In situ spectroscopic investigation of phase change dynamics in deeply quenched quantum liquids with nanosecond time resolution
Zusammenfassung der Projektergebnisse
The exciting possibility that quantum effects might play a role in the dynamics of a supercooled liquid has been suggested theoretically over the past years. However, with the only exception of recent studies of nanometer-sized hydrogen complexes, possible experimental investigations of this behavior have so far been precluded by the difficulties in supercooling a (quantum) bulk liquid to very low temperatures below melting. Within the funded period it was our aim to overcome these experimental challenges by employing the cryogenic liquid microjet technique developed recently in our laboratory at the University of Frankfurt to investigate by Raman scattering the crystallization process of supercooled isotopic mixtures of the zero nuclear spin species para-hydrogen (pH2) and ortho-deuterium (oD2). In experiments carried out in Prof. Salvador Montero’s Laboratory of Molecular Fluid Dynamics at CSIC in Madrid, Spain, we observed a complex and unexpected dependence of the crystallization rate on composition: starting with the pure pH2 system, the crystal growth was found to slow down considerably with increasing amount of oD2, up to three times in mixtures with about 20-50% oD2, to then become faster again in oD2-enriched mixtures. This behavior is quite surprising if one keeps in mind the isotopic nature of the pH2-oD2 mixtures for which the pair interactions are isotope independent. Classical simulations of supercooled model binary mixtures of particles of different sizes have reported, on the other hand, a correlation between composition and crystallization kinetics that is strikingly similar to that observed in our experiments. Are these similarities fortuitous? The answer is no, as path integral numerical simulations of the supercooled pH2-oD2 mixtures performed by Dr. Davide Galli and coworkers at the University of Milano, Italy, have clearly shown: quantum fluctuations increase the effective radius of the pH2 and oD2 molecules, yet the strength of this effect differs for pH2 and oD2 due to their masses, resulting in a particle size ratio different from unity. Additional experiments on Raman scattering from supercooled binary liquids formed by mixing either pH2 or oD2 with small amounts of the much heavier neon atoms confirmed the decisive role played by the interplay of composition and particle size ratio in the crystallization process, offering in particular a suggestive trend: the degree of the observed slowdown of crystal growth upon mixing appear to be directly correlated with the ratio of the effective sizes of the impurity and solvent particles. Our results provide firm experimental evidence for the subtle role played by composition and particle size ratio in the crystallization of simple molecular binary mixtures. But what is the microscopic mechanism that correlates composition and size ratio to slower crystallization kinetics? Answering this question represents a formidable task, whose resolution should allow one to identify the principles that govern the stability of supercooled liquids against crystallization with profound implications; for example, it has been recognized that it would be crucial to clarify the nature of the glass transition. With our work we have not only established a promising experimental route to address this latter fundamental issue, but most notably we have provided evidence for slowdown of crystallization that is purely of quantum origin. In fact, in our supercooled quantum liquid binary mixtures it is only the difference in the mass-induced quantum delocalization of the two components that ultimately introduces a degree of frustration of crystallization.
Projektbezogene Publikationen (Auswahl)
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Observation of crystallization slowdown in supercooled parahydrogen and orthodeuterium quantum liquid mixtures. Phys. Rev. B 89, 180201(R) (2014)
M. Kühnel, J. M. Fernández, F. Tramonto, G. Tejeda, E. Moreno, A. Kalinin, M. Nava, D. E. Galli, S. Montero, and R. E. Grisenti
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Mixing effects in the crystallization of supercooled quantum binary liquids. J. Chem. Phys. 143, 064504 (2015)
M. Kühnel, J. M. Fernández, F. Tramonto, G. Tejeda, E. Moreno, A. Kalinin, M. Nava, D. E. Galli, S. Montero, and R. E. Grisenti
