Branching of negative streamers in free flight

We recently have shown that a negative streamer in a sufficiently high homogeneous field can branch spontaneously due to a Laplacian instability, rather than approach a stationary mode of propagation with fixed radius. In our previous simulations, the streamer started from a wide initial ionization seed on the cathode. We here demonstrate in improved simulations that a streamer emerging from a single electron branches in the same way. In fact, though the evolving streamer is much more narrow, it branches after an even shorter propagation distance.Comment: 4 pages, 3 figure

Stability of negative ionization fronts: regularization by electric screening?

We recently have proposed that a reduced interfacial model for streamer propagation is able to explain spontaneous branching. Such models require regularization. In the present paper we investigate how transversal Fourier modes of a planar ionization front are regularized by the electric screening length. For a fixed value of the electric field ahead of the front we calculate the dispersion relation numerically. These results guide the derivation of analytical asymptotes for arbitrary fields: for small wave-vector k, the growth rate s(k) grows linearly with k, for large k, it saturates at some positive plateau value. We give a physical interpretation of these results.Comment: 11 pages, 2 figure

Spontaneous Branching of Anode-Directed Streamers between Planar Electrodes

Non-ionized media subject to strong fields can become locally ionized by penetration of finger-shaped streamers. We study negative streamers between planar electrodes in a simple deterministic continuum approximation. We observe that for sufficiently large fields, the streamer tip can split. This happens close to Firsov's limit of `ideal conductivity'. Qualitatively the tip splitting is due to a Laplacian instability quite like in viscous fingering. For future quantitative analytical progress, our stability analysis of planar fronts identifies the screening length as a regularization mechanism.Comment: 4 pages, 6 figures, submitted to PRL on Nov. 16, 2001, revised version of March 10, 200

Some perspectives in electron momentum spectroscopy experiments

The basic challenges of electron momentum spectroscopy (EMS) as it has guided our research efforts at Carleton University are outlined. These challenges are : (a) Are the spectroscopic factors obtained in EMS equivalent to the spectroscopic factors observed in photoelectron spectroscopy ? (b) Can experimental momentum profiles be related to the chemical reactivity of complex molecular systems ? (c) Are the basic assumptions of (e,2e) reaction theory valid for complex molecular systems ? Insights based on our efforts to address these questions are presented

VIBRATIONAL ENERGY SURVIVAL PROBABILITY IN MOLECULAR BEAM SCATTERING: NO(v equals 1)/LiF(100).

Summary form only given. The authors report here the survival probability for the scattering of a well-characterized molecular beam of vibrationally excited NO from a LiF(100) surface. Laser spectroscopic techniques are combined with molecular beam techniques to perform state-to-state scattering experiments. In these experiments, two laser beams intersect a supersonic molecular beam of NO. The state-to-state scattering experiments reported here give details of the scattering from an initial single electronic, vibrational, and rotational state with a narrow velocity distribution into specific final channels on collision with a clean well-defined surface and permit a more careful comparison with theoretical calculations than is possible with static gas sample techniques