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Abstract: The rotationally resolved spectrum of the (060)0–(000) band in the system of HCBr has been recorded in absorption using a frequency-modulated c.w. tunable dye laser. Both 79Br and 81Br isotopomer spectra were rotationally assigned and effective rotational constants determined. The spectrum contains both perpendicular and parallel rotational sub-band structures. The latter are partially due to axis-switching in the transition and Coriolis coupling in the excited state, but are relatively more intense and show a different J-dependence to that expected on the basis of these mechanisms. Intensity borrowing from a predicted but unobserved shorter wavelength transition explains the observations. We have calculated the energy and structure of HCBr in the state at the MRCI/cc-pVTZ level and find . The band system should be experimentally observable. [Copyright &y& Elsevier]

Doppler-free transition frequencies for v 4 - and v 5 -excited hot bands have been measured in the v 1 + v 3 band region of the spectrum of acetylene using saturation dip spectroscopy with an extended cavity diode laser referenced to a frequency comb. The frequency accuracy of the measured transitions, as judged from line shape model fits and comparison to known frequencies in the v 1 + v 3 band itself, is between 3 and 22 kHz. This is some three orders of magnitude improvement on the accuracy and precision of previous line position estimates that were derived from the analysis of high-resolution Fourier transform infrared absorption spectra. Comparison to transition frequencies computed from constants derived from published Fourier transform infrared spectra shows that some upper rotational energy levels suffer specific perturbations causing energy level shifts of up to several hundred MHz. These perturbations are due to energy levels of the same rotational quantum number derived from nearby vibrational levels that become degenerate at specific energies. Future identification of the perturbing levels will provide accurate relative energies of excited vibrational levels of acetylene in the 7100–7600 cm −1 energy region. [ABSTRACT FROM AUTHOR]

Highlights: [•] We report two-color sub-Doppler saturation spectra of CN in the red A–X system. [•] Hyperfine constants derived for υ =1 and 2 of the CN A 2Π state. [•] Offset lock of probe laser to a cavity that tracks the bleach laser fluctuations. [Copyright &y& Elsevier]

Abstract: Measurements of multiple rotational lines in the (1,0) band of the A 2Π i − X 2Σ+ “red” system of the cyanogen radical (CN) at sub-Doppler resolution are reported. The CN radical was produced by 193nm photodissociation of NCCN (ethane dinitrile) and detected with a Ti:sapphire ring laser operating near 10 900cm−1. The sample was exposed to a weak, frequency-modulated probe beam and a strong, counterpropagating bleach laser beam. Demodulated probe laser signals display absorption and dispersion features derived from Doppler-free saturation of the hyperfine components as the laser scans across the central region of a Doppler-broadened rotational line spectrum. Hyperfine-resolved transition frequencies were combined with known ground-state X 2Σ hyperfine term values to determine A 2Π state hyperfine term values, which were analyzed in terms of an effective Hamiltonian for the A 2Π state. All the expected hyperfine and 14N quadrupolar parameters were determined and their values analyzed in terms of a simple molecular orbital picture of the bonding in the radical. Higher sensitivity obtained with 400kHz amplitude modulation of the bleach laser and additional phase-sensitive detection allowed hyperfine splittings in some rotational lines of 13C14N to be observed in natural abundance. Excited state hyperfine splittings were determined for a selection of rotational states, but not enough to determine the 13C hyperfine parameters. [Copyright &y& Elsevier]

Spectra of both the A˜(0, 0, 0)–X˜(0, 1, 0) and A˜(0, 0, 0)–X˜(0, 0, 1) bands in the A˜–X˜ band system of HCBr have been recorded and rotationally assigned. The HCBr radical was formed by 193 nm excimer laser photolysis of bromoform. We found no signals from HCBr when the photolysis laser wavelength was changed to 248 nm, although strong CH product emission was observed by eye at both wavelengths. Most of the spectra were recorded in absorption at ambient temperature in a long-path absorption cell using a single frequency Ti:sapphire laser as the light source, but some congested sections were recorded under jet-cooled conditions. Analysis yielded accurate values for the vibrational fundamentals and rotational constants for the vibrationally excited ground state levels determined. A combination of experimental measurements and ab initio vibration–rotation constants was used to estimate equilibrium parameters and a ground state structure for HCBr. [Copyright &y& Elsevier]

Abstract: We report the observation of levels in the state of CH2 via optical–optical double resonance spectroscopy. Direct transitions between the lowest singlet state and the state are allowed by symmetry, but weak because they correspond to a two electron excitation in the single configuration approximation to the electronic wavefunction. The observed transitions involve sequential single photon absorptions at visible and near infrared wavelengths using state intermediate levels. Recent ab initio results (S.N. Yurchenko et al., J. Mol. Spectrosc. 208 (2001), 136) predicted the positions of some of the levels which are confirmed by the present results. The new spectra provide accurate energies for rotational levels in the , l =0 level of the state. [Copyright &y& Elsevier]

We report the observation and analysis of spectra in part of the near-infrared spectrum of C 2 H, originating in rotational levels in the ground and lowest two excited bending vibrational levels of the ground X ̃ 2 Σ + state. In the analysis, we have combined present and previously reported high resolution spectroscopic data for the lower levels involved in the transitions to determine significantly improved molecular constants to describe the fine and hyperfine split rotational levels of the radical in the zero point, v 2 = 1 and the 2 Σ + component of v 2 = 2 . Two of the upper state vibronic levels involved had not been observed previously. The data and analysis indicate the electronic wavefunction character changes with bending vibrational excitation in the ground state and provide avenues for future measurements of reactivity of the radical as a function of vibrational excitation. [ABSTRACT FROM AUTHOR]

