Core-to-Rydberg band shift and broadening of hydrogen bonded ammonia clusters studied with nitrogen K-edge excitation spectroscopy.

Nitrogen 1s (N 1s) core-to-Rydberg excitation spectra of hydrogen-bonded clusters of ammonia (AM) have been studied in the small cluster regime of beam conditions with time-of-flight (TOF) fragment-mass spectroscopy. By monitoring partial-ion-yield spectra of cluster-origin products, "cluster" specific excitation spectra could be recorded. Comparison of the "cluster" band with "monomer" band revealed that the first resonance bands of clusters corresponding to N 1s → 3sa(1)/3pe of AM monomer are considerably broadened. The changes of the experimental core-to-Rydberg transitions ΔFWHM (N 1s → 3sa(1)/3pe) = ~0.20/~0.50 eV compare well with the x ray absorption spectra of the clusters generated by using density functional theory (DFT) calculation. The broadening of the core-to-Rydberg bands in small clusters is interpreted as being primarily due to the splitting of non-equivalent core-hole N 1s states caused by both electrostatic core-hole and hydrogen-bonding (H(3)N···H-NH(2)) interactions upon dimerization. Under Cs dimer configuration, core-electron binding energy of H-N (H-donor) is significantly decreased by the intermolecular core-hole interaction and causes notable redshifts of core-excitation energies, whereas that of lone-pair nitrogen (H-acceptor) is slightly increased and results in appreciable blueshifts in the core-excitation bands. The result of the hydrogen-bonding interaction strongly appears in the n-σ* orbital correlation, destabilizing H-N donor Rydberg states in the direction opposite to the core-hole interaction, when excited N atom with H-N donor configuration strongly possesses the Rydberg component of anti-bonding σ* (N-H) character. Contributions of other cyclic H-bonded clusters (AM)(n) with n ≥ 3 to the spectral changes of the N 1s → 3sa(1)/3pe bands are also examined.

A study to control chemical reactions using Si:2p core ionization: site-specific fragmentation.

In an aim to create a "sharp" molecular knife, we have studied site-specific fragmentation caused by Si:2p core photoionization of bridged trihalosilyltrimethylsilyl molecules in the vapor phase. Highly site-specific bond dissociation has been found to occur around the core-ionized Si site in some of the molecules studied. The site specificity in fragmentation and the 2p binding energy difference between the two Si sites depend in similar ways on the intersite bridge and the electronegativities of the included halogen atoms. The present experimental and computational results show that for efficient "cutting" the following conditions for the two atomic sites to be separated by the knife should be satisfied. First, the sites should be located far from each other and connected by a chain of saturated bonds so that intersite electron migration can be reduced. Second, the chemical environments of the atomic sites should be as different as possible.

Specific fragmentation of the K-shell excited/ionized pyridine derivatives studied by electron impact: 2-, 3- and 4-methylpyridine.

Fragmentation of the pyridine ring upon K-shell excitation/ionization has been studied with gaseous 2-, 3- and 4-methylpyridine by the electron-impact method. Ab initio molecular orbital (MO) calculations were also carried out to explore electronic states correlating with specific fragments. Some specific fragmentation channels were identified from the ionic fragments enhanced characteristically at the N 1s edge. Yields of the C(2)HN(+) and C(5)H(5)(+)/C(5)H(6)(+) ions show that the fission of the N-C2 and C4-C5/C5-C6 bonds of the ring is likely to occur after the N 1s excitation and ionization. Ab initio MO calculations for the 2-methylpyridine molecule indicate that the dissociation channels to produce these ions are only accessible through the excited states of the parent molecular dication, which can be formed by Auger decays after the N 1s ionization. Fragment ions via hydrogen rearrangement are produced as well, but the rearrangement is not a phenomenon specific to the K-shell excitation/ionization.

Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge.

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH.H+ and H3O+ cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s(C[double bond]O/OH) electrons into pi(CO)* orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s(C[double bond]O)-->pi(CO)* transition by approximately 0.2 eV and a redshift of the O1s(OH)-->pi(CO)* by approximately 0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer andor trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO+ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD2O+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD.D+ (protonated HCOOH.H+) cations within the clusters.

Theoretical study of ion desorption from poly-(methyl methacrylate) and poly-(isopropenyl acetate) thin films through core excitation.

Site-specific chemical reactions following core excitation of poly-(methyl methacrylate) (PMMA) and poly-(isopropenyl acetate) (PiPAc) thin films were investigated. New x-ray absorption spectra of PMMA and PiPAc at the C and O K edges and theoretical spectra within the framework of density functional theory using model molecules were reported, and some new peak assignments were proposed for these spectra. Core-hole excited state molecular dynamics simulations were performed to discuss dissociation dynamics for the target systems, and some specific reaction mechanisms were discussed and explained theoretically; for example, the amount of CH3 ion fragments for PMMA was enhanced at the C and O K edges through the existence of the repulsive sigma*(O-CH3) excited state.

Unimolecular decomposition of formic acid in the gas phase--on the ratio of the competing reaction channels.

The thermal decomposition of formic acid was reinvestigated in the gas phase using two types of shock tubes. It was confirmed that the unimolecular decomposition proceeds through a main channel of dehydration (k1) and a minor decarboxylation channel (k2). This result is in good agreement with our previous study (J. Chem. Phys. 1984, 80, 4989). Furthermore, it was confirmed that the dehydration process is in the second-order region and that the decarboxylation is in the falloff region, in the temperature range of 1300-2000 K and over the total density of (0.5-2.5) x 10(-5) mol cm(-3). The experimental ratios between the two channels, k2/k1, are compared with those of theoretical calculations by conventional transition state theory and the Rice-Ramsperger-Kassel-Marcus theory.