M. A. Anderson

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.

The millimeter-wave spectrum of the MgOH radical (X 2Σ+)

Abstract The pure rotational spectrum of the ground electronic state (X 2 Σ + ) of the MgOH radical has been observed using millimeter/sub-millimeter direct absorption spectroscopy, as well as that of MgOD, and the less abundant magnesium isotopically-substituted species 25 MgOH and 26 MgOH. The free radicals were created in a Broida-type oven by the reaction of metal vapor with hydrogen peroxide. The spin—rotation splitting was readily observed in these data; however, the hyperfine structure was only resolved for 25 MgOH. The rotational and spin—rotation constants of the radicals have been determined from a non-linear least-squares fit using a 2 Σ Hamiltonian. The 25 Mg hyperfine constants are also estimated.

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.

The millimeter-wave spectrum of the SrOH and SrOD radicals

Abstract The pure rotational spectra of the X 2 Σ + ground states of the SrOH and SrOD radicals have been observed in the laboratory using millimeter/submillimeter direct absorption spectroscopy. The lesser abundant isotopically substituted species, 86 SrOH, has been observed as well. The molecules were produced in a Broida-type oven by the reaction of Sr metal vapor with either H 2 O 2 or D 2 O 2 . The rotational and spin—rotation constants were determined for the radicals from a nonlinear least-squares fit to the data using a 2 Σ Hamiltonian. The millimeter-wave measurements support the picture that strontium hydroxide is linear in its ground electronic and vibrational state.

The millimeter-wave spectrum of25MgNC and 26MgNC: bonding in magnesium isocyanides

Abstract Rotational spectra of the 25 Mg and 26 Mg isotopomers of MgNC have been recorded in the frequency range 298–386 GHz using millimeter/sub-millimeter direct absorption spectroscopy. The species were created in their natural isotopic ratios in a dc discharge of cyanogen gas and magnesium vapor. Rotational and fine structure parameters were determined for these radicals, as well as magnesium 25 hyperfine constants for 25 MgNC. For this isotopomer, b F ≈ −298(16), indicating a large electron density at the nucleus and an ionic type Mg + NC − structure, as expected for a linear configuration. R 0 bond lengths were also derived, which are in good agreement with theoretical calculations.

The millimeter and sub-millimeter spectrum of the BaOH radical

The pure rotational spectrum of the X2Σ+ ground electronic states of the BaOH and BaOD radicals has been observed using millimeter/sub-millimeter direct absorption spectroscopy. The lesser abundant isotopically substituted species, 136BaOH and 137BaOH, have been detected as well. The radicals were created by reacting barium metal vapor, produced in a Broida-type oven, with either H2O2 or D2O2. The rotational and spin—rotation constants were determined for the molecules from a nonlinear leastsquares fit to the data, using a 2Σ Hamiltonian. Hyperfine constants were also derived for 137BaOH, the one species where hyperfine structure was resolved. These measurements confirm a linear structure for BaOh.

Laboratory detection and millimeter spectrum of the MgCCH radical

The pure rotational spectrum of the magnesium monoacetylide radical, MgCCH, has been recorded in the laboratory using millimeter/sub-mm direct absorption spectroscopy. These measurements constitute the first time this molecule has been observed in the laboratory by any spectroscopic method. Seventeen rotational transitions were observed in the frequency range 210-370 GHz for MgCCH, which appears to be a linear molecule with a (sup 2)Sigma ground electronic state. Rotational and fine structure constants were determined for this radical from a nonlinear least squares fit to the data. The rotational rest frequencies measured here will enable astronomical observations to be carried out for MgCCH towards IRC +10216, where the magnesium compounds MgCN and MgNC, as well as many acetylide species, are present.

NEW ROVIBRATIONAL DATA FOR MgOH AND MgOD, AND THE INTERNUCLEAR POTENTIAL FUNCTION OF THE GROUND ELECTRONIC STATE

Abstract Using millimeter wave absorption spectroscopy we have recorded pure rotation spectra of MgOH in its ground vibrational state and in six excited vibrational states. We have also determined a large number of bending vibrational energies, and the MgO stretching frequency, using dispersed fluorescence studies of both MgOH and MgOD. By a weighted least-squares optimized fitting to this data, using the vibrational approach MORBID (Morse oscillator rigid bender for internal dynamics), we have derived the full ground state potential surface in an analytical form. The equilibrium structure is determined to be linear but with a flat predominantly quartic bending potential.

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.

Millimeter‐wave spectroscopy of MgF: Structure and bonding in alkaline–earth monofluoride radicals

The pure rotational spectrum of the MgF radical in its ground electronic state (X 2Σ+) has been recorded using millimeter/submillimeter direct absorption techniques. Transitions arising from the v=0, 1, 2, and 3 vibrational modes of the main magnesium isotopic species, 24MgF, have been observed. In addition, spectra of the isotopomers 26MgF and 25MgF in the natural abundances of magnesium have been detected. Rotational and fine structure constants have been determined for these species, as well as hyperfine parameters for the fluorine nucleus (I=1/2). For 25MgF, the hyperfine structure was also resolved arising from the magnesium nucleus, which has I=5/2, yielding the 25Mg hyperfine and quadrupole constants. Comparison of these hyperfine parameters with those of the heavier alkaline–earth monofluorides and the free 25Mg+ atom suggests that there is an increase in covalent bonding in MgF vs its heavier fluoride counterparts. This behavior is also apparent in the hybridization of the wave function of the un...