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The unimolecular photodissociation dynamics of acetone spanning the entire S1 ← S0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window λ = 311.2–305.8 nm. From λ = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C–C bond cleavage on the lowest excited triplet state, T1. At higher energy (λ < 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 ± 4 kJ mol−1, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S0 reaction, in this higher energy region of the first electronic absorption band. [ABSTRACT FROM AUTHOR]

We implement a new observational method for mapping the aliphatic hydrocarbon content in the solid phase in our Galaxy, based on spectrophotometric imaging of the 3.4  |$\mu$| m absorption feature from interstellar dust. We previously demonstrated this method in a field including the Galactic Centre cluster. We applied the method to a new field in the Galactic Centre where the 3.4  |$\mu$| m absorption feature has not been previously measured and we extended the measurements to a field in the Galactic plane to sample the diffuse local interstellar medium, where the 3.4  |$\mu$| m absorption feature has been previously measured. We have analysed 3.4  |$\mu$| m optical depth and aliphatic hydrocarbon column density maps for these fields. Optical depths are found to be reasonably uniform in each field, without large source-to-source variations. There is, however, a weak trend towards increasing optical depth in a direction towards b  = 0° in the Galactic Centre. The mean value of column densities and abundances for aliphatic hydrocarbon were found to be about several |$\rm \times 10^{18} \, cm^{-2}$| and several tens × 10−6, respectively for the new sightlines in the Galactic plane. We conclude that at least 10–20 per cent of the carbon in the Galactic plane lies in aliphatic form. [ABSTRACT FROM AUTHOR]

The 3³Πgg state of the dicarbon molecule, C2, has been identified for the first time by a combination of resonant ionization spectroscopy, mass spectrometry, and high-level ab initio quantum chemical calculations. This marks the discovery of the final valence triplet state of C2 spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 4³Πgg state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 3³Πgg state of C2 is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d³Πgg state. [ABSTRACT FROM AUTHOR]

Resonant two-photon threshold ionization spectroscopy is employed to determine the ionizationenergy of C2 to 5 meV precision, about two orders of magnitude more precise than the previously accepted value. Through exploration of the ionization threshold after pumping the 0-3 band of the newly discovered 4³Πg←a³Πu band system of C2, the ionizationenergy of the lowest rovibronic level of the a³Πu state was determined to be 11.791(5) eV. Accounting for spin-orbit and rotational effects, we calculate that the ionizationenergy of the forbidden origin of the a³Πu state is 11.790(5) eV, in excellent agreement with quantum thermochemical calculations which give 11.788(10) eV. The experimentally derived ionizationenergy of X¹Σg+X¹Σg+ state C2 is 11.866(5) eV. [ABSTRACT FROM AUTHOR]

In the interstellar medium, the cosmic elemental carbon abundance includes the total carbon in both gas and solid phases. The aim of the study was to trial a new method for measuring the amount and distribution of aliphatic carbon within interstellar dust over wide fields of view of our Galaxy. This method is based on the measurement of the 3.4- |$\mu$| m absorption feature from aliphatic carbonaceous matter. This can readily be achieved for single sources using infrared (IR) spectrometers. However, making such measurements over wide fields requires an imaging IR camera, equipped with narrow-band filters that are able to sample the spectrum. While this cannot produce as good a determination of the spectra, the technique can be applied to potentially tens to hundreds of sources simultaneously, over the field of view of the camera. We examined this method for a field in the centre of the Galaxy, and produced a map showing the variation of 3.4- |$\mu$| m optical depth across it. [ABSTRACT FROM AUTHOR]

A high-resolution single vibronic level emission study from the à 2Π3/2 state of the HC4S radical is reported. Ground state density functional theory frequencies have been used to assign ground state vibronic levels involving three stretching modes ν2, ν3, and ν5 in the region of 0–3250 cm-1, while the frequency of ν4 remains speculative. Tentative assignments are given for the complicated structures arising from Renner-Teller and spin-orbit interactions within the bending energy levels. From analysis of the dispersed emission spectra, Fermi resonances involving pairs of bands have been identified in the à 2Π3/2←X 2Π3/2 laser induced fluorescence spectrum. [ABSTRACT FROM AUTHOR]

