Origin of Two Distinct Peaks of Ice in the THz Region and Its Application for Natural Gas Hydrate Dissociation

Abstract

For liquid water in the far-infrared spectrum, phonons of molecular vibrations constitute two bands with a narrow gap at around 30 meV. Interestingly, there are two distinct peaks for ice in this gap. We demonstrated that the two peaks come from two kinds of translational modes. Considering different O–O–O bending constants, we yielded two frequencies based on the ideal model of ice Ic. These two kinds of vibrational modes do not exist in liquid water due to the collapse of the rigid tetrahedral structure. Thus, a window remains for ice resonance absorption with minimum energy loss in liquid water. A new method to decompose gas hydrates was proposed by supplying two terahertz radiation energies at ∼6.8 and 9.1 THz. This is also applicable to flow assurance in gas pipelines, aircraft deicers, and so on. Experimental measurements are expected to verify this finding along with the rapid development of a THz laser.

Computational Analysis of Hydrogen Bond Vibrations of Ice III in the Far-Infrared Band

“…We first found the two kinds of intrinsic HB vibrational modes in H-ordered ice Ic, which has only two molecules per primitive cell [21]. Then, we confirmed that the two distinct HB peaks in ice Ih originated from these two kinds of HB modes [19]. By now, we have investigated almost all the ice phases, thereby determining that this is a general rule for ice because of its local tetrahedral structure [18].…”

“…To determine the two kinds of HB vibrational modes in ice III, we employed a selfcompiled program to classify the normal modes in the translation band [19]. The criterion for classification is based on the included angle between the vibrational direction and the H-O-H angle bisector.…”

“…Two kinds of intrinsic HB vibrational modes, denoted two-bond and four-bond modes, were found for the ideal ice Ic [21]. A series of investigations of the ice phases has revealed that this is a general rule for the ice family and may successfully explain their far-IR band [18][19][20]. In the present study, we theoretically analyzed the vibrational modes in the translation band to identify the characteristic HB vibrations of ice III in comparison with those of the H-ordered ice IX.…”

“…Since two distinct peaks in the far-infrared (IR) region of ice were first probed in the 1990s by inelastic neutron scattering (INS) experiments [15][16][17], the origin of these two vibrational peaks has remained a controversial issue [18][19][20]. Using first-principles density functional theory (DFT), we developed an approach to determine the lattice dynamic process of the crystal vibrational spectrum.…”

A Strategy for the Analysis of the Far-Infrared Vibrational Modes of Hydrogen-Disordered Ice V

“…The vibrational frequency of the two-bond mode is weaker than that of the four-bond mode. The dynamic processes of these modes are described in refs and .…”