PHOTON-INDUCED FIELD-IONIZATION MASS-SPECTROMETRY OF ETHYLENE ON SILVER

Photon-induced field ionization is investigated for ethylene adsorbed on a silver field emitter. Laser pulses (2 to 5 ns pulse length, repetition rate up to 100 pulses/s, energy density 105 to 106 W/cm2 pulse, wave lengths 300, 435 and 600 nm) are used to induce ion desorption with correlated time-of-flight measurements for mass determination. At temperatures T < 100 K, photon induced ion formation is observed in two different regions: Below the onset of normal field ionization (F < 0.93 V/ A ̊) hydrocarbon ions, like CnH+2n (2 ≤ n ≤ 6), are observed. They are products of a field-induced polymerization and dehydrogenation reaction. At field strengths above onset of normal field ionization (F > 0.93 V/ A ̊) only C2H+4 ions are obtained. Here, photon induced ions appear only in small intensities (down to 10-6 of total ion intensity). Ion intensities are measured as a function of the emitter temperature, field strength, laser photon energy, laser pulse repetition rate, laser photon density, and gas pressure. At F < 0.93 V/ A ̊ a direct electronic excitation mechanism is involved, at F > 0.93 V/ A ̊ a small, superimposed temperature increase( T ≈ 10 K), caused by the laser pulse, is responsible for photoninduced field ionization. © 1979.