We report on the dynamics of multiphoton excitation and dissociation of NO2 at wavelengths between 395 and 420 nm and intensities between 4 and 10 TW cm-2. The breakup of the molecule is monitored by NO A 2Σ+n′=1,0→X 2Πrn″=0 fluorescence as a function of time delay between the driving field and a probe field which depletes the emission. It is found that generation of n′=0 and 1 NO A 2Σ+ results in different fluorescence modulation patterns due to the intense probe field. The dissociation dynamics are interpreted in terms of nuclear motions over light-induced potentials formed by coupling of NO2 valence and Rydberg states to the applied field. Based on this model, it is argued that the time and intensity dependences of A 2Σ+n′=0→X 2Πrn″=0 fluorescence are consistent with delayed generation of NO A 2Σ+n′=0 via a light-induced bond-hardening brought about by the transient coupling of the dressed à 2B2 and Rydberg 3sσ 2Σg+ states of the parent molecule. The increasingly prompt decay of A 2Σ+n′=1→X 2Πrn″=0 fluorescence with increasing intensity, on the other hand, is consistent with a direct surface crossing between the X 2A1 and 3sσ 2Σg+ dressed states to generate vibrationally excited products. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The B [SUP3]Σ[SUP-,SUBg] ← X[SUP3]Σ[SUP-,SUBg] transitions of HC[SUB13]H and HC[SUB19]H have been measured in the gas phase, exhibiting broad, Lorentzian shaped bands. More extensive A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] spectra have been observed for HC[SUB2n+1]H (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 [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], and C [SUP3]Γ[SUBu] ← X[SUP3]Σ[SUP-,SUBg], located, for the smaller members of the series, in the visible, UV and VUV range, respectively. The A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system is found to be of medium intensity and the B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] transition is predicted to be very strong. This is a result of configuration mixing in the excited states. The oscillator strength of the lowest energy transition is not strongly dependent on the length of the chain, but that of the B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system increases monotonically with size. The C [SUP3]Γ[SUBu] state is Rydberg in character. The astrophysical implications are considered and an upper limit of the column densities of these carbon chains in diffuse clouds has been estimated as 10[SUP13] cm[SUP-2], based on calculated oscillator strengths. [ABSTRACT FROM AUTHOR]

The mid-IR spectrum of the interstellar medium contains both aromatic and aliphatic hydrocarbon features. These are generally attributed to carbonaceous dust. The aliphatic component is of particular interest because it produces a significant 3.4 μm absorption feature. The optical depth of this feature is related to the number and type of aliphatic carbon C–H bonds in the line of sight. It is possible to estimate the column density of aliphatic carbon from quantitative analysis of the 3.4 μm interstellar feature, providing that the absorption coefficient of interstellar aliphatic hydrocarbon is known. We produced interstellar dust analogues with spectra closely matching astronomical observations. Using a combination of FTIR and 13C NMR spectroscopy, we determined an integrated absorption coefficient of the aliphatic component. The results thus obtained permit direct calibration of astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the ISM. [ABSTRACT FROM AUTHOR]

