THE MILLIMETER-WAVE SPECTRUM OF HIGHLY VIBRATIONALLY EXCITED SIO

The millimeter-wave rotational spectra of SiO in high vibrational states (v = 0-40) in its electronic ground (chi1-sigma-+) state were measured between 228 and 347 GHz in a laboratory discharge through SiH4 and CO. On ascending the vibrational ladder, populations decline precipitously for the first few levels, with a vibrational temperature of about 1000 K; at v almost-equal-to 3, however, they markedly flatten out, and from there to v almost-equal-to 40 the temperature is of the order of 10,000 K. With the Dunham coefficients determined here, the rotational spectrum of highly vibrationally excited SiO can now be calculated into the far-infrared to accuracies required for radioastronomy (i.e., greater-than-or-similar-to 1 km s-1). Possible astronomical sources of highly vibrationally excited SiO are certain stellar atmospheres, ultracompact H II regions, very young supernova ejecta, and dense interstellar shocks.