Synchrotron infrared spectroscopy at megabar pressures: Vibrational dynamics of hydrogen to 180 GPa

Abstract

We have developed new techniques for measuring infrared spectra at megabar pressures using synchrotron radiation and applied them to study the Q\(\), Q\(\)+S\(0), and QRU) vibrational transitions of solid hydrogen to 180 GPa. The frequency difference between the Q\(\) infrared and Raman vibrons increases from 3 cm" 1 (zero pressure) to 510 cm" 1 (180 GPa), indicating a dramatic increase in intermolecular coupling with pressure. A negative frequency shift is observed for the infrared vibron above 140 GPa. A significant increase in frequency and LO-TO splitting of the lattice phonon is also documented. PACS numbers: 78.30.Hv, 62.50.+p, 63.20.-eThe density dependence of intermolecular and intramolecular interactions in solid hydrogen is fundamental to understanding the range of structure, dynamics, and phase transitions in this material, from quantumsolid behavior of the low-density molecular solid to theoretically predicted monatomic phases at ultrahigh pressures [1]. One of the critical issues is the mechanism responsible for the decrease in the frequency of the Raman-active (intramolecular) vibron at 30 GPa in hydrogen and 50 GPa in deuterium [2-4]. The negative pressure shift continues to higher pressures above 100 GPa [5-7]. This result has been ascribed to the development of increasing intermolecular and decreasing intramolecular bonding with pressure, and has been regarded as a possible precursor to dissociation. In the megabar range (> 100 GPa), profound changes in interactions are expected due to the onset of band-overlap metallization and related transitions [8]. Further evidence for changes in intermolecular forces with pressure has been obtained from calculations of pressure-volume relations using gasphase (or low-density) intermolecular potentials for hydrogen [9], which show a significant departure from accurate equations of state determined by diffraction techniques [10]. Moreover, the phonon frequencies calculated from such potentials differ significantly from direct measurements by Raman spectroscopy [8,11]. These results demonstrate that although appreciable increases in intermolecular interactions occur with increasing density, the form of the potentials and a quantitative understanding of the vibrational dynamics in the high-density solid are not yet in hand.Additional types of vibrational spectroscopic measurements are required to evaluate separately the intramolecular and intermolecular interactions. Raman studies of H2 and D2 molecules isolated in rare-gas matrices and of hydrogen isotopic mixtures provide such information, but these have been limited to moderate pressures [12,13]. The infrared absorption spectrum of solid hydrogen consists of vibrational and rotational transitions induced by fluctuating dipole moments that arise from intermolecular interaction in condensed phase [1]. The infrared absorption spectrum of solid hydrogen at ambient pressure has been measured in detail by Gush et al. [14]. Because of the sensitivity of the spectra to the range and extent of intermolecular interact...

Synthesis of lithium polyhydrides above 130 GPa at 300 K

“…positive pressure-induced frequency shift of these peaks does not match with the infrared absorption spectra of pure solid H 2 (19). The latter is at higher frequency and exhibits a negative pressure dependence.…”

Inelastic x-ray scattering at ultrahigh pressures

“…For instance, the development of electrical conductivity and magnetic susceptibility [22] microprobes has led to measurement of the record-high superconducting temperature (164 K) [23] and the discoveries of the high-pressure chalcogen superconductors (among which the sulfur becomes a record-high elemental superconductor at 17 K and 1.5 Mbar (150 GPa) [17]). The developments of laser [24, 251, optical, and infrared spectroscopies [26,27], which probe vibrational, electronic, and magnetic states in molecules and solids at high pressures, has led to the discoveries of quantum and classical orderings of solid hydrogen [28] and symmetrization of ice [29, 3 01. The developments of synchrotron x-radiation for diffraction, scattering, and spectroscopy has lnelped to characterize samples under extreme conditions [3 l-331 and led to discoveries of myriad new phenomena [34,35] and new classes of materials [4, 10, 14, 361.…”

Calculation of the turnover in the vibrational frequencies of solid hydrogen at high pressures

“…1 Later it was recognized that the effect was due to resonance coupling ͑RC͒ between the neighboring molecules. 3,4 Several authors have tried to calculate the Raman frequency, but the results at high pressures were rather disappointing, [5][6][7] moreover, the RC was not addressed in this work. Recently, also the IR spectrum has been measured up to 180 GPa.…”

“…The vibrational properties of solid hydrogen at ultrahigh pressures continue to draw enormous attention [1][2][3][4][5][6][7][8] because of the suggested implications for the metallization. Until recently the Raman branch of the vibron had been experimentally determined up to 150 GPa, 1 but the infrared ͑IR͒ branch only up to 60 GPa.…”