Abstract

This paper describes the advances in step-scan FT-IR time-resolved spectroscopy (TRS) and its application to the study of liquid crystal reorientation dynamics. The most important advantage of step-scan interferometry lies in the fact that the optical retardation of the interferometer is held constant during the sampling of interferogram elements, and consequently the spectral multiplexing is decoupled from the time dependence of data collection. This feature of step-scan interferometry allows us to perform both time-domain (one-dimensional time-resolved spectroscopy: 1D TRS) and frequency-domain (two-dimensional frequency correlation spectroscopy: 2D IR) dynamic experiments without the need to deconvolute the time dependence of the sample response from that of the data collection process. The design of the step-scan FT-IR spectrometer used in this study (Bio-Rad FTS60A/896), the experimental setup for 1D and 2D TRS measurements, and the results of a performance test are detailed. The FT-IR TRS techniques applied to the dynamic analysis of liquid crystals have revealed new information that enables us to penetrate into detailed sub-molecular mechanisms of the electrically induced liquid crystal reorientation. The results include the following: (1) 1D FT-IR TRS with microsecond time resolution has been able to follow the real-time transition dynamics of each individual functional group in the molecule; (2) 2D FT-IR TRS, capable of analyzing spatial and temporal correlations between reorientational motions of different sub-molecular segments, has shown that a flexible chain appended to a rigid core of the liquid crystalline molecule undergoes a fast local motion in addition to the rotational relaxation motion of the entire molecule; and (3) 2D frequency correlation analyses have been able to isolate a hidden absorption band and have suggested a possible assignment of this new band. It is emphasized that all these results have been obtained by taking the advantage of <i>time-resolved spectroscopy</i> that provides both <i>temporal</i> and <i>spectral</i> information simultaneously. The results presented in this paper should illustrate the potential applicability of FT-IR TRS to the study of a wide variety of time-dependent phenomena.

Time-resolved spectral characterization of ring cavity surface emitting and ridge-type distributed feedback quantum cascade lasers by step-scan FT-IR spectroscopy

Markus Brandstetter, Andreas Genner, Clemens Schwarzer, Elvis Mujagic, Gottfried Strasser, and Bernhard Lendl
Opt. Express 22(3) 2656-2664 (2014)

High-information time-resolved step-scan Fourier interferometer

Georges Durry and Guy Guelachvili
Appl. Opt. 34(12) 1971-1981 (1995)

Accurate infrared transmittance measurements on optical filters using an FT-IR spectrometer

David A. C. Compton, John Drab, and Howard S. Barr
Appl. Opt. 29(19) 2908-2912 (1990)

Ultrafast nonequilibrium Fourier-transform two-dimensional infrared spectroscopy

Carlos R. Baiz, Matthew J. Nee, Robert McCanne, and Kevin J. Kubarych
Opt. Lett. 33(21) 2533-2535 (2008)

The dynamic influence of photoisomerization on optical reorientation in absorbing isotropic liquid crystals

Xiu Liu, Pei Yang, Liyong Ji, Liying Liu, and Lei Xu
Opt. Express 14(24) 11709-11714 (2006)