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The 101–000 and 202–101 rotational transitions of a monohalomethylene, HCI, are observed in a discharge plasma of diiodomethane by Fourier-transform microwave (FTMW) and FTMW-microwave double-resonance spectroscopy. The effective rotational constant ( B + C ) / 2 , the centrifugal distortion constant Δ J , and the quadrupole coupling constant of the iodine nucleus are determined from the observed line frequencies. The nuclear spin-rotation coupling constant C bb is required to be included into the set of fitting parameters for a satisfactory analysis of the observed line frequencies.

Pure rotational transitions of cis-OSOO are identified in a discharge plasma of a SO2 and O2 gaseous mixture by Fourier-transform microwave (FTMW) and FTMW-MW double-resonance spectroscopy. Both a- and b-type transitions are observed for this molecule, and the rotational and centrifugal distortion constants are precisely determined from the observed line positions. The inertial defect calculated from the determined rotational constants indicates that the cis-OSOO molecule has a planar equilibrium geometry in the ground vibronic state.

Fourier transform microwave spectra for the J = 1 ← 0 and 2 ← 1 rotational transitions of the SiCl+ ion were observed for two isotopologues ( 35 Cl and 37 Cl) in the ground and the first excited vibrational states of the ground 1 Σ + electronic state. Thanks to the high resolution of the FTMW spectrometer, hyperfine structures due to the quadrupole moment of the chlorine nucleus and the nuclear spin-rotation interaction were fully resolved. The observed FTMW spectra were combined with previously reported MMW and diode laser spectra in an analysis to determine the mass-independent Dunham coefficients U k , l as well as a mass scaling parameter Δ 01 Cl = - 0.856 ( 30 ) . The equilibrium bond length of SiCl+ determined is r e = 1.9439729 ( 10 ) A and the nuclear quadrupole coupling constant of Si 35 Cl+ is eQq e = - 11.8788 ( 23 ) MHz.

Measurement of pure rotational transitions for the vinylthio radical, CH2CHS, previously observed by Fourier-transform microwave spectroscopy (Nakajima et al., 2007), are extended to a number of fine and hyperfine component lines split by the unpaired electron spin and the three non-equivalent protons. All the measured transition frequencies have been analyzed by a Hamiltonian for doublet asymmetric top molecules with hyperfine interaction terms for three nuclear spins, yielding precise molecular constants of CH2CHS in the ground state. Discussions on the electronic structure of CH2CHS are given based on the determined molecular constants.

The high resolution rotational spectroscopic observation of 1,1-diiodoethane is investigated using a pulsed jet, cavity Fourier transform microwave (FTMW) spectrometer over the frequency range 11.5–18 GHz for the first time. The rotational constants, the centrifugal distortion constants, the nuclear spin-rotation coupling constants, and the complete tensor components of the nuclear quadrupole coupling for both iodine nuclei have been determined and reported. The fitted rotational constants are A = 4548.320446(47), B = 625.629141(55), C = 558.798939(43) MHz and the nuclear quadrupole coupling constants are χaa = −1089.8125(7), χbb − χcc = −542.3162(13), |χab| = 1215.7505(10), χbc = 340.8983(14), and |χac| = 562.4206(19) MHz. No A-E splittings due to the methyl group internal rotation were observed. Many dipole-forbidden/electric quadrupole coupling allowed transitions were observed in the spectrum due to the large iodine quadrupole coupling effect. Quantum chemical calculations were performed at the CCSD(T)/aug-cc-pVTZ-pp level of theory. The calculated rotational constants, centrifugal distortion constants, and hyperfine constants were used to guide the data analysis.

Pure rotational transitions of three conformers of epichlorohydrin were observed. Among the three possible conformers expected for this species, spectra of the two chlorine isotopologues, 35Cl and 37Cl, have been detected for the two gauche-conformers, while only the 35Cl isotopologue was detected for the cis-conformer which is much higher in energy. Well resolved chlorine nuclear hyperfine splittings were observed for all the conformers and isotopologues, yielding precisely determined molecular constants.

Pure rotational transitions of the anti-form of the simplest alkyl-substituted Criegee intermediate, anti-CH3CHOO, were observed by Fourier-transform microwave spectroscopy. The observed transition frequencies in the A and E levels of the ground vibrational state were analyzed together, and molecular constants including parameters which characterize the internal rotation of the methyl rotor were determined. The experimentally determined barrier height of the three-fold potential for the methyl internal rotation is 399.1 cm−1, less than a half of that of the other conformer, syn-CH3CHOO (Nakajima and Endo, 2014).

