4506
J. Phys. Chem. 1984,88, 4506-4509
Electron Bombardment Ionization and Fragmentation of van der Waals Clusters D. R. Worsnop, S. J. Buelow, and D. R. Herschbach* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received: June 6, 1983)
Mass spectrometric analysis and velocity and angular distribution measurements for XeAr, clusters formed from reactions of Xe atoms with Ar, clusters in crossed supersonic beams give evidence of severe fragmentation of clusters by electron bombardment. The observed ions exhibit m > n for much of the product distributions, as a consequence of fragmentation. Kinematic analysis shows that marked changes in the XeAr, angular distributions with source pressure likewise result from fragmentation. Threshold measurements and other features suggest that ionization of XeAr, clusters may involve “internal Penning ionization” in which electronic excitation of argon induces ionization of xenon within the cluster. The ionization thresholds for larger clusters (m or n > 6) are lower than those for the corresponding bare atoms (Xe or Ar) but greater than those determined for the atoms in bulk argon matrices. Similar, but less-pronouncedeffects are seen with the analogous Kr system. Also reported are distributions of ions from Ar,, Kr,, and Xe, cluster beams for a wide range of source pressures. Our results provide evidence that the “magic numbers” found in such distributions indicate the special stability of certain fragment ions rather than the neutral parent clusters.
Introduction The supersonic beam techniques championed by Fenn’ have made feasible studies of a host of collisions processes involving van der Waals clusters.* However, since mass spectroscopy remains the most generally applicable detection method, the fragmentation induced by the ionization process may become an acute problem for such weakly bound molecular clusters. At present, quantitative estimates of fragmentation by electron impact are available for only two examples. For argon dimers, Lee and Fenn3 found by comparing total ion flux with total beam flux that the Ar2+/Ar+ intensity ratio is about 3 or 4 times smaller than the corresponding Ar2/Ar ratio. For carbon monoxide dimers, Gough and Miller4 found by use of infrared laser-bolometer detection that (CO),+/CO+ is likewise about 3 times smaller than (CO),/CO. For larger clusters, there is qualitative evidence for substantial fragmentati~n.~,~ Thus, for strongly polymerized argon beams, changes in the Arz+ signal indicate contributions from fragmentation of higher clusters. Yet such evidence has not been regarded as compelling and thus “cascading” contributions have been ignored in the interpretation of many experiments,’ including recent assignments of “magic numbers” to rare gas cl~sters.8.~ This paper describes information about fragmentation derived from a crossed beam study of reactions of Xe atoms with Ar, clusters. The single-collision character of the experiment imposes kinematic constraints that are independent of the ionization process. These constraints enabled us to resolve unequivocally Ar, reactionlo and to demonstrate conditions that the Xe produce drastic fragmentation. The same strategy should be useful in many collision experiments with cluster beams. The electron energy dependence for ionization was also measured to examine the variation with cluster size and the effect of inserting a single xenon atom into an argon cluster. For comparison with other mass spectra were also obtained for cluster beams of argon, krypton, and xenon over a wide range of source pressures. The structure previously attributed to “magic number” stability of certain neutral xenon clusters becomes much weaker for krypton and vanishes for argon. It also becomes much weaker as the extent
+
(1) Anderson, J. B.; Andres, R. P.; Fenn, J. B. In “Molecular Beams”; Ross, J., Ed.; Wiley: New York, 1966; pp 275-317. (2) See, for example, Surf.Sci. 1981, 106, 1-608. (3) Lee, N.; Fenn, J. B. Rev. Sci. Instrum. 1978, 49, 1269. (4) Gough, T. E.; Miller, R. E. Chem. Phys. Lett. 1982, 87, 280. (5) van Deursen, A.; van Lumig, A.; Reuss, J. Inr. J . Mass Spectrom. Ion Phys. 1975, 18, 129. (6) Behrens, R.;Freedman, A.; Herm, R.R.;Parr, T. P. J . Chem. Phys. 1975. 63. 4622. (7j Golomb, D.; Good, R.E.; Bailey, A. B.; Busby, M. R.; Dawbarn, R. J . Chem. Phys. 1972,57, 3844. (8) Echt, 0.;Sattler, K.; Recknagel, E. Phys. Rev. Lett. 1981, 47, 1121. (9) Ding, A.; Hesslick, J. Chem. Phys. Lett. 1983, 94, 54. (10) Worsnop, D. R.;Buelow, S. J.; Herschbach, D. R. J. Phys. Chem. 1981,85, 3024.
