Introduction to Terahertz Spectroscopy of Amorphous Molecular Materials

In this video Professor Axel Zeitler gives a tutorial on the basic principles of using terahertz time-domain spectroscopy (THz-TDS) to explore the materials properties of organic molecular materials in their disordered, amorphous state.

The talk outlines the link between the spectra of organic molecular crystals, which exhibit sharp vibrational modes in the far-infrared at terahertz frequencies, and the vibrational density of states (VDOS) that is observed instead for disordered materials when such molecular crystals have been turned into amorphous glasses. By varying the temperature of the sample during terahertz spectroscopy experiments it is possible to explore the vibrational dynamics using THz-TDS. The talk shows how THz-TDS links with dielectric spectroscopy at lower frequencies and (far-)infared spectroscopy at higher frequencies.


00:00 Introduction
00:44 Vibrational Spectroscopy at Terahertz Frequencies
08:31 Relaxation Processes and Vibrational Dynamics
13:36 Measuring Molecular Mobility
25:44 Conclusions

References:

S. Capaccioli, K. L. Ngai, M. S. Thayyil, and D. Prevosto, Coupling of Caged Molecule Dynamics to JG β-Relaxation: I, J. Phys. Chem. B, 119(28):8800-8808, July 2015.
https://doi.org/10.1021/acs.jpcb.5b04408

M. Goldstein, Viscous Liquids and the Glass Transition: A Potential Energy Barrier Picture, J. Chem. Phys., 51(9):3728, 1969.
https://doi.org/10.1063/1.1672587

S. Kastner, M. Köhler, Y. Goncharov, P. Lunkenheimer, and A. Loidl, High-frequency dynamics of type B glass formers investigated by broadband dielectric spectroscopy, J. Non-Cryst. Sol., 357(2):510-514, January 2011.
https://doi.org/10.1016/j.jnoncrysol....

E. P. J. Parrott and J. A. Zeitler, Terahertz Time-Domain and Low-Frequency Raman Spectroscopy of Organic Materials, Appl Spectrosc, 69(1):1-25, January 2015.
https://doi.org/10.1366/14-07707

M. T. Ruggiero, J. Sibik, J. A. Zeitler, and T. M. Korter, Examination of l-Glutamic Acid Polymorphs by Solid-State Density Functional Theory and Terahertz Spectroscopy., J. Phys. Chem. A, 120(38):7490-7495, September 2016.
https://doi.org/10.1021/acs.jpca.6b05702

M. T. Ruggiero, M. Krynski, E. O. Kissi, J. Sibik, D. Markl, N. Y. Tan, D. Arslanov, W. van der Zande, B. Redlich, T. M. Korter, H. Grohganz, K. Löbmann, T. Rades, S. R. Elliott, and J. A. Zeitler, The significance of the amorphous potential energy landscape for dictating glassy dynamics and driving solid-state crystallisation., Phys. Chem. Chem. Phys., 19(44): 30039-30047, November 2017.
https://doi.org/10.1039/C7CP06664C

J. Sibik, S. R. Elliott, and J. A. Zeitler, Thermal decoupling of molecular-relaxation processes from the vibrational density of states at terahertz frequencies in supercooled hydrogen-bonded liquids, J. Phys. Chem. Lett., 5(11):1968-1972, 2014.
https://doi.org/10.1021/jz5007302

J. Sibik and J. A. Zeitler, Direct measurement of molecular mobility and crystallisation of amorphous pharmaceuticals using terahertz spectroscopy., Adv. Drug Del. Rev., 100:147-157, May 2016.
https://doi.org/10.1016/j.addr.2015.1...