Pure rotational transitions of the simplest Criegee intermediate, CH2OO, were observed for several excited vibrational levels by Fourier-transform microwave spectroscopy combined with a double-resonance technique, and precise rotational constants of vibrationally excited CH2OO were determined. Comparing the experimentally determined vibration–rotation constants with those derived from a high-level ab initio potential energy surface, vibrational assignments were made for the observed levels. The observed intensities of the 101–000 rotational lines suggested that the COO bending mode is significantly excited.

Excitation spectra of the d 3 Π g - c 3 Σ u + band system of the C2 molecule were observed at room temperature by laser-induced fluorescence spectroscopy. Molecular constants of the c 3 Σ u + state were determined for the v = 0, 1, 2, 3, and 5 levels, including the centrifugal distortion constant. A level crossing between the v = 1 level of the c 3 Σ u + state and the v = 7 level of the a3Πu state was unambiguously identified by simultaneously observing the 3–1 band of the d 3 Π g - c 3 Σ u + system and the 3–7 band of the Swan system. Rotational level shifts observed in these vibronic levels could be explained by the perturbations arising from the L±S∓ and J±L∓ terms of the spin-orbit and rotational Hamiltonians. De-perturbed molecular constants for the c 3 Σ u + ( v = 1 ) and a3Πu(v = 7) states are presented, and the possibility of an additional perturbation by another level, A1Πu(v = 2), is discussed.

Pure rotational transitions of the vinoxy radical in the ground vibronic state have been observed by Fourier-transform microwave spectroscopy in the cm-wave region and double resonance spectroscopy in the mm-wave region. Total of 6 rotational transitions are newly observed in the present work, which are least squares fitted together with the mm-wave data observed previously (Endo et al., 1985). The hyperfine coupling constants for the third proton in the –CHO moiety of the vinoxy radical have been determined precisely for the first time. Four b -type transitions, that escaped to be detected in the previous study in the mm-wave region, have provided a more accurate set of molecular constants for the ground vibronic state.

Electronic excitation spectra of the B 2 A ″ - X 2 A ″ transition of the trans-2-methylvinoxy radical were observed in the region of 29 000–32 000 cm−1 by laser-induced fluorescence and fluorescence depletion spectroscopy. Fourteen vibronic bands observed in the depletion spectrum were unambiguously identified to be originating from the radical. Dispersed fluorescence spectra from three different vibronic levels were also obtained, and a total of 69 emission bands were assigned, yielding the ground state vibrational constants.

Rotationally resolved spectra of the A ˜ 2 A ′ - X ˜ 2 A ″ system for the HSO radical were observed by laser induced fluorescence spectroscopy. The radical was produced in a discharged supersonic jet of a gas mixture of dimethyl disulfide and O2 diluted in Ar. The observed spectra were assigned to ro-vibronic transitions of the K a ′ ← K a ″ = 0 ← 1, 1 ← 0, and 2 ← 1 subbands of A ˜ ( 00 v 3 ′ ) - X ˜ (0 0 0) with v 3 ′ = 0–8, and those of A ˜ (0 1 3) − X ˜ (0 0 1), yielding the rotational and spin–rotation interaction constants for the excited electronic state. For the vibronic origin band, the band origin and the rotational constants are determined to be Tv = 14383.2533(28) cm−1, A = 287316(33), (B + C)/2 = 16712.8(48), and (B − C)/2 = 532.0(70) MHz. Dispersed fluorescence spectra of the HSO radical were also observed, where the fundamental vibrational frequencies of the ν1 and ν2 modes for the X ˜ state have been determined experimentally for the first time.

Pure rotational spectra of S235Cl2 and S235Cl37Cl have been observed using a Fourier-transform microwave spectrometer. An analysis of the hyperfine structure made by considering the nuclear spin statistics showed that S2Cl2 has C2 symmetry, where the hyperfine splittings due to the two Cl nuclei were analyzed precisely. The nuclear quadrupole coupling constants including the off-diagonal (χab, χac, χbc) components and the nuclear spin–rotation interaction constants associated with the two Cl nuclei have been determined for the first time. We have shown that the nuclear quadrupole interaction plays an important role in the ortho–para mixing.