of polymerization in the beam decreases. This indicates that the “magic number” structure arises from fragmentation of higher clusters to form smaller cluster ions with special stability. Experimental Procedure The reactant beams intersect at 90’ and angular and velocity distributions of scattered products are measured with a rotatable mass spectrometer equipped with a time-of-flight analyzer.” For the Xe atom beam the supersonic nozzle diameter was typically 200 pm, stagnation pressure 50 torr, and nozzle temperature 300 K; for the Ar, Kr, and Xe cluster beams the corresponding quantities were 120 pm, 25 to 700 torr, and 77 to 170 K. Both nozzles were made by grinding down a drawn out glass tube and thus have a “converging”profile. The mass spectrometer contains a Weiss type ionizer (Extranuclear 041-1) and quadrupole mass filter. The electron energy of the ionizer is scanned by varying the filament/grid voltage while keeping the emission current fixed (0.3 mA for electron energies 12 eV). Ionization potentials can be measured with an accuracy of -0.2 eV from the threshold voltage for the appearance of ion signals.12 The electron energy scale was calibrated by measuring the ion yield curves for Ar, Kr, and Xe; normalization of the observed thresholds to the known ionization potentials serves to compensate for the electron energy spread and any contact potentials within the ionizer. For measurements of cluster distributions in the parent beams, a 250-jtm aperture was placed over the entrance to the detector in order to reduce the mass flow and thereby avoid distortions from effects of space charge.
-
Results and Discussion In our crossed beam experiments, the xenon beam contained no detectable clusters, whereas the argon beam contained a wide range. The various possible reaction processes thus have the form Xe Ar, XeAr, + ArP Ar, ..,
+
-
+
+
For single-collisionevents, n > m + 1 is ordinarily required. The other possibility, n = m, only becomes possible when the cluster produced is so large that it can accommodate the kinetic energy deposited in the collision without dissociation during flight to the detector. The neutral reaction products must also obey the kinematic constraints imposed by conservation of energy and momentum, but fragmentation may cause the observed ionic species to disobey these constraints. Kinematic Analysis. Figure 1 shows for three reactions the velocity vector diagrams which define the range of laboratory
~
(1 1) Lee, Y. T.;McDonald, J. D.; LeBreton, P. R.;Herschbach, D. R.Rev. Sci. Instum. 1969, 40, 1402; J. Chem. Phys. 1972, 56, 169. (12) Honig, R. E. J. Chem. Phys. 1947, 16, 105; “Methods of Experi-
mental Physics”, Vol. 3, “Molecular Physics”; Williams, D., Ed.;Academic Press: New York, 1962; Chapter 5.
0022-3654/84/2088-4506$01.50/0 0 1984 American Chemical Society
Ar,
~~:
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4507
Fragmentation of van der Waals Clusters
(90")
L
.-e c
e
9
0 C
.-CT v,
IO2
IO
2
3
4
5
6
7
Cluster Size n
Xe (0") H 100 rn/sec
Figure 1. Velocity vector diagram showing initial asymptotic velocities of the Ar, and Xe beams and the relative velocity vector (hypotenuse). Circles indicate loci of the most probable final velocity vectors for XeAr, (upper left) from the reactions Xe + Ar8 XeAr, + Ar and Xe + Ar9 XeAr, + Ar2and XeAr (lower right) from the reaction Xe + Ar2 XeAr + Ar. The circles determine the allowed laboratory angular ranges (cf. labeled angles) of the XeAr, produced in those reactions.
