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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.

Doppler-limited absorption spectra of dicarbon are recorded for Phillips bands ( A 1 Π u – X 1 Σ g + ) involving v ′ = 9 –11 at a room temperature. Small energy shifts of rotational levels are identified in the v = 10 level of the A 1 Π u state. The spin-orbit interaction between A 1 Π u ( v = 10 ) and c 3 Σ u + ( v = 7 ) is responsible for the level shifts. In addition to the perturbations in A 1 Π u ( v = 10 ) , those previously identified in the v = 2 and 6 levels of A 1 Π u are re-investigated. The magnitude of the spin-orbit interaction between the A 1 Π u and c 3 Σ u + states is evaluated to be approximately 4 cm−1 from perturbation analyses for these vibronic levels.

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.

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).

Doppler-limited excitation spectra of the C2 anion were observed for the (2,4), (3,5), and (4,6) bands of the B 2 Σ u + – X 2 Σ g + system by laser-induced fluorescence spectroscopy at room temperature. Doublet peaks which are expected for rotational lines of a 2 Σ – 2 Σ transition were resolved in the spectra. The vibrational term values and molecular constants of the v = 4–6 levels in the X 2 Σ g + state were experimentally determined.

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.

Pure rotational transitions of weakly bound complexes of the simplest Criegee intermediate CH2OO with Ar, CO, and N2 are observed by Fourier-transform microwave (FTMW) and FTMW-MW double-resonance spectroscopy. Many of the observed lines show doublet peaks associated with the tunneling splitting between the two equivalent configurations of the complexes. The intermolecular distances of the complexes are derived from the precise rotational constants determined in this study. The separations between two components of the doublet peaks observed for c-type transitions of CH2OO–Ar are satisfactorily demonstrated by a one-dimensional hindered rotor model for the internal rotation of the CH2OO monomer.

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.

The Ã(2)Σ(+)-X̃(2)Πi band system of l-SiC3H in the region 14,700-16,300 cm(-1) was re-investigated by laser induced fluorescence (LIF) and fluorescence depletion spectroscopy. Rotational analyses were made for three intense bands 0(0)(0), 4(0)(1), and 6(0)(1)7(0)(1) by observing high-resolution LIF excitation spectra. The determined rotational constants demonstrate that SiC3H is linear in the à state, as is the case in the X̃ state, and the observed band types are consistent with the vibrational assignments. The ν3(″) (C1-C2 stretch) level was identified in a newly observed dispersed fluorescence spectrum from the zero-vibrational level of the à state.

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.

In a discharged supersonic jet of carbon disulfide, a band system of a new species is observed by laser-induced fluorescence (LIF) spectroscopy. Rotationally resolved spectra show that the observed electronic transition is 2Π3/2–2Π3/2. Since both sulfur and carbon atoms have even numbers of electrons, it is concluded that the spectral carrier is an ionic species. Furthermore, the spectral carrier is confirmed to have two equivalent carbon atoms from the LIF spectra of the 13C-substituted species. With a help of ab initio calculations, the carrier is identified as the linear negative ion SCCS−, where the observed electronic transition is considered to be A 2Πg–X 2Πu. A mass spectrometric measurement also gives evidence that SCCS− is produced in the discharged jet. The vibrational structure for the upper electronic state is determined by both LIF and fluorescence depletion spectroscopy. A dispersed fluorescence spectrum is also observed to determine the vibrational structure of the lower electronic state. ...

Laser-induced fluorescence (LIF) excitation spectra of the SiH 2 A (1,0,0) – and A (1,1,0)–X (0,0,0) bands, and dispersed fluorescence (DF) spectra from the A (1,0,0) and (1,1,0) levels were measured in the supersonic-free jet. From the rotational structures of the excitation spectra, the rotational constants in A (1, v 2 ,0), v 2 =0∼2, were determined. The fluorescence decay profiles of the single ro-vibronic levels were also measured, and the fluorescence lifetimes were determined. Based on the lifetimes, it was found that the predissociation depending on the vibronic level starts between the A (1,4,0) and (1,5,0) levels, and the predissociation depending on the rotational level in the A state is suppressed by the ν 1 mode excitation and is promoted by the ν 2 mode. The latter one is qualitatively explained by considering a second-order predissociation mechanism and a rotation-vibration constant, α i A .

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.

A new type of microwave-detected microwave-optical double-resonance (MODR) spectroscopy has been developed using a Fourier-transform microwave spectrometer and a tunable pulsed dye laser. In this method, a free-induction decay (FID) signal was detected instead of the microwave (MW) absorption. To demonstrate the performance, we measured the MODR spectra of the CCS and C 4 H radicals in supersonic jets generated by a pulsed-discharge nozzle. Since the pulsed sources are employed for both the optical and microwave radiations, it is possible to control the relative timing of irradiations of the MW and optical pulses. We were able to obtain two different types of spectra; one is the ordinary population labeling spectrum, and the other is a spectrum obtained by breaking the coherence of molecules. In the latter case, more than 50% of depletion of the FID signal was observed, which is unable to be attained when noncoherent phenomena are used to detect the double-resonance signal.

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.

Two silicon-bearing carbon chain radicals, SiC2N and SiC3N, were detected in the laboratory by Fourier transform microwave spectroscopy. Molecular constants including the hyperfine coupling constants have been determined for the two radicals in the ground electronic states. The SiC2N and SiC3N radicals have linear structures in the (2)Π ground electronic states with inverted and regular fine structures, respectively, as are the cases for their isoelectronic radicals, SiC3H and SiC4H, indicating that the SiC(n)N radicals have similar electronic structures to the SiC(n +1)H radicals. The electronic structures of SiC2N and SiC3N in the ground states are discussed on the basis of the experimentally determined molecular constants.

