Matthew J. Nee

High resolution photodetachment spectroscopy of negative ions via slow photoelectron imaging.

A technique for high resolution anion photodetachment spectroscopy is presented that combines velocity map imaging and anion threshold photodetachment. This method, slow electron velocity-map imaging, provides spectral line widths of better than 1 meV. Spectra over a substantial range of electron kinetic energies are recorded in a single image, providing a dramatic reduction of data acquisition time compared to other techniques with comparable resolution. We apply this technique to atomic iodine and the van der Waals cluster I.CO2 as test systems, and then to the prereactive Cl.D2 complex where partially resolved structure assigned to hindered rotor motion is observed.

Two-dimensional infrared spectroscopy detected by chirped pulse upconversion

A two-dimensional (2D) infrared spectrum of Mn2(CO)10 is measured by using chirped-pulse upconversion (CPU) of the nonlinear signal field plus a reference local oscillator. By converting the spectrum to the visible, a silicon CCD camera can be used. The method offers an attractive alternative to direct IR detection due to the technological maturity of silicon and its greater intrinsic detectivity over HgCdTe. Using CPU, we acquired a rephasing 2D IR spectrum in a few seconds.

Multilevel vibrational coherence transfer and wavepacket dynamics probed with multidimensional IR spectroscopy.

Multidimensional infrared (MDIR) spectroscopy of a strongly coupled multilevel vibrational system Mn(2)(CO)(10) (dimanganese decacarbonyl) in cyclohexane solution reveals fully resolved excited vibrational state coherences that exhibit slow 0.25-0.50 ps(-1) decay constants. Detailed analysis of the waiting-time dependence of certain cross-peak amplitudes shows modulation at multiple frequencies, providing a direct signature of excited vibrational coherences resulting from coherence transfer. A new signature of coherence transfer is observed as temporally modulated cross-peak amplitudes with more than one modulation frequency. The relative importance of different coherence transfer paths is considered in the context of the orientational response of a system which includes two vibrational modes with parallel dipole moments. Since MDIR spectroscopy enables spectral isolation of individual excited vibrational coherences (i.e., coherences between fundamental excitations), these experiments report directly on the frequency-frequency correlation functions of the excited states relative to each other as well as relative to the ground state. These results highlight the rich information contained in fully exploring three-dimensional third-order spectroscopy, particularly regarding chemically relevant slower dynamics and the importance of intramolecular interactions leading to dephasing by optically dark or low-frequency modes of the molecule.

Ultrafast nonequilibrium Fourier-transform two-dimensional infrared spectroscopy

We present what we believe to be the first implementation of nonequilibrium two-dimensional IR spectroscopy (2DIR) combining electronic excitation within the Fourier transform (FT) approach. Nonequilibrium 2DIR spectra of Mn2(CO)10 and its photoproducts are obtained in two modalities: photoexcitation at 400 nm, either before a 2DIR probe or during the waiting time of the FT 2DIR measurement. Extending FT 2DIR to nonequilibrium systems offers insight into complex condensed-phase reaction dynamics.

Slow electron velocity-map imaging photoelectron spectra of the methoxide anion

High resolution anion photodetachment spectra are presented for the methoxide anion and its fully deuterated counterpart. The spectra were obtained with slow electron velocity-map imaging. Improved electron affinities are determined for CH3O as 1.5690+/-0.0019 eV and for CD3O as 1.5546+/-0.0019 eV. The spectra resolve many features associated with spin-orbit and vibronic coupling that were not seen in previous photodetachment studies. Photoelectron angular distributions taken as a function of detachment wavelength for the ground vibronic state transitions are recorded and are consistent with the removal of a nonbonding, p-type electron localized on the oxygen atom. Several hot bands and sequence bands are observed for the first time, providing insight into the vibrational structure of the methoxide anion. The results are compared to recent calculations of the anion photoelectron spectra that incorporate bilinear coupling terms among the methoxy vibrational modes and are found to be in reasonable agreement.