-
-
-+
Figure 3. Cluster ion distributions for A,r: (XeArPl)', (HIAr,,)', and (KrArPl)', obtained from an argon expansion at 90 torr and 85 K. Data for the mixed clusters are shifted to the left one unit to correlate n and rn in the text. The Ar,' points were obtained from elastic scattering of the neutral clusters from He, observed at 6 O from the Ar,, beam; this Ar; distribution is virtually identical with that measured by directly viewing the Ar, beam. Points for mixed clusters were obtained from integrations over all laboratory angles (cf. Figures 1 and 2). The curves are normalized at n = 2 except for (HIArPl)' which has the same ordinate scale as (XeArPl)+. The nominal electron bombardment energy is 35 eV. dissociation. This constraint limits the laboratory velocity of each product species to a distinct, narrow band near the outer edge of the kinematic circle. In this case, the velocity distribution measurements (analyzed in detail elsewherelo) permit unequivocal identification of the Xe Ar2 reaction for mild conditions and again reveal the major contribution from fragmentation of larger clusters when the parent beam is highly polymerized. Distribution of Cluster Zons. Figure 3 compares the observed distributions of Ar,+ and (XeAr,)+ ions. The comparison involves a difference in normalization, since Ar,+ is detected in the parent beam (or after elastic scattering of the neutral clusters from He) whereas the (XeAr)' comes from ionizing the reaction products scattered over a wide range of angles. For a given initial Ar, distribution in the parent beam, the shape of the Ar,+ distribution depends only on the ionization and fragmentation processes. The (XeAr,)' distribution depends also on the relative cross sections for reactions of Xe atoms with the various Ar, clusters and the fraction of precursor clusters that fall within the detector scan. Interpretation of Figure 3 in terms of fragmentation thus requires estimating the variation of reaction cross sections and ionization cross sections with cluster size. We first assume that the primary process forming XeAr, is
+
' 0
30.
60 .
9' 0
Laboratory Scattering Angle Figure 2. Laboratory angular distributions for (XeAr)' (dashed curve) and (XeAr7)+(dotted curve) observed with Ar expansion at 90 torr at 85 K and electron bombardment energy of 35 eV; for (XeAr)' (solid curve) under the same conditions except only 15-eV electron energy; and for (XeAr)' (dot-dashed curve) at 75 torr and 120 K and 35 eV. The ordinate scale for the first three curves is the same (except that the 15-eV data are multiplied by 100); the scale for the fourth curve is arbitrary. angles and speeds accessible to the corresponding product clusters (with m = 1 or 7), as determined by the kinematic constraints. The observed angular distribution of (XeAr)' ions, shown in Figure 2, becomes consistentlo with formation of XeAr from Xe + Ar2 only when the Ar expansion is mild (75 torr and 120 K). Under less mild conditions much of the XeAr+ appears in an angular region inaccessible to XeAr from the Xe Ar2 reaction. These XeAr' ions must come from reactions involving larger Ar, clusters. They can come from either (1) ionization of XeAr produced by Xe Ar, XeAr Ar,, ...
+
+
-
+
+
or (2) from fragmentation of larger XeAr, clusters. Comparison with the data shown in Figure 2 for larger cluster ions, such as (XeAr7)+,suggests that (2) becomes the major source of (XeAr)+ when conditions are other than mild. In the same fashion, velocity analysis of the reaction products provides a further test of parentage. For a weakly bound product species such as XeAr, the dissociation energy is substantially less than the initial kinetic energy of the collision. Most of the kinetic energy must then appear in the relative velocity of the product species; otherwise internal excitation of the cluster would induce
Xe
+ Ar,
-
XeAr,
+ Ar
so m = n - 1. The reaction probability is taken as proportional to the target area of the Ar,,cluster, or n2/3.Kinematic calculations (akin to Figure 1) indicate that the amount of XeAr, falling within the detector scan goes roughly as m1I3.The ionization probabilities for Ar, clusters are assumed to be proportional to n. Those for XeAr, clusters are taken as either (i) proportional to m 3, or (ii) roughly independent of m , to span the range of plausible estimates for the mixed clusters. As the net effect of these factors, the intensity ratio (XeAr,{)+/Ar,+ depends only weakly on cluster size; we obtain
+
~ n ~ /- ~i)1/3[i ( n or n
+ 2]/n
for (i) or (ii), respectively. This indicates that the omitted normalization factors vary with cluster size in about the same way. Thus the difference of several orders of magnitude between the slopes of the Ar,+ and (XeAr,)' curves offers evidence that fragmentation of large clusters is much more extensive for argon than for the mixed clusters. Changing the initial assumption from m = n - 1 to m < n - 1 would increase the difference between the curves.
