J. Phys. Chem. 1991, 95, 8402-8409
8402
cm2. partially dissociated H 2 0 layer, (3-0.6) X Concbions The results are summarized as follows: UV laser irradiation (6.4 eV) of the water bilayer adsorbed on Pd(ll1) results in the sequential dissociation to form hydroxyl groups and later atomic oxygen. In the first step of photodissociation ( H 2 0 hv OH H), the dissociating hydrogen atom is retained on the surface, while in the second step (OH hv 0 H*) it recombines with high probability with another surface hydroxyl group forming a desorbing water molecule (H* OH(a) H20(g)). Molecular water desorption is initially dominated by direct photostimulated desorption of adsorbed molecular water, but after sufficient accumulation of hydroxyl groups strong water desorption from the "hot H atom" mechanism is evident. In either case water desorbs with nonthermal translational energies.
+
-+ +
+
+
+
-
On c(4X2)CO and p(2X2)O covered Pd( 11 l), water photodesorbs nonthermally with a cross section of -2.5 X lezo cm2, whereas photon-stimulated dissociation is not observed. The results presented here corroborate the conclusion drawn in a previous study on the H 2 0 photochemistry on Pd( 1 11) that was attributed to the interaction with generated photoelectrons or hot electrons, most probably via dissociative electron attachment.
Acknowledgment. We thank Professor A. Cassuto for stimulating discussions and Dr.M. T. Paffett for generously loaning the Pd(ll1) crystal. M.W. acknowledges the hospitality during his stay in Austin. This work was supported in part by the National Science Foundation, Grant CHE 9015600. Registry NO. HzO,7732-18-5; Pd,7440-05-3;0,17778-80-2;CO, 630-08-0; H,12385-13-6; OH,3352-57-6.
Resilience of AlCCarbon Molecules Cdo,Cto, and C8,: A Surface-Scattering Time-of-f Hght Investigation Rainer D. Beck, Pamela St. John, Marcos M. Alvarez, Fransois Diederich, and Robert L. Whetten* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569 (Received: April 2, 1991)
Ion beam scattering experiments on the larger carbon molecules (Cso*,C,o+, CS4+)demonstrate their exceptionally high stability with respect to impact-induced fragmentation proccsses. The charged molecules are formed by ultraviolet laser desorption of high-purity molecular samples into a pulsed helium jet. Extracted ions impact Si(100) or graphite(0001) in a high-resolution ion beam/surface collider with mass time-of-flight and angular analysis. Collisions are highly inelastic processes: A large fraction of the entire perpendicular momentum component is lost, and 60 f 20% of the parallel component is either lost or exchanged. No more than 10%of the incident ions are returned, which is attributed to neutralization during the collision event. In contrast to all molecular ions (benzene and naphthalene cations) and clusters (alkali-metal halides), these molecules exhibit no evidence for impact-induced fragmentation, even at impact energies exceeding 200 eV. In the case of Cm-, both the intact parent ion and ejected electrons are detected, with the latter becoming dominant above 120 eV impact energy. Cm+is found to have an exceptionally low energy threshold for inducing sputtering processes of adsorbed overlayers on graphite. Some of these results may be interpretablein terms of the unique structural-energeticcharacteristics of the fullerene family. The results are compared to recent computer simulations of the impact event, which predict high resilience for these molecules.
I. Introduction The recent discovery of methods of isolating' and purifying2-" large quantities of molecular allotropic forms of carbon has made it possible to determine the physical and chemical properties of these fascinating new m~lecules.~Prior to this development, we carried out preliminary surfacecollisionexperiments on a number of charged carbon molecules, CN+, as obtained from laser ablation of graphite in a helium-jet source. More recently, starting with pure molecular solids' CN (N= 60, 70, 76,84, ...) in hand, we7 and have worked to obtain pulsed, mass-selected beams (1) KrHtschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. fhys. Lett. 1990,170. 167. Kriltschmer, W.; Lamb. L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347,354. (2) Taylor. R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H.W. J. Chem. Soc., Chem. Commun. 1990, 1423-5. (3) Bethune, D. S.;Meijer, G.; Tang, W. C.; Rosen, H. J. Chem. Phys. Lett. 1990, 174. 219. (4)Aijc, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropouloe, K.; Huffman, D. R.; Kriltschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, K.; Whetten, R. L. J. fhys. Chem. 1990,94,8630. (5) For a collection of early reports, ace: Mater. Res. Soc. Roc. 1991,206. (6) Dicderich, F.; Ettl, R.; Rubin, Y.; Whetten, R. L.; Beck, R. D.; Alvarez, M. M.; Anz, S. J.; Sensharma, D.; Wudl, F.; Khemani, K. C.; Koch, A. Science 1991. 252. 548. (7)Whetten, R. L.;Alvarez, M. M.; Anz, S. J.; Schriver, K. E.; Beck, R. D.; Diederich, F. N.; Rubin, Y.; Ettl. R.; Foote, C. S.; Damanyan, A. P.; Arbogast, J. W. Mater. Res. Soc. Roc. 1991. 206. 639.
