Felix Güthe

Electronic spectra of the chains HC2nH (n=8–13) in the gas phase

The intense 1Σu+←X1Σg+ electronic transition of the chains HC2nH (n=8–13) has been recorded in the gas phase using resonant two color two photon ionization spectroscopy. The gas phase frequencies lie ∼1300 cm−1 to the blue of the values in a neon matrix. These polyynes appear to be linear in both the ground and the excited state and, as for the polyynes, spectral and theoretical evidence for large bond alternation in the ground state is found indicating localized π orbitals on the C≡C units. These species are the longest carbon chains observed spectroscopically in the gas phase up to now.

Gas phase electronic spectra of the linear carbon chains HC2n+1H (n=3-6,9)

The B 3Σu−←X 3Σg− transitions of HC13H and HC19H have been measured in the gas phase, exhibiting broad, Lorentzian shaped bands. More extensive A 3Σu−←X 3Σg− spectra have been observed for HC2n+1H (n=3–6) than before with many new vibronic bands identified. The spectra were obtained by means of a mass selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The electronic structures of this series of molecules (n=2–9) in both the ground and excited states have been investigated using DFT, MP2, and state-averaged CASSCF theories. The three lowest dipole allowed electronic transition systems are A 3Σu−←X 3Σg−, B 3Σu−←X 3Σg−, and C 3Πu←X 3Σg−, located, for the smaller members of the series, in the visible, UV and VUV range, respectively. The A 3Σu−←X 3Σg− system is found to be of medium intensity and the B 3Σu−←X 3Σg− transition is predicted to be very strong. This is a result of configuration mixing in the excited states. The oscillator strength of the lowest energ...

The Reheat Concept: The Proven Pathway to Ultralow Emissions and High Efficiency and Flexibility

Reheat combustion has been proven now in over 80 units to be a robust and highly flexible gas turbine concept for power generation. This paper covers three key topics to explain the intrinsic advantage of reheat combustion to achieve ultralow emission levels. First, the fundamental kinetic and thermodynamic emission advantage of reheat combustion is discussed, analyzing in detail the emission levels of the first and second combustor stages, optimal firing temperatures for minimal emission levels, as well as benchmarking against single-stage combustion concepts. Second, the generic operational and fuel flexibility of the reheat system is emphasized, which is based on the presence of two fundamentally different flame stabilization mechanisms, namely, flame propagation in the first combustor stage and autoignition in the second combustor stage. This is shown using simple reasoning on generic kinetic models. Finally, the present fleet status is reported by highlighting the latest combustor hardware upgrade and its emission performance.

Gas phase electronic spectrum of C3H in the visible

The electronic spectrum of C3H has been observed by means of a resonant two color two photon ionization technique sampling a supersonic plasma source. On the basis of ab initio calculations, vibrational and rotational analysis, the complex vibronic system observed in the visible and near UV is assigned to three electronic transitions, A 2A′←X 2Π, B 2A″←X 2Π and C 2A″←X 2Π of C3H. Potential curves along the CCH bending coordinate have been obtained because of its role for the electronic spectrum and dynamics of C3H.

Photodetachment Spectrum of l-C3H2-: The Role of Dipole Bound States for Electron Attachment in Interstellar Clouds

The electronic spectrum of the linear propadienylidene anion l-C3H has been recorded at 50 K by resonance-enhanced two-photon detachment spectroscopy. This resembles the absorption spectrum to the dipole bound state involving bound-bound and bound-free transitions. The wavelength of the K = 1 ← 0 component for the origin and three vibrational bands match the weak and narrow diffuse interstellar bands. To explain the abundance of this carrier in interstellar clouds, the ratio of anions to neutrals is estimated for small polar molecules with high electron affinities using chemical considerations. An appreciable amount of anions can result in diffuse clouds because the electron attachment to polar molecules is enhanced via dipole bound states.

