L. M. Ziurys

A millimeter/submillimeter spectrometer for !high resolution studies of transient molecules

A design is presented for a millimeter/submillimeter direct absorption spectrometer for studies of the pure rotational spectra of metal‐bearing free radicals. The spectrometer operates in the frequency range of 65–550 GHz with an instrumental resolution of 200–1000 kHz and an absorption sensitivity of a few ppm. The instrument utilizes phase‐locked Gunn oscillators as the tunable, coherent source of radiation from 65–140 GHz. Higher source frequencies are obtained with Schottky diode multipliers. The gas cell and optics path are designed utilizing Gaussian beam optics to achieve maximum interaction between molecules and the mm‐wave radiation in the reaction region. Scalar feedhorns and a series of PTFE lenses are used to propagate the source signal. The gas cell is a cylindrical tube 0.5 m in length with a detachable Broida‐type oven. The detector for the spectrometer is a helium‐cooled InSb hot electron bolometer. Phase‐sensitive detection is achieved by FM modulation of the Gunn oscillators and use of a lock‐in amplifier. Spectra are recorded by electrical tuning of the Gunn oscillator, which is done under computer control. The millimeter and sub‐mm rotational spectra of several free radicals have been observed for the first time using this instrument, including CaOH, MgOH, CaH, MgF, and BaOH.

Detection of nitric oxide in the dark cloud L134N

We report the first detection of interstellar nitric oxide (NO) in a cold dark cloud, L134N. Nitric oxide was observed by means of its two 2 pi 1/2, J = 3/2 --> 1/2, rotational transitions at 150.2 and 150.5 GHz, which occur because of lambda-doubling. The inferred column density for L134N is N(NO) approximately 5 x 10(14) cm-2 toward the SO peak in that cloud. This value corresponds to a fractional abundance relative to molecular hydrogen of f(NO) approximately 6 x 10(-8) and is in good agreement with predictions of quiescent cloud ion-molecule chemistry. NO was not detected toward the dark cloud TMC-1 at an upper limit of f(NO) < or = 3 x 10(-8).

Confirmation of Interstellar HOC+: Reevaluating the [HCO+]/[HOC+] Abundance Ratio

The identification of interstellar HOC+ has been confirmed through new observations of this species at 3, 2, and 1.2 mm using the NRAO 12 m telescope. Toward Sgr B2 (OH), the J = 2 → 1 and J = 3 → 2 lines of this ion at 179 and 268 GHz were detected for the first time. In addition, the J = 1 → 0 line of HOC+ at 89 GHz was observed toward several new sources, including Orion-KL. In conjunction with past observations of the J = 1 → 0 transition toward Sgr B2 (OH), these data clearly establish the presence of this species in the interstellar medium. The column density toward Sgr B2 (OH) is estimated to be Ntot ~ 3 × 1012 cm-2, which corresponds to a fractional abundance relative to H2 of f ~ 10-12. Measurements of HCO+ were also carried out toward Sgr B2 (OH), resulting in an abundance ratio of [HCO+]/[HOC+] ~ 140-360 at gas densities of n(H2) > 105 cm-3. This ratio contradicts past predictions of ion-molecule models by several orders of magnitude and suggests that the reaction HOC+ + H2 → HCO+ + H2 is not as fast as previously thought.

Millimeter‐wave spectroscopy of vibrationally excited ground state alkaline‐earth hydroxide radicals (X 2Σ+)

Pure rotational spectra of the alkaline‐earth monohydroxides have been recorded for vibrationally excited states (0 1 0), (0 2 0), (0 3 0), and (1 0 0) of the ground electronic state (X 2Σ+) using millimeter‐wave absorption spectroscopy. The radicals MgOH, CaOH, SrOH, and BaOH were studied. The data for CaOH, SrOH, and BaOH were analyzed with a linear 2Σ+ model, but with the addition of two terms to account for contamination of the v2=1 2Π and v2=2 2Δ vibronic levels with 2Π and 2Δ electronic states. The data for MgOH, however, did not fit well to this linear model and is additional evidence that this species is quasilinear.

High resolution spectroscopy of MgOH (X 2Σ+) in its V2 mode: Further evidence for quasilinearity

Pure rotational spectra of the MgOH and MgOD radicals have been recorded in the v2 bending vibration of their X 2Σ+ ground electronic states using millimeter-wave direct absorption spectroscopy. Multiple rotational transitions arising from the v2l=11, 22, 20, 31, 33, 42, and 44 substates have been measured in the frequency range 240–520 GHz for these species. Both the spin-rotation and l-type doubling interactions have been resolved in the spectra. The complete data sets for MgOH and MgOD have been analyzed using a linear model for the Hamiltonian which takes into account higher order (l=±4) l-type interactions. The global analyses were adequate, but anomalous behavior was apparent in both molecules. In particular, the Bv vs v2 relation was found to be highly nonlinear, large variations in the l-type doubling constant q were observed with vibrational level, and r0, rs, and re structures determined differed substantially. Such findings suggest that MgOH is highly quasilinear, comparable to HNCO. The compe...

