Probing a Quantum Solid with Deuterated Acetylene

Strom, Aaron
The condensed phase is often thought of as a rigid, bulk composition of particles with severely restricted degrees of freedom – perhaps limited only to vibrational modes. In 1930, meditations on molecular rotation within crystals led Linus Pauling to the deduction that this behavior is merely natural, so long as the molecule in consideration possesses the appropriate mass and rotational quantum numbers. Little did he know, dynamical phenomena proliferate through other states of matter, like the quantum solid: a highly ordered array of particles capable of relatively large scale zero point motion about their mean lattice positions. Helium-4 is well known for superfluidity below the Lambda point, but a quantum solid will emerge with pressure and cooling. It turns out that there are only two known quantum solids, the second being solid parahydrogen (pH2); "normal hydrogen" comprises of a 3:1 ortho-/para-H2 ratio as a consequence of nuclear-spin statistics. Homonuclear molecular hydrogen lacks a transition dipole moment, however, the infrared signature of solid pH2 arises due to subtle anisotropic crystal field interactions that distort its spherical symmetry. In situ matrix isolation spectroscopy of a molecular probe such as dideutero-acetylene (DCCD) dispersed in crystalline pH2 via a rapid vapor deposition technique illuminates perturbations influencing the potential energy surfaces of the probe and host of this condensed phase environment. In this work, high-resolution FTIR spectra of DCCD-doped pH2 crystals recorded in the low-temperature regime (1.6–4.3 K) are presented. In particular, the rotational motion of DCCD about a pH2 vacancy will be elucidated.
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