Core excitation, specific dissociation, and the effect of the size of aromatic molecules connected to oxygen: phenyl ether and 1,3-diphenoxybenzene.

Near-edge X-ray absorption fine structure (NEXAFS) spectra of phenyl ether at the carbon K-edge and 1,3-diphenoxybenzene at both the carbon and oxygen K-edges were measured in the total ion yield mode using X-rays from a synchrotron and a reflectron time-of-flight mass spectrometer. Time-dependent density functional theory was adopted to calculate the carbon and oxygen K-edge NEXAFS spectra of phenol, phenyl ether, and 1,3-diphenoxybenzene. The assignments and a comparison of the experimental and calculated spectra are presented. The mass spectra of ionic products formed after X-ray absorption at various excitation energies are also reported. Specific dissociations were observed for the 1s → π* transition of phenyl ether. In comparison with phenol and phenyl ether, the dependence of the fragmentation on the excitation site and destination state was weak in 1,3-diphenoxybenzene, likely as a result of delocalization of the valence electrons and rapid randomization of energy.

Photodissociation of S atom containing amino acid chromophores.

Photodissociation of 3-(methylthio)propylamine and cysteamine, the chromophores of S atom containing amino acid methionine and cysteine, respectively, was studied separately in a molecular beam at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed for 3-(methylthio)propylamine, including (1) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)SCH(2)CH(2)CH(2)NH+H, (2) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)+SCH(2)CH(2)CH(2)NH(2), (3) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)S+CH(2)CH(2)CH(2)NH(2), and (4) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)SCH(2)+CH(2)CH(2)NH(2). Two dissociation channels were observed from cysteamine, including (5) HSCH(2)CH(2)NH(2)-->HS+CH(2)CH(2)NH(2) and (6) HSCH(2)CH(2)NH(2)-->HSCH(2)+CH(2)NH(2). The photofragment translational energy distributions suggest that reaction (1) and parts of the reactions (2), (3), (5) occur on the repulsive excited states. However, reaction (4), (6) occur only after the internal conversion to the electronic ground state. Since the dissociation from an excited state with a repulsive potential energy surface is very fast, it would not be quenched completely even in the condensed phase. Our results indicate that reactions following dissociation may play an important role in the UV photochemistry of S atom containing amino acid chromophores in the condensed phase. A comparison with the potential energy surface from ab initio calculations and branching ratios from RRKM calculations was made.

Carbon-carbon bond cleavage in the photoionization of ethanol and 1-propanol clusters.

Tunable VUV laser was used to initiate the ion-molecule reactions in the clusters of ethanol and 1-propanol by photoionization in the region between 10.49 to 10.08 eV. Ionic products were detected by the time-of-flight mass spectrometer. In addition to the protonated clusters from proton transfer reactions, the products corresponding to beta carbon-carbon bond cleavage were found to be one of the major products for small sizes of clusters. A comparison with photoionization of methanol clusters and the results of ab initio calculation has been made.

Dynamics of photodissociation of ethylene and its isotopomers at 157 nm: branching ratios and kinetic-energy distributions.

We investigated the photodissociation of ethylene and its isotopomers at 157 nm in a molecular-beam apparatus using photofragment translational spectroscopy combined with synchrotron-based photoionization. The time-of-flight (TOF) spectra of all photofragments H, H(2), C(2)H(2), C(2)H(3), and their deuterium isotopic variants were recorded, from which kinetic-energy distributions P(E(t)) and branching ratios were obtained. Most C(2)H(3) spontaneously dissociates to C(2)H(2)+H and only C(2)H(3) with small internal energy survives. The C(2)H(2) fragment due to H(2) elimination is observed leading the C(2)H(2) fragment due to 2H elimination in TOF distribution because the former process has more kinetic-energy release. An analogous result is observed for C(2)D(4) photolysis. That elimination of molecular hydrogen is site-specific and is revealed from photolysis of three dideuterated ethylene isotopomers, in which an isotopic effect plays a significant role. Observations of C(2)D(2)+2H and C(2)H(2)+2D product channels in the photolysis of 1,1-CH(2)CD(2) provide evidence for migrations of H and D atoms. A comparison with previous experimental and theoretical results is made.