4508 The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
Worsnop et al. t A ‘n
t
‘ie
P=500Torr T=87K
I
25 Af
P=250Torr
x 25 Krt P= 7 0 0 Torr T= I24 K
I
I IT
K r$ P s 300
Torr
T=‘24K
P=300Torr
d 5
Electron Energy
(ev)
Figure 4. Dependence of cluster ion signals on electron bombardment energy: (a) Arnt elastically scattered by He at 6 O (expansion conditions: 140 torr at 85 K); (b) (XeAr,)+ at 1 5 O (110 torr at 85 IC); (c) (XeAr)+ at 1 5 O and 5 5 O (110 torr at 85 K). Curves show least-squares hyperbolic polynomial fits to data, normalized to full scale at 35 eV. Points are shown only for Arzt and (XeAr)+ to avoid clutter. Data for the larger clusters have been combined to improve the signal, since differences between them are smaller than the apparatus resolution. In each panel, the dotted and short-dashed curves show Xet and Art calibration curves, respectively. Other curves are (a) for argon clusters: Arz+ (-); Ar3+ (---); Ar4+(----); Ar5++ Ar6’ + Ar7+(----); Ar8++ Argt + Arlo+ (----). (b) For mixed clusters: (XeAr)+ (-); (XeAr,)+ (---); (XeAr3)’ + (XeAr4)+(--.); (XeAr# + (XeAr6)++ (XeAr7)’ (---.); (c) for (XeAr)+ at 1 5 O and 5 5 O as labeled. Solid arrow in (a) indicates the photoemission threshold for Ar in an Ar bulk matrix (ref 16). Open arrow in (a) indicates the reported threshold for forming Arzt from Ar2 by electron bombardment (ref 15).
Electron Energy Thresholds. Figure 4 shows the dependence of the Arn+and (XeAr,,,)+ ion signals on the electron bombardment energy, for Ar source pressures well above the “mild” regime.13 The calibration curves for Ar+ and Xe+ using cluster free beams are included; these represent least-square fits to the data points using a hyperbolic polynomial form.14 With the energy scale normalized to place the Ar+ threshold a t the known ionization potential, the Kr+ and Xe+ thresholds likewise fall at the expected values (within 0.2 eV). Also, the Arz+ threshold is well below that for Ar+, consistent with the measured ionization potential of the Ar dimer15 (open arrow in Figure 4a). For the larger Ar,+ (13) The expansion conditions of Figure 3 are stronger than in Figure 1 in order to increase the signal to compensate for the low emission currents required to determine electron energy thresholds; also, the dynamic range of 3-4 orders of magnitude shown in Figure 3 requires intense signals. The result is that larger clusters than those shown in Figure 3 are detectable. (14) The hyperbolic polynomial form used is
I/log (y) = a#
+ u+,x”-l + ... + aIx + a,,x
where n = 3-5 in Figure 3. This fitting is a generalization of a cubic polynomial fitting used by Honig.’* (15) Two values have been reported, one measured from electron bombardment energy threshold (15.2 eV) and from photoionization (14.54 eV). The former (the open arrow in Figure 3a) is in good agreement with electron energy threshold measured here. The references are, respectively, Helm, H.; Stephan, K.; Mark, T. D. Phys. Rev. A 1979, 19, 2154. Ng, C. Y.;Trevor, D. J.; Mahan, B. H.;Lee, Y . T. J . Chern. Phys. 1973,66,446.
