Joshua P. Darr

Spectroscopic Identification of Higher-Order Rare Gas-Dihalogen Complexes with Different Geometries: He2,3···Br2and He2,3···ICl

Rovibronic transitions of multiple conformers of the He(2)...(79)Br(2)(X, v = 0), He(3)...(79)Br(2)(X, v = 0), He(2)...I(35)Cl(X, v = 0), and He(3)...I(35)Cl(X, v = 0) complexes stabilized in a pulsed, supersonic expansion are observed in action spectra recorded in the B-X region of the dihalogens. In addition to features associated with He(2)...(79)Br(2) and He(2)...I(35)Cl complexes with the rare gas atoms localized in the toroidal potential well lying in a plane perpendicular to the dihalogen bond, those associated with a ground-state conformer that has one He atom localized in the toroidal potential and the other He atom localized in the linear well at the end of the dihalogen moiety are also identified. Transitions of at least three conformers of the He(3)...Br(2) complex and two conformers of the He(3)...ICl complex are also observed. The relative populations of the different conformers are found to depend on where along the supersonic expansion the spectra are recorded, and thus on the local temperature regime sampled. The He(2)...(79)Br(2) and He(2)...I(35)Cl conformers with one He atom in each well are found to be the more stable conformers.

A combined experimental/theoretical investigation of the near-infrared photodissociation of IBr−(CO2)n

We report the collaborative experimental and theoretical study of the time-resolved recombination dynamics of photodissociated IBr(-)(CO(2))(n) clusters. Excitation of the bare anionic chromophore to the dissociative A() (2)Pi(1/2) state yields only I(-) and Br products. Interestingly, however, the addition of a few solvent molecules promotes recombination of the dissociating chromophore on the X (2)Sigma(1/2)(+) ground state, which correlates asymptotically with Br(-) and I products. This process is studied experimentally using time-resolved, pump-probe techniques and theoretically via nonadiabatic molecular dynamics simulations. In sharp contrast to previous I(2)(-) studies where more kinetic energy was released to the photofragments, the observed recombination times increase from picoseconds to nanoseconds with increasing cluster size up to n=10. The recombination times then drop dramatically back to picoseconds for cluster sizes n=11-14. This trend, seen both in experiment and theory, is explained by the presence of a solvent-induced well on the A() state, the depth of which directly corresponds to the asymmetry of the solvation about the chromophore. The results seen for both the branching ratios and recombination times from experiment and theory show good qualitative agreement.

Photofragmentation dynamics of ICN−(CO2)n clusters following visible excitation

Photodissociation of ICN(-)(CO2)n, n = 0-18, with 500-nm excitation is investigated using a dual time-of-flight mass spectrometer. Photoabsorption to the (2)Π(1/2) state is detected using ionic-photoproduct action spectroscopy; the maximum absorption occurs around 490 nm. Ionic-photoproduct distributions were determined for ICN(-)(CO2)n at 500 nm. Following photodissociation of bare ICN(-) via 430-650 nm excitation, a small fraction of CN(-) is produced, suggesting that nonadiabatic effects play a role in the photodissociation of this simple anion. Electronic structure calculations, carried out at the MR-SO-CISD level of theory, were used to evaluate the potential-energy surfaces for the ground and excited states of ICN(-). Analysis of the electronic structure supports the presence of nonadiabatic effects in the photodissociation dynamics. For n ≥ 2, the major ionic photoproduct has a mass corresponding to either partially solvated CN(-) or partially solvated [NCCO2](-).

Probing the dependence of long-range, four-atom interactions on intermolecular orientation: 3. Hydrogen and iodine.

Two-laser, action spectroscopy experiments have been performed in the I(2)B-X, υ-0 spectral region on H(2)···I(2) and D(2)···I(2) complexes to investigate the dependence of the H(2)/D(2) + I(2) intermolecular interactions on orientation. The spectra contain features associated with at least two different conformers of the ground-state H(2)/D(2)···I(2)(X,υ = 0) complexes; one conformer has a preferred T-shaped geometry with the H(2)/D(2) moiety localized in a potential minimum that is orthogonal to the I-I bond axis, and the second conformer has a linear geometry with the H(2)/D(2) moiety positioned in minima at either end of the I(2) molecule, along the bond axis. Those features associated with complexes containing para-H(2)(j = 0), ortho-H(2)(j = 1), ortho-D(2)(j = 0), and para-D(2)(j = 1) are also assigned. The linear conformers are found to be more strongly bound than the T-shaped conformers with binding energies of 118.9(1.9) cm(-1) versus 91.3-93.3 cm(-1) for the ortho-H(2)···I(2) complexes and 144.2(2.1) cm(-1) versus 107.9 cm(-1) for the para-D(2)···I(2) complexes, respectively. Electronic structure calculations of the complexes containing ICl and I(2) with H(2), He, Ne, and Ar were performed to reveal the nature of the interactions and to shed insight into the origins of the different binding energies. The most stable minima in the H(2)/D(2) + I(2)(B,υ) excited-state potentials have T-shaped geometries. Calculated energies and probability amplitudes of the excited-state levels provide insight into the different excited-state intermolecular vibrational levels accessed by transitions of the two ground-state conformers.

The Hydropathy Scale as a Gauge of Hygroscopicity in Sub-Micron Sodium Chloride-Amino Acid Aerosols

Sodium chloride, NaCl, is commonly used as a proxy for sea spray aerosols. However, field work has demonstrated that sea spray aerosols also often contain a significant organic component. In this work, we examine the effect of amino acids on the hygroscopic properties of NaCl aerosols using a Fourier transform infrared spectrometer coupled to a flow-cell apparatus. It is found that the effect can be drastically different depending on the nature of the amino acid. More hydrophilic amino acids such as glycine lead to continuous hygroscopic growth of internally mixed NaCl-amino acid aerosols generated from an equimolar precursor solution. However, more hydrophobic amino acids such as alanine do not significantly alter the hygroscopicity of NaCl aerosols. The hydropathy scale is found to be a good qualitative diagnostic for the effect that an amino acid will have on the hygroscopicity of NaCl.