Terahertz Time-Domain Spectroscopy of Low-Concentration N2O Using Long-Range Multipass Gas Cell

We developed a long-range multipass gas cell that extends the beam path to 18.61 m, for which a THz beam shows multiple reflections within a diameter of only 32 cm and a height of 54 cm. Due to the very long propagation THz beam path in the multipass gas cell, the absorbance and absorption coefficients of nitrous oxide can be measured very sensitively from 0.2 to 1.2 THz bandwidth. The gas concentration can be determined from the measured THz pulses for the absorption. As the gas concentration increased sequentially from 1.0% to 8.3% at atmospheric pressure, the absorbance lines also increased sequentially. The resulting absorptions and frequencies of transitions showed good agreement between experiment and calculations performed with the HITRAN database.