Showing papers by "Richard J. Saykally published in 1984"

Laser magnetic resonance rotational spectroscopy of 2Σ radicals: Ethynyl (CCH)

Abstract: The first terrestrial measurement of the free ethynyl radical (CCH), made by far‐infrared laser magnetic resonance, is described. The N=6→7 rotational transition was observed for the lowest vibrational level of the 2Σ+ ground state. Because of the very weak spin coupling in this state, the LMR spectrum is complex and badly overlapped. A theoretical formalism for the prediction and analysis of such weakly coupled 2Σ states is presented, in which frequencies, linewidths, and intensities of all transitions are computed as a function of magnetic flux density, and the total absorption coefficient is computed at each field point in order to simulate the magnetic resonance spectrum. This formalism is used to analyze the LMR spectra of CCH. A combined least squares analysis of existing microwave, astronomical, and LMR data was carried out to determine an improved set of molecular parameters for this important interstellar molecule.

A high resolution study of the ν3 band of the ammonium ion (NH+4) by velocity modulation laser absorption spectroscopy

Abstract: Over 200 transitions in the ν3 band of the ammonium ion (NH+4 ) have been measured near 3 μm with a color center laser spectrometer to an accuracy of 0.005 cm−1 using the technique of velocity modulation laser absorption spectroscopy. These have been assigned to strongly allowed Q0, P+ and R− branches in the triply degenerate asymmetric stretching mode of a spherical top molecule, and analyzed by nonlinear least–squares methods with an effective sixth order Hamiltonian. Ground state constants were fixed at values obtained with a recent ab initio molecular potential surface. Twenty‐six molecular parameters were determined in the fit. The fact that terms in the Hamiltonian beyond fourth order are required to fit transitions with J as low as 12 indicates the existence of a perturbation involving the ν3=1 levels. It is suggested that a Fermi resonance between ν3 and ν2+ν4 is an important source of this perturbation, as in the case of CH4. The frequencies of ‘‘forbidden’’ transitions, required in order to dete...

The high resolution infrared spectrum and molecular structure of the superacid H2F+ by velocity modulation laser absorption spectroscopy

Abstract: The gas phase infrared spectrum of the fluoronium ion (H2F+) was recorded with Doppler‐limited resolution between 3080 and 3520 cm−1 by velocity modulation laser absorption spectroscopy of a hydrogen plasma containing 5% HF. One hundred and nine of the observed transitions were assigned to the symmetric stretch (ν1) and 217 to the asymmetric (ν3) stretch. A least squares analysis of ν3 transitions up to J=11 and Ka=6 and ν1 transitions up to Ka=4 with Watson’s S‐reduced Hamiltonian yielded rotational constants and centrifugal distortion constants for the vibronic ground state and for the two excited states. Small Coriolis interactions between ν1 and ν3 levels were observed, but were not treated explicitly. The band origins for ν3 and ν1 are 3334.6895(26) and 3348.7078(36) cm−1, respectively. The r0 structure of H2F+ was determined as: rHF=0.9577 A

A study of the ν1 fundamental and bend‐excited hot band of DNN+ by velocity modulation absorption spectroscopy with an infrared difference frequency laser

Abstract: The combined application of the ac discharge velocity modulation technique with cw tunable difference frequency infrared generation and dual beam subtraction of laser amplitude noise produces an extremely sensitive (10−6 absorbance/Hz1/2) direct absorption method for observing vibrational spectra of molecular ions. In this study we report observation of the fundamental ν1 (D–N stretch) of DNN+ in both P and R branches from J=0 to 24, as well as the 30‐fold weaker hot band transitions originating from the (0110) bending state. l‐doubling in these latter transitions is measured interferometrically with a precision of ±10 MHz. Spectroscopic constants for (0000), (1000), (0110), and (1110) states are reported and compared with ab initio calculations and previous microwave measurements for DNN+.

An investigation of the laser optogalvanic effect for atoms and molecules in recombination-limited plasmas

Abstract: In this paper the nature, the characteristics, and the potential applications of the optogalvanic effect are explored in a broad set of experiments. The optogalvanic effect was investigated, for the Ar, Ne, He, Ar +, He +, Cu and Na atoms and N2, H~, CO, CN, NH2, He2, and CuO molecules in recombination-limited hollow cathode plasmas, with the use of dye lasers. Comparative studies of the time dependence of transitions from metastable and non-metastable states of neutrals and non-metastable states of ions were made. Autoionizing transitions in Cu(I) were found to yield very strong OGE signals. An examination of the various processes known to be important in such plasmas indicates that associative ionization is a very effective means for coupling light energy into the plasma. Measurement of Rydberg transitions in the helium dimer suggests that optogalvanic spectroscopy in recombination plasmas may be useful as a way to study such molecules with repulsive ground states by generating them 'from the top down ' by recombination.

Velocity modulation infrared laser spectroscopy of molecular ions: The ν1 and ν3 bands of fluoronium (H2F+)

Abstract: The infrared absorption spectrum of the fluoronium ion (H2F+) has been observed in the gas phase using velocity modulation laser absorption spectroscopy of a H2/HF plasma. Over 300 transitions were measured in the frequency range 3080–3500 cm−1 and assigned to the ν3 and ν1 bands of H2F+. The band centers are 3348.710(20) cm−1 for ν1 and 3334.679(5) cm−1 for ν3. 150 transitions in ν3 have been fit with a least squares procedure yielding rotational constants and centrifugal distortion constants for the ground and first excited ν3 states. The r0 structure of fluoronium has been determined: rH–F=0.968(9) A; αHFH=113.9(1.0)°.

Measurement of the rotational spectrum of hf+ by laser magnetic resonance

Abstract: Rotational transitions in the X 2Π state of the HF+ molecular ion were measured by laser magnetic resonance spectroscopy. Hyperfine splittings from both nuclei were resolved. An analysis of the J = 3/2 → 5/2 transitions in the Ω = 3/2 spin sublevel is presented.

A search for interstellar silicon nitride.

Abstract: We have searched for the N = 2-l rotational transI-tion of silicon nitride, SiN, at 87 GHz. Upper limits to the SiN column densities are NL/sub siN/ < or = l0/sup 12/-l0/sup 15/ cm-2. The ratios of the abundances of SiN to CN toward the four sources observed are SiN/CN < or = 0.002-0.08, significantly less than the silicon-to-carbon ratI-o of 0.l. These ratios suggest that either silicon is depleted onto grains where CN is present, or that ion-molecule chemistry does not favor the production of SiN. Upper limits to the SiN/SiO ratios are significantly less than the nitrogen-to-oxygen ratio for Orion-KL and IRC + l0216 indicating that the chemistry responsible for producing SiO does not correspondingly produce SiN. We report detection of /sup 30/SiS toward IRC + l02l6, and comparison with /sup 28/SiS and /sup 29/SiS yields terrestrial isotopic abundance ratios for silicon.