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Abstract No.: MoP-064
Session: Fundamentals: Theory and Experiment
Presentation date: Mon, Aug 28, 2006
Presentation time: 14:30 – 16:00

Characterisation of Laser Ablation Plumes by Time of Flight Quadrupole Mass Spectrometry

Rebecca Suzanne Sage1, Nicholas Roger Walker1, Michael N. R. Ashfold1

1 University of Bristol, Bristol, United Kingdom

Correspondence address: Rebecca Suzanne Sage, University of Bristol, School of Chemistry, Cantocks Close, Bristol, BS8 1TS United Kingdom.

Keywords: Energy, Kinetic-, Distribution; Laser Ablation; Mass Spectrometry, Quadrupole; Mass Spectrometry, Time of Flight.

Novel aspect: Experimental and modelling studies illustrating the use of a novel time-of-flight quadrupole mass spectrometer for direct kinetic energy measurements of mass selected ions and neutrals in a laser ablation plume.

 

Pulsed Laser ablation (PLA) is used to deposit films in a variety of applications, including the production of hard coatings and semiconductors. Optimal control of film growth requires an understanding of the physical and chemical properties of species within the plume of material ablated by the laser. In order to measure the kinetic energies of ejected ions and neutrals, a vacuum chamber containing a laser vaporisation target has been coupled to a quadrupole mass spectrometer. The central axis of the mass spectrometer is perpendicular to the ablation target, i.e. coaxial with the centre of the expanding plume. The second harmonic (532 nm) of a pulsed, Nd:YAG laser is used to ablate material from the surface. The ablated material is sampled by a skimmer placed between the ablation chamber and the mass spectrometer. This is configured to record time of flight (TOF) data of mass selected species which are used to deduce the kinetic energy (KE) of particles in the plume. The experiments are supported by Faraday Cup measurements and computer modelling using SIMION 7.0. Objectives include measurements on ground and excited state neutral atoms and ions.

Current studies are focussed on identifying ionic species generated in the PLA of various elemental targets, and their TOF distributions. Effects of varying target material, laser wavelength and laser power are being investigated. Preliminary studies have been carried out on the ablation of an iron target. A mass scan shows the predominant ion species to be Fe+ (isotopes 56 and 54) and Fe2+. At incident pulse energies of about 1-2 mJ/pulse the KE distribution of Fe+ is seen to span the range 0-100 eV, whereas the Fe2+ distribution stretches to over 150 eV. Decreasing the laser power causes a decrease in both the maximum KE and the width of the distributions. The wider TOF distributions show some structure which we attribute to the relative transmission efficiency of ions of different energies through the quadrupole. SIMION models were used to determine the KE dependence of the transmission function.