The work functions of ${}^7\mathrm{Li}$ and ${}^6\mathrm{Li}$ metals have been measured as a function of temperature, by using photoionization of pure isolated metal nanoparticles in a beam. These data reveal a marked isotope effect in the temperature variation of these work functions. Furthermore, for both isotopes the curvature of this temperature variation is found to be significantly larger than may be ascribed purely to a change in the electron-gas density. These findings enhance the characterization of lithium as a quantum material in which the interplay between electronic and ionic degrees of freedom is nontrivial, and call for a microscopic understanding beyond simple models. Additionally, the slope of the work-function curves was observed to vanish in the low-temperature limit, as had been predicted on the basis of the Third Law of Thermodynamics.

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• Electron emission
• Isotope effect
• Thermal expansion
• Alkali metals
• Nanoclusters
• Nanoparticles
• Photoelectron techniques
• Photoionization
• Time-of-flight mass spectrometry