Electronic spectra of the C2n+1H(n=2–4) radicals in the gas phase

The visible electronic spectra of the linear l-C2n+1H (n=2–4) radicals have been measured in the gas phase. These have been obtained by means of a mass-selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The observed spectra are assigned to the A 2Δ←X 2Π, B 2Σ−←X 2Π, and C 2Σ+←X 2Π electronic transitions arising from σ→π electron promotion. The assignments are based on ab initio calculations, wavelength dependence of the 000 transition on size, and isotopic substitution. The lifetime broadening of the bands and effects due to vibronic coupling are associated with the carbon skeleton bending modes. The detection of these carbon chains in the diffuse interstellar medium appears to be more favorable by radio astronomy rather than by electronic spectroscopy.

Ignition Delay Time and Laminar Flame Speed Calculations for Natural Gas/Hydrogen Blends at Elevated Pressures

Applications of natural gas and hydrogen co-firing have received increased attention in the gas turbine market, which aims at higher flexibility due to concerns over the availability of fuels. While much work has been done in the development of a fuels database and corresponding chemical kinetics mechanism for natural gas mixtures, there are nonetheless few if any data for mixtures with high levels of hydrogen at conditions of interest to gas turbines. The focus of the present paper is on gas turbine engines with primary and secondary reaction zones as represented in the Alstom and Rolls Royce product portfolio. The present effort includes a parametric study, a gas turbine model study, and turbulent flame speed predictions. Using a highly optimized chemical kinetics mechanism, ignition delay times and laminar burning velocities were calculated for fuels from pure methane to pure hydrogen and with natural gas/hydrogen mixtures. A wide range of engine-relevant conditions were studied: pressures from 1 to 30 atm, flame temperatures from 1600 to 2200 K, primary combustor inlet temperature from 300 to 900 K, and secondary combustor inlet temperatures from 900 to 1400 K. Hydrogen addition was found to increase the reactivity of hydrocarbon fuels at all conditions by increasing the laminar flame speed and decreasing the ignition delay time. Predictions of turbulent flame speeds from the laminar flame speeds show that hydrogen addition affects the reactivity more when turbulence is considered. This combined effort of industrial and university partners brings together the know-how of applied, as well as experimental and theoretical disciplines.

Photodetachment spectroscopy of the C2nH−(n=2–4) anions in the vicinity of their electron detachment threshold

The electronic spectra of the C2nH(D)−, n=2–4, anions have been observed in the gas phase using photodetachment spectroscopy. These are assigned to 1Π←X 1Σ+ electronic transitions. The 1Π excited states possess a dipole bound character indicated by the energetic proximity between the origin of the transitions and electron affinities. The dipole bound states are related to the X 2Π ground states of C6H and C8H whereas for C4H, which has an X 2Σ+ ground state, the 2Π is an excited state. Vibronic coupling through a bending motion of the carbon skeleton is inferred to be the reason of the stabilization of this state.

Diagnosis of a benzene discharge with a mass-selective spectroscopic technique

Abstract The products formed in a benzene discharge have been probed by resonant two photon ionization spectroscopy. The molecules formed are cooled in a supersonic expansion and mass-analysis combined with spectroscopic information is used to identify the species. By this means styrene, phenylacetylene, methylstyrene, indene, fluorene and tolane are recognized. No six-ring polycyclic aromatic hydrocarbon could be detected. With this method the plasma is sampled at an early stage where the ethynylated and cyclopentafused polycyclic aromatic hydrocarbons are more abundant than the more stable six-ring ones. The coupling of a discharge source to a REMPI detection system enables electronic spectra of neutral molecules to be obtained which can be used to study their role in combustion processes, plasmas as well as in astrophysics.

Electronic spectra of carbon chain anions: C2nH− (n=5–12)

The electronic absorption spectra of mono-hydrogenated carbon chain anions C2nH− (n=5–10) have been measured in the gas-phase and in 6 K neon matrices (n=8–12). The techniques of resonant two-color electron photodetachment in the gas-phase and absorption spectroscopy of mass-selected anions in neon matrix were used. A homologous series is observed, with band system origins shifting from 304 nm for C10H− to 590 nm for C20H−. In conjunction with ab initio calculations the band systems are attributed to a 1Σ+←X 1Σ+ transition of linear acetylenic anions. Another near lying electronic transition due to a second isomer is also apparent for C10H− up to C24H−. Comparison with tables of the known diffuse interstellar bands indicates possible matches for the origin bands of the C18H− and C20H− isomers.