ITALIAN PHYSICAL SOCIETY PROCEEDINGS OF THE INTERNATIONAL SCHOOL OF PHYSICS «ENRICO FERMI» COURSE CVII edited by G. SCOLES Director of the Course VARENNA ON LAKE COMO VILLA MONASTERO 28 June - 7 July 1988 The Chemical Physics of Atomic and Molecular Clusters 1990 NORTH-HOLLAND AMSTERDAM - OXFORD - NEW YORK - TOKYO INDICE G. SCOLES and S. STRINGARI - Preface pag. xvn Gruppo fotografico dei partecipanti al Corso fuori testo PART I. - THEORY R. S. BERRY - Structure and dynamics of Clusters: an introduction. 1. What are Clusters? pag. 3 2. Simulations and diagnostics » 8 21 » 8 2*2 T... » 15 R. S. BERRY - Structure and dynamics of Clusters: phase equilibrium and phase change. 1. Solid and liquid Clusters: their equilibrium » 23 ri » 25 1*2 » 27 2. Simulations » 32 2*1 » 32 2*2 » 37 3. The future » 39 J. JORTNER, D. SCHARF, N. BEN-HORIN, U. EVEN and U. LANDMAN - Size effects in Clusters. Prologue » 43 1. Energetic and thermodynamic size effects » 44 1' 1. Energetic size effects » 44 1*1.1. From molecules to Clusters » 44 1*1.2. From Cluster to bulk Condensed matter » 45 1*2. Isomerization and melting of Clusters » 50 1*3. Experimental interrogation of Cluster isomerization » 58 v VI INDICE 2. Dynamic size effects in electronically excited rare-gas Clusters pag. 64 2"1. Reactive and nonreactive relaxation » 64 2'2. Application of classical molecular-dynamics method » 67 2'3. Analysis of the molecular-dynamics data » 71 2'3.1. Size analysis » 71 2'3.2. Energetics » 72 2'3.3. Configurational relaxation » 72 2'3.4. Mass transport of electronic excitations » 73 2'4. Computational results » 73 2'4.1. Atomic-impurity excitations in Xe*Ar12 and Xe*Ar54 Clusters » 73 2'4.2. Excimer excitations in Xe|Arn and XefAr53 Clusters ... » 81 2'5. Discussion » 91 2'5.1. Dynamics of atomic-impurity excitations » 91 2'5.2. Dynamics of excimer excitations » 92 F. G. AMAR - A structural approach to the analysis of Cluster dynamics. PART I - Introduction » 99 PART II - Melting and isomerization of argon Clusters » 100 1. Background » 100 2. Melting loops » 101 3. Isomers of Ar7 » 103 4. Molecular dynamics and quenching » 104 5. Residence times » 107 6. Lifetime distributions » 111 7. Ar7 melting: summary » 113 PART III - Cooling and dynamic quenching of Ar13 » 113 1. Introduction » 113 2. Brownian dynamics » 114 3. Other quenching techniques » 115 4. Results » 115 5. Conclusions » 119 PART IV - Photodissociation dynamics of Br^ in (C02)TC and Ar„ Clusters » 119 1. Background » 119 2. A model for negative-ion photodissociation » 120 3. Variable-charge calculation » 123 4. Minimum-energy structures » 124 5. Photoexcitation dynamics of minimum-energy structures » 125 6. Thermal averages » 129 7. Caging fraction » 130 8. Discussion » 131 PART V - Summary » 132 INDICE VII R. J. LE ROY - Spectra, phase transitions and «dynamical isomeriza- tions» of mixed Van der Waals Clusters. 1. Introduction pag. 137 2. Methodology, potential-energy surface and equilibrium structures .... » 138 3. Simulating the Cluster spectra » 141 4. Dynamical simulations and phase change behaviour » 146 5. Spontaneously reversing isomerization of an isolated SF6-(Ar)9 Cluster » 150 6. Discussion and speculations » 154 W. ANDREONI - The Car-Parrinello method and its application to microclusters. 