A laser-induced fluorescence excitation spectrum of the cyclohexadienyl ( c -C 6 H 7 ) radical was observed in the region between 550 and 530 nm under jet-cooled conditions. A partly rotationally resolved spectrum of the vibronic origin band was obtained with a high-resolution laser scan. The observed rotational structure of the origin band is that of a b -type transition, being not discrepant with the A ∼ 2 A 2 – X ∼ 2 B 1 electronic transition which has been indicated by a few theoretical calculations. The origin band position and the rotational constants in the upper and lower states were determined from a rotational analysis of the spectrum. The determined molecular constants agreed very well with results of our ab initio calculations. The theoretical vibrational frequencies for the upper and lower electronic states were also calculated for comparison with the experimental results.

In a discharged supersonic jet of Cl2, transitions of the D′ 2g(3P2)–A′ 3Π(2u) system for 35Cl2 were observed directly by laser induced fluorescence spectroscopy. By a discharge in Cl2, the Cl2 molecules were populated into the A′ state, which is a metastable and optically forbidden state, from the X 1 Σ g + ( 0 g + ) state. An ultraviolet laser radiation excites the molecules to the D′ ion-pair state. A set of Dunham parameters for the A′ state is determined from a global least-squares fitting for 59 vibronic bands with v″ = 0–7. In the fitting, the previously reported data, T(v) and B(v) for the v = 14 and 15 bands of the A′ state [T. Ishiwata, A. Ishiguro, K. Obi, J. Mol. Spectrosc. 147 (1991) 300–320], were included. Y00 = 57295.723(5) cm−1 of the D′ state [J.-H. Si, T. Ishiwata, K. Obi, J. Mol. Spectrosc. 147 (1991) 334–345] was also included in the global fitting in order to determine the absolute position of the A′ state. The determined parameters of the A′ state are Y00 = 17171.506(14), Y10 = 255.915(85), Y20 = −4.465(70), Y30 = −8.7(23) × 10−2, Y40 = 6.3(35) × 10−3, Y50 = −4.9(26) × 10−4, Y60 = 1.43(69) × 10−5, Y01 = 0.16282(15), Y11 = −2.363(68) × 10−3, Y21 = −5.01(93) × 10−5, and Y31 = −3.01(36) × 10−6 (in cm−1 and one standard deviations of the fit in parentheses). The absolute position of the A′ state is determined with good accuracy.

Pure rotational transitions of a carbon-chain free radical, CCCCCl, have been observed by Fourier-transform microwave spectroscopy. The radical was produced in a supersonic free jet by a pulsed discharge in a 0.2%/0.2% mixture of acetylene and carbon tetrachloride diluted in Ne. Transitions with a 2 Σ type spectral pattern were observed in the regions from 11.6 GHz for N = 4–3 to 23.3 GHz for N = 8–7. The hyperfine structures due to the Cl nucleus were clearly resolved, and the rotational, spin-rotation, and hyperfine coupling constants have been precisely determined.

In a discharged supersonic jet of a CS2 and C2H2 mixture, a vibronic band system of a new radical species was observed in the energy region 21 800–23 000 cm−1 by laser-induced fluorescence (LIF) spectroscopy. The LIF excitation spectrum shows progressions with 490 and 80 cm−1 separations. The vibronic structure of a dispersed fluorescence (DF) spectrum, obtained by tuning a probe laser to the vibronic origin band, also consists of progressions with 520 and 100 cm−1 separations. A high-resolution laser scan provided a rotationally resolved LIF excitation spectrum for the vibronic origin band, showing the rotational structure of a-type transitions of a near-prolate top. Several chemical tests indicate that the spectral carrier contains sulfur atom(s), one hydrogen atom and more than one carbon atoms. Electronic transition energy, vibrational frequencies, and rotational constants of this species are similar to those of SCCS− [M. Nakajima, Y. Yoneda, Y. Sumiyoshi, T. Nagata, Y. Endo, J. Chem. Phys. 119(2003)7805–7813.], and the spectral carrier was assigned as an isoelectronic radical, bent HSCCS. Ab initio geometrical optimizations supported the spectral carrier to be HSCCS. The observed electronic transition was assigned to be the B ∼ 2 A ″ – X ∼ 2 A ″ transition, which corresponds to the Π–Π transition in the limit of linear geometry. The observed vibrations in the excitation and DF spectra were assigned as the symmetric C S stretching (ν5) and SCC bending (ν7) modes by comparing the results of theoretical calculations.

