Submillimeter-wave spectral lines of negative ions (SH− and SD−) identified by their Doppler shift

For the first time submillimeter-wave transitions of an anion have been firmly identified. We observed one transition of SH− (J=1←0, 564 421.869 MHz) and two of SD− (J=1←0, 292 359.129 MHz; J=2←1, 584 629.519 MHz) created in the positive column of an electric discharge set up in a mixture of H2S and argon. By studying the Doppler shift brought about by the motion of charged species in the electric field of the discharge we were able to distinguish between positively and negatively charged ions and neutrals. We have hence shown that velocity modulation techniques commonly used for infrared spectroscopy could also be successfully applied for the study of ions in the submillimeter-wave region.