IO
15
20
5
io
T=170K
i5
20
Cluster Size n Figure 5. Cluster ion distributions for Ar, Kr, and Xe beams produced at high (a, b, c) and low (a’, b’, c’) nozzle stagnation pressures. Electron bombardment energy is 35 eV. In each plot, the monomer peak ( n = 1) is off scale. cluster ions, the thresholds decrease further but remain significantly above the ionization potential measured for a bulk argon matrix16 (solid arrow in Figure 4a). The (XeAr,)+ thresholds show that the electron removed by ionization comes from a level near the ionization potential of the Xe atom. The energy dependence of the ionization cross section differs noticeably from that for bare Xe. This suggests that the electron may not be removed directly from the Xe atom. Instead, “internal Penning ionization” may occur, in which electronic excitation of argon induces ionization of Xe in the cluster. An analogous process appeared in photoionization studies” of mixed dimers such as XeAr, which showed large yields of Xe+ ions at energies corresponding to excitation of Rydberg states of the Ar atom. Again, for the (XeAr,)+ clusters the thresholds decrease with increase in size of the cluster (although this trend is less pronounced than for Ar,’), but the thresholds remain higher than the ionization potential of Xe in a bulk Ar matrix16 (10.2 eV). The variation in the electron energy dependence of (XeAr)’ with laboratory scattering angle elucidates further the fragmentation problem. The marked difference in the shape of 15 and 55O curves (shown in Figure 4c) is significant in view of the different kinematic situation at these angles. In Xe Ar, collisions, the centroid velocity vector (cf. Figure 1 ) shifts toward the argon beam as the cluster size increases. This changes the kinematic diagrams so that large XeAr, clusters tend to scatter at angles near the argon beam, whereas small XeAr, clusters will appear at relatively wide angles. At 15O, the observed (XeAr)’ curve (and to a lesser extent those for m = 2, 3, 4 in Figure 4b) has a much shallower energy dependence than at 55O, which shows the “normal” shape, essentially identical with that for Xe+. The kinematic bias suggests that this difference appears because a
+
(16) Photoemission thresholds are reported for bulk Ar matrices by Jort-
ner, J. “Proceedings of the IV International Conference on Vacuum Ultraviolet
Radiation Physics (Hamburg, 1974)“; Koch, E.-E.; Haensel, R. H.; Kunz, C. Ed.; Pergamon: 1975; Chapter 4. (17) Ng, C. Y.;Tiedermann, P. W.; Mahan, B. H.; Lee, Y.T. J . Chern. Phys. 1977, 66, 5737.
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4509
Fragmentation of van der Waals Clusters major part of the (XeAr)’ at 15’ comes from fragmentation of larger XeAr, clusters. Other Reaction Systems. Cluster ions (XAr,)+ derived from products of several other X Ar, reactions have also been examined, with X = Kr, HI, Oz, N2, CO,, CH4, and CzH4. The only reactants tried for which no signal could be detected were X = He and Ne. The results for Kr showed the same trends seen for Xe although they were less pronounced. For instance, the decrease in intensity of (KrAr,)’ with increasing cluster size (included in Figure 3) is steeper and thus intermediate between the (XeAr,)+ and Ar,+ distributions. For the molecular reactants, the distribution of cluster ions is rather flat and contains less intensity in the small cluster region. The data for (HIAr,)+ included in Figure 3 are typical. A plausible interpretation is that small clusters tend to predissociate due to excitation of rotation or vibration in the molecular component; large clusters may have enough heat capacity to accommodate such excitations. Under the mild conditions that permit the X Arz reaction to predominate, the (XAr,)+ signals are very weak. Question of “Magic Numbers”. Figure 5 shows sample distributions of cluster ions obtained for beams of argon, krypton, and xenon. The distributions were measured for a range of Po, the source pressure (200-700 torr), and Eo, the electron bombarding energy (9-160 eV). As Po is increased, the contour of these X,+ distributions shifts to yield more ions with large n. However, there is little variation with Eoas long as it exceeds the ionization potential of the monomer. In agreement with previous work,8 the Xe,+ intensity distribution shows distinct breaks for particular cluster sizes. The enhancements a t n = 13 and n = 19 correspond to the first two “magic numbers” for sphere packings.I8 These enhanced peaks become less distinct as Po is lowered. They also become less pronounced for Kr,+ and the enhancement typically disappears for Ar,,+. For all three systems a t any Po, however, the n = 20 peak appears distinctly weak compared to its neighbors. A previous study of the Xe,+ case attributed the n = 13 and 19 enhancements to particularly stable neutral clusters.