REACTIVE COLLISIONS OF C6H6.-ASSEMBLED MONOLAYER FILMS PREPARED ON GOLD FROM N-ALKANETHIOLS AND A FLUORINATED ALKANETHIOL - THE INFLUENCE OF CHAIN-LENGTH ON THE REACTIVITY OF THE FILMS AND THE NEUTRALIZATION OF THE PROJECTILE( AND C6D6.+ AT SELF)

Low-energy ion-surface collisions were used to probe the reactivity, e lectron barrier properties, and relative degree of order of self-assem bled monolayer films after transfer of the films from solution to vacu um (10(-7) Torr). Mass-selected polyatomic projectiles (ionized benzen e, benzene-d6, and fluorobenzene) collided with self-assembled monolay er films at collision energies in the range of 20-70 eV, and the resul ting ions were mass-analyzed and detected. The surfaces were prepared by the spontaneous assembly of n-alkanethiols (CH3(CH2)(n)SH, n = 3, 1 1, 17), perdeuterioeicosanethiol (CD3(CD2)19SH), and 2-(perfluorooctyl )ethanethiol (CF3(CF2)7CH2CH2SH) on both gold foil and vapor-deposited gold. Chemical reactions between the benzene molecular ion and the mo nolayer films (e.g., H, CH3, D, CD3, F, and CF3 additions) are evident from deuterium labeling results. Ion-surface collision spectra for io nized benzene are sensitive to (i) the chemical composition of the mon olayer, (ii) the chain length of the alkanethiol used to prepare the f ilm, (iii) the preparation of the gold surface prior to reaction (mech anically polished gold foil vs vapor-deposited gold vs plasma-cleaned vapor-deposited gold), and (iv) the exposure time between the alkaneth iol solution and the gold. For comparison with our experimental result s, ab initio calculations have also been carried out to predict the en ergetics of the loss of H and H-2 from selected ion-surface reaction a dducts (H, F, and CH3 addition products). Experimental data not direct ly available from the tandem mass spectra (i.e., total ion signals and surface currents) are used to characterize the relative degree of neu tralization of the projectiles at the films. The data suggest that the relative electron barrier properties of the films in vacuum mimic tho se reported for electrochemistry experiments in solution. The mass spe ctra and the current measurements indicate that the relative degree of order of the monolayers is retained upon transfer of the films from s olution to vacuum.