J. Phys. Chem. 1989, 93, 1136-1 139
1136
of the electron dissociative attachment rate constant for these fluoride compounds should prove useful in the modeling of the xenon fluoride excimer laser using NF3 as the halogen donor. In addition, in separate experiments, the nitrogen difluoride radical has also been shown to support XeF laser action when it alone was used as the halogen donor in an electron beam pumped laser device (see Figure 3).
Acknowledgment. I thank Ron Brochu for his many contributions to this experiment. Also, many helpful discussions with J. Jacob, M. Rokni, and Professor F. Kaufman regarding the operable kinetics for this excimer system are sincerely acknowledged. Registry No. NF,, 3744-07-8.
Electron Attachment to CO, Clusters by Collisional Charge Transfer Edward L. Quitevist and Dudley R. Herschbach* Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 (Received: June 8, 1988)
Electron transfer from a seeded supersonic beam of alkali-metal atoms to a crossed molecular beam of C02 clusters was found to form predominantly (C02); anions, with n = 2-16 under our experimental conditions. The mass spectrum has especially prominent anion peaks at “magic numbers” of n = 4, 7, and 14. Threshold energies for formation of the anion clusters with n = 2-10 were determined by measuring the yield as a function of collision energy. The nominal electron affinities corresponding to these threshold energies are consistent with an impulsive model which postulates that the electron attachment occurs to a subunit within the clusters. This implies that cluster anions larger than the subunit moiety are produced with substantial vibrational excitation. As interpreted with the impulsive model, the threshold data give an approximate value (uncertain by about 10.2 eV) for the adiabatic electron affinity of the subunit moiety, but do not unequivocally identify its size. Thus we find EA = 0.8 eV if the electron attachment occurs to a dimer and EA = 0.1 eV if to a trimer.
Introduction Atomic and molecular cluster anions are prominent in many diverse phenomenona, including solvation and reaction processes in solution, ion-molecule chemistry in the upper atmosphere, and nuc1eation.l The electron affinity (EA) of the corresponding neutral cluster serves as the customary measure of the stability of a cluster anion, equivalent to the ionization potential (IP) of the anion, but as yet EA values have been determined or estimated for only a few cluster species. This prompted molecular beam experiments in our l a b o r a t ~ r y to ~ - produce ~ cluster anions by electron transfer from a fast alkali-metal atom to neutral clusters A+ + X;. In such an endoergic charge-transfer via A X, process, the electron attachment occurs isoenergetically a t threshold, and thus cluster fragmentation is minimized (in contrast to electron bombardment, which excites and dissociates the target clusters). From the threshold collision energy ET, the nominal electron affinity of the neutral cluster can be obtained by EA(X,) = IP(A) - ET (1)
+
-
where IP(A) is the ionization potential of the alkali-metal atom. Since the collisional time scale is subpicosecond, these nominal EA’S do not pertain to adiabatic attachment to the parent cluster and are interpreted in terms of an approximate, impulsive model., Also, the method yields only modest accuracy, about 1 0 . 2 eV, because the threshold energies must be evaluated by extrapolating cross sections for ion-pair formation. Despite these limitations, the method offers a useful means to examine the electrophilic character of molecular clusters. In this paper we report a study of COzclusters. Anions of CO, clusters have been produced in several other experimentseI0 and theoretical electronic structure have explored the geometry and stability of the (CO&- dimer. The (CO,), clusters are able to bind an electron despite the fact that the adiabatic EA for the monomer is quite negative (-0.6 f 0.2 eV).I3 Recently, Bowen and co-workers1° have obtained a lower bound (>2.4 eV) to the vertical detachment energy of the (CO,),- anion, but no other measurements relating to the electron affinity of COz clusters are available. Here we report threshold energies for Present address: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409.
0022-365418912093-1136$01.50/0
producing COzcluster anions with n = 2-10 via collisional charge transfer from alkali-metal atoms. These data prove consistent with the impulsive model, which assumes that the electron attachment occurs to a subcluster moiety, while the rest of the cluster is merely a startled spectator during the very brief collision duration. The model relates the observed ET values via a reduced mass factor to the adiabatic EA of the active subcluster. For this we find EA = 0.8 eV if the subcluster is a dimer and EA = 0.1 eV is the subcluster is a trimer.
