Production of 0.1–3 eV reactive molecules by laser vaporization of condensed molecular films: A potential source for beam-surface interactions

Abstract

A versatile, repetitively pulsed source of translationally fast, reactive molecules is described that is suitable for materials processing experiments. The pulsed beams are generated by excimer laser vaporization of cryogenic molecular films that are continuously condensed on transparent substrates. The generation of fast, energy variable pulsed molecular sources of Cl₂ and NO is demonstrated. The most probable translational energies of Cl₂ and NO molecules can be reproducibly varied monotonically by adjusting the laser fluence or film thickness. Here, the most probable translational energy is quoted as the energy corresponding to the maximum of the time-of-flight trace. Using laser fluences of 2–25 mJ cm⁻² from a 193 nm excimer laser, the most probable translational energies of Cl₂ are 0.4–2 eV. Significant fractions of molecules with translational energies greater than 3 eV are observed at the leading edges of the distributions. Very similar results are obtained by vaporizing Cl₂ with 248 and 351 nm radiation. Pulses of translationally fast NO molecules are generated in a similar manner; most probable energies from 0.1–0.4 eV, with the fastest molecules up to 0.8 eV, are obtained using laser fluences of 1–11 mJ cm⁻² at 193 nm. Approximately 10¹³−10¹⁴ molecules per cm² of the film are vaporized per laser pulse, depending on film thickness and laser fluence.