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New data are presented on the resonant Auger decay of iodobenzene (C6H5I) in the region of the I 4d−1 ionization threshold. The excited molecules decay by participator and spectator processes to populate single-hole valence states and two-hole, one-particle excited states of the cation, providing new information on the structure of C6H5I+. Excitation of dissociative C6H5I (I 4d5/2,3/2−1)σ* resonances can, in principle, result in ultrafast dissociation to C6H5 + I** and the subsequent autoionization of I**, but no evidence for this process is observed. The results are compared with our recent study of the resonant Auger decay of methyl iodide (CH3I). [ABSTRACT FROM AUTHOR]

Resonant Auger processes provide a unique perspective on electronic interactions and excited vibrational and electronic states of molecular ions. Here, new data are presented on the resonant Auger decay of excited CH3I in the region just below the I 4d−1 ionization threshold. The resonances include the Rydberg series converging to the five spin–orbit and ligand-field split CH3I (I 4d−1) thresholds, as well as resonances corresponding to excitation from the I 4d5/2,3/2 orbitals into the σ* lowest unoccupied molecular orbital. This study focuses on participator decay that populates the lowest lying states of CH3I+, in particular, the X ̃ 2E3/2 and 2E1/2 states, and on spectator decay that populates the lowest-lying (CH3I2+)σ* states of CH3I+. The CH3I (I 4d−1)σ* resonances are broad, and dissociation to CH3 + I competes with the autoionization of the core-excited states. Auger decay as the molecule dissociates produces a photoelectron spectrum with a long progression (up to v3+ ∼ 25) in the C–I stretching mode of the X ̃ 2E3/2 and 2E1/2 states, providing insights into the shape of the dissociative core-excited surface. The observed spectator decay processes indicate that CH3I+ is formed on the repulsive wall of the lower-lying (CH3I2+)σ* potentials, and the photon-energy dependence of the processes provides insights into the relative slopes of the (4d−1)σ* and (CH3I2+)σ* potential surfaces. Data are also presented for the spectator decay of higher lying CH3I (I 4d−1)nl Rydberg resonances. Photoelectron angular distributions for the resonant Auger processes provide additional information that helps distinguish these processes from the direct ionization signal. [ABSTRACT FROM AUTHOR]

Electronic spectra are measured for mass-selected C 2 n + (n = 6–14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N2 molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C 2 n + (n = 6–14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C 22 + and larger clusters. The C 2 n + electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C 12 + , C 16 + , C 20 + , C 24 + , and C 28 + are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C 14 + , C 18 + , C 22 + , and C 26 + are narrower, consistent with slower non-radiative deactivation. [ABSTRACT FROM AUTHOR]

Fluorinated species have a pivotal role in semiconductor material chemistry and some of them have been detected beyond the Earth's atmosphere. Achieving good energy accuracy on fluorinated species using quantum chemical calculations has long been a challenge. In addition, obtaining direct experimental thermochemical quantities has also proved difficult. Here, we report the threshold photoelectron and photoion yield spectra of SiF and CF radicals generated with a fluorine reactor. The spectra were analysed with the support of ab initio calculations, resulting in new experimental values for the adiabatic ionisation energies of both CF (9.128 ± 0.006 eV) and SiF (7.379 ± 0.009 eV). Using these values, the underlying thermochemical network of Active Thermochemical Tables was updated, providing further refined enthalpies of formation and dissociation energies of CF, SiF, and their cationic counterparts. [ABSTRACT FROM AUTHOR]

N -heterocycles are suspected to play an important role in the chemical origin of life. Despite their detection in meteorites and in Titan's atmosphere, their extra-terrestrial chemical formation networks remain elusive. Furthermore N -heterocyclics are undetected in the interstellar medium. This paper assesses the photostability of protonated N -hetero(poly)acenes after ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. It provides information on their ability to retain the N atom into the cycle to generate larger N -containing species or functionalized N -heterocyles. Protonated N -hetero(poly)acenes were generated using electrospray ionization and injected into a linear ion trap where they were irradiated by radiation of 4.5 to 10 eV using the DESIRS beamline at the synchrotron SOLEIL. The photodissociation action spectra of protonated pyridine, quinoline, isoquinoline, and acridine were measured by recording the photofragment yields as a function of photon energy. The four systems exhibit dissociation channels associated with H2 and HCN/HNC loss but with different branching ratios. The results indicate that increasing the size of the N -hetero(poly)acenes increases the chance of retaining the N atom in the larger fragment ion after photodissociation but it remains that all the protonated N -hetero(poly)acenes studied lose their N atom at part of a small neutral photofragment, with high propensity. Therefore, protonated N -hetero(poly)acenes in interstellar space are unlikely precursors to form larger N -containing species. However, protonated pyridine, quinoline, isoquinoline, and acridine are most likely to retain their N atoms in planetary atmospheres where UV radiation at the planet's surface is typically restricted to wavelengths greater than 200 nm – suggesting such environments are possible substrates for prebiotic chemistry. [ABSTRACT FROM AUTHOR]

Polyacetylene cations (HC2 n H+) play important roles in combustion processes and in the chemistry of planetary atmospheres and interstellar clouds. Here we report the electronic spectrum for the triacetylene cation (HC6H+) recorded over the 300–610 nm range by photodissociating mass-selected ions tagged with argon atoms in a tandem mass spectrometer. The spectrum shows three band systems that are assigned to CH18508_IE1.gif (origin transition 16 665 cm−1), CH18508_IE2.gif (origin transition 23 916 cm−1), and CH18508_IE3.gif (origin transition 29 920 cm−1). Although the CH18508_IE4.gif band system is well known, the CH18508_IE5.gif and CH18508_IE6.gif band systems are observed for the first time in the gas phase. In addition, the CH18508_IE7.gif electronic spectrum of the protonated triacteylene cation tagged with an argon atom (HC6 CH18508_IE8.gif -Ar) is reported, providing the first gas-phase spectrum for this species. Polyacetylene cations play an important role in combustion and in the chemistry of planetary atmospheres and interstellar clouds, where they have been suspected as carriers of the diffuse interstellar bands. Here we present gas-phase electronic spectra of the triacetylene cation and protonated triacetylene by photodissociation of mass-selected, argon-tagged ions in a tandem mass spectrometer. [ABSTRACT FROM AUTHOR]