Experimental Results The crossed-molecular beam apparatus, experimental procedure, and data analysis have been described in detail e l ~ e w h e r e . ~The ,~ cluster anions were produced by crossing a Rb atom beam with neutral COz clusters. The fast R b atoms were generated from a seeded supersonic jet,14 and the cluster beam by the expansion of neat COz at a stagnation pressure of 23.5 psig through a 0.08-mm-diameter nozzle at room temperature. The relative kinetic energy E,, was varied by changing the temperature and diluent gas of the seeded Rb atom beam. The product anions were ( I ) See: Mark, T. D.; Castleman, A. W. Jr. Adv. A t . Mol. Phys. 1984, 20, 65 and references cited therein. (2) Bowen, K. H.; Liesegang, G . W.; Sanders, R. A,; Herschbach, D. R. J . Phys. Chem. 1983, 87, 557. (3) Quitevis, E. L.; Bowen, K. H.; Liesegang, G . W.; Herschbach, D. R. J . Phys. Chem. 1983,87, 2076. (4) Klots, C. E.; Compton, R. N. J . Chem. Phys. 1977, 67, 1179. (5) Klots, C. E.; Compton, R. N. J . Chem. Phys. 1978, 69, 1636. (6) Stamatovic, A,; Leiter, K.; Ritter, W.; Stephan, K.; Mark, T. D. J . Chem. Phys. 1985,83, 2942. (7) Kondow, T.; Mitsuke, K. J . Chem. Phys. 1985,83, 2612. (8) Knapp, M.; Kreisle, D.; Echt, 0.; Sattler, K.; Recknagel, E. Surf. Sci. 1985, 156, 313. (9) Alexander, M. L.; Johnson, M. A,; Levinger, N. E.; Lineberger, W. C. Phys. Rev. Lett. 1986, 57, 916. (10) Coe, J. V.; Snodgrass, J. T.; McHugh, K. M., Freidhoff, C. B.; Bowen, K. H. J . Chem. Phys., rn press. (11) Rossi, A. R.; Jordan, K. D. J . Chem. Phys. 1979, 70, 4422. (12) Fleishman, S. H.; Jordan, K. D. J . Phys. Chem. 1987, 91, 1300. (13) Compton, R. N.; Reinhardt, P. W.; Cooper, C. D. J . Chem. Phys. 1975, 63, 3821. (14) Larsen, R. A,; Neoh, S. K.; Herschbach, D. R. Rev. Sci. Instrum. 1974, 45, 1 5 1 1 .
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 3, 1989
Electron Attachment to C 0 2 Clusters
1137
TABLE I: Observed Thresholds and ImDulsive-Model Analvsisa
EA, eV
cluster n 2 3 4
KO; rlcql.
111
*
2
4
6
e
J
10
Figure 1. Signal-averagedmass spectrum of negative ions produced by charge exchange from Rb atoms to (CO,), clusters. Signal for n = 4 peak is about 2000 counts in 2 min. Experimental conditions: Rb beam velocity = 4.3 km/s; flux (monitored by surface ionization detector) = 6.4 X lo-*A;carrier gas, H2.Cluster beam source temperature To = 25 OC; stagnation pressure Po = 1.6 atm (23.5psig); nozzle diameter d = 0.075mm.
5 6 7 8 9 10
ET,eV
k =n
k=1
k =2
k =3
3.41 i 0.14 4.09 f 0.10 4.53 f 0.15 4.90 f 0.19 5.04 i 0.27 5.26 f 0.24 5.51 f 0.08 5.46 i 0.16 5.55 f 0.18
0.77 0.09 -0.35 -0.72 -0.86 -1.08 -1.33 -1.28 -1.37
1.89 1.89 1.89 1.89 1.91 1.89 1.85 1.92 1.92
0.77 0.76 0.76 0.72 0.80 0.77 0.66 0.81 0.81
0.09 0.09 0.05 0.13 0.10 0.02 0.15 0.15
'Electron affinities calculated from eq 1, 2, and 3 using IP(Rb) = 4.18 eV. I
"
'
I
"
/
.-. >
0)
b
17 K W
z
W
' ' / A /
..-
* /
/
EA o
k=l
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
k-2
RELATIVE KINETIC ENERGY (eV)
Figure 2. Determination of threshold energies for formation of (CO,); cluster anions with n = 2-10. To avoid overcrowding, data are shown only for n = 2, 3, 4, 7, 10;different symbols in each plot pertain to different runs. Lines obtained from least-squares fits to all points for each cluster.
