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The lowest energy rotational transitions of CD and 13CH in their 2Πr ground electronic states have been directly measured using submillimeter direct absorption spectroscopy. These two radicals were produced in an electrical discharge of either CD4 (or CH4 and D2) or 13CH4. The J = 3/2← 1/2, N = 1← 1 transition at 439 GHz and the J = 3/2← 3/2 and 5/2← 3/2 fine structure lines of the N = 2← 1 rotational transition near 885 GHz and 916 GHz were recorded for CD (Hund's case b notation), each of which consist of lambda doublets. In addition, hyperfine splittings due to the deuterium nuclear spin of I = 1 were measured in several doublets, although some hyperfine components were blended together at higher frequency. For 13CH, the lambda doublets of the fundamental N = 1← 1 line near 532-536 GHz were recorded; in this case, hyperfine interactions arising from both 13C and H nuclei were resolved. These data were fit with a case b effective Hamiltonian, and spectroscopic parameters were derived. In particular, the deuterium hyperfine constants for CD were improved by about an order of magnitude, while for 13CH the hydrogen Fermi contact and dipolar terms were established for the first time. These measurements will enable definitive searches for CD and 13CH in interstellar gas, in particular with the upcoming Herschel Space Observatory.

We have identified vibrationally excited CH3CH2CN (ethyl cyanide) for the first time in the interstellar medium. Using the Caltech Submillimeter Observatory, the Berkeley-Illinois-Maryland Association Array, and the Caltech Millimeter Array, we have detected several transitions from both the vb = 1 in-plane bending state and the vt = 1 torsional state in the Sgr B2(N-LMH) source. Because these excited states lie near 300 K above ground, vibrationally excited CH3CH2CN is potentially a useful probe of the hottest regions of dense, dusty molecular cloud cores where massive star formation is occurring. In addition, this identification will help account for many unidentified and misidentified spectral lines observed in hot molecular cores.

The laboratory submillimeter wave rotational spectrum of the 13CH3CN, CH3C13CN, and CH3C15N isotopomers of methyl cyanide has been observed in natural abundance in the 294 to 607 GHz region. The maximum J and K values are 34 and 14, respectively. Fifteen additional CH3CN transitions up to K = 21 were also measured. The transitions of all four species are fitted to a symmetric top Hamiltonian, and the rotation and distortion constants are determined. The 14N quadrupole and spin rotation coupling constants are also calculated and presented. Suggested values for many other parameters, which could not be directly determined from the isotope spectra, are calculated from the normal species values and isotope relationships. The determined and calculated constants should predict the spectrum of the three isotopomers to well over 1 THz accurately enough for astronomical assignments.

The rotational spectrum of H_2NCO^+, the ground-state isomer of protonated HNCO, has been measured in a molecular beam in the centimeter band with a Fourier transform microwave spectrometer and in a low-pressure laboratory discharge in absorption in the millimeter band. Spectroscopic constants, including the nitrogen-14 hyperfine coupling constant, derived from 30 a-type transitions between 20 and 367 GHz with J ≤ 18 and K_a ≤ 3 allow the principal rotational transitions to be calculated to 1 km s^(–1) or better in equivalent radial velocity well into the far IR. Two low-lying rotational transitions of H_2NCO^+ in the centimeter band (0_(0,0)-1_(0,1) and 1_(1,0)-2_(1,1)) were tentatively identified in absorption in the PRIMOS spectral line survey of Sgr B2(N) with the Green Bank Telescope. The lines of H_2NCO^+ arise in a region of the Sgr B2(N) halo whose density is low (n < 1 × 10^4 cm^(–3)). The derived column density of (6-14) × 10^(11) cm^(–2) implies that the fractional abundance is ~10^(–12). Owing to the ubiquity of HNCO in galactic molecular clouds, H_2NCO^+ is a good candidate for detection in sources spanning a wide range of physical conditions.

Ethyl cyanide, CH3CH2CN, is an important interstellar molecule with a very dense rotational-vibrational spectrum. On the basis of new laboratory data in the range of 17-605 GHz and ab initio calculations, two new vibrational states, ν12 and ν20, have been detected in molecular clouds of Orion. Laboratory data consist of Stark spectroscopy (17-110 GHz) and frequency-modulated spectrometers (GEM laboratory in Valladolid: 17-170, 270-360 GHz; Toyama: 26-200 GHz; Emory: 200-240 GHz; Ohio State: 258-368 GHz; and JPL: 270-318, 395-605 GHz). More than 700 distinct lines of each species were measured in J up to 71 and in Ka up to 25. The states were fitted with Watson's S-reduction Hamiltonian. The two new states have been identified in the interstellar medium toward the Orion Nebula (Orion KL). The ground state, the isotopologues of CH3CH2CN, and the vibrationally excited states have been fitted to obtain column densities and to derive vibrational temperatures. All together, ethyl cyanide is responsible for more than 2000 lines in the observed frequency range of 80-280 GHz.

To facilitate studies of water in the interstellar medium and late-type stars, the frequencies of 30 new millimeter- and submillimeter-wave transitions of H2O-16 have been measured, which lie between 100 GHz and 600 GHz. This represents almost a doubling of the number of water lines that have been observed in the laboratory in this spectral region at high resolution. All of the newly observed lines are highly excited, lying between 2400 and 4200/cm above the ground level. Some of these have large excitation energies because of their high rotational states and others because they lie in excited vibrational states. These lines are potentially of substantial astrophysical significance because they are related to the study of interstellar masers and because their high excitation eliminates the atmospheric self-absorption associated with the more well-known water lines.

Detection du radical C 4 D par decharge incandescente dans un melange de C 2 D 2 et d'H 2 . 19 transitions rotationnelles entre 88 et 282 GHz ont pu etre mesurees au laboratoire