Terahertz Time-Domain Spectroscopy of Nitrogen Ice

We have used terahertz time-domain spectroscopy from 0.1-1.6 THz to study thin films of solid N$_2$ from 10-25 K. A temperature dependent absorption line shift was observed near 1.46 THz as the temperature increased from 10 to 25 K, where the center frequency of the absorption line decreased with temperature. We can fit these data to a model assuming a standard Lennard-Jones potential with the addition of a quadrupole-quadrupole interaction. We modeled the shift in the resonant absorption with a lattice expansion that includes previously published thermal expansion coefficients in N$_2$ ice, the gas-phase Lennard-Jones parameters, and the gas-phase quadrupole moments.