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The methylene radical (C[H.sub.2]) was very important to Gerhard Herzberg. We have carried out high-resolution spectroscopic measurements on two energetically low-lying, pure rotational transitions of methylene in its ground vibrational--electronic state at frequencies near 2 THz. One of the transitions--the [N.sub.KaKc] = [2.sub.11] [left arrow] [2.sub.02] multiplet--belongs to ortho-C[H.sub.2] and is centered at 1.954 THz. The other rotational transition--the [N.sub.KaKc] = [l.sub.10] [left arrow] [l.sub.01] multiplet--belongs to para-C[H.sub.2] and is centered at 1.915 THz. Since the ground electronic state of methylene has [sup.3][B.sub.1] symmetry, the rotational transitions are split into three fine-structure components, while the ortho transitions are split additionally by hyperfine structure. In total, we have measured 29 new lines, six for the para transition and the remainder for the ortho transition. The newly measured lines can be added to high-resolution spectral data obtained by ourselves and others at lower frequencies. Progress towards a global fit is discussed. Key words: spectroscopy, THz frequencies, radicals, methylene, interstellar medium.

Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion–molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In the case of ammonia, the detection of its triply deuterated isotopologue hints at high abundances of the deuterated intermediate nitrogen radicals, ND, NHD, and ND2. So far however, only ND has been detected in the interstellar medium. In this paper, to constrain the formation of ammonia, we aim at determining the NHD/NH2 and ND2/NHD abundance ratios, and compare them with the predictions of both pure gas-phase and grain-surface chemical models. We searched for the fundamental rotational transitions of NHD and ND2 towards the class 0 protostar IRAS16293−2422, towards which NH, NH2 and ND had been previously detected. Both NHD and ND2 are detected in absorption towards the source. The relative abundance ratios NH2:NHD:ND2 are close to 8:4:1. These ratios can be reproduced by our gas-phase chemical model within a factor of 2–3. Statistical ratios as expected from grain-surface chemistry are also consistent with our data. Further investigations of the ortho-to-para ratio in ND2 , both theoretical and observational, could bring new constraints to better understand nitrogen hydride chemistry. [ABSTRACT FROM AUTHOR]

Pure rotational spectra of the bromomethyl radical, CH2Br, were measured by using a Fourier transform microwave (FT-MW) spectrometer in order to fully resolve hyperfine structures arising from both the bromine and hydrogen nuclei. We detected a total of 124 lines for the 79Br and 81Br isotopomers, including Ka=0 (ortho species) and Ka=1 (para species). No hyperfine splitting due to the hydrogen nuclei was observed for the para species, directly confirming the planarity of the radical. We conducted a global analysis of our present FT-MW results and previous measurements in the millimeter-wave region and obtained an exhaustive list of molecular constants. The sign of the Fermi constant of the bromine nucleus was unambiguously determined to be positive, which is opposite to that found in previous work in the millimeter-wave region and in electron spin resonance experiment on this radical. The present study permitted a systematic comparison to be made of the hyperfine coupling constants of both the halogen and hydrogen nuclei for CH2X-type compounds, where X=F, Cl, and Br. [ABSTRACT FROM AUTHOR]

The pure rotational spectrum of the isocyanomethyl radical, CH2NC, was measured for the first time by using a Fourier transform microwave spectrometer. The molecule was produced by a discharge of isocyanomethane, CH3NC, diluted in Ar or Ne. The spectral lines due to the N=1-0 and 2-1 transitions were recorded near 22 and 44 GHz, respectively. The observed spectrum showed a complicated fine and hyperfine structure because of the same order of interaction energies. Among the 39 spectral lines detected and assigned, the transitions with Ka=1 show no hyperfine splitting due to the hydrogen nuclei, suggesting planarity for the molecule. Molecular constants such as rotational and spin-rotational parameters including centrifugal effects and hyperfine coupling constants due to both the nitrogen and the hydrogen nuclei were accurately determined. The structure and the astronomical implications of the molecule are discussed. [ABSTRACT FROM AUTHOR]

