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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]

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]

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]

The final state distribution of carbon monoxide produced in the photodissociation of the formyl (HCO) radical has been studied both experimentally and theoretically. Renner–Teller coupling between the excited HCO A state and the ground state leads to dissociation and yields H and CO. Vibrational and rotational distributions have been measured for CO produced after excitation to specific vibrational levels on the A←X transition of HCO cooled in a supersonic expansion. The strongest transitions are for excitation to vibrational states with six to 16 quanta in the bending mode, and dissociation from these states produces inverted CO rotational distributions with average rotational quantum numbers = in the 22–33 range. The value of = increases monotonically with the vibrational quantum number describing the bend of the excited triatomic. Experiments involving excitation of one quantum of the C–H stretching motion have revealed that this vibration results in increased rotational excitation of the product CO with values of = as high as 41. In contrast, experiments indicate that the C–O stretching mode of HCO acts nearly as a spectator during the dissociation process. Excitation of HCO states with one quantum of C–O stretch yields vibrationally excited CO as the dominant dissociation product, but with a rotational distribution similar to that for CO(ν=0) produced following the excitation of HCO states without the quantum of C–O stretch. Classical trajectory calculations on an ab initio potential energy surface have modeled many of the experimental features and trends of the CO product distributions. There are, however, some discrepancies in the positions of rotational maxima and in the efficiency of the coupling of the C–O vibration of HCO to the dissociation coordinate. It is not clear whether these are due to approximations made in the modeling or inaccuracies in the potential energy surface. [ABSTRACT FROM AUTHOR]

The vibrational and rotational distributions of CO(1Σ+g) produced in the 157 nm photodissociation of CO2 have been determined by measuring vacuum-ultraviolet laser-induced fluorescence spectra of the CO photoproduct. The photodissociation of CO2 is known to occur via two pathways; one yielding O(1D) and the other yielding O(3P). Spin conservation and previous experimental studies confirm that dissociation via the O(1D) channel is the dominant process. The available energy for this channel is sufficient to populate only the ground and first excited vibrational levels of CO. We measured the rotational distributions for CO in v=0 and v=1 and found them to be nonBoltzmann. In fact, a highly structured distribution with distinct peaks at J=10, 24, 32, and 39 is observed for CO in v=0. A less structured population is displayed by molecules in v=1. The relative vibrational population (v=0/v=1) was determined to be 3.7±1.2. Doppler spectra of individual rovibronic transitions were also recorded. The profiles have widths in accord with the available translational energy, display the expected v⊥J correlation, and are best described by an isotropic distribution of the velocity vectors with respect to the polarization direction of the dissociation light. [ABSTRACT FROM AUTHOR]

The dissociation of carbon suboxide by single photon absorption at 157.6 nm has been studied under the collisionless environment of a molecular beam. The primary products are 2CO+C [3P(97%) or 1D(3%) ]. The spin–orbit levels of the 3P carbon are statistically distributed. The CO rotational populations in the first four vibrational levels are found to be well described by Boltzmann distributions with temperatures 3430, 4120, 4670, and 2340 K for υ=0,1,2,3, respectively. A second low temperature component in the υ=0 rotational distribution is attributed to CO produced in coincidence with C(1D). Significant population is found in the first four vibrational levels with less than 3% estimated in the higher levels; a vibrational temperature of 3700 K fits the distribution. Analysis of the Doppler profiles of the CO and carbon suggest that the dissociation is stepwise; the first dissociation appears to be described by an anisotropy parameter near β=2, while the second appears to be isotropic. The mean CO fragment speeds were nearly constant for all rotational levels, though slightly faster for υ=1 than υ=0. From the translational energetics of the CO at least a small amount of stable C2O is inferred to exist. The overall energetics place the stable C2O quantum yield under 2% assuming that excited C2O is not radiatively stabilized. We were unable to detect C2O directly in any electronic state. The dissociation of C3O2 into C(3P)+2CO appears to be best described as a stepwise reaction that produces a nearly statistical partitioning into all fragment degrees of freedom. The best agreement is obtained for an intermediate C2O electronic state in the vicinity of the b state (e.g., b, a or A); a ground state C2O intermediate is unlikely. The singlet to triplet crossing most likely occurs in the C2O system on a time scale longer than a rotation (a few picoseconds). [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]

Presents a study that investigated an analytical model for concentration profiles of reactive gas-phase species adjacent to surfaces, determined by optical probe techniques. Measurements of carbon-fluoride detected by laser-induced fluorescence; Conditions under which the model is applicable; Interpretation of the measurements.

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]

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]