1. Introduction » 159 2. The Car-Parrinello method » 159 3. Computational procedure » 162 4. Some test examples » 164 5. Some results: structural and finite-7" properties » 167 6. Final remarks » 173 R. KAWAI, I. L. GARZÖN, X. P. LONG and J. H. WEARE - Simulation of metallic and impure van der Waals Clusters. Introduction » 177 1. Structural and dynamical properties of impure van der Waals Sys­ tems » 178 1" 1. Low-temperature structures » 180 11.1. The A1SB System » 180 1*1.2. The A55B System » 181 1*1.3. The A13B13 system » 182 1*2. The finite-temperature behavior » 182 1*2.1. Heating in the A12B, A1SB and A55ß Systems » 182 1*2.2. Heating in the A1SB13 System » 186 2. Car and Parrinello methods for metallic Clusters » 187 S. STRINGARI - Clusters of quantum liquids. 1. Introduction » 199 2. Bulk and surface properties of 4He and 3He » 200 2*1. Bulk properties » 200 2*2. Surface properties » 203 3. Ground-state properties of helium Clusters » 205 3' 1. The density functional approach » 205 3*2. Ground-state properties of 4He Clusters » 208 VIII INDICE 3'3. Ground-state properties of 3He Clusters pag. 212 3'4. 3He impurities on 4He Clusters » 219 4. Collective excitations in helium Clusters » 223 4' 1. Collective modes in 4He Clusters \ » 223 4' 1.1. Compression modes » 225 4'1.2. Surface excitations » 227 4'2. Collective modes in 3He Clusters » 227 4*2.1. Density excitations » 228 4'2.2. Magnetic excitations » 229 5. Conclusions » 233 X. CAMPI - Fragmentation of Clusters. 1. Introduction » 237 2. Survey of experimental data » 237 2'1. Fragmentation of nuclei » 237 2'2. Fragmentation of atomic Clusters » 244 3. Elements of Cluster fragmentation theory » 248 31. Partitions >» 248 3'2. Geometrical modeis » 250 3'3. Statistical equilibrium » 256 3"4. Rate equation theory » 257 4. Signals of scaling in Cluster fragmentation » 258 4'1. Critical behaviour and critical exponents » 259 4'2. Finite-size scaling » 265 Summary » 266 PART IL - SPECTROSCOPY AND PHOTODISSOCIATION R. 0. WATTS - Infrared spectroscopy of large Clusters. 1. Introduction » 271 2. Experimental methods » 272 3. Experimental results » 276 4. Vibrational-spectroscopy theory » 287 5. Semi-classical calculations of band shapes » 299 6. Quantum Simulation » 311 7. Summary » 326 D. J. LEVANDIER, M. MENGEL, J. MCCOMBIE and G. SCOLES - Infrared spectroscopy in and on argon Clusters: matrix and surface spectroscopy in the gas phase. 1. Introduction » 331 2. Experimental background » 332 INDICE IX 3. Clusters as microreactors: complex formation between CH3F and HCl in argon pag. 334 4. The surface of Clusters: a unique environment for spectroscopy and chemical dynamics » 338 5. Clusters and glasses » 345 6. Phase coexistence in Clusters: hot solids and cold liquids » 351 7. Clusters as microsolutions: will the would-be solute be admitted in or left on the surface? » 352 G. DELACRETAZ, P. FAYET, J. P. WOLF and L. WÖSTE - Spectroscopy, reactivity and photodynamics of size-selected metal Clusters. 