electrostatically focused into a quadrupole mass spectrometer (1600 amu mass range; mass resolution, m/Am I1000) that viewed the collision zone. The mass-selected ions were detected by a channeltron electron multiplier, augmented by pulse counting and signal averaging instrumentation. Cluster ion signals as a function of Ere,were converted to cross sections. Figure 1 shows a typical anion mass spectrum. The predominant species produced in the charge-transfer collisions are the negative ion clusters, (CO,);, with n I16 under our conditions. Dissociative electron attachment to produce solvated carbonate anions, C03-(C02),, was clearly a minor process (in contrast to typical electron bombardment experiments4). The abundance of (Cot),,- cluster anions with n = 4, 7, 14 is notably larger than that of adjacent cluster ions. These same so-called "magic numbers", attributed to variations in relative ion stabilities, have been previously observed in the mass spectra of (C02); anions produced in collisional ionization by high Rydberg rare gas atoms' and by low-energy electron beam bombardment.*s9 The dependence of the ion yields on the stagnation pressure Po of C02in the source was measured (from about 1 to 2 atm) for several cluster anions, for collision energies close to threshold (within -0.3 eV). For anions with n = 2-5, we found the yields proportional to Poa, and the pressure exponent a matched the cluster polymer number n within fO. 15 or better. For anions with n = 6-9, however, we found that a was in each case substantially smaller than n; the values obtained were a = 4.4, 4.7, 5.3, and 5.6, respectively. This offers some evidence that at least the anions with n = 2-5 are formed chiefly by electron attachment to the corresponding neutral clusters rather than by fragmentation on attachment to larger clusters. Figure 2 illustrates the determination of threshold energies for appearance of the anion clusters with n = 2-10 by linear ex-
o
k=n
j / l l / l l l l l 1
5 CLUSTER
10
SIZE, n
Figure 3. Variation with cluster size of the maximu relative kinetic energy (dashed curve), the observed threshold energies, and values of adiabatic electron affinity derived from analysis employing impulsive model. For the monomer ( n = l), the indicated threshold (denoted by X) is computed from the known electron affinity (-0.6eV).
trapolation of the cross sections as functions of the collision energy. For each n, the line shown was obtained from a least-squares fit to all the points. The threshold energies, listed in Table I, increase steadily with n from about 3.4 eV for the dimer anion to about 5.5 eV for the decamer anion. Figure 3 displays this trend. The threshold energy for producing the monomer anion C02- from the neutral monomer is 4.8 eV, which is greater than the maximum available relative kinetic energy of 2.8 eV in our experiments. We therefore attribute the CO; seen in Figure 1 to fragmentation from larger clusters. This is consistent with the transit time through the quadrupole mass spectrometer from the collision zone to the channeltron, which was 30 p. Although COT is metastable and has a lifetime of =90 lrs,13 70% of the initially formed CO; could survive to reach the channeltron. Discussion Table I includes electron affinities derived from four variants of the impulsive collision modeL2 The nominal values (listed in the column designated by k = n) are obtained from eq 1 and hence pertain to the threshold kinetic energy for the alkali-metal atom relative to the full cluster. This "full cluster" model assumes that all the monomer units participate in the electron attachment process. The newly formed negative ions may contain vibrotational
1138 The Journal of Physical Chemistry, Vol. 93, No. 3, 1989
excitation, but at least some of this is relaxed by Coulombic interaction with the sibling alkali-metal cation.15 If the relaxation were complete, the threshold data would yield adiabatic electron affinities. However, the adiabatic electron affinities should increase with cluster size.16 In contrast, we find that the nominal EA values for (CO,), decrease with increasing cluster size and indeed become negative for n > 3. This trend is attributed to incomplete configurational relaxation of the clusters during the extremely short collision duration, which is in the subpicosecond range (=0.3 ps). The threshold energy then does not correspond to an adiabatic EA because the collision duration is much briefer than the characteristic relaxation time for monomer units in the cluster to reorient into a configuration which stabilizes the excess electron.2 From rates for electron solvation by alcohols in s o l ~ t i o n , we '~ estimate that these relaxation times exceed 10 ps. Under these conditions the nominal electron affinities cannot pertain to adiabatic attachment to the full cluster but may correspond to attachment to a subcluster. In the subcluster variants of the impulsive model: the collision of an alkali-metal atom A with a cluster X, involves electron transfer to subcluster Xk via
