The dynamics of CO production from the photolysis of acetone across the whole S1 ← S0 absorption spectrum: Roaming and triple fragmentation pathways.

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.

Role of Surface Capping Molecule Polarity on the Optical Properties of Solution Synthesized Germanium Nanocrystals.

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.

First observation of the 3Πg3 state of C2: Born-Oppenheimer breakdown.

The 33Πg 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 43Πg state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 33Πg state of C2 is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d3Πg state.

The eΠg3 state of C2: A pathway to dissociation.

The lowest 13 vibrational levels, v = 0-12, of the eΠg3 state of the C2 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 C2, 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 C2 between 6.1803 and 6.2553 eV, in agreement with the Active Thermochemical Tables.

The ionization energy of C2.

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.

Infant botulism: a case series and review of the literature.

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.

Laser-induced fluorescence spectrum of 3-vinyl-1H-indene.

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.

Experimental and theoretical investigation of the dispersed fluorescence spectroscopy of HC4S.

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.

Intense-field modulation of NO2 multiphoton dissociation dynamics.

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.