Knowledge Bank

omment{Please schedule this talk immediately preceding the related talk by Susanna Widicus Weaver.} \hspace{0.25in} For over twenty years, the technique of velocity modulation spectroscopy} has been a workhorse method for the study of molecular ions, because it discriminates between the absorptions of ions and those of neutral molecules, which are many orders of magnitude more abundant in typical positive column plasmas. Using velocity modulation, it has been possible to study molecular ions as large as C$2$H$\mathrm{3+}$,} but the study of larger molecular ions such as CH$\mathrm{5+}$ or C$3$H$\mathrm{3+}$ has been difficult due to the large partition functions of these molecules at the relatively high ($\sim$300\; K)\; temperatures\; in\; these\; plasmas.\; Supersonically\; expanding\; plasmas\; can\; produce\; molecular\; ions\; at\; much\; lower\; temperatures,\; but\; to\; our\; knowledge\; no\; techniques\; have\; yet\; been\; developed\; to\; eliminate\; the\; absorptions\; of\; neutral\; molecules\; in\; these\; plasmas.\; \backslash hspace\{0.25in\}\; A\; promising\; alternative\; to\; these\; techniques\; is\; direct\; absorption\; spectroscopy\; of\; a\; fast\; ion\; beam,\; as\; pioneered\; by\; the\; Saykally\; group\}.\; Molecular\; ions\; can\; be\; produced\; in\; a\; variety\; of\; plasma\; sources\; (including\; a\; supersonic\; expansion)\; and\; then\; extracted,\; accelerated,\; and\; focused\; using\; electrostatic\; ion\; optics.\; The\; resulting\; fast\; ion\; beam\; can\; then\; be\; turned\; 90$^irc$ by an electrostatic quadrupole, allowed to drift through a field-free region, and then turned another 90 into a mass spectrometer. The beam in the drift region can be spectroscopically probed in a collinear configuration, which offers the advantages of a reasonable path length, a very narrow linewidth, and a mass-dependent Doppler splitting (when bidirectional radiation is employed). We will describe the development and commissioning of our ion beam system, which we call SCRIBES (Sensitive, Cooled, Resolved Ion BEam Spectroscopy). Spectroscopic studies with this instrument will be discussed in the following talk.