There is considerable uncertainty as to the amount of carbon incorporated in interstellar dust. The aliphatic component of the carbonaceous dust is of particular interest because it produces a significant 3.4 μm absorption feature when viewed against a background radiation source. The optical depth of the 3.4 μm absorption feature is related to the number of aliphatic carbon C–H bonds along the line of sight. It is possible to estimate the column density of carbon locked up in the aliphatic hydrocarbon component of interstellar dust from quantitative analysis of the 3.4 μm interstellar absorption feature provided that the absorption coefficient of aliphatic hydrocarbons incorporated in the interstellar dust is known. We report laboratory analogues of interstellar dust by experimentally mimicking interstellar/circumstellar conditions. The resultant spectra of these dust analogues closely match those from astronomical observations. Measurements of the absorption coefficient of aliphatic hydrocarbons incorporated in the analogues were carried out by a procedure combining FTIR and 13C NMR spectroscopies. The absorption coefficients obtained for both interstellar analogues were found to be in close agreement [4.76(8) × 10−18 cm group−1 and 4.69(14) × 10−18 cm group−1], less than half those obtained in studies using small aliphatic molecules. The results thus obtained permit direct calibration of the astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the interstellar medium. [ABSTRACT FROM AUTHOR]

Spectroscopic investigations of PAHs have been conducted for many years, commencing with solid and solution studies and more recently including gas phase characterisation of a broad range of different species. Through the development of new, efficient methods of production and more sensitive spectroscopic techniques, fresh data are becoming available for not only neutral species, but also for radicals, ions and clusters. [ABSTRACT FROM PUBLISHER]

Context. Recent attention has been directed on closed-shell aromatic cations as potential carriers of the diffuse interstellar bands. The spectra of mass-selected, matrix-isolated benzylium, and tropylium cations were recently reported. The visible spectrum of benzylium exhibits a large Franck-Condon (FC) envelope, inconsistent with diffuse interstellar band carriers. Aims. We perform a computational analysis of the experimentally studied benzylium spectrum before extending the methods to a range of larger, closed-shell aromatic cations to determine the potential for this class of systems as diffuse interstellar band carriers. Methods. Density functional theory (DFT), time-dependant ((TD)DFT), and multi-configurational self-consistent field second-order perturbation theory (MRPT2) methods in concert with multidimensional FC analysis is used to model the benzylium spectrum. These methods are extended to larger closed-shell aromatic hydrocarbon cations derived from resonance-stabilized radicals, which are predicted to show strong S0 → Sn transitions in the visible region. The ionization energies of a range of these systems are also calculated by DFT. Results. The simulated benzylium spectrum was found to yield excellent agreement with the experimental spectrum showing an extended progression in a low frequency (510 cm-1) ring distortion mode. The FC progression was found to be significantly quenched in the larger species: 1-indanylium, 1-naphthylmethylium, and fluorenium. Excitation and ionization energies of the closed-shell cations were found to be consistent with diffuse interstellar band carriers, with the former lying in the visible range and the latter straddling the Lyman limit in the 13-14 eV range. Conclusions. Large closed-shell polycyclic aromatic hydrocarbon cations remain viable candidate carriers of the diffuse interstellar bands. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons are a class of molecules of broad interest that has long been explored by various spectroscopic techniques. The electronic spectroscopy of these species is of particular interest since it provides a framework for the understanding of the electronic structure of large polyatomic molecules. Such studies also allow the systematic investigation of electronic relaxation mechanisms in large molecules. In this review, we focus on the gas-phase experimental work on such systems and present the latest progress. We also underline the challenges that remain to be tackled. A focus on the understanding of the electronic relaxation pathways at work in gas-phase PAHs will also be presented, as well as their possible manifestation in space. [ABSTRACT FROM AUTHOR]

The class of compounds known as the all-benzenoid polycyclic aromatic hydrocarbons (ABPAHs) are treated with time-dependent density functional theory (TDDFT) and a modified Hückel theory to determine how large an ABPAH must be before it is found to absorb strongly in the visible region that corresponds to the bluest of the diffuse interstellar bands (DIBs). The first six ABPAHs are treated with TDDFT using the B3LYP functional to determine their excitation energies. A two-parameter Hückel theory is adopted to deal with those systems too large to be addressed with TDDFT. The transitions are calculated and fitted to experimental data. An ABPAH system with 84 carbons and 14 all-benzenoid rings is found to be the first carrier to absorb strongly in the region where DIBs are observed. [ABSTRACT FROM AUTHOR]