A
+ Xk...X,pk
-+
A+ + Xk-...X,pk
where the X,,+ portion of the cluster is merely a spectator. The threshold collision energies are then given by where fk is the ratio of the reduced mass for A, Xk to that for A, X,, which can be written as
= - [(n - k ) / n l [ m A / ( M A + kmX)l (3) The corresponding subcluster electron affinities are again obtained from eq 1, using the threshold energies of eq 2 for the appropriate k value. For an ideal impulsive model involving the Xk subcluster, the EA values derived from thresholds for all clusters with n L k should be equal. Despite this criterion, in practice the impulsive model lacks an unequivocal means to identify the size of the electrophilic subcluster. As seen in Table I and Figure 3, we find that the EA values obtained for attachment to a specified subcluster (with k = 1, 2, or 3) indeed prove to be constant for the series of larger target clusters ( n > k ) within experimental error. For the monomerimpulsive model ( k = l ) , the average EA is 1.9 eV. This value cannot be reconciled with the adiabatic EA of -0.6 eV for the monomer and thus indicates that attachment actually occurs to a larger subunit. For a dimer-impulsive model ( k = 2 ) , the series of clusters extending up to n = 10 yields an average EA of 0.76 eV with a standard deviation of 0.05 eV, a result close to the nominal EA of 0.8 f 0.2 eV obtained from the threshold data for the dimer. If this value corresponds approximately to the adiabatic EA, the dimer anion is much more stable than suggested by ab initio electronic structure calculations'* (which predict the adiabatic EA -0.3 eV). Although the photodetachment spectrum for the dimer anion recently obtained by Bowen and co-workers1° provides a lower bound (>2.4 eV) to the vertical detachment energy, this bound appears compatible with both our EA 0.8 eV and the electronic structure calculations for the dimer species. We also examined a trimer-impulsive model ( k = 3) and again found the results are internally consistent. The average EA 0.09 eV is now much smaller and thus subject to relatively large scatter but agrees with the nominal EA obtained from the threshold for the trimer. Qualitative extrapolation of the electronic structure results for the dimer suggests that the adiabatic EA for the trimer should be of this magnitude.I2 Since the impulsive model implies that electron attachment is nonadiabatic for full clusters with n > k , the corresponding vibrotational excitation AE(n,k) of the cluster anions at threshold fk
-
-
-
(15) Havernann, U.; Zulicke, L.;Nikitin, E. E.; Zembekov, A. A. Chem. Phys. Lett. 1974; 25, 487. (16) See for example: Coe, J. V.; Sncdgrass, J. T.;Freidhoff, C. B.; McHugh, K. M.; Bowen, K. H. J . Chem. Phys. 1987, 87, 4302. (17) Kenney-Wallace, G. A. Ace. Chem. Res. 1978, / I , 433.
Quitevis and Herschbach can be derived from the model as AE(%k) = (1 -fk)ET(A&) = EA(Xk) - EA(&)
(4) This excitation is thus given simply by the difference between the EA'S for the subcluster and full cluster variants. From Table I, we see that at threshold (for n > k ) the excitation per COz moiety implied by the model, AE(n,k)/n, is in the range 0.21-0.28 or 0.11-0.17 eV for the dimer or trimer variants, respectively. Such excitation (particularly above threshold) will foster evaporation of neutral COz units from the cluster anions, especially for the larger clusters. The observed pressure dependence for the anion cluster mass spectrum, noted in our comments on Figure 1, suggests that evaporation is not important for small clusters with n = 2-5 but might cause the weakening we saw for n = 6-10. For larger cluster anions, other studies have clearly demonstrated that evaporation has a major role and produces the "magic numbers" evident in the mass From our estimates of EA'S for the dimer and trimer we may derive corresponding ion-neutral bond dissociation energies by means of a thermochemical identity, D(X--Xj) = EA(X,+l) - EA(X)
+ D(X-X,)
(5)
