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Biomedical subjects

A W Castleman

Publications and source records attributed to A W Castleman.

26 records · Page 2Linked to original sources
PubMed2004

Appearance of bulk properties in small tungsten oxide clusters.

Contrary to the conventional understanding that atomic clusters usually differ in properties and structure from the bulk constituents of which they are comprised, we show that even a dimer of tungsten oxide (WO(3))(2) possesses bulklike features and the geometry of a small cluster containing only 4 tungsten and 12 oxygen atoms bears the hallmarks of crystalline tungsten oxide, WO(3). This observation, based on a synergistic approach involving mass distributions under quasisteady state conditions, photoelectron spectroscopy, and first principles molecular orbital theory, not only illustrates the existence of a class of strongly covalent or ionic materials whose embryonic forms are tiny clusters but also lends the possibility that a fundamental understanding of complex processes such as catalytic reactions on surfaces may be achieved on an atomic scale with clusters as model systems.

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PubMed2004

The influence of cluster formation on the photodissociation of sulfur dioxide: excitation to the E state.

A femtosecond pump-probe technique was employed to study the dissociation dynamics of sulfur dioxide and sulfur dioxide clusters in real time. Dissociation is initiated by a multiphoton scheme that populates the E state. The SO(2) (+) transient is fit to a biexponential decay comprising a fast and a slow component of 230 fs and 8 ps, respectively. The SO(+) transient consists of a growth component of 225 fs as well as a subsequent decay of 373 fs. The pump-probe response obtained from the monomer clearly shows the predissociative cleavage of a S-O bond. Upon cluster formation, a sequential increase in the fast decay component is observed for increasing cluster size, extending to 435 fs for (SO(2))(4) (+). The transient response of cluster dissociation products SO(SO(2))(n) (+), where n=1-3, reflects no growth component indicating that formation proceeds through the ion state. Therefore, cluster formation results in a caging effect, which impedes the dissociation process. Further direct evidence for our proposed mechanism is obtained by a technique that employs a comparison of the amplitude coefficients of each respective component of the fit. This method makes possible the determination of branching ratios of competing relaxation processes and thereby the influence of cluster formation on each can be resolved. The caging effect is attributed to a steric hindrance placed on the SO(2) chromophore, preventing it from attaining a linear geometry necessary for dissociation.

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PubMed2002

Dynamics of hydrogen bromide dissolution in the ground and excited states.

The dissolution of acids is one of the most fundamental solvation processes, and an important issue is the nature of the hydration complex resulting in ion pair formation. We used femtosecond pump-probe spectroscopy to show that five water molecules are necessary for complete dissolution of a hydrogen bromide molecule to form the contact ion pair H+.Br-(H2O)n in the electronic ground state. In smaller mixed clusters (n < 5), the ion pair formation can be photoinduced by electronic excitation.

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PubMed1999

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer.

Presented here are femtosecond pump-probe studies on the water-solvated 7-azaindole dimer, a model DNA base pair. In particular, studies are presented that further elucidate the nature of the reactive and nonreactive dimers and also provide new insights establishing that the excited state double-proton transfer in the dimer occurs in a stepwise rather than a concerted manner. A major question addressed is whether the incorporation of a water molecule with the dimer results in the formation of species that are unable to undergo excited state double-proton transfer, as suggested by a recent study reported in the literature [Nakajima, A., Hirano, M., Hasumi, R., Kaya, K., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Phys. Chem. 101, 392-398]. In contrast to this earlier work, our present findings reveal that both reactive and nonreactive dimers can coexist in the molecular beam under the same experimental conditions and definitively show that the clustering of water does not induce the formation of the nonreactive dimer. Rather, when present with a species already determined to be a nonreactive dimer, the addition of water can actually facilitate the occurrence of the proton transfer reaction. Furthermore, on attaining a critical hydration number, the data for the nonreactive dimer suggest a solvation-induced conformational structure change leading to proton transfer on the photoexcited half of the 7-azaindole dimer.

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