A new technique is presented that makes it possible, with a single laser pulse, to determine the three‐dimensional spatial distribution of state‐selected photoproducts. Initially, absorption of a photon from a laser beam causes fragmentation of a molecule. Multiphoton ionization is used to select the internal state of a desired fragment without perturbing its velocity. Following a short delay, the three‐dimensional spatial distribution caused by the fragment velocities is projected onto two dimensions by accelerating the state‐selected fragment ions into the surface of a channel plate particle multiplier. Electrons emerging from the multiplier are imaged onto a phosphorescent screen for analysis by a digital‐image processing device such as a two‐dimensional optical multichannel analyzer. The three‐dimensional spatial distribution is reconstructed by taking the Hankel transform of the Fourier transform of the projection. The technique is illustrated by recording the spatial distribution of methyl fragments produced in their vibrational ground state by the 266 nm photodissociation of CH3I. From this study it is determined that the fraction of CH3(v=0) formed in coincidence with I(2P1/2) is greater than 0.95, the rest being formed in coincidence with I(2P3/2) ground state.

Two‐dimensional imaging of state‐selected photodissociation products detected by multiphoton ionization

Control of unimolecular reactions using coherent light

Sputtering by ion bombardment theoretical concepts

This article is a brief review of dry etching as applied to pattern transfer, primarily in silicon technology. It focuses on concepts and topics for etching materials of interest in micromechanics. The basis of plasma-assisted etching, the main dry etching technique, is explained and plasma system configurations are described such as reactive ion etching (RIE). An important feature of RIE is its ability to achieve etch directionality. The mechanism behind this directionality and various plasma chemistries to fulfil this task will be explained. Multi-step plasma chemistries are found to be useful to etch, release and passivate micromechanical structures in one run successfully. Plasma etching is extremely sensitive to many variables, making etch results inconsistent and irreproducible. Therefore, important plasma parameters, mask materials and their influences will be treated. Moreover, RIE has its own specific problems, and solutions will be formulated. The result of an RIE process depends in a non-linear way on a great number of parameters. Therefore, a careful data acquisition is necessary. Also, plasma monitoring is needed for the determination of the etch end point for a given process. This review is ended with some promising current trends in plasma etching.

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A survey on the reactive ion etching of silicon in microtechnology

Photochemistry at adsorbate/metal interfaces

The excitation of the umbrella mode of CH3 and CD3 formed from photodissociation of CH3I and CD3I at 248 nm

Our previous model on dimer sputtering is extended to the formation of larger clusters. The general formalism for the calculation of the momentum distribution function of these clusters is given. A simple analytical expression is obtained for the distribution function of any cluster in the high energy limit. Comparison of this expression with experimental data on the energy and angle distribution of Wk + clusters at high energies shows good agreement. The experimental results on K2 and KI sputtering with low energies are recompared with the theory. For the ionic crystal KI, evidence is obtained that sputtering of non adjacent K and I atoms from the lattice gives an important contribution to dimer formation. The role of the binding energy Eb , of the particles with respect to the surface is discussed.

On the energy distribution of sputtered clusters

A theoretical expression is obtained for the energy distribution of sputtered dimers from crystal surfaces. The derivation is based on a model where the atoms which constitute a dimer, are sputtered independently from the crystal according to their respective single particle energy distribution functions. Two neighbouring atoms are then supposed to form a dimer if the sum of their initial relative kinetic energy and potential energy is less than zero. A comparison with experimental results for K2 and KI sputtered from polycrystalline K and KI surfaces respectively, shows a good agreement.

On the energy distribution of sputtered dimers

Photofragmentation of CH3Br in the A band

Time-of-flight (TOF) spectra and the angular distributions of H atoms resulting from the photofragmentation of HI at 248 nm, 222 nm and 193 nm have been measured. The ratio of I(2P 1 2 ) and I(2P 3 2 ) formation and the symmetries of the repulsive excited states were obtained. From this information the potential-energy curves of the relevant excited states were calculated by means of the modified δ approximation. With these potential curves the I(2P 1 2 )/I(2P 3 2 ) ratio was deduced for DI. The thus calculated ratios for DI are in excellent agreement with the values known from the literature.

A.E. De Vries

Papers

The excitation of the umbrella mode of CH3 and CD3 formed from photodissociation of CH3I and CD3I at 248 nm

On the energy distribution of sputtered clusters

On the energy distribution of sputtered dimers

Photofragmentation of CH3Br in the A band

Photofragmentation of HI in the first continuum