This calls for adiabatic electron affinities, but if a nonadiabatic value is used for EA(X,,,) the right-hand side will underestimate the energy required to extract X- from the Xi+' cluster. Using EA(X) = -0.6 f 0.2 eV for the COz monomer13and D(X-X) = 0.06 eV for the neutral dimer,I8, we find that the impulsive model result for EA(X2) = 0.8 f 0.2 eV requires that D(X--X) > 1 eV. Likewise, our result for EA(X3) 0.1 eV requires that D(X--X2) > 0.3 eV. These lower bounds may be compared with ion-neutral bond dissociation energies D(X--Y) obtained from thermochemical studies of binary anion complexes by Castleman and co-w,orkers.19 For Y = C 0 2 and X- = C1-, I-, NO2-,SO3-, they find values in the range D(X--Y) = 0.24 - 0.40 eV. For large C 0 2 clusters, the energy dependence of photon-induced evaporation gives an upper bound of 0.2 eV for the binding energy of a neutral C 0 2 onto the cluster anion, as shown by Lineberger and c o - w ~ r k e r s .These ~ comparisons emphasize how strikingly large is our result for the dimer electron affinity. Although our estimate for EA(X2) must be regarded as tentative until tested by another method, it is a direct consequence of the threshold we find for electron transfer to the dimer (n = 2 entry in Table I); the values derived from the larger clusters using the impulsive model merely offer corroborating evidence. The only apparent means to discount the observed threshold for the dimer is to assume that the dimer anion comes predominantly from fragmentation of larger clusters. That appears incompatible both with the quadratic pressure dependence we find for the yield of dimer anions and the observations of dimer anions in several other s t ~ d i e s . ~ -In' ~particular, lifetime measurements by Klots and Comptons indicate that (unlike the monomer anion) the dimer anion is not metastable. If we nevertheless assume that the trimer rather than the dimer is the smallest subcluster capable of binding an electron, our data indeed give EA(X,) and D(X-Xz) small enough to be compatible with the ab initio electronic structure calculations by Fleishman and Jordan.I2 However, since for anions electron correlation has a major role, the energetic aspects of such calculations may not yet be reliable. Fleishman and Jordan included the leading correlation terms but found EA is negatiue for all of the geometrical isomers of (CO,), that they examined, in contradiction to the experimental observations of free dimer anions. Recently Lindholm found that the semiempirical HAM method,20which in principle includes electron correlation and has given reasonable agreement with electron affinities for many molecules (often within -0.3 eV), yields large positive values of the order EA -1 eV for some plausible (CO,),) structures.21
-
(18) Murthy, C. S.;Singer, K.; McDonald, 1. R. Mol. Phys. 1981,44, 135. (19) Keesee, R. G.; Castleman, A. W. Jr. J . Phys. Chem. R e j Data 1986, 15, 1011. (20) Lindholm, E . J. Chem. Phys. 1986,83, 1484 and work cited therein.
J . Phys. Chem. 1989, 93, 1139-1 144
(o,c.oj+
0-c-0 0-c,-0
0-C-0
+
0-C-0
C02+C0,*e-
\\\2.9a I I . C ~ ~
\,
'\
'\
0-c
-0
Figure 4. Estimates of energies and structures pertaining to carbon dioxide dimer neutral and anion. Double-headed arrows show vertical detachment energies in electron volts (from ref 10) and estimates of adiabatic electron affinities (from ref 5 for monomer, from present work for dimer). Corresponding geometrical structures are indicated, inferred from theory for monomer and dimer anions (ref 12) and from experiment for neutral dimer (ref 23-25). While definitive results for electron attachment await improved experiments and calculations, we summarize in Figure 4 properties derived from several studies pertaining to (C02)2or its anion. For the neutral dimer, experiments and theory are in accord concerning both the structural parameters and dissociation energy."25 There are two geometrical isomers of similar stability, both planar and involving nearly linear COz monomer units: (I) a polar, T-shaped, C , isomer and (11) a nonpolar, "staggered side-by-side" C2 isomer which is the prevalent f ~ r m . ~For * ~the ~ dimer anion, experiments (21) Lindholm, E., private communication (University of Stockholm, December, 1986). (22) Bohm, H. J.; Ahlrichs, R.; Scharf, P.; Schafer, H. J . Chem. Phys. 1984. 81. 1389.
(23) Walsh, M. A.; England, T. H.; Dyke, T. R.; Howard, B. J. Chem. Phys. Lett. 1987, 142, 265. (24) Jucks, K. W.; Huang, Z. S.; Dayton, D.; Miller, R. E.; Lafferty, W. J. J. Chem. Phvs. 1987. 86. 4341. (25) Illies, A. J.; McKee,.M. L.; Schlegel, H. B. J. Phys. Chem. 1987,91, 3489.
1139
and theory agree about some properties but not others. Evidence for two isomeric dimer anion species has been found by Margrave and co-workersZ6in infrared spectra of products from reaction of Li with C 0 2in argon matrices. One species is ascribed to an adduct (Li+CO