1. Introduction » 359 2. Spectroscopic approaches to metal Clusters » 359 2' 1. Cluster beam sources for performing spectroscopy » 359 2'2. Eesonant two-photon ionization and depletion spectroscopy » 363 2'3. As an example: spectroscopy of Na3 » 372 2*3.1. The ground State of Na3 » 372 2'3.2. The 675 nm System (A-state) » 372 2'3.3. The510nmSystem(ß-state) » 373 2'3.4. The560nmSystem(ß'-state) » 374 2'3.5. The 475nm System (C-state) »> 375 2'3.6. The predissociated 420 nm System (D-at&te) » 376 3. Chemical reactions of metal Clusters » 378 3'1. Production of size-selected metal Cluster ions » 378 3'2. Experimental set-up » 381 3'3. Cluster-molecule reactions » 384 4. Photodynamics of metal Clusters » 389 M. MOSKOVITS - Size-dependent properties in small metal and semiconductor Clusters » 397 W. D. KNIGHT - Structures of metal Clusters. 1. Introduction » 413 2. Shell structure » 414 3. Experimental properties » 419 3'1. Abundance and stability » 419 3'2. Ionization potential » 420 3'3. Static electric polarizability » 422 3'4. Giant dipole resonance » 422 4. Conclusions and future directions » 424 1. X INDICE T. D. MARK, A. STAMATOVIC, F. HOWORKA, P. SCHEIER and G. WALDER - Interaction between visible laser light and expanding van der Waals Cluster beams. 1. Introduction pag. 427 2. Experimental technique » 428 3. Results » 429 4. Experimental tests » 440 4' 1. Laser power dependence » 440 4'2. Laser wavelength dependence » 443 4'3. Laser focus dependence » 443 4'4. Dependence on irradiation geometry » 444 4'5. Various further tests and observations » 444 5. Analysis (cross-section) and Interpretation (dissociation mechanism) .. » 445 D. RAY, N. E. LEVINGER and W. C. LINEBERGER - Spectroscopy and dynamics of «van der Waals» Cluster ions. 1. Introduction » 451 2. Discussion » 452 2' 1. Experimental method » 452 2'2. Optical absorption in «van der Waals» Cluster ions » 454 2'3. Photodissociation dynamics of «van der Waals» Cluster ions ..... » 457 2'4. Photochemistry in Cluster ions » 461 3. Conclusion » 463 S. T. ARNOLD, J. V. COE, J. G. EATON, C. B. FREIDHOFF, L. KIDDER, G. H. LEE, M. R. MANAA, K. M. MCHUGH, D. PATEL- MISRA, H. W. SARKAS, J. T. SNODGRASS and K. H. BOWEN - Photodetachment spectroscopy of negative Cluster ions. 1. Introduction » 467 2. Experimental » 469 3. Results and discussion » 470 3*1. Ion-molecule complexes: Cluster anions with localized excess negative charges » 470 3'2. More complicated cases: Cluster anions with excess Charge dispersal » 479 3'3. Alkali metal Cluster anions » 484 3'4. Water Cluster anions » 486 C. BRECHIGNAC and PH. CAHUZAC - Excitation of free metal Clusters. 1. Introduction » 491 2. Photoexcitation of alkali Cluster ions » 491 INDICE KI 2*1. Unimolecular dissoeiation following electronic excitation pag. 491 2'1.1. Experimental procedure » 493 2'1.2. Energetic of the unimolecular dissoeiation » 494 2'1.3. Dynamics ofthe unimolecular dissoeiation » 496 2'2. Photoinduced dissoeiation: a probe to follow electronic excitation » 500 3. Photoionization of metal Clusters » 502 3'1. Direct ionization » 503 3'1.1. Charge exchange between Na^" Cluster and Cs atoms: generation of neutral mass-selected Cluster beam » 503 3' 1.2. Photoionization of mass-selected neutral Clusters » 506 3'2. Autoionization process » 507 PART III. - MASS SPECTROSCOPY SCATTERING AND CHEMICAL PROP- ERTIES G. TORCHET, J. FARGES, M. F. DE FERAUDY and B. RAOULT - Electron diffraction studies of Clusters produced in a free jet expansion. 1. Introduction » 513 2. Experimental methods » 514 2' 1. Cluster beam » 514 2'2. Electron diffraction » 515 2'3.