Rotationally resolved fluorescence depletion spectra have been observed for the vibronic bands of the A ˜ 2 Π g – X ˜ 2 Π u system of SCCS − . The observed line widths varied from 0.03 to ∼2 cm −1 due to the lifetime broadening in the upper levels of the vibronic bands. The excited state lifetimes have been estimated from the spectral line widths, which show a distinct vibrational mode dependence.

Rotational transitions of the carbon-chain radicals, NC 2 O and NC 3 O, have been observed for the first time by Fourie-transform microwave spectroscopy. The radicals were produced in a supersonic free jet by a pulsed discharge of acetylcyanide (CO(CH 3 )(CN)) diluted in Ar for NC 2 O or a gas mixture of HC 3 N and O 2 diluted in Ar for NC 3 O. Pure rotational transitions from N =1–0 to N =3–2 for NC 2 O and those from N =2–1 to N =6–5 for NC 3 O, both with fine and hyperfine structures, were observed. The rotational, spin-rotation, and hyperfine coupling constants for the radicals have been precisely determined. Ab initio calculations at the RCCSD(T) level of theory considering the correlation of core electrons using the cc-pCVTZ basis sets have been performed. The present observations and the ab initio calculations revealed that both the radicals have bent structures in the ground states.

Rotational spectrum of jet-cooled 9-cyanoanthracene has been observed in the 4–8 GHz region with a Fourier-transform microwave spectrometer. The present observation of 25 low- J transitions with J ′′ ⩽11 has confirmed the previous results on the rotational constants of the ground state determined by rotational coherence spectroscopy [J. Phys. Chem. A. 105 (2001) 1131] and provided the values with significantly improved precision. An accurate set of hyperfine splitting constants is also reported for the 14 N nuclear quadrupole coupling. The electric dipole moment was determined from Stark effect measurements on several split components: μ b (= μ )=4.406(7) D.

An improved potential energy surface that is able to reproduce all the observed rotational transition frequencies of the radical complexes, Ar–SH and Ar–SD, is reported, where several potential parameters are constrained to the values obtained from ab initio calculations using the RCCSD(T) theory with the aug-cc-pVTZ basis set including additional bond functions located at the mid point of the vdW bond. The present study is an extension of a previous report on a determination of a two-dimensional intermolecular potential energy surface of Ar–SH( 2 Π i ) using the data obtained by Fourier-transform microwave spectroscopy [J. Chem. Phys. 113 (2000) 10121]. To reproduce all the observed transition frequencies of Ar–SH and Ar–SD within the experimental accuracy, it was required to introduce an effective correction term to express a small difference of the potentials for the SH and SD species. The determined values for the correction term are discussed by considering the difference of the amplitudes of the zero-point vibrations for SH and SD in the complex.

New vibronic bands were observed in the 17 000–17 600 cm −1 region by laser induced fluorescence (LIF) spectroscopy in the discharged products of a mixture of C2H2 and CS2 in a supersonic jet. The effective rotational constants and the position of the vibronic band origin have been determined to be B ″ eff =0.01910(2) cm −1 , B ′ eff =0.01891(2) cm −1 , and T 0 =16961.5161(5) cm −1 , respectively. The spectral carrier of the bands has been confirmed to be the HC6S radical from the chemical behavior of the products and the determined rotational constant.

Vibronic band systems for the HC 4 S and DC 4 S radicals were observed in supersonic jets for the first time by laser induced fluorescence (LIF) spectroscopy. The rotational structures of the bands were resolved by high-resolution scans. It was found that all the observed bands are 2 Π 3/2 – 2 Π 3/2 transitions. Band origins and effective rotational constants of the origin bands are determined to be T 0 =19 980.687(1) and B ′ eff =0.046580(4) cm −1 for HC 4 S, T 0 =20 034.8748(9) and B eff ′ =0.044294(3) cm −1 for DC 4 S. Ab initio calculations were carried out to interpret the vibronic transitions.

The Kr–HCO+ ionic complex and its isotopomers have been detected for the first time by Fourier transform microwave spectroscopy, where pure rotational transitions have been observed for six isotopomers under a jet-cooled condition. The rotational and the centrifugal distortion constants of the six isotopomers have been determined by least squares fittings. The complex has been determined to have a linear proton-bound form, with a Kr–H distance to be 2.222 A and an estimated intermolecular stretching frequency of 129 cm−1. Results of Kr–HCO+ are compared with those of Ar–HCO+ and neutral complexes containing Kr.