Molecular materials may overcome some of the difficulties for making an intermediate band solar cell by storing energy in long-lived triplet states. Both the problems of fast nonradiative interband relaxation and selective photon absorption can be solved by this approach. A practical implementation of the molecular intermediate band solar cell is considered with a symmetric band alignment, resting on a proven triplet-triplet annihilation process. The limiting power conversion efficiency for this system exceeds that of a single bandgap device over a broad range, peaking at 40.6% for 1.9 eV under 1 sun illumination. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to an article by C. Boehme and J. Lupton related to the research in organic spintronics discussing the spin-based phenomena in organic semiconductors that was published in a previous 2013 issue.

Resonant two-photon threshold ionization spectroscopy is employed to determine the ionizationenergy of C2 to 5 meV precision, about two orders of magnitude more precise than the previously accepted value. Through exploration of the ionization threshold after pumping the 0-3 band of the newly discovered 4³Πg←a³Πu band system of C2, the ionizationenergy of the lowest rovibronic level of the a³Πu state was determined to be 11.791(5) eV. Accounting for spin-orbit and rotational effects, we calculate that the ionizationenergy of the forbidden origin of the a³Πu state is 11.790(5) eV, in excellent agreement with quantum thermochemical calculations which give 11.788(10) eV. The experimentally derived ionizationenergy of X¹Σg+X¹Σg+ state C2 is 11.866(5) eV. [ABSTRACT FROM AUTHOR]

The unimolecular photodissociation dynamics of acetone spanning the entire S 1 ← S 0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S 0 state after internal conversion. A roaming mechanism forming C 2 H 6 + CO is active in the window λ = 311.2-305.8 nm. From λ = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C-C bond cleavage on the lowest excited triplet state, T 1 . At higher energy (λ < 262 nm), an increasing fraction of CH 3 CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH 3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH 3 radical, allowing us to determine the height of the energetic barrier to CH 3 CO decomposition as 68 ± 4 kJ mol -1 , which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S 0 reaction, in this higher energy region of the first electronic absorption band.

The role surface capping molecules play in dictating the optical properties of semiconductor nanocrystals (NCs) is becoming increasingly evident. In this paper the role of surface capping molecule polarity on the optical properties of germanium NCs (Ge NCs) is explored. Capping molecules are split into two groups: nonpolar and polar. The NCs are fully characterized structurally and optically to establish the link between observed optical properties and surface capping molecules. Ge NC optical properties altered by surface capping molecule polarity include emission maximum, emission lifetime, quantum yield, and Stokes shift. For Ge NCs, this work also allows rational tuning of their optical properties through changes to surface capping molecule polarity, leading to improvements in emerging Ge based bioimaging and optoelectronic devices.

The lowest 13 vibrational levels, v = 0-12, of the eΠg3 state of the C 2 molecule have been measured by laser-induced fluorescence of new bands of the Fox-Herzberg system. The newly observed levels, v = 5-12, which span the eΠg3 electronic state up to and beyond the first dissociation threshold of C 2 , were analyzed to afford highly accurate molecular constants, including band origins, and rotational and spin-orbit constants. The spin-orbit coupling constants of the previously published lowest five levels are revised in sign and magnitude, requiring an overhaul of previously published molecular constants. The analysis is supported by high level ab initio calculations. Lifetimes of all observed levels were recorded and found to be in excellent agreement with ab initio predicted values up to v = 11. v = 12 was found to exhibit a much reduced lifetime and fluorescence quantum yield, which is attributed to the onset of predissociation. This brackets the dissociation energy of ground state XΣg+1 C 2 between 6.1803 and 6.2553 eV, in agreement with the Active Thermochemical Tables.

The 3 3 Π g state of the dicarbon molecule, C 2 , has been identified for the first time by a combination of resonant ionization spectroscopy, mass spectrometry, and high-level ab initio quantum chemical calculations. This marks the discovery of the final valence triplet state of C 2 spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 4 3 Π g state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 3 3 Π g state of C 2 is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d 3 Π g state.

