cluster behind, threshold energies as low as 1 eV are possible. However, the mechanism of formation of the large cluster ion by condensation of neutral water molecules around an H30+ion suggests that the cluster ion has its charged buried in the core of the cluster. Thus the removal of an H30+from the surface requires migration of charge on a proton from the center to the surface of the
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
