Investigating the Liquid State of Carbon

Carbon materials have a many contemporary applications and new carbon allotropes are being discovered. However, while graphite and diamond are well understood, very little is known about the liquid state of carbon due to the high temperatures (above 5,000 K) and pressures (above 10 MPa) required for its formation. Initial studies used electrical heating to determine the melting point of graphite and the resistivity of liquid carbon. More recent studies used non-thermal laser melting to generate a metastable liquid that was studied with visible reflectivity and X-ray spectroscopies. Shock waves have also been used to transiently generate liquid carbon. Theoretical calculations of liquid carbon initially suggested the possibility of a liquid-liquid phase transition, but later ab initio quantum mechanical simulations showed only a continuous change in liquid coordination as its density increased./nIn this dissertation, extreme-UV (EUV) reflectivity and chirped coherent anti-Stokes Raman spectroscopy (c-CARS) were used to study non-thermally melted liquid carbon. Femtosecond laser pulses at 250 nm with a fluence of 0.45 J/cm2 (3.5 x 1012 W/cm2 intensity) were used to generate liquid carbon from an amorphous carbon substrate and the time evolution of EUV reflectivity was probed. EUV wavelengths from 20 to 42 nm were used with both s and p polarizations. The reflectivity decreased at all wavelengths probed as the material expanded and ablated. For wavelengths below 32 nm, the reflectivity decay time was less than ~2 ps. This time constant describes the lattice dynamics after melting, while above 32 nm, the reflectivity is also sensitive to the hot electron plasma generated by the melting pulse. From these results and equations for the behavior of a shock wave in a material, the electron temperature of the melted material was found to be 0.30 ± 0.6 eV. The reflectivity at two different polarizations was also used to calculate the complex refractive index of the material as it evolved over time. C-CARS spectra were obtained for highly ordered pyrolytic graphite (HOPG) and glassy carbon using CARS pump wavelengths of 400 nm and 800 nm. These spectra showed strong G peak resonance (1580 cm-1), corresponding to the relative vibrations of sp2 carbons in the material. The D peak (~1350 cm-1) resonance seen in Raman scattering of disordered graphite films was not observed in the CARS spectra. As this mode occurs when the excited electron scatters from a defect or phonon, it could be that the stimulated Stokes emission that occurs during the CARS process prevents such scattering. The sample was melted with an 800 nm, 90 fs laser pulse with fluences from 0.40 to 0.85 J/cm2 (intensities of 4.4 x 1012 to 9.4 x 1012 W/cm2). Delay times of less than 500 fs and as long as 100 ps all showed no broadening or shifting of the G peak, as would be expected for damaging and disordering of the material; only an intensity change is seen as the material ablates. Microscope images show permanent damage to the substrate and the fluences and times studied were comparable to those used in published reflectivity studies of liquid carbon./nTo advance the study of liquid carbon, a soft X-ray second harmonic generation (SHG) technique was developed and explored. X-ray absorption provides element-specific information on the electronic structure of a material that is sensitive to the environment around the element. Combining this with the interface specificity of SHG, provides a useful technique for studying solid-solid interfaces that are difficult to study otherwise. Our first soft X-ray SHG experiments on graphite films showed that the technique was indeed highly interface specific. The technique was also sensitive to resonance amplification when the input photons were at or above the carbon K-edge. A second experiment compared the boron/vacuum interface to a buried boron/carbon (Parylene-N) interface. The technique was sensitive to interface effects, showing larger SHG intensity at the boron K-edge for the boron/Parylene-N interface compared to the boron/vacuum interface. Ab initio quantum simulations were used to calculate the soft X-ray SHG spectra of these systems, verifying the interface sensitivity of the technique.

Advisor: Saykally, Richard J