A BaGa4Se7 crystal based pulsed mid-infrared light source with a narrow linewidth in 4-12 µm.

We present a BaGa4Se7 (BGSe) crystal based coherent pulsed light source for high resolution mid-infrared (MIR) spectroscopy in the 4-12 µm region. The all-solid-state system consists of an injection seeded optical parametric generator (OPG) and an optical parametric amplifier (OPA) using two KTiOPO4 crystals. The idler output of OPG-OPA and the fundamental output (1064 nm) of a wavelength stabilized Nd:YAG laser are employed for difference frequency generation of MIR pulses in the BGSe crystal. Pulsed MIR radiation in the 4-12 µm range is obtained with typical pulse energies higher than 100 µJ and pulse durations of ∼5 ns. By measuring H2O absorption lines in the 8 µm region with this MIR light source and a cavity ring-down spectrometer, the linewidth of the MIR source is inferred as 120 ± 10 MHz, which is very close to the Fourier-transform limited linewidth of 5 ns laser pulses.

High-Resolution Electronic Spectrum of the 1,4,6-Heptatrienyl Radical in the Gas Phase.

We report the gas-phase identification of the 1,4,6-heptatrienyl (C7H9) radical via its A~2B1-X~2A2 electronic transition spectrum. The optical absorption spectrum in the 590-630 nm region is recorded using cavity ring-down spectroscopy in combination with a supersonic plasma jet. An analysis of the rotationally resolved 000 origin band spectrum has allowed an accurate determination of spectroscopic constants for both the X~2A2 electronic ground and A~2B1 excited states of this radical. Ab initio calculations at the CASPT2/cc-pVTZ level have been performed to predict the radical structure and molecular constants that are in good agreement with the experimental results. By combination with available experimental data for the allyl and 1,4-pentadienyl radicals, we extrapolate an excitation energy gap between the ground and first excited states of a long C2n-1H2n+1 polyenyl chain converging to a residual energy gap, suggesting the presence of residual nonuniform C-C bond lengths in a long polyenyl chain.

Gas-Phase Optical Spectra of the Indenyl Radical and Its Quantitative Detection in a Jet-Stirred Reactor.

Resonance-stabilized radicals (RSRs), such as the indenyl radical (C9H7), are proposed to be initiator radicals in soot inception and growth in hydrocarbon combustion processes, but spectroscopic data for many RSRs are still lacking. In this work, the gas-phase optical absorption spectra of the B̃2A2-X̃2A2 electronic transition of indenyl were identified in a supersonic indene/argon plasma jet. Spectroscopic parameters, including the transition energy, rotational constants, and upper-state lifetime broadening, were obtained from analysis of the experimental spectra. The results were readily applied to the quantitative detection of indenyl produced from high-temperature reactions in a jet-stirred reactor. This study now makes indenyl optically accessible in further reaction kinetics studies and in situ spectroscopic diagnostics of hydrocarbon combustion processes.

High-resolution laser spectroscopy of the trans- and cis-1-vinylpropargyl radicals.

Rotationally resolved spectra of the Ã2A''-X̃2A'' origin bands for both trans- and cis-conformers of 1-vinylpropargyl radical (1VPR) are experimentally studied. Rotational constants for both ground and electronically excited states are experimentally determined. The stability of the Ã2A'' excited state of the cis-1VPR is found to be higher than that of trans-1VPR, which is likely due to additional π-overlap and increased pπ electron delocalization in the excited state of cis-1VPR.

High-Resolution Laser Spectroscopic Survey of the H3Σu--X3Σg- Electronic Transition of Si2.

Rotationally resolved spectra of the H3Σu--X3Σg- electronic transition bands of Si2 have been experimentally studied using laser-induced fluorescence in the 380-520 nm range. Si2 molecules are produced in a supersonically expanding planar plasma by discharging a silane/argon gas mixture. In total, 44 bands belonging to the H3Σu--X3Σg- electronic transition system of the most abundant isotopologue 28Si2 are experimentally recorded. With a spectral resolution of ∼0.04 cm-1, the triplet spin-splitting structures in individual rotational transition lines are fully resolved. Detailed analyses on the high-resolution spectra have yielded an accurate determination of spectroscopic constants for both X3Σg- and H3Σu- states. The spin-spin interaction constants for the two triplet states are found to be comparable (λ ≈1.5 cm-1), which may originate from the 3p atomic orbital interaction in the triplet Si2 molecule. The measured isotopologue spectra of 29Si28Si and 30Si28Si indicate that the H3Σu--X3Σg- transition system of 29S28S and 30S28S can be reasonably reproduced by the isotope mass-scaling rule. Spectroscopic parameters, including the Franck-Condon factors, the Einstein coefficients, and the oscillator strengths, are also determined from the experimental results and the Rydberg-Klein-Rees (RKR) calculations. The agreement between the experimentally measured and calculated dispersed fluorescence spectra indicates that the RKR calculations with the molecular constants determined in this work can accurately reproduce the diatomic potentials of both states. These molecular data provide a benchmark in high-level theoretical studies on Si2 and likely other small silicon clusters.