Pure rotational spectra of 14 N 2 ··CO, 15 N 14 N ··CO, and 15 N 2 ··CO have been observed using a Fourier transform microwave spectrometer. Two sets of transitions with J=2←1, 3←2, and 4←3 were observed for 14 N 2 ··CO and 15 N 2 ··CO; transitions with smaller effective rotational constants B were stronger than those with larger B for the 14 N 2 ··CO complex, while the opposite was observed for 15 N 2 ··CO. It was concluded that for 14 N 2 ··CO the first set corresponds to the K=0 stack of ortho-N2 and the second to K=1 of para-N2 based on a previous infrared analysis, hyperfine splittings, and the spin statistical weights. In the case of 15 N 14 N ··CO, only one set was observed because of the lack of symmetry. The electric quadrupole coupling constant of 14 N is much smaller, while the centrifugal distortion constant is much larger, than those of related complexes.

The triplet HC4N radical in a linear carbon-chain form has been detected by Fourier transform microwave spectroscopy for the first time. Rotational transitions with fine and hyperfine structures, about 130 lines in total, were observed in a pulsed-discharge-nozzle supersonic expansion of the HC3N sample diluted in Ar buffer gas. The spectrum was assigned to the linear HC4N radical by various combinations of discharge gases, by the rotational, fine and hyperfine structures, and most decisively, by comparison of the determined molecular constants with those of the HCCN radical.

Rotational spectra of the molecular ions HOCO + and HOCS + , and the ion complexes, D 3 + Ar and sym -D 2 H + Ar were observed in a supersonic-jet expansion by using a Fabry-Perot type Fourier-transform microwave spectrometer cooperated with a pulsed discharge nozzle. Ion-formation efficiency for HOCS + relative to the parent molecule under applied conditions was estimated to be ≈ 10 −4 . Tunneling splitting in the lowest rotational transition of D 3 + -Ar was not resolved within the experimental linewidth of ≈ 100 kHz.

LIF spectra of the A 2 Π i − X 2 Π i transition of the HCCS radical were observed in a supersonic jet generated by using a pulsed-discharge nozzle. Adiabatic cooling yielded definitive rotational assignments for ≈ 40 vibronic bands. The fluorescence from single rovibronic levels exhibited a quantum beat in most of the observed bands. The number of beat components increased gradually with internal energy, approaching the limiting case for a ‘large molecule’. The observed quantum interference is ascribed, based on the estimated densities of background states, to the internal conversion coupling with highly excited vibrational levels in the X 2 Π i state.

The lowest rotational transitions J = 1 ← 0 of NaBH 4 and KBH 4 were observed, using a Fourier transform microwave spectrometer combined with a laser ablation apparatus. Hyperfine structure has been observed at good resolution yielding precise values of the nuclear quadrupole coupling constants eQq for both the alkali metal and boron atoms. The eQq values of the alkali metals, −3.385(31) and −4.256(29) MHz for M = 23 Na and 39 K in M 11 BH 4 , respectively, are close to those in Na 35 Cl (−5.6698 MHz) and K 35 Cl (−5.656 MHz), indicating that NaBH 4 and KBH 4 are also ionic molecules. The boron eQq ’s are small, −0.670(53) and −0.722(59) MHz for Na 11 BH 4 and K 11 BH 4 , respectively, much smaller in magnitude than those reported for known boron-containing compounds, indicating that the BH 4 group retains a structure of approximate T d symmetry.

