Terahertz absorption spectrum of D2O vapor
Introduction
Heavy water (D2O) is important for many applications in the nuclear field. Deuterium (D) is an isotope of hydrogen which contains one proton and one neutron. D2O is naturally present in water at a low concentration of about 1 part in 5000. Heavy water is one of the two principal moderators which allows a nuclear reactor to operate with natural uranium as its fuel. Studies with D2O have shown large isotope effects with some properties, such as the temperature of maximum density occurring at ∼277 K for liquid H2O and ∼284.4 K for liquid D2O, while other properties, such as the static dielectric constant, showing little difference for the two isotopes [1], [2], [3], [4]. Liquid H2O and D2O are perhaps the most studied chemical system using various experimental and theoretical methods in most of the electromagnetic spectral region from the UV to the far-infrared [3], [4]. The Debye relaxation time for both D2O and H2O liquids at 300 K is ∼7.5 ps [4]. The main vibrational modes of D2O are denoted by ν1 (1 0 0), ν2 (0 1 0) and ν3 (0 0 1) modes. The infrared spectrum of D2O molecule gives three strong bands at 2671.46, 1178.7, and 2788.05 cm−1 which are associated with ν1, ν2 and ν3, respectively [5]. The ν2 level can be populated at room temperature. The absorption lines of D2O in 700–7000 cm−1 have been assigned to the overtones of ν1, ν2, and ν3[6], [7]. Although the far-infrared absorption spectra of D2O vapor have been studied by microwave techniques [8] and the Berkeley terahertz laser sideband spectrometer [9], [10], the absorption properties and dynamics of D2O vapor are still far from being completely understood on a molecular level in the THz region.
In this article, the absorption spectra, linewidths, and collisional times of D2O vapor associated with the rotational modes in the ground vibrational state in the frequency region 0.2–2.0 THz were measured using the temporal THz-TDS technique and assigned to the populated rotational modes at room temperature (296 K). The intense absorption lines were assigned to resonance rotational modes. To determine the collision broadened linewidth, the lines were fit numerically to a Lorentzian profile convolved with a sinc function that was determined by the measurement window. The temperature dependence of Δν at different temperatures was measured and fitted to ∼T−a, where a = 3/4, which gives information on the collisional interaction between D2O molecules.
Section snippets
Experiment
Following the pioneering works of Zhang [11] and Grischkowsky [12], a THz-TDS setup is used to measure THz absorption of D2O and H2O. A Ti:sapphire laser system is used to produce a pulse of 200 fs in duration with a repetition rate of 250 kHz at wavelength 800 nm. Ninety two percent of the power is used as a pump beam to generate the THz pulses via optical rectification in a χ(2) nonlinear medium. The remaining power was used to probe the THz signal with free-space electro-optic sampling. The
Results and discussion
Fig. 1 shows changes of the profiles of the transmitted pulse in the time-domain upon passing through D2O vapor as compared with nitrogen (N2) at room temperature (296 K). The additional fast oscillations in Fig. 1(b) for D2O are caused by the combined action of the dispersion and absorption of the D2O molecules. The uncertainty of measurement of delay time is ±0.01 ps. The power spectrum of the pulses ranging from 0.2 to 2.0 THz upon transmission through D2O vapor and the reference gas (N2
Conclusion
The THz absorption spectrum and linewidths of 26 rotational modes of D2O vapor in the frequency region 0.2–2.0 THz is reported from temporal measurements which is much more modes observed than H2O vapor. The experimental technique of THz-TDS using a modest gas path length permits the simultaneous measurement of many rotational lines over a broad frequency region to determine the line center positions, linewidths and collision dephasing times.
Acknowledgement
This work is supported in part by NASA, and New York State Center for Advanced Technology for Photonic Applications.
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