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

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Abstract

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 PT 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.

Introduction

Studies of simple molecular systems at high pressure and temperature are motivated by the great importance of these materials to the development of fundamental physics and chemistry, the search for new energy sources and for the improvement of our understanding of planetary interiors. Molecules under extreme conditions ultimately dissociate as a consequence of the electronic charge redistribution that follows from the increasing overlap of the molecular wave functions, valence charge transfer and the modification of the intramolecular potential (see e.g., Ref. [1]). This process may be associated with the presence of intermediate (e.g., ionized) states, and can be either abrupt or continuous. Elucidation of the detailed nature of these processes represents an important problem of condensed matter physics, and has implications for the ongoing search for metallic hydrogen as well as new energetic materials. Due to the large energetic barriers that may be associated with such transformations, one may expect a large PT regions for metastability of the dissociated state that make possible technological applications under less extreme conditions (e.g., energetic materials, high-temperature superconductors). Moreover, a knowledge of the chemistry involved in the high P–T behavior of molecular materials, thermal equations of state and electric conductivities are crucial to developing planetary models that are consistent with astronomical and geophysical observations. Such information is also important for precisely predicting the performance of existing high-energy density materials [2].

The laser-heated diamond anvil cell (LHDAC) allows measurements up to multimegabar pressures and several thousand degrees (e.g., Ref. [3]), yet generating and maintaining extreme conditions in reactive and mobile materials like hydrogen at least long enough for conventional measurements is challenging [4], [5], [6], [7]. Here, we present novel approaches to reach extreme conditions of simultaneous high pressure and temperature with reactive materials and to probe these materials using spectroscopic techniques.

Section snippets

Experiment

Our combined Raman and continuous wave (CW) laser-heating technique have been described previously [8], [9]. The apparatus includes a CW Nd: YLF (or Nd: YAG) laser for the laser heating, an argon ion laser operating at 457–488 nm for the Raman scattering excitation, and dedicated spectrographs equipped with CCD detectors for Raman and thermal radiation acquisition. The setups built at LLNL and the Geophysical Laboratory is very similar, and both were used in these experiments.

For the

Results and discussion

We first present several results obtained from diatomic molecular materials. In one particular experiment hydrogen was loaded into a DAC that was equipped with alumina-coated anvils. Fig. 5 shows the Raman spectra recorded at 35 GPa at room and high temperatures (approximately 1000 K on the basis of the appearance of a weak thermal “glow” in the sample cavity) with CW laser heating. The qualitative spectral changes at high temperature agree with those observed in our previous study [7], in which

Conclusions

Both CW and pulsed laser-heating techniques in combination with CW and time-resolved optical spectroscopic methods provide useful tools for the study of simple molecular materials under conditions of simultaneous high pressure and temperatures. New methods of sample loading and preparation allow measurements under simultaneous conditions of temperature and static pressure higher than previously attained. Use of a metallic coupler in combination with pulsed heating opens new possibilities to

Acknowledgments

We acknowledge support by DOE/BES, DOE/NNSA (CDAC), NSF, Carnegie Institution of Washington, and the W. M. Keck Foundation. This work at LLNL was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract no. W-7405-Eng-48. We thank S. Gramsch for comments.

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