* Our observations suggest that these peaks instead correspond to particularly stable cluster ions, with intensity augmented by cascading from fragmentation of larger clusters during ionization.Ig Direct supporting evidence comes from a recent study of metastable mass peaks resulting from the unimolecular decomposition of ion clusters.z0 This shows that
.---_
+
5
z 2Lz-.-l z
-
Arig+
+ Ar
occurs to form a particularly stable species. The technique appears to work only if atomic or molecular rearrangement is involved, which suggests that the n = 19 ion may indeed correspond to a “magic number” configuration for sphere packing. In any case, this reaction nicely accounts for the observed depletion of the n = 20 ion yield. An analogous situation occurs in electron bombardment ionization of water clusters,*’ which produces a prominent fragment peak, (H20)21H+.As shown by Searcy and Fenn, this correlates with a major cluster ion peak produced by solvation of a proton beam.zz Again, the metastable peak analysisz0 confirms that (Hz0)21H+is a particularly stable cluster ion. Figure 6 displays for each of the rare gas systems the dependence on Po of the intensity ratio I,,/I,,+, of adjacent peaks near n = 13 and n = 19. For low Po, these ratios all exceed unity and for higher Po most approach unity. The ratios also exhibit clearly the decrease in enhancement for n = 13 as Xe Kr Ar and the prevalence of weakness for n = 20 as the pressure increases.
--
(18) Sattler, K.; Muhlbach, J.; Rechagel, E.; Reyes Flotte, A. J. Phys. E 1980, 13, 673. (19) Milne, T. A.; Greene, E. T.; Beachey, J. E. J . Chem. Phys. 1972,56, 5340. (20) Stace, A. J.; Moore, C. Chem. Phys. Lett. 1983, 96, 80. (21) Castleman, A. W., Jr.; Kay, B. D.; Hermann, V.; Holland, P. M.; Mark, T. D. Chem. Phys. Lett. 1981, 179. Dreyfuss, D.; Wachman, H. Y. Ibid. 1981, 178. (22) Searcy, J. Q.;Fenn, J. B. J. Chem. Phys. 1974, 61, 5282.
Kr
T=124K
>> I t , the third term is small and the observed I,,’ peaks will not appear to be enhanced. For higher Po, where I> I,“, the relative magnitudes of the a,,,,, coefficients will govern the shape of the I,,+ distribution. If the primary decomposition path for I,+ involves loss of only one atom, then the Zml+ peak should appear enhanced. If it is not, the “extra” intensity must be spread out over several clusters so that none appear enhanced. In particular, our data indicate that n = 20 is less stable than its neighbors and that downward cascading from fragmentation of large clusters increases Kr Ar. substantially as Xe Electron bombardment will produce cluster ions with a range of internal energies, but this excitation need not depend appreciably on the kinetic energy Eo of the incident electron. The excitation may depend strongly on which atom is ionized. For instance, the most stable configuration of the cluster ion might have the charged atom located at the center and solvated by the surrounding neutral atoms. Formation of this cluster ion by ionizing one of the outer atoms would then induce a rearrangement to the most stable configuration and thereby endow the cluster ion with internal excitation. As long as Eo is sufficient to remove an electron, such processes should produce energy distributions that are not strongly dependent on the electron bombardment energy.
+
-
- -
Acknowledgment. This study was prompted by lively discussions enjoyed by D.R.W. at the Minisymposium on Clusters included in the IXth Symposium on Molecular Beams (Cannes, June, 1981). We gratefully acknowledge support of this work by the Air Force Geophysical Laboratory under Contract F 19628-78-C-0100. Registry No. Xe, 7440-63-3; Ar, 7440-37-1; Kr, 7439-90-9.
