Laser-heating diamond anvil cell studies of simple molecular systems at high pressures and temperatures

We report the application of new laser-heating techniques and sample preparation procedures for simple molecular materials (diatomic molecules and water) under high pressure in the diamond anvil cell (DAC). Both continuous and pulsed laser heating was employed. We probed the materials using Raman spectroscopy and also by analyzing the time evolution of the temperature of the metallic coupler that is used to absorb laser radiation and heat the sample. Raman measurements of H2, D2, N2, H2O and O2 show a broadening of intramolecular vibrations at high P−T conditions, indicating a decreasing molecular lifetime, and hence suggest an increasing molecular dissociation. In diatomic molecules the intramolecular bonding can be further probed by observations of sidebands corresponding to vibrational transitions from excited states; the energies of these sidebands imply intramolecular potentials that become increasingly less anharmonic as pressure is increased. We also show that the pulsed heating technique combined with instantaneous radiative temperature measurements provides a useful tool for studies of thermochemical properties and phase transformation boundaries.