The C-H stretching fundamental of the free radical HCCN has been investigated under high resolution using infrared kinetic spectroscopy. This transient molecule was produced by the flash photolysis of dibromoacetonitrile (Br$\sb2$HCCN) at 193 nm and its transient infrared absorption spectrum probed using a color center laser. Spectra of the molecule were collected between 3182 and 3274 cm$\sp{-1}$. The rotational analysis of the $\nu\sb1$ fundamental places its origin at 3246.657 cm$\sp{-1}$. Four hot bands associated with the bending vibrations have been observed; the vibrational and rotational assignment of three of these bands $\nu\sb1 + \nu\sb5 - \nu\sb5$, $\nu\sb1 + \nu\sb4 - \nu\sb4$, and $\nu\sb1 + 2\nu\sb5\sp{\pm 2} - 2\nu\sb5\sp{\pm 2}$ is definite. The $\nu\sb5$ energy was obtained by measuring the intensity of a line of the $\nu\sb1 + \nu\sb5 - \nu\sb5$ band relative to one of the fundamental and calculating the energy assuming a room temperature Boltzmann distribution. A comparison of this energy with that of quasilinear fulminic acid, HCNO, and with theoretical calculations suggests a similar floppy HCX bending potential.

Ab initio SCF Hartree-Fock calculations have been carried out on all the fullerene isomers of C$\sb{20}$ to C$\sb{36}.\ C\sb{20},\ C\sb{24},$ and C$\sb{26}$ have only one fullerene isomer each, of $C\sb{2h},\ D\sb6$ and $D\sb{3h}$ symmetry respectively. C$\sb{28}$ has two distinct fullerene isomers, C$\sb{30}$ has three, C$\sb{32}$ and C$\sb{34}$ have six, and C$\sb{36}$ has fifteen. Their lowest energy structures were found to be of $T\sb{d}\ C\sb{2v},\ D\sb3,\ C\sb2,$ and $D\sb{2d}$ symmetry respectively. All ground-state isomers have closed-shell electronic configurations except C$\sb{26}$-$D\sb{3h}$ (open-shell $\rm\sp5A\sp\prime\sb1)$ and C$\sb{28}$-$T\sb {d}$ (open-shell $\sp5{\rm A}\sb2).$ A new mechanism, called "peeling", is proposed in order to explain the end of the C$\sb2$ loss fragmentation pattern at C$\sb{32}$ observed in photodissociation studies. It consists of opening the fullerene surface and excising long carbon chains. MNDO calculations show the "peeling" channel to be more competitive than the C$\sb2$ loss fragmentation process for C$\sb{32}.$

The photophysics of higher fullerenes remain poorly characterized, partly because they may exist in many isomeric structures that are difficult to isolate. This project concerns the properties of C84 isomers. Using recycling HPLC methods, we separated a sample of purified C84 into three fractions. The first contains a mixture of the three most abundant isomers---D 2 (IV), D2d (II), and Cs (a). The second and third peaks were found to contain the C2 and Cs (b) isomers, respectively. A kinetics-based method was used to find the effective molar absorptivities of these fractions. Sensitive transient absorption measurements following optical excitation reveal widely differing triplet state lifetimes among these isomers. Quenching studies indicate that one C84 isomer has a triplet state energy below that of oxygen's 1Delta g state, while the others have higher energies. Further results, including fluorescence spectra and variable-temperature triplet decay kinetics, will also be presented.

Even when using parametrized semiempirical methods, quantum chemical calculations on molecules containing more than a few hundred atoms become prohibitively expensive due to O (N3) time and memory costs where N is the number of atoms. I implemented methods to allow the CPU time cost of semiempirical methods to scale linearly with system size enabling semiempirical calculations on large biological systems such as proteins and nucleic acids. The cost of forming the initial guess density matrix was reduced by replacing the O (N3) diagonalization of the Huckel Hamiltonian with an approach which uses localized molecular orbitals based on the Lewis dot structure to build the density matrix. The Fock matrix build was reduced from O (N2) to linear scaling in CPU time using atom-atom distance cutoffs. The diagonalization step was replaced by several linear scaling methods described in the literature: conjugate gradient density matrix search (CGDMS), purification of the density matrix (PDM), pseudodiagonalization (PD), and the Chebyshev expansion method (CEM). While in my semiempirical implementation all of these methods demonstrated linear scaling, CGDMS, PDM and PD required about the same amount of CPU time for calculations on water clusters and polyglycine chains but CEM was found to be about three times as expensive as the other methods. However, CGDMS stands out among the other methods by having the added property of enhancing self-consistent field (SCF) convergence in cases where diagonalization has convergence difficulties. Finally, to demonstrate the effectiveness of the linear scaling semiempirical method on a realistic system we performed the first-ever semiempirical geometry optimization using PM3 implemented with CGDMS on a 1226 atom kringle 1 of plasminogen.

