arXiv subjects

Ioan B. Magdău

Publications and source records attributed to Ioan B. Magdău.

2 recordsLinked to original sources

arXiv2022

Calculating the Raman Signal Beyond Perturbation Theory for a Diatomic Molecular Crystal

We calculate the eigenstates of a diatomic molecule in a range of model mean-field potentials, and evaluate the evolution of their associated Raman spectra with field strength. We demonstrate that dramatic changes in the appearance of the Raman spectrum for a diatomic molecule occur without any associated change in the symmetry of the surrounding potential. The limiting cases of the quantum eigenstates correspond, in the classical sense, to free rotation, and libration of well-oriented molecules. However, there are also many mixed modes which are neither rotons nor librons. The consequence for Raman spectroscopy is a series of complications - the non-harmonic potential splits the Raman active modes, and breaks the selection rules on forbidden modes. The mass-dependence of the various states is different - rotors, oscillators and reorientations have $1/m$, $1/\sqrt{m}$ and weaker mass dependence respectively. This may allow one to identify the character of the mode with isotope spectroscopy. However it is complicated by mixed modes and transitions between two different eigenstates with different character. We conclude that significant changes in the Raman spectrum of molecular systems are insufficient to demonstrate a phase transition since such changes can also occur in a fixed symmetry potential upon increasing field strength.

cond-mat.mtrl-sci↗

arXiv2017

Infrared Peak-Splitting from phonon localization in Solid Hydrogen

We show that the isotope effect leads to a completely different spectroscopic signal in hydrogen-deuterium mixtures, compared to pure elements that have the same crystal structure. This is particularly true for molecular vibrations, which are the main source of information about the structure of high-pressure hydrogen. Mass disorder breaks translational symmetry, meaning that vibrations are localized almost to single molecules, and are not zone-center phonons. In mixtures, each observable IR peak corresponds to a collection of many such molecular vibrations, which have a distribution of frequencies depending on local environment. Furthermore discrete groups of environments cause the peaks to split. We illustrate this issue by considering the IR spectrum of the high pressure Phase III structure of hydrogen, recently interpreted as showing novel phases in isotopic mixtures. We calculate the IR spectrum of hydrogen/deuterium mixtures in the $C2/c$ and $Cmca$-12 structures, showing that isotopic disorder gives rise to mode localization of the high frequency vibrons. The local coordination of the molecules leads to discrete IR peaks. The spread of frequencies is strongly enhanced with pressure, such that more peaks become resolvable at higher pressures, in agreement with the recent measurements.

cond-mat.mtrl-sci↗