Abundances of hydrogen sulfide in star-forming regions

Interstellar hydrogen sulfide (H2S) and its isotopic variant H2 34S have been observed toward several star-forming regions via their 1(10)-1(01) transitions at 2 mm, using the FCRAO telescope. In sources where both isotopic species H2S and H2 34S were observed, column densities of approximately 10(16) cm-2 were measured. Column density lower limits of approximately 10(14) cm-2 for H2S were found for other sources, where only the main isotopic line was observed. The fractional abundances of H2S relative to molecular hydrogen appear to be enhanced by at least an order of magnitude relative to quiescent cloud values (approximately 10(-9)) for many of the observed sources. Such enhancement toward star-forming clouds suggests that some process involving elevated temperature aids in producing this species; this could be gas-phase reactions, grain-related processes, or both.

The Millimeter/Submillimeter Spectrum and Rotational Rest Frequencies of MgCH3(X̃ 2A1)

The pure rotational spectrum of the free radical MgCH3( 2A1) has been measured in the laboratory for the first time using millimeter/submillimeter direct absorption spectroscopy. The molecule was made in a DC discharge by the reaction of magnesium vapor and dimethyl mercury. Ten rotational transitions of this species were observed in the range 176-374 GHz. MgCH3 is a symmetric top molecule with net spin angular momentum. Consequently, both K structure and spin-rotation interactions were observed in every rotational transition. The data were analyzed with a2A1 Hamiltonian and rotational, centrifugal distortion, and fine-structure parameters were accurately determined. These measurements will now enable a search for this radical in interstellar and circumstellar sources to be conducted using millimeter/submillimeter telescopes.

The millimeter-wave spectrum of CaH (X 2Sigma(+))

The millimeter-wave spectrum of the CaH and CaD radicals (X 2Sigma(+)) has been observed in the laboratory via direct absorption spectroscopy. The species were generated by the reaction of calcium vapor, produced in a Broida-type oven, with hydrogen atoms created in a microwave discharge. The six hyperfine components of the N = 0 yields 1 rotational transition of CaH near 254 GHz were resolved and their frequencies determined to an accuracy of +/- 100 kHz. The main hyperfine lines originating in the N = 0 yields 1, 1 yields 2, and 2 yields 3 rotational transitions of CaD were also detected. Spectroscopic constants were determined for both radicals from a nonlinear least squares fit to the data, using a 2Sigma Hamiltonian. These measurements constitute the first laboratory observation of CaH at high spectral resolution, and are essential to the detection of this radical in interstellar space.

The Millimeter and Submillimeter Rotational Spectrum of CaCH3(X̃ 2A1)

The pure rotational spectrum of the calcium monomethyl radical, CaCH3( 2A1), has been measured in the laboratory using millimeter/submillimeter direct absorption techniques. CaCH3 was produced by the reaction of calcium vapor and an organometallic methyl compound. Seventeen rotational transitions in the frequency range 136-377 GHz were recorded for this free radical, which is a symmetric top species with one unpaired electron. In every transition, several K-components were observed, as well as fine-structure splittings. The data were analyzed using a2A1 Hamiltonian, and rotational and spin-rotation parameters were determined. These high-resolution measurements for CaCH3 will now enable a search for this molecule in the interstellar medium.

The millimeter-wave spectrum of the CaF radical (X(sup 2)Sigma(+))

The pure rotational spectrum of the CaF radical has been measured in the laboratory using millimeter/sub-mm direct absorption spectroscopy. Fourteen rotational transitions have been detected originating in the v = 0 mode of the species. Rotational lines of the vibrationally excited v = 1, 2, and 3 states have also been observed. Spin-rotation splittings, as well as hyperfine interactions arising from the nuclear spin of the fluorine atom, were resolved in the CaF spectra. Rotational, fine-structure, and in some cases, hyperfine parameters were determined for the various vibrational modes of the molecule from a nonlinear least-squares fit to the data, using a (sup 2)Sigma Hamiltonian, and are in agreement with past measurements. The newly measured rest frequencies for CaF, which are accurate to at least + or - 100 kHz, will enable astronomical searches to be conducted for the molecule in interstellar and circumstellar gas. Given the recent detection of AlF in IRC + 10216, metal fluoride species may be more abundant than previously thought.