The nature of metal and semi-conductor bonding has been investigated via UV photoelectron spectroscopy of cluster anions. Electrons from negative cluster ions of the desired mass are photodetached and then collected with the aid of judiciously arranged magnetic fields. Pulsed operation allows the use of time of flight techniques on both the parent ions and the photoelectrons. Photodetaching with UV grants access to the details of valence electronic structure. Our experiments show marked variations in electronic structure as a function of cluster size and composition. Cu cluster UPS show an even-odd alternation in electron affinities, predicted by cluster shell models. The UPS of sixty atom C Clusters corroborates the proposed soccerball structure for some C$\sb{60}$. Semi-conductor cluster UPS indicate shell closings for certain cluster sizes.

A new technique for obtaining continuously scanned Raman excitation profiles (REP's) is described. The technique relies on simultaneous stepping of the laser wavelength (excitation frequency), the angle of a frequency-doubling crystal and spectrometer grating angle under control of a single computer. The method is applied to the REP's of four vibrational bands of iodobenzene ($\rm 265 cm\sp{-1}, 530 cm\sp{-1}, 1065 cm\sp{-1}$ and 1575 cm$\sp{-1}$) excited in the B-continuum (216-233 nm). The results are interpreted in terms of a dynamical model of the ground and excited $\rm\tilde{B}\ \sp{1}A\sb1$ electronic states.

The rate and extent of intramolecular vibrational redistribution (IVR) following optical excitation in an isolated naphthalene molecule has been measured using the technique of supersonic free-jet spectroscopy. Naphthalene, exicted to its first (('1)B(,3u)) or second (('1)B(,2u)) excited singlet state is found to undergo a rapid vibrational redistribution which is quite global in nature. The rate of the IVR process is measured by observation of the fluorescence excitation linewidths as a function of vibrational energy (E(,v)). A smooth, monotonic increase in the vibronic linewidths as vibrational energy is increased is observed, corresponding to redistribution rates ranging from 9 x 10('10) sec('-1) at E(,v) = 3068 cm('-1) to 7 x 10('11) sec('-1) at E(,v) = 5200 cm('-1). The extent of the redistribution is monitored via single vibronic level fluorescence spectra of naphthalene taken in a free-jet. At low vibrational excitation the fluorescence spectra display sharp, well resolved features, indicating that little redistribution has occurred on the time-scale of the fluorescence (nsec). However, at energies above E(,v) = 2122 cm('-1) the spectra become diffuse. Computor modeling of these spectra shows that many zero-order levels must be involved in the redistribution process, at least all of the quasi-resonant levels of the same symmetry as the originally pumped level. Therefore, even at moderate vibrational energies, an isolated molecule the size of naphthalene undergoes a rapid flow of vibrational energy among much of the energetically available vibrational phase space, following optical excitation.

The uses of resonance enhanced two-photon ionization (R2PI) for studying unimolecular energy redistribution of molecules in a supersonic molecular beam is explored. The R2PI process relies on initially exciting a molecule to a state that is stable with respect to the up-pumping rate of an ionizing laser field. For such situations, the ion signal produced is proportional to the population of the intermediate state. It can therefore be used in place of the observation of emitted light for measuring the population of an excited state. This is particularly useful for states that do not radiate. The pulsed valve used to create the molecular beam is described in detail. Several experiments, undertaken to characterize the types of information that might be obtained, are described. From these it was determined that the R2PI technique has a detection efficiency of almost 100%. In addition, at laser fluences less than those required to saturate the first, resonant step, only the parent ion is observed. The technique has been shown to be useful for recording electronic absorption spectra with 1 cm('-1) precision, both for molecules with long singlet lifetimes (longer than the 4 nsec laser pulse width) and for molecules with very short singlet lifetimes (up to three orders of magnitude shorter than the laser pulse width). The ionization threshold for a large polyatomic molecule, naphthalene, has been determined to a precision greater than previously reported. A two-laser experiment is also described in which the decay of unperturbed singlet and unrelaxed triplet states are observed. This experiment can easily be extended to provide sufficient information, with the addition of a single independently determined quantity, to completely characterize the intramolecular energy redistribution in molecules.