Laser induced fluorescence spectra of the C1B2X1A1 band of SO2 in the 235-210 nm region were measured under jetcooled conditions. By high-resolution (≈ 0.08 cm−1) measurements of the vibronic transitions, which were well separated from each other due to rotational and vibrational cooling, the rotational constants and term values for the 33 vibrational levels were determined. For the other four levels, only the term values were determined. A total of 573 ro-vibronic transition wavenumbers are presented. It was found that the C rotational constants exhibit considerable fluctuation over the whole observed energy range due to the C-axis Coriolis interaction. The observed vibrational term values were utilized for the extension of the secure vibrational assignments to the higher energy region. However, due to the strong 1:2 Fermi interaction between v1 (symmetric stretch) and v3 (anti-symmetric stretch), definitive vibrational assignments for the transitions to the vibrational levels above 2000 cm−1 were found to be intrinsically impossible except for the C (v 1 , v 2 , v 3 ) = (1, n, 0) , (2, n, 0), (3, n, 0) and (1, n, 2) vibrational levels. By constructing the three-dimensional Hamiltonian with a vibrational potential expanded to the fourth-power of the normal coordinates, the expansion coefficients were determined by the least-square fitting to the observed term values and those of several known lowlying vibrational levels. The Franck-Condon intensity pattern calculated using the vibrational wavefunctions derived as eigenfunctions was consistent with the observed pattern below the C(1, 4, 2) band, above which the predissociation occurs. The derived vibrational eigenfunctions showed that the v1 and v3 modes mix with each other significantly in the higher energy region above 2000 cm−1, which is consistent with the difficulty encountered in assigning the definitive v1 and v3 vibrational quantum numbers. The vibrational wavefunctions were further utilized to analyze the C-axis Coriolis interaction, and the counterpart levels of the perturbation having an odd v3 quantum number were identified for the lowlying vibrational levels. By measurements of the hot-band transitions from X(0, 1, 0), the v2 (bend) fundamental wavenumber of the electronic ground X1A1 state was determined to be 517.90(3) cm−1.

The rotational spectrum of 24 Mg 35 Cl in its X 2 Σ + (ν=0 and 1 ) state has been observed in the 14 GHz region by using a Fourier- transform microwave spectrometer combined with a laser-ablation source. The radical was produced by the reaction of atomic Mg vaporized by 532 nm laser light with Cl 2 diluted in Ar. The present observation of the lowest N transition by a high-resolving power instrument has provided accurate hyperfine coupling constants associated with the 35 Cl nucleus of this molecule.

The rotational spectrum of a short-lived free radical, HCCO, has been observed by using a Fabry-Perot-type Fourier-transform microwave spectrometer combined with a pulsed-discharge nozzle. HCCO has been produced by a discharge of a mixture of C 2 H 2 and O 2 diluted in Ar and subsequently cooled to a few Kelvin in a supersonic expansion. All the fine and hyperfine components have been observed for the N = 1-0 rotational transition at 21 GHz. The lines have been analyzed with previously reported data in the submillimeter wave region, leading to a set of the effective constants for the K a = 0 state. The precisely determined magnetic hyperfine constants for the hydrogen nucleus have been discussed in connection with the electronic and geometrical structures of the radical. Transition frequencies for the hyperfine components of several low- N lines have been tabulated with calculated line strengths, as an aid for future astronomical searches of the radical.

Rotational spectra of a transient molecule, CCCS, and its isotopic species, 13CCCS, C13CCS, and CCC34S, have been observed by using a Fabry-Perot-type Fourier-transform microwave spectrometer combined with a pulsed discharge nozzle. C3S has been produced by a discharge of a mixture of CS2 and C2H2 diluted in Ar, and subsequently cooled to a few kelvin in a supersonic expansion. From the determined rotational constants for the four isotopic species, the following substitution structure has been derived: rs(C1C2) = 1.2724, rs(C2C3) = 1.3028, and r s ( C 3 S 4 ) = 1.5323 A . The possibility of applying the present experimental setup to studies of other transient species and free radicals is briefly discussed.

The pure rotational spectrum of the benzene—15N2 complex has been observed by a pulsed-nozzle Fourier-transform microwave spectrometer. The observed spectrum consists of two internal-rotation states and evidently shows the existence of an almost-free rotational motion of N2 within the complex. The rotational and centrifugal-distortion constants for the ground internal-rotation state have been determined precisely. An analysis of the centrifugal-distortion constants has been carried out to derive information on the intermolecular vibrations.

The structure of N,N-dimethylaminobenzonitrile (DMABN) was determined using microwave and high-resolution laser spectroscopy. In the ground state, the molecule is found to be nearly planar with the inversion angle being about 15°. In the excited state, however, the dimethylamino group rotates by 30° from the benzene plane while the inversion angle is zero. The structure of 1:1 DMABN-Ar and 1:1 DMABN-H 2 O complexes was also estimated using high-resolution spectra. The Ar atom was found to lie above the benzene ring whereas the water molecule is located within the benzene plane, about 3.5 A to the side of the CN group.

Homogeneous linewidths of the individual rotational transitions in the AX system of ammonia were determined precisely by a microwave—optical double-resonance technique, where a single inversion transition in the ground state was selected by microwave radiation, and subsequent electronic transitions were observed by changes in the microwave absorption. The linewidths, which directly correspond to the A state predissociation rates, increase with J but are nearly independent of K . Small but distinct asymmetry in the line shape was observed for the first time.