The hyperfine resolved rotational spectrum of the CH[sub 2]CP radical in the &Xtilde;²B[sub 1] ground electronic state has been observed for the first time using a Fourier transform microwave spectrometer in combination with a pulsed discharge nozzle. The radical was produced by discharging a mixture of PH[sub 3] and C[sub 2]H[sub 2] diluted in either Ar or Ne. A total of 25 hyperfine components of the 2[sub 02]-1[sub 01] and 3[sub 03]-2[sub 02] transitions have been measured which enabled us to precisely determine hyperfine coupling constants for both phosphorus and hydrogen nuclei. Spin densities on the phosphorus and β-carbon atoms, estimated from the hyperfine coupling constants, suggest that the radical forms an allenic structure (CP double bond) that is modified by a phosphoryl structure (CP triple bond), which is consistent with the theoretical estimation obtained previously by an ab initio calculation. The nature of the CP chemical bond in the radical is investigated in comparison with the corresponding nitrogen bearing counterparts. [ABSTRACT FROM AUTHOR]

High resolution two-color threshold photoelectron spectra of tropolone and 9-hydroxyphenalenone (9-HPO) isolated in a free jet expansion have been measured to study the proton tunneling phenomena in the cation ground state (D0). The tunneling splitting widths of the D0 zeroth vibrational level for the both molecules lie within our experimental accuracy (2 cm-1), indicating that the proton tunneling is inhibited when compared to that obtained in their respective neutral states. By means of a comparative study of these two molecules, the tunneling inhibition has been explained in terms of a large contribution from the electronic factor which represents the changes in charge distribution of the π electrons upon ionization. The geometric factor, which is associated with the ring planarity is less important in determining the rate of tunneling in the D0 state. In the case of tropolone, we have found that the measurement of an out-of-plane skeletal vibration in various electronic states make it possible to describe the tunneling path at a higher level than a simple one-dimensional description. Moreover, the adiabatic ionization energies of 9-HPO and the deuterated 9-HPO have been determined accurately to be 65 338±5 cm-1 (8.1009±0.0006 eV) and 65 350±5 cm-1 (8.1024±0.0006 eV), respectively. [ABSTRACT FROM AUTHOR]

In this work, the molecules styrene (ST) and phenylacetylene (PA), as well as their argon complexes ST–Ar and PA–Ar, have been investigated with (1+1’) resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy (TES). The first adiabatic ionization energies of ST, PA, ST–Ar, and PA–Ar have been measured as 68 267±5, 71 175±5, 68 151±5, and 71 027±5 cm-1, respectively. For both ST–Ar and PA–Ar, the first photoelectron band shows structure in the lowest frequency van der Waals (vdW) bending mode in the ground ionic state, with νvdW being measured as 15 cm-1 in each case. For each molecule excitation to a particular vibrational level of the S1 state followed by ionization, allows structure in that mode to be observed in the threshold photoelectron spectrum. This has been achieved for three modes in both styrene and phenylacetylene. The experimental ionic vibrational frequencies thus obtained, have been compared with those known for the S0 and S1 states. [ABSTRACT FROM AUTHOR]

Measurements of mass-selected ion-current and threshold photoelectron spectra of jet-cooled aniline–Arn van der Waals complexes (n=1 and 2) have been carried out with a two-color resonantly enhanced multiphoton ionization (REMPI) technique using a high-resolution threshold photoelectron analyzer developed in this laboratory. From our (1+1’) REMPI experiments via the respective excited S1 states, we have obtained photoelectron spectra with well-resolved vibrational progressions due to ‘‘low-frequency van der Waals modes’’ of the cations; νvdW=16 cm-1 (n=1) and νvdW=11 cm-1 (n=2). From Franck–Condon calculations, we have assigned these low-frequency vibrations to the ‘‘van der Waals bending’’ of the cations. We have also found that the angles of the van der Waals bonds in the cations are changed by 8.2 (n=1) and 8.8 (n=2) degrees with respect to the S1 states. The adiabatic ionization potentials (Ia) of aniline and the aniline–Arn complexes (n=1 and 2) have been determined as 62 268±4 cm-1 (aniline), 62 157±4 cm-1 (n=1), and 62 049±4 cm-1 (n=2). Their shifts ΔIa are 111 cm-1 (n=1) and 219 cm-1 (n=2) with respect to aniline. Spectral shifts due to complex formation have been observed for a total of 13 ring modes of the cations. [ABSTRACT FROM AUTHOR]

From our high resolution REMPI threshold photoelectron measurements of jet-cooled tropolone, it has been found that no spectral doublet showing ‘‘proton tunneling’’ in the cation ground state (D0) is observed within an experimental accuracy of 2 cm-1. The adiabatic ionization potential was obtained as Ia=68 365±5 cm-1. [ABSTRACT FROM AUTHOR]