Enormous Yield of Photoelectrons from Small Particles

Abstract

A new ac bridge technique allows detailed photoelectric studies on very small particles suspended in gases. The photoelectric yield F near threshold 4 still follows the Fowler-Nordheim law F=c{h&-4); yet. c is generally much larger compared to surfaces of 2. 0 extended solids. For Ag particles of radius 20 A in an airlike mixture of N2 and 02, c 100cp where co applies to a macroscopic Ag surface in the same environment. PACS numbers: 79.60.-i, 82.70.Rr, 92.60.Mt, 94.10.NhThe purpose of this Letter is to report on a large enhancement of the yield F of photoelectrons per incident photon if ultrafine particles with radii 3 -50 A are chosen as photoemitters.The results were obtained with Ag and %Q, by use of a novel technique in which the density Z of the particles does not have to be known.The particles are suspended in an N, -Q, mixture similar to air. The photoelectric size effect to be reported here has wide implications including photoemission fr om inter stellar dust gr ains by starlight, ' atmospheric electricity, ' nucleation, ' and smog photochemistry, " simply because photoemission from small particles has been underestimated so far. It also may become the physical basis for characterizing and measuring very small particles suspended in gases.For the determination of I' and the particle radius R, the aerosol is illuminated with monochromatic light from a Xe high-pressure arc and the conductivity arising from photoelectron emission by the particles is measured. For this photo-conductivity measurement an alternating electric field (amplitude, 30 V/cm; frequency, 90 s ') is applied to the aerosol in two cylindrical condensers, one of which is exposed to uv light. Here, an in-phase current component is caused by the oscillatory motions of the charge carriers produced by photoemission. ' The differential conductivity in the two condensers is measured in an ac bridge circuit by means of a phase-sensitive amplifier.In this way, the capacitive component, typically 10' times higher than the photoconductivity, can be suppressed and the aerosol conductivity arising from ionization by cosmic rays and natural radioactivity is eliminated.Aerosol photoconductivity Z arises from the negative small ions formed by the photoelectrons' and the particles left behind with a positive charge.With ultrafine particles (A~3 0 A), small ion reattachment is negligible. ' Multiple photoelectric particle charging can also be neglected with the low photon energies applied here, because of the strong Coulomb attraction between a doubly

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