The D(1)((2)A("))-D(0)((2)A(")) electronic transition of the resonance-stabilized 1-phenylpropargyl radical, produced in a jet-cooled discharge of 3-phenyl-1-propyne, has been investigated in detail by laser-induced fluorescence excitation and dispersed single vibronic level fluorescence (SVLF) spectroscopy.The transition is dominated by the origin band at 21,007 cm(-1), with weaker Franck-Condon activity observed in a(') fundamentals and even overtones and combinations of a(") symmetry. Ab initio and density functional theory calculations of the D(0) and D(1) geometries and frequencies were performed to support and guide the experimental assignments throughout. Analysis of SVLF spectra from 16 D(1) vibronic levels has led to the assignment of 15 fundamental frequencies in the excited state and 19 fundamental frequencies in the ground state; assignments for many more normal modes not probed directly by fluorescence spectroscopy are also suggested. Duschinsky mixing, in which the excited state normal modes are rotated with respect to the ground state modes, is prevalent throughout, in vibrations of both a(') and a(") symmetry.

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were observed in the 4080-4530 Angstrom range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4264 Angstrom is estimated to have an oscillator strength of f=1.4x10^-3, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4x10^12 cm^-2, accounting for a maximum of ~0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future., 9 pages, 1 figure. Fixed a typo on the frequency of the 'b' band

Pure rotational transitions of the Ar-CS van der Waals complex have been observed by Fourier Transform Microwave (FTMW) and FTMW-millimeter wave double resonance spectroscopy. Rotational transitions of v(s) = 0, 1, and 2 were able to be observed for normal CS, together with those of C(34)S in v(s) = 0, where vs stands for the quantum number of the CS stretching vibration. The observed transition frequencies were analyzed by a free rotor model Hamiltonian, where rovibrational energies were calculated as dynamical motions of the three nuclei on a three-dimensional potential energy surface, expressed by analytical functions with 57 parameters. Initial values for the potential parameters were obtained by high-level ab initio calculations. Fifteen parameters were adjusted among the 57 parameters to reproduce all the observed transition frequencies with the standard deviation of the fit to be 0.028 MHz.

Pure rotational spectra of the allenyloxy radical (CH2=CCHO) were observed by Fourier transform microwave (FTMW) and FTMW-millimeter wave double-resonance spectroscopy. Molecular constants including the hyperfine interaction constants of CH2=CCHO in the (2)A(″) ground electronic state were precisely determined. Ab initio calculations indicate that CH2=CCHO has a linear C-C-C backbone with Cs symmetry, where the formyl group is in the Cs plane and perpendicular to the methylene group. The determined rotational constants and the inertial defect agree well with those derived from the calculations, implying that the calculated molecular structure is reasonable. The fine and hyperfine constants also agree with those derived from the calculated spin density, where the unpaired electron is located mainly on the central carbon atom. The ground state CH2=CCHO can, thus, be described as taking the formylvinyl (CH2=Ċ-CH=O) form rather than as the allenyloxy (CH2=C=CH-Ȯ) form.

Pure rotational spectra of the H2O-trans-HOCO complex have been observed by Fourier transform microwave (FTMW) spectroscopy and millimeter-wave FTMW double resonance spectroscopy. The complex was produced in a supersonic jet by discharging a mixture gas of CO and H2O diluted in Ar. The observed rotational lines consist of two groups of transitions with different hyperfine patterns. This is explained by considering the internal rotation of the H2O monomer in the complex. The molecular constants including the fine and hyperfine coupling constants have been determined for the two groups of lines. The hydrogen bond distance between H2O and the trans-HOCO monomer has also been determined with other structural parameters fixed to ab initio values. The hydrogen bond distance, 1.794 Å, is much shorter than that of the water dimer, and similar to those of water-acid complexes. The Fermi coupling constant of the proton of HOCO is compared with that of the trans-HOCO monomer, leading to the conclusion that there is an induced effect on the spin density on the proton of HOCO by the complex formation.

Gas-phase laser excitation spectra of the jet-cooled CCS radical were observed for the 2 (3)Sigma(-)-X (3)Sigma(-) electronic transition in a discharge of a gas mixture containing acetylene and H(2)S in Ar. The observed excitation spectra show complicated rotational structures, consisting of a much larger number of rotational lines than those expected for this size of linear molecule. The excited vibronic levels emit fluorescence, the lifetime of which is much longer than that estimated theoretically, indicating that the upper levels are strongly mixed with many "dark" background levels. Dispersed fluorescence spectra from single vibrational levels of the 2 (3)Sigma(-) state show long progressions of the C-S stretching mode, enabling observations of many vibrational levels in the ground electronic state.

An electronic excitation spectrum of the B–X transition of the CH2CHS (vinylthio, thiovinoxy) radical was observed in the range of 21 800–23 400 cm−1 by fluorescence depletion spectroscopy. Vibrational assignments for the depletion spectrum were made with the help of a theoretical excitation spectrum computed from results of ab initio calculations. The previously observed emission spectrum [M. Nakajima et al., J. Chem. Phys. 126, 044307 (2007)] was also reassigned based on the present theoretical study. Rotational band contours observed in the depletion spectrum show a large variation due to a vibrational-level dependence of the excited state lifetime. However, no obvious vibrational mode dependence of the lifetime was observed.