“Forbidden” rotational transitions of NH 3 with the selection rules Δ J =0 and k =±1←∓2 were measured in the submillimeter-wave region. The C o rotational constant for the lower component of the inversion doublet in the ground vibrational state is obtained from the observed spectrum as 186 695.815±0.053 MHz. The observed line intensity agreed well with that calculated from the centrifugal-distortion-induced dipole moment.

The ClBO molecule was detected by a 60-Hz discharge in a mixture of O2 and BCl3; the ν3 BCl stretching vibration-rotation band was observed by infrared diode laser spectroscopy. Subsequently, the pure rotational spectrum was observed by microwave spectroscopy for eight isotopic species: 35Cl11B16O, 35Cl11B18O, 35Cl10B16O, 35Cl10B18O, 37Cl11B16O, 37Cl11B18O, 37Cl10B16O, and 37Cl10B18O. By using all the infrared and microwave data the molecular constants were obtained for the eight species in the ground state, for 35Cl11B16O and 37Cl11B16O in the v2 = 1 state, and for 35Cl11B16O, 37Cl11B16O, and 35Cl10B16O in the v3 = 1 state. The ν3 band origin was determined to be 676.0368(3), 668.5912(27), and 680.7642(30) cm−1 for the last three isotopic species. From the observed ground-state rotational constants the rs structure parameters were calculated as follows: r s (ClB ) = 1.68274(19) A and r s (BO ) = 1.20622(21) A , where the values in parentheses denote the ranges of deviations among isotopic combinations used.

The rotational spectrum of the SF radical in the 2 Π 1 2 state was observed by using a source-modulation microwave spectrometer with a 1-m-long discharge cell. The SF radical was generated directly in the cell by a dc discharge in an OCS CF 4 mixture. A previous measurement of the microwave spectrum in the 2 Π 3 2 state was also extended to higher-J transitions. The least-squares analysis of all the observed spectral lines gave the B0 rotational constant and the D0 centrifugal distortion constant to be 16 576.9140(46) and 0.02924(10) MHz, respectively, where the values in parentheses denote 2.5 times the standard deviations. The Λ-doubling constant pv was found to be extremely small, 3.409(44) MHz, and it was presumed that contributions of 2Σ+ and 2Σ− states to the Λ doubling cancel each other. All the four fluorine hyperfine coupling constants were also determined, from which the spin density on the F atom was calculated to be 0.13.

The pure rotational spectrum of the a 1 Δ SO radical in the vibrational states of ν up to 5 has been observed in the submillimeter-wave region of 300 to 400 GHz. The SO radical was produced by the excimer laser photolysis of Cl 2 SO at 193 nm, and its absorption signal was detected by a transient digitizer followed by a computer for signal processing. The present method is discussed as a means of studying short-lived molecules.

The microwave spectrum of cyclopropane-1,1- d 2 has been observed in the frequency region from 320 to 400 GHz using a source-frequency modulation microwave spectrometer. Sixty-four observed a -type R -branch transitions were analyzed by the least-squares method to determine the rotational and centrifugal distortion constants to be A = 18 835.662(18), B = 16 370.2703(70), C = 11 409.2285(67), Δ J = 0.011246(12), Δ JK = 0.005087(35), Δ K = 0.00706(12), δ J = 0.0030280(79), and δ K = 0.005561(27), in MHz with three standard errors in parentheses. The observed rotational constants of the 1,1- d 2 species and the B 0 and C 0 constants of C 3 H 6 and the B 0 constant of C 3 D 6 available in the literatures were corrected for harmonic terms to derive an average structure: r z (CH) = 1.0797(34) A , r z (CC) = 1.5157(23) A , θ z (HCH) = 115.47(38)°, and r z (CH) − r z (CD) = 0.00029(16) A with standard deviation of the fit in parentheses. Using the vibration-rotation constants reported, the equilibrium structure was also estimated, where only two “diagonal” third-order anharmonic terms were taken into account.