Time-of-flight detected resonant two photon ionization (R2PI) in supersonic beams is investigated. The advantages of the method are the high sensitivity, spectral selectivity and the lack of extensive fragmentation in the mass spectra. To obtain these properties the molecular system must meet several restrictions. Stable intermediate electronic states, which are greater than half of the adiabatic ionization energy generally are detected with these advantages, with a single photon field supplying the energy to populate the intermediate state and to form the photoions. Modulation of the ionization efficiency is observed when a rapid decay channel is available to the intermediate state, which is no longer coupled to the photon field. These properties are illustrated by the behavior of aniline, bromobenzene, and metal carbonyl systems. Two-color ionization removes many of the restrictions inherent to one-color R2PI. High efficiency is maintained for a broader class of molecules by allowing the ionization photon to provide the energy spanned by the intermediate state and the ionization continuum levels. Experiments which scan the ionization laser energy, provide information regarding the nature of the ionization step in benzene and napthalene. The predominantly step-like direct ionization structure observed, indicates that this is the dominant ionization process. Thus the relevant selection rules governing the propensity of an intermediate state to ion transitions are largely of a vibrational nature. The two-color ionization method has been specifically applied to studies of collision-free vibrationally excited triplet state levels formed in the molecular beam. By varying the time delay between the pump and ionization lasers, the decay of excited state populations can be monitored. This time evolution data has been unavailable through conventional methods due to the experimental difficulties imposed by the long-lived, non-fluorescent nature of these states. As the decay rate of these states are highly sensitive to excess vibrational energy, the presence of collisional perturbation tends to distort the true non-radiative decay rates. In the molecular beam environment, the decay of triplet levels, isoenergetic with initially pumped S(,1) vibronic states, have been monitored as a function of the excess vibrational energy in several aromatic systems. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UMI

This dissertation presents the results of chemical research efforts directed in three areas: (1) the final design of an ultraviolet-visible absorption and emission matrix isolation apparatus is described, (2) by employing this apparatus, molecular scale interactions of the 1:1 metal-water adduct are investigated utilizing electronic absorption spectroscopy, and (3) molecular orbital and electronic state-to-state correlations are invoked to interpret the chemical reaction dynamics of the adducts along ground and excited potential energy surfaces. The final design of this matrix isolation apparatus incorporated unique features that abbreviated data acquisition time and obviated several experimental problems that have plagued previous matrix isolation studies. The investigations of the molecular interactions and reactions of the Group IIA metal atoms (Mg, Ca, Sr, Ba), the Group IIIA metal atoms (Al, Ga, In), and the Group IVA metal atoms (Si, Ge, Sn, Pb) with water molecules isolated in rare gas matrices at 15 K are reported. In most instances, the strength of the metal-water interaction is sufficiently strong to perturb significantly the electronic structure of the metal atom which results in a unique band structure for the adduct that is red-shifted from the metal atomic resonance transition. Selective photolysis studies contributed to a better understanding of the electronic structure of the adduct. Molecular orbital theory is invoked to interpret the nature of the ground and excited states of the metal-water adduct. By resorting to molecular orbital and electronic state correlation methods, the qualitative features of the metal-water interaction potential energy surfaces are derived which predicate the chemical reaction dynamics that, in turn, result in a fundamental understanding of relative reactivities, photochemical pathways, and chemiluminescent processes. In addition, this dissertation reports studies of the electronic structures of the Group IIIA metal suboxides (Al(,2)O, Ga(,2)O, In(,2)O) in absorption and emission. Progressions in the symmetric stretching and bending modes for the ground and excited states are observed. Finally, previously undocumented electronic structures of several metal dimers (Al(,2), Ba(,2), Ge(,2)) are reported and discussed.