Carbon in its many forms has proven to have a wide range of properties that has made it critical in a wide variety of scientific and technological applications. The element’s great versatility stems from its ability to form single, double, and triple bonds, allowing for a wide array of bonding motifs, all with distinct and, often, very useful properties. With carbon being a material of such fundamental interest, it is strange then that, at present, very little is known about the properties of its liquid state. Extreme pressures (~10 MPa) and temperatures (~5000 K) are required to produce the liquid and these conditions are near to impossible to achieve at equilibrium in a laboratory. As such, it is impossible to isolate liquid carbon and most of its stated properties are speculative. In an effort to resolve some of the many controversies regarding the liquid, we have set out to make direct experimental measurements of the liquid’s properties. To surmount the need for high temperature and pressure, we produce the liquid through a non-equilibrium process called non-thermal melting [2,3]. In this process, an intense, ultrashort laser pulse is used to promote a significant portion (10-20%) of the electron density into the conduction band. This large degree of excitation weakens the carbon-carbon bonds to the point that the bonds break even at room temperature, resulting in a fluid material that has properties analogous to those of the liquid. The produced liquid remains at the initial material density for a time on the order of picoseconds due to a process known as inertial confinement before ablating away. The non-thermally melted carbon can therefore be probed with any ultrafast technique, yielding information on the liquid’s properties and structure.
Above: G. Zhao et al. Phys. Scri. 88, 045601 (2013).
Above: Bundy, F. et al. Carbon. 34, 141 (1996).
At present, we have probed liquid carbon using a wide variety of techniques, including coherent anti-Stokes Raman spectroscopy (CARS), second harmonic generation (SHG), resonant inelastic x-ray scattering (RIXS), and reflectivity. In the future we plan to use techniques including x-ray emission to learn more about the liquid’s electronic structure and the evolution of temperature and density in the material. In addition to our work on liquid carbon, our group is also interested in exploring other metastable states of carbon with our ultrafast spectrocopy techniques. One such state is Q-carbon, a recently discovered state thought to be prepared from an undercooled liquid state. The resulting material is reportedly a room temperature ferromagnet that is electrically conductive and possesses a hardness greater than that of diamond . There has also been increasing interest in warm dense carbon , as well as a new set of carbon allotropes involving regions of sp2 carbons interspersed with regions of sp3 carbons which has been recently predicted from theoretical calculations .
References:  Hull, C. J., Raj, S. L., Saykally, R. J. "The Liquid State of Carbon" Chem. Phys. Lett., 749 (137341), (2020).  Jeschke, H. O., Garcia, M. E. & Bennemann, K. H. Theory for the Ultrafast Ablation of Graphite Films. Physical Review Letters 87, (2001).  Reitze, D. H., Ahn, H. & Downer, M. C. Optical properties of liquid carbon measured by femtosecond spectroscopy. Physical Review B 45, 2677 (1992).  Raj, S. L., Devlin, S. W., Mincigrucci, R., Schwartz, C. P., Principi, E., Bencivenga, F., Foglia, L., Gessini, A., Simoncig, A., Kurdi, G., Masciovecchio, C., Saykally, R. J. "Free Electron Laser Measurement of Liquid Carbon Reflectivity in the Extreme Ultraviolet”. Photonics 7 (2), 35 (2020).  Narayan, J. & Bhaumik, A. Novel phase of carbon, ferromagnetism, and conversion into diamond. Journal of Applied Physics 118, 215303 (2015).  Burchfield, L. A., Fahim, M. A., Wittman, R. S., Delodovici, F. & Manini, N. Novamene: A new class of carbon allotropes. Heliyon 3, e00242 (2017).  Kraus, D. et al. The complex ion structure of warm dense carbon measured by spectrally resolved x-ray scattering. Physics of Plasmas 22, 056307 (2015).