Undercooling of gold nanodroplets levitated in a quadrupole ion trap in high vacuum*

A nanoparticle levitated in vacuum has minimal thermal contact with its surroundings and can be heated efficiently with a laser beam. Precise control of the temperature of a levitated particle could facilitate thermodynamic measurements of materials with very high melting points. To establish such a measurement technique using a well-characterized material, we levitate a charged 200 nm gold nanoparticle in an AC quadrupole electric field trap in a vacuum chamber and use a 532 nm laser to illuminate and heat it. Accurate measurements of the mass of the particle are used to deduce the internal temperature of the particle from the rate of mass erosion. We have observed undercooling of gold particles, a phenomenon in which a melted droplet remains liquid when cooled below the usual freezing temperature. In our preliminary measurements, particles have remained liquid to temperatures around 190 K below the freezing point and have persisted in this state in excess of two minutes without refreezing. We will discuss progress toward the precise control of the temperature of a gold nanoparticle and prospects for the measurement of other materials.