D. P. Hydutsky

The ultrafast dynamics of HBr–water clusters: Influences on ion-pair formation

The ultrafast dynamics of HBr–water clusters have been investigated using pump–probe spectroscopy coupled with reflectron time-of-flight mass spectrometry. HBr clusters, mixed HBr–water clusters, and protonated water clusters are observed in the mass spectra. Dynamic studies reveal that when an HBr chromophore of a cluster with less than five solvent molecules is excited electronically, solvent reorganization occurs to form the solvent separated ion-pair [S. M. Hurley et al., Science 298, 202 (2002)]. The present paper focuses on the influence of clustering on the dynamics of the C and D states of HBr. In addition, further evidence is presented which confirms that complete dissolution of HBr requires five solvent molecules in the isolated species found in complexes comprised of pure water or HBr/H2O mixtures.

Photoinduced ion-pair formation in the (HI)m(H2O)n cluster system

The temporal behavior of the photoinduced ion-pair formation process in the (HI)m(H2O)n (n=1-6 for m=1 and n=1-4 for m=2) cluster system has been studied via the coupling between the g 3Sigma- Rydberg and V 1Sigma+ valence states. Comparison of the time constants obtained to those measured in previous experiments for the analogous process in HBr-water clusters, along with a detailed analysis of the signal intensity as a function of laser-pulse power, provides new insight into and confirmation of the previously proposed ion-pair formation mechanism.

- Photodissociation of SO 2 at 200 nm: Evidence of an anisotropy effect

This chapter deals with the ultrafast dynamics of SO 2 , SO 2 clusters, and SO 2 clustered with water. The experiments investigate both bound and dissociative processes in the monomer and cluster species. Currently, the interest lies in sulfur isotope effects in the photodissociation of SO 2 excited by 200 nm pulses to the C/D state. The scientific community is interested in isotope effects in heavy atoms for fundamental reasons as well as for the information inferred about the earths early atmosphere. A preliminary investigation of SO 2 yields evidence for anisotropy effects. The polarization of the pump and probe beam are found to have a profound effect on the ability to observe the neutral state dynamics of SO fragments that result from the photodissociation of SO 2 with 200 nm light. The evidence for these observations as well as plausible explanations of the results is presented.

Chapter 3 - Cluster size effects on bound excited states of SO 2

Sulphuric acid resulting from the oxidation of atmospheric SO 2 is a major constituent of acid rain. The oxidation process is influenced by factors such as clouds and aerosol particles, light intensity, O 3 , and peroxides. Thus, the nature of the factors leading to the alteration of atmospheric SO 2 chemistry may be clarified by experimentally studying the excited state dynamics of SO 2 solvated in SO 2 clusters. It appears that the size of a particle containing SO 2 may alter its reactivity since the dynamics of (SO 2 )n clusters show a substantial dependence on n. The findings of the experiments presented in this chapter indicate that the time required for the transition from the initially excited 1 B 1 state to the double wells that result from the crossing of the 1 A 2 and 1 B 1 states is significantly greater than is found from a theoretical treatment of the transition. The study of the SO 2 F band provides experimental evidence that the SO 2 molecule remains in a bound excited state for a substantial time indicating that coupling to dissociative sates is not operative in this region of the potential energy surface. Thus, the experiments presented in this chapter provide previously unknown information about the properties of the little studied F band of SO 2 .