Abstract: Bands in the transition between 12500 and 13000cm−1 were recorded at Doppler-limited resolution using a transient frequency-modulation (FM) laser absorption spectrometer. Rotational levels in seven upper vibronic states: , , and were assigned with the assistance of optical–optical double resonance (OODR) and ground state combination differences. Perturbations due to rotation-vibration coupling, anharmonic couplings, ℓ-resonance, as well as the Renner–Teller effect are discussed as sources of the irregular rotational patterns observed in the , and vibronic levels. [Copyright &y& Elsevier]

Abstract: We report measurements of self- and nitrogen-pressure broadening of the P(11) line in the ν 1 + ν 3 combination band of acetylene at 195 739.649 5135(80)GHz by absorption of radiation emitted by an extended cavity diode laser referenced to a femtosecond frequency comb. Broadening, shift and narrowing parameters were determined at 296K. For the most appropriate, hard collision, model in units of cm−1/atm, we find 0.146317(27), 0.047271(104) and −0.0070819(22) for the acetylene self-broadening, narrowing and shift, and 0.081129(35), 0.022940(74) and −0.0088913(25) respectively, for the nitrogen-broadening parameters. The uncertainties are expressed as one standard deviation (in parenthesis) in units of the last digit reported. These parameters are 2–3 orders of magnitude more precise than those reported in previous measurements. Similar analyses of the experimental data using soft collision and simple Voigt lineshape models were made for comparison. [Copyright &y& Elsevier]

Abstract: Based on measurements made by mass-resolved 1+1′+1″ resonance-enhanced multiphoton ionization spectroscopy, we have determined new molecular constants describing the rotational and fine structure levels of the B, D, E, and F states of the most abundant isotopic variant 208Pb19F, and we summarize the spectroscopic constants for all the know electronic states of the radical. Many spectroscopic constants for the isotopologues 206Pb19F and 207Pb19F have also been determined. The symmetry of the D-state is found to be 2 Π 1/2, and the F-state is found to be an Ω =3/2 state. [ABSTRACT FROM AUTHOR]

The rotationally resolved A˜(010)–X˜(000) spectrum of DCCl between 12 880 and 12 964 cm−1 was measured using frequency-modulated laser absorption spectroscopy of jet-cooled and ambient temperature samples. Transitions to levels with Ka′=0 and 1 were assigned, and their analysis leads to improved accuracy of both the ground state rotational constants of DCCl and, when combined with existing data for HCCl, the geometry of the radical. In addition to the expected perpendicular band structure, a number of parallel (Δ Ka=0) subbands were observed. Their intensity derives from a combination of c-type Coriolis coupling and axis-switching (J. T. Hougen and J. K. G. Watson, 1965, Can. J. Phys.43, 298) resulting from the change in geometry between the two states. The two contributions have been calculated for the (010)–(000) band of DCCl and previously recorded data for HCCl. Satisfactory agreement with experimental measurements was obtained. The Coriolis contributions are small for these transitions, but may add to or subtract from the axis-switching. For higher bending excitation in the upper state, Coriolis coupling is predicted to make larger contributions to the parallel subband intensities. [Copyright &y& Elsevier]

The (0,0) and (1,0) bands of the E3Π–X3Δ transition of TiO in the near-infrared have been recorded by frequency modulated laser absorption spectroscopy in a laser ablation/free jet expansion source. The observed linewidths (FWHM) varied from 300 to 500 MHz according to the expansion conditions and are dominated by residual Doppler broadening in the unskimmed source. Data for the (0,0) band have been obtained for TiO molecules containing all the naturally occurring Ti isotopes but, for the weaker (1,0) band, only for 48TiO. Rotational constants for the two upper state vibrational levels were derived by fitting the data to an effective Hamiltonian; equilibrium parameters have been calculated. The experimental results are compared to the results of ab initio calculations on the E–X system. Ab initio results for the b–a system and for the lowest 3Σ− state are also presented. They indicate that the D3Σ− state is not a very low-lying state. [Copyright &y& Elsevier]

• New high resolution spectra of ethynyl radical in the 1.65 μm region. • Improved parameters for the lowest few levels of the radical are obtained. • The spectra will potentially be useful for monitoring CCH in chemical systems Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 μ m. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground X ̃ 2 Σ + state and its first excited bending vibrational level of 2 Π symmetry. The upper states, of 2 Σ + symmetry at 6055.6 cm−1 and 2 Π symmetry at 6413.5 cm−1, respectively, had not previously been observed and the data were analyzed in terms of an effective Hamiltonian representing their rotational and fine structure levels to derive parameters which can be used to calculate rotational levels up to J = 37/2 for the 2 Π level and J = 29/2 for the 2 Σ one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (0 2 0), level of 2 Σ symmetry, to a previously observed state of 2 Π symmetry near 6819 cm−1. These strong absorption bands at convenient near-IR laser wavelengths will be useful for monitoring CCH radicals in chemical systems. [ABSTRACT FROM AUTHOR]