Indirect observation of molecular disassociation in solid benzene at low temperatures

TL;DR

This study reveals a low-temperature molecular disassociation process in solid benzene through dielectric measurements, highlighting isotope effects and quantum tunneling phenomena in a prototypical molecular crystal.

Contribution

It provides the first indirect observation of many-body tunneling events in solid benzene at low temperatures, expanding understanding of quantum mechanics in molecular crystals.

Findings

Identified a minimum in dielectric loss at 17.9 K

Observed isotope shift in critical temperature with deuterated benzene

Indicated continued proton and deuteron tunneling at low temperatures

Abstract

The molecular dynamics of solid benzene are extremely complex; especially below 77 K, its inner mechanics remain mostly unexplored. Benzene is also a prototypical molecular crystal that becomes energetically frustrated at low temperatures and usually unusual phenomena accompanies such scenarios. We performed dielectric constant measurements on solid benzene down to 5 K and observed a previously unidentified minimum in the imaginary part of the dielectric constant at Tm=17.9 K. Results obtained on deuterated solid benzene (C6D6, where D is deuterium) show an isotope effect in the form of a shift of the critical temperature to Tm'=18.9 K. Our findings indicate that at Tm, only the protons without the carbon atoms continue again to undergo rotational tunneling about the hexad axes. The deuterons appear to do the same accounting for an indirect observation of a continued and sustained 12-body tunneling event. We discuss how similar experiments performed on hydrogen-based molecular crystals can be exploited to help us obtain more insight on the quantum mechanics of many-body tunneling.