No abstract

A trochoidal electron monochromator has been interfaced to a mass spectrometer to perform electron capture negative ion mass spectrometric (ECNIMS) analyses of environmentally relevant chemicals. The kinetic energy of the electron beam can be varied from 0.025 to 30 eV under computer control. No reagent gas is used to moderate the electron energies. An electron energy spread of +/- 0.1 to +/- 0.4 eV full width at half-maximum (fwhm) can readily be obtained at a transmitted current of 2 x 10(-6) A, improving to +/- 0.07 eV at 5 x 10(-7) A. Comparisons of ECNI results from the electron monochromator/mass spectrometer system with those from a standard instrument that uses a moderating gas show similar spectra for heptachlor but not for the s-triazine herbicides, as for example, atrazine. This compound shows numerous adduct ions by standard ECNIMS that are eliminated by using the electron monochromator to generate the mass spectra. Isomeric tetrachlorodibenzo-p-dioxins show distinct differences in the electron energies needed to produce the maximum amount of parent and fragment anions. Multiple resonance states resulting in stable radical anions (M.-) are easily observed for nitrobenzene and for polycyclic aromatic hydrocarbons. Ionic products of dissociative electron capture invariably occur from several resonance states.

No abstract

A surface dipole layer due to adsorbates on a fine mesh can produce localized electric fields capable of ionizing highly excited atoms. This effect is observed when an atomic beam containing high-Rydberg states of Li is incident on a degassed Au mesh having square openings 6 pm across. During the formation of an adlayer on the surface, the adsorbate field is analyzed through measurements of the excited-state atomic-beam flux transmitted through the mesh. The beam-transmission experiments confirm that the excited-state survival probability is surface-layer dependent. Two distinct dips are observed in the transmission as a function of time, during submonolayer adsorption of H&O and Li on the mesh surface. A cusp between the dips is interpreted as evidence for a change in the sign of the net surface dipole-moment density. These results are compatible with a Langmuirtype model for the survival probability of excited atoms subject to electric-field ionization as they pass through the mesh. Demonstrating that high-Rydberg atoms may be useful as probes for localized electric fields, the experiments serve as feasibility studies for a method of analysis which can readily determine surface dipole orientations and which may have the ability to detect changes in adsorbate coverage at the level of 10 -10 ' monolayer.

When does the Schrödinger equation have bound-state solutions? This question is pursued by a variational method with simple trial wave functions. An arbitrary one- or two-dimensional attractive potential is shown to have at least one discrete energy level, and estimates of the binding energy are obtained for shallow potential wells. In other problems, sufficient conditions are established for the existence of bound states.