A millimeter-wave spectroscope for the detection of triatomic gases has been constructed and characterized for frequencies between 230 and 325 GHz (H-band). The achieved results demonstrate a high sensitivity and low threshold detection. A circular lensed horn antenna transmits millimeter- waves into a gas-filled vacuum tube and excites triatomic gas molecules to a higher energy level, if the rotational resonance frequency of the molecule matches with the excitation frequency. At the other end of the tube a second lensed horn antenna receives the propagated electromagnetic wave and the millimeter-wave power is measured by a heterodyne receiver. By sweeping the radiated transmit frequency, the molecules' specific absorption can be detected. The measured absorption results are superimposed by standing wave effects within the tube. To eliminate the standing wave effects, spectroscopy on the basis of rotational spontaneous millimeter-wave emission was examined. This kind of spectroscopy decouples the transmitted from the received signal, whereby independent excitation and detection of the molecules are realized. The use of additional absorbers at the end of the gas tube decreases the decay time of the radiated wave inside the gas cell. In this paper, the detection of spontaneous emission of triatomic gas molecules with the use of a pulse-controlled transmitter and receiver is shown. Optimizations improved the stability and reproducibility of the measurements, and the detection threshold of nitrous oxide could be decreased to a ratio of 1/400. Furthermore, the implementation of a differential measurement method reduces the measurement time by a factor of 150 and simultaneously decouples of environmental influences.