Resonant two-photon threshold ionization spectroscopy is employed to determine the ionization energy of C2 to 5 meV precision, about two orders of magnitude more precise than the previously accepted value. Through exploration of the ionization threshold after pumping the 0-3 band of the newly discovered 4(3)Πg ← a(3)Πu band system of C2, the ionization energy of the lowest rovibronic level of the a(3)Πu state was determined to be 11.791(5) eV. Accounting for spin-orbit and rotational effects, we calculate that the ionization energy of the forbidden origin of the a(3)Πu state is 11.790(5) eV, in excellent agreement with quantum thermochemical calculations which give 11.788(10) eV. The experimentally derived ionization energy of X(1)Σg(+) state C2 is 11.866(5) eV.

The laser-induced fluorescence spectrum of 3-vinyl-1H-indene was recorded between 33,000 and 33,800 cm(-1). An origin band was observed at 33,455 cm(-1) along with several low-frequency modes. With the aid of density functional theory and configuration interaction calculations, the electronic transition was assigned as S1 <-- S0 and the short progression in an 80 cm(-1) mode was identified as a vinyl group torsion. Theoretical, spectroscopic, and thermochemical considerations suggest that the 3-vinyl-1H-indene spectrum results from excitation from both conformational isomers with the vinyl and indene double bonds in trans and cis arrangements. The results are discussed in the context of the identification of species arising from the discharge of benzene in argon.

A high-resolution single vibronic level emission study from the A (2)Pi(32) state of the HC(4)S radical is reported. Ground state density functional theory frequencies have been used to assign ground state vibronic levels involving three stretching modes nu(2), nu(3), and nu(5) in the region of 0-3250 cm(-1), while the frequency of nu(4) remains speculative. Tentative assignments are given for the complicated structures arising from Renner-Teller and spin-orbit interactions within the bending energy levels. From analysis of the dispersed emission spectra, Fermi resonances involving pairs of bands have been identified in the A (2)Pi(32)<--X (2)Pi(32) laser induced fluorescence spectrum.

We report on the dynamics of multiphoton excitation and dissociation of NO(2) at wavelengths between 395 and 420 nm and intensities between 4 and 10 TW cm(-2). The breakup of the molecule is monitored by NO A (2)Sigma(+)n(')=1,0-->X (2)Pi(r)n(")=0 fluorescence as a function of time delay between the driving field and a probe field which depletes the emission. It is found that generation of n(')=0 and 1 NO A (2)Sigma(+) results in different fluorescence modulation patterns due to the intense probe field. The dissociation dynamics are interpreted in terms of nuclear motions over light-induced potentials formed by coupling of NO(2) valence and Rydberg states to the applied field. Based on this model, it is argued that the time and intensity dependences of A (2)Sigma(+)n(')=0-->X (2)Pi(r)n(")=0 fluorescence are consistent with delayed generation of NO A (2)Sigma(+)n(')=0 via a light-induced bond-hardening brought about by the transient coupling of the dressed A (2)B(2) and Rydberg 3ssigma (2)Sigma(g) (+) states of the parent molecule. The increasingly prompt decay of A (2)Sigma(+)n(')=1-->X (2)Pi(r)n(")=0 fluorescence with increasing intensity, on the other hand, is consistent with a direct surface crossing between the X (2)A(1) and 3ssigma (2)Sigma(g) (+) dressed states to generate vibrationally excited products., ((c) 2004 American Institute of Physics)

Little has been written about infant botulism in the emergency medicine literature, despite increasing references in the pediatric journals. We describe three cases diagnosed at The Children's Hospital in Denver, Colorado, over an 8-month period. A review of the literature follows, to discuss the clinical manifestations, differential diagnosis, diagnostic workup and treatment of the disease.