Coupled-channel calculations and the accuracy of the sudden approximation for atom—surface scattering

Exact coupled-channel calculations are presented for the scattering of Ne from W(110) and He from LiF(001), using symmetry to partly decouple the scattering equations. The results are used to test the recently proposed sudden approximation. For Ne/W(110), typical of all metals, the sudden approximation gives excellent quantitative accuracy. For the very unfavorable system He/LiF(001) good semiquantitative agreement is found with the exact results. It is concluded that the sudden approximately provides an efficient and accurate tool for atom—surface scattering calculations.     

Femtosecond ionization and fragmentation of free silicon nanoparticles

Femtosecond-laser spectroscopy is used to study the photoionization and photofragmentation of large neutral silicon clusters in a beam. Silicon clusters Si_n with sizes up to n≈6000, corresponding to nanoparticles with diameters up to 6 nm, are generated in a laser vaporization source. Nanosecond- and femtosecond-laser ionization are employed to characterize the free silicon nanoparticles. Excitation with intense femtosecond-laser pulses leads to prompt formation of doubly and triply charged Si_n clusters. Additionally, strong fragmentation of charged clusters occurs by Coulomb explosion, resulting in high released kinetic energies. Multiply charged atoms up to Si⁴⁺ with initial kinetic energies in the range of 500 eV are observed for laser intensities of about 10¹³ W/cm². Pump–probe spectroscopy yields decay times of the intermediate resonances of the nanoparticles. Received: 22 January 2000 / Published online: 7 August 2000     

Phosphonic systems: 5. Prototropic equilibria of diethyl alkenylphosphonates

Base-catalyzed prototropic equilibria for nine pairs of diethyl 1- and 2-alkenylphosphonates have been determined. The results, together with those reported elsewhere for three other pairs, demonstrated that the diethoxyphosphoryl group PO₃Et₂ has only a very weakly stabilizing effect on the adjacent carbon–carbon double bond. The decisive factors are the number of carbon atoms attached to the double bond and the number of hydrogen atoms available for hyperconjugation.