Using a newly developed optoelectronic source of well-collimated beams of subpicosecond pulses of terahertz radiation to excite a N2O vapor cell, we have observed the subsequent emission from the vapor of a coherent pulse train extending as long as 1 ns. The individual subpicosecond terahertz pulses of the train are separated by 39.8 ps, corresponding to the frequency separation between adjacent rotational lines of the excited manifold of more than 50 lines. From these observations the coherent relaxation time T2 is obtained as a function of vapor pressure, even for the case of overlapping lines. In addition, from the pulse repetition rate in the train the frequency separation between the rotational lines is determined. Finally, from the changing individual pulse shapes in the train the anharmonicity factor for the N2O molecule is evaluated. © 1991 Optical Society of America.