0022-3654/91/2095-8402$02.50/0
with orders of magnitude greater intensity and stability using ultraviolet laser desorption of the solid molecular films. Because of the unique shapes and bonding of these molecules-cage structures, like those shown in Figure 1, that can be loosely thought of as closed graphitic Sheets'O-and their reputed ultrahigh stability under extreme energization," it seemed likely that their response to impact with solid surfaces would be very unusual and interesting. In the new experiments reported here, in which Cm*, Cfo+,and C8,+ are collided against graphite and silicon over a range of energies, this supposition is confirmed. In concurrent research, Callahan et al.'* have collided Cas against polished stainless steel, and Mowrey et al.I3 have conducted extensive simulations of the collisions of (neutral) Cm with graphite and with H-terminated diamond. In the latter work, it has been predicted that collision (8) Haufler, R. E.; Wang, LA.;Chibante, L. P. F.; Jin, C.; Conceicao, J. J.; Chai, Y.; Smalley, R. E. Chem. fhys. Lett., in press. (9)Bethune, D. S.;deVries, M.; et al., private communication. (IO) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. E.Nature 1985,318, 162. For reviews, sa: Smalley, R. E.; Curl, R. F. Scfence 1988,242,1017. Kroto, H.W.Science 1988,242, 1139. (11) OBrien, S.C.; Heath, J. R.; Curl, R. F.; Smalley, R. E. J. Chem. fhys. 1988,88, 220. (12) McElvanv. S.W.; Ross, M. M.; Callahan, J. H. Mafer. Res. Soc. Prdc. iwi,206,697. (13) Mowrey, R. C.;Brenncr, D. W.; Dunlap, B. I.; Mintmire, J. W.; White, C. T. Mater. Res. Soc. Proc. 1991.206. 351: J . fhvs. Chem. 1991. 95, 7138. Mowrey, R. C. Private communication.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 21, 1991 8403
Resilience of All-Carbon Molecules
Figure 1. Structures of the all-carbon molecules CWand C70(known) and Cu (proposed).
events are highly inelastic, but largely nonreactive and nonfragmenting, although the long-time consequences of such heating could not be studied. These simulations predict a remarkable resilience of the structure of Cm: despite collision energies of 150 eV that during impact deform the structure until it is nearly planar, the hollowcage structure is retained upon recoil. It will now prove possible to establish some experimental correspondences to these results. 11. Experimental Method Molecular Carbon Samples. These have been prepared as described p r e v i o u ~ l y . ~ *Briefly, ~ * ~ ~ a high-quality graphite rod is evaporated by resistive heating under an helium atmosphere. The collected material is dissolved in boiling toluene, and the insoluble carbon fraction is separated by filtration. The resulting yield of several hundred milligrams of soluble material consists of Cm, C70, and higher molecules,6which can each be obtained in pure form by repeated liquid-gravity chromatography on alumina. The resulting pure molecular solids are soluble (several mg/cm3) to a limited extent in a variety of solvents, so that a thick film of material, suitable as a laser-desorption target, can be formed by dipping the substrate in the solution and rapidly evaporating the s o l ~ e n t . ' ~ Laser-DesorptionMass Spectrometry. Thick films of pure or mixed molecular carbon are deposited on a 3 mm diameter steel or tantulum rod. The rod is mounted in a high-vacuum chamber (Figure 2 ) within a short nozzle assembly,l5which in turn mounts on the faceplate of a pulsed gas valve. The rod is continuously (14) Allemand, P.-M.; Koch, A.; Wudl, F.; Rubin,
Y.;Diederich, F.;
Alvarez, M. M.; Anz, S.J.; Whetten, R. L. J. Am. Chem. SOC.1991, 113, 1050. (15) Schriver, K.
E.,Ph.D. Thesis, University of California, 1990. Diederich, F.; Rubin, Y.; Knobler, C. B.; Whetten, R. L.; Schriver, K. E.; Houk, K. N.; Li, Y. Science 1989, 245, 1088.
rotated and translated by a motor mounted exterior to the vacuum chamber, in order to continually expose fresh surface to the laser. To obtain a laser-desorptionmass spectrum, the radiation (
