Vibrational spectra of nucleosides studied using terahertz time-domain spectroscopy

doi:10.1016/j.vibspec.2003.12.004

Vibrational Spectroscopy

Volume 35, Issues 1–2, 17 June 2004, Pages 111-114

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Abstract

We demonstrate that terahertz (THz) time-domain spectroscopy is a powerful technique for measuring temperature-dependent and spatially-resolved vibrational spectra in the frequency range 0.2–3.0 THz. A number of well resolved absorption peaks are observed for poly-crystalline nucleosides, and we have mapped the evolution of these absorption features continuously between 4 and 295 K. The resonance frequency is found to shift with temperature and an empirical expression describing this frequency shift is given. In addition, we present spectrally-resolved THz images based on these vibrational modes, accessing both spatial and compositional information of subsurface biochemical materials.

Introduction

For many years, Fourier-transform infrared (FTIR) spectroscopy has been used successfully to study the chemical composition of materials. More recently, with the introduction of focal plane array detectors, FTIR imaging systems have been developed and led to the acquisition of spatially-resolved spectral data in the mid-infrared frequency range [1], [2], [3]. In the far-infrared spectral range, however, the poor performance of FTIR spectrometers, owing to the lack of suitable sources, makes it impractical to develop FTIR imaging systems. Here we demonstrate that terahertz (THz) time-domain spectroscopy can be used to obtain spatially-resolved and temperature-dependent vibrational spectra in the far-infrared spectral range.

The THz region of the electromagnetic spectrum spans the frequency range between the mid-infrared and the millimeter/microwave. The relatively unexplored central part of the THz region (0.3–3 THz) comprises frequencies lower than those corresponding to most internal vibrations of isolated small molecules. Instead, spectra contain information on motions associated with coherent, delocalized movements of large numbers of atoms and molecules. Such collective modes are sensitive to perturbation by inter-molecular interactions with surrounding molecules as well as to changes of intra-molecular interactions through structural fluctuations [4], [5]. The measurement of these low-frequency vibrations over a wide temperature range is thus important in understanding the structural and functional properties of biomolecules.

In this paper, we present THz time-domain spectroscopy [6], [7], [8] studies of the vibrational spectra of poly-crystalline nucleosides, monitoring the evolution of the resonance absorption features from 4 to 295 K. We also demonstrate that spectrally-resolved THz imaging is capable of accessing both spatial and compositional information of subsurface biochemical materials.

Section snippets

Experiment

Fig. 1 shows the THz time-domain spectroscopy apparatus used in our experiments [9]. In brief, a Ti:sapphire laser provides visible/near-infrared pulses of 12 fs duration at a center wavelength of 790 nm with a repetition rate of 76 MHz. The output is split into two parts: a 300 mW pump beam is focused onto the surface of a biased GaAs photoconductive emitter for THz generation, and a 25 mW beam serves as the probe beam for electro-optic detection [10], [11] using a 1 mm thick ZnTe crystal. The

Results and discussion

Fig. 2a shows a typical temporal THz waveform obtained from the apparatus shown in Fig. 1. Fourier-transforming this time-domain signal gives the frequency response of the THz spectroscopy system (Fig. 2b). The useful bandwidth is 0.1–3.5 THz, and is mainly limited by the frequency response of the 1 mm thick ZnTe detector. Using a 20 μm thick ZnTe crystal as detector, the useful bandwidth has recently been extended to over 20 THz [14], but at the expense of a reduced signal-to-noise ratio. Fig. 2b

Conclusion

We have measured the low-frequency vibrational spectra of nucleosides in the temperature range 4–290 K, using THz time-domain spectroscopy. Sharp absorption features were observed in the frequency range 0.1–3.0 THz, and were used for spectrally-resolved THz imaging. In addition, a number of vibrational modes were found to become more intense and shift to higher frequencies as the temperature was reduced, and these were fitted by an empirical formula.

Acknowledgements

The authors thank A. Moss and M. Vickers for X-ray diffraction measurements. This work was supported by the EPSRC, the Royal Society (AGD), and Toshiba Research Europe Ltd. (EHL). PCU the Association of Commonwealth Universities.

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Vibrational spectra of nucleosides studied using terahertz time-domain spectroscopy

Abstract

Introduction

Section snippets

Experiment

Results and discussion

Conclusion

Acknowledgements

Chem. Phys.

J. Pharm. Sci.

Anal. Chem.

Anal. Chem.

Appl. Spectrosc.