Orientational Dynamics of Transient Molecules Measured by Nonequilibrium Two-Dimensional Infrared Spectroscopy

Transient two-dimensional infrared (2DIR) spectroscopy is applied to the photodissociation of Mn2(CO)10 to 2 Mn(CO)5 in cyclohexane solution. By varying both the time delay between the 400 nm phototrigger and the 2DIR probe as well as the waiting time in the 2DIR pulse sequence, we directly determine the orientational relaxation of the vibrationally hot photoproduct. The orientational relaxation slows as the photoproduct cools, providing a measure of the transient temperature decay time of 70 +/- 16 ps. We compare the experimental results with molecular dynamics simulations and find near quantitative agreement for equilibrium orientational diffusion time constants but only qualitative agreement for nonequilibrium thermal relaxation. The simulation also shows that the experiment probes an unusual regime of thermal excitation, where the solute is heated while the solvent remains essentially at room temperature.

Experimental and theoretical study of the infrared spectra of BrHI− and BrDI−

Gas phase vibrational spectra of BrHI- and BrDI- have been measured from 6 to 17 microm (590-1666 cm(-1)) using tunable infrared radiation from the free electron laser for infrared experiments in order to characterize the strong hydrogen bond in these species. BrHI-.Ar and BrDI-.Ar complexes were produced and mass selected, and the depletion of their signal due to vibrational predissociation was monitored as a function of photon energy. Additionally, BrHI- and BrDI- were dissociated into HBr (DBr) and I- via resonant infrared multiphoton dissociation. The spectra show numerous transitions, which had not been observed by previous matrix studies. New ab initio calculations of the potential-energy surface and the dipole moment are presented and are used in variational ro-vibrational calculations to assign the spectral features. These calculations highlight the importance of basis set in the simulation of heavy atoms such as iodine. Further, they demonstrate extensive mode mixing between the bend and the H-atom stretch modes in BrHI- and BrDI- due to Fermi resonances. These interactions result in major deviations from simple harmonic estimates of the vibrational energies. As a result of this new analysis, previous matrix-isolation spectra assignments are reevaluated.

A Novel n-Type Organosilane–Metal Ion Hybrid of Rhodamine B and Copper Cation for Low-Temperature Thermoelectric Materials

An n-type organosilane-metal ion hybrid of Rhodamine B-silane and copper cation (Cu-RBS) was investigated as a low-temperature thermoelectric material. Computational analysis revealed the most likely localized binding site of Cu2+ was to the Rhodamine B core and provided predictions of molecular orbitals and electrostatic potentials upon complexation. The concentration-dependent optical absorption and emission spectra confirmed the effective metal-ligand charge transfer from Cu2+ to the xanthene core of RBS, indicating the potential for improved electrical properties for the complex relative to RBS. The electrical conductivity and Seebeck thermoelectric (TE) behavior were evaluated and compared with its precursor complex of Rhodamine B and copper cation. While a moderately high electrical conductivity of 4.38 S m-1 was obtained for the Cu-RBS complex, the relatively low Seebeck coefficient of -26.2 μV/K resulted in a low TE power factor. However, compared to other organic doped materials, these results were promising toward developing n-type thermoelectric materials with no doping agents. Both phase segregation and thin film heterogeneity remain to be optimized; thus, the balance between Cu2+ domains and RBS domain phases will likely yield higher Seebeck coefficients and improved power factors.

Microbubble Fabrication of Concave-porosity PDMS Beads.

Microbubble fabrication (by use of a fine emulsion) provides a means of increasing the surface-area-to-volume (SAV) ratio of polymer materials, which is particularly useful for separations applications. Porous polydimethylsiloxane (PDMS) beads can be produced by heat-curing such an emulsion, allowing the interface between the aqueous and aliphatic phases to mold the morphology of the polymer. In the procedures described here, both polymer and crosslinker (triethoxysilane) are sonicated together in a cold-bath sonicator. Following a period of cross-linking, emulsions are added dropwise to a hot surfactant solution, allowing the aqueous phase of the emulsion to separate, and forming porous polymer beads. We demonstrate that this method can be tuned, and the SAV ratio optimized, by adjusting the electrolyte content of the aqueous phase in the emulsion. Beads produced in this way are imaged with scanning electron microscopy, and representative SAV ratios are determined using Brunauer-Emmett-Teller (BET) analysis. Considerable variability with the electrolyte identity is observed, but the general trend is consistent: there is a maximum in SAV obtained at a specific concentration, after which porosity decreases markedly.

Triggered-exchange Two-dimensional Infrared Spectroscopy of Metal Carbonyl Photodissociation Dynamics

We present an ultrafast study of the dissociation dynamics of a metal carbonyl complex using transient Fourier transform 2DIR spectroscopy.