The microwave spectrum of 28Si35Cl, the most abundant isotopic species of the silicon monochloride radical, was observed in both the 2 Π 1 2 and 2 Π 3 2 spin states of the ground vibronic state. The SiCl radical was produced in a flow cell by a dc discharge in SiCl4. The observed transitions were J = 7 2 ← 5 2 up to 21 2 ← 19 2 for both the spin states, and the observed frequencies were subjected to the least-squares analysis to yield accurate molecular constants as follows: B0 = 7652.3048(23), D0 = 0.007017(14), AJ = −0.8392(16), p0 + 2q0 = 138.660(98), q0 = 0.20(17), a + (b + c) 2 = 37.50(28) , a − (b + c) 2 = 49.84(73) , b = 9(12), d = 46.40(94), and eQq1 = −23.13(96), all in MHz with 3σ in parentheses. The positive sign of the Λ-doubling constant, p0, indicates that the contributions of 2Σ− states dominate over those of 2Σ+ states. The spin and orbital averages of the unpaired-electron distribution were calculated from the observed hyperfine coupling constants and were discussed in terms of the electronic structure of the molecule.

The CCl radical was generated by a dc glow discharge in CCl4, and rotational transitions of C35Cl in both 2 Π 1 2 and 2 Π 3 2 were observed in the mm-wave region by using a source modulation microwave spectrometer. From an analysis of the observed spectra the magnetic and electric quadrupole hyperfine coupling constants of the 35Cl nucleus were determined precisely. The Λ-doubling constant p0 was determined to be positive, indicating that unknown 2Σ− excited electronic states are making dominant contributions to the Λ doubling in the ground state. The previous diode-laser result [J. Mol. Spectrosc.85, 416–426 (1981)] was reanalyzed by using improved wavelength standards and by fixing the ground-state parameters to the present microwave values including the B0 rotational constant 20 797.1725(38) MHz. The vibration-rotation constant αB and the equilibrium bond length re were thus determined to be 0.006 768 2(48) cm−1 and 1.645 218(16) A, respectively, with 2.5 times standard deviations in parentheses. The spin density on the chlorine atom was estimated to be about 0.20 from the observed hyperfine coupling constants.

15NO2 (88 lines) in the ground state has been observed in the microwave spectral region up to 170 GHz. The least squares analysis gave the higher order distortion terms of rotational and spin-rotational Hamiltonian.

The pure rotational spectra of three deuterated ethylenes, CH2CD2, CH2CHD, and cis-CHDCHD, were observed by microwave spectroscopy, and the rotational and centrifugal distortion constants were determined precisely. The dipole moment of CH2CD2 was calculated from the Stark effects to be 0.0091 ± 0.0004 D. From the observed rotational constants the average structure was calculated to be r z (CC) = 1.3391 ± 0.0013 A , r z (CH) = 1.0869 ± 0.0013 A , θz(CCH) = 121.28 ± 0.10°, and r z (CH) - r z (CD) = 0.00137 ± 0.00037 A , where the errors include one standard deviation in the fitting and errors due to an uncertainty (±0.03°) in θz(CCH) - θz(CCD).

The ν1 (CO stretching) and ν2 (CF stretching) bands of the FCO radical were observed with Doppler-limited resolution by an infrared diode laser spectrometer with Zeeman and source modulation. The FCO radical was generated by a 60-Hz discharge in one of the following three gas mixtures: O2 + C2F4, CO + SF6, and CO + C2F4, all diluted with He. The observed spectra were analyzed to determine the rotational constants, the centrifugal distortion constants, and the spin-rotation interaction constants. The band origins, 1861.6372(1) and 1026.1283(1) cm−1 [with standard errors in parentheses], which were obtained, were found to agree well with matrix data, 1857 and 1023 cm−1, respectively. The assignment of the observed spectra to the FCO radical was further supported by observing the ν1 band of F13CO, which was obtained from 13CO and SF6. The molecular structure and the force field of FCO are briefly discussed by using molecular constants obtained from the observed spectra.

The pure rotational spectrum of the DCO radical was observed in the millimeter- and submillimeter-wave regions for both a-type and b-type transitions. Spin splittings were clearly resolved, whereas deuterium hyperfine splittings were partially resolved. Analysis of the observed spectrum yielded precise ground state molecular constants of DCO: A = 441 714.02(16), B = 38 417.03(27), C = 35 116.48(27), ϵaa = 7130.59(40), ϵbb = 5.5(17), ϵcc = −174.2(16), |ϵab + ϵba|2 = 74.7(18), aF = 58.413(74), Taa = 2.191(93), Tbb = 0.16(12), and χaa = 0.22(21) in megahertz with 2.5 standard deviations in parentheses.