Louis H. Haber

Nanoscopic interchain aggregate domain formation in conjugated polymer films studied by third harmonic generation near-field scanning optical microscopy

The electronic structure of conjugated polymer films is of current interest due to the wide range of potential applications for such materials in optoelectronic devices. A central outstanding issue is the significance of interchain electronic species in films of these materials. In this paper, we investigate the nature of interchain species in films of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) both before and after thermal annealing. Our investigation employs a combination of third harmonic generation (THG) and near-field scanning optical microscopy to measure the wavelength and spatial dependence of the THG efficiency. These chemically selective imaging measurements reveal new, low-energy absorption features in nanometer-scale spatially distinct regions of annealed films that are only infrequently observed prior to annealing. This suggests that the polymer strands in annealed MEH-PPV films pack together closely enough that significant ground-state wave function overlap can occ...

Fluorescence, Phosphorescence, and Chemiluminescence

Noureen Siraj,† Bilal El-Zahab,‡ Suzana Hamdan,† Tony E. Karam,† Louis H. Haber,† Min Li, Sayo O. Fakayode, Susmita Das, Bertha Valle, Robert M. Strongin, Gabor Patonay, Herman O. Sintim, Gary A. Baker, Aleeta Powe, Mark Lowry, Jan O. Karolin, Chris D. Geddes, and Isiah M. Warner*,† †Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States ‡Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33174, United States Process Development Center, Albemarle Corporation, Baton Rouge, Louisiana 70805, United States Department of Chemistry, Winston-Salem State University, Winston-Salem, North Carolina 27110, United States Department of Civil Engineering, Adamas Institute of Technology, Barasat, Kolkata 700126, West Bengal India Department of Chemistry, Texas Southern University, Houston, Texas 77004, United States Department of Chemistry, Portland State University, Portland, Oregon 97207, United States Department of Chemistry, Georgia State University, Atlanta, Georgia 30302-4098, United States Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States Department of Chemistry, University of Missouri Columbia, Columbia, Missouri 65211-7600, United States Department of Chemistry, University of Louisville, Louisville, Kentucky 40208, United States Institute of Fluorescence, University of Maryland Baltimore County, Baltimore, Maryland 21202, United States

Shape control of near-field probes using dynamic meniscus etching

Dynamic etching methods for fabricating fibre optic tips are explored and modelled. By vertically translating the fibre during etching by an HF solution under an organic protective layer, a variety of tip shapes were created. The probe taper lengths, cone angles and geometrical probe shapes were measured in order to evaluate the dynamic meniscus etching process. Fibre motion, etching rate, meniscus distortion and etching time were all found to be important variables that can be used to control the final probe shape.

Molecular Excited-State Relaxation Dynamics at the Colloidal Microparticle Interface Monitored with Pump–Probe Second Harmonic Generation

Time-resolved second harmonic generation is used to monitor the excited-state relaxation dynamics of molecules adsorbed to the surface of colloidal microparticles suspended in solution. The cationic organic dye, malachite green (MG), is adsorbed to the negatively charged surface of polystyrene sulfate microparticles in water. MG is photoexcited to the S1 excited state by a 615 nm pump pulse. The time-dependent change of the S0 ground-state depletion is probed by second harmonic generation of an 800 nm pulse as a function of pump-probe delay to obtain a lifetime of 5.7 ± 0.4 ps. This excited-state lifetime is approximately three times longer than the corresponding lifetime at the air/water interface, showing the significant effect of the negatively charged surface on local friction, which is important in the energy relaxation of photoexcited MG.

Ultrafast predissociation of superexcited nitrogen molecules

Superexcited states of neutral nitrogen molecules, with excited ion cores, are prepared above the N2 ionization limit with femtosecond pulses of 23.1 eV photons, produced by high-order harmonic generation. Time-resolved predissociation of the superexcited nitrogen molecules is observed by detection of electronically excited atomic N*(4p) and/or N*(3d) product formation, by photoionization of the excited atoms with a delayed 805 nm pulse. An upper limit lifetime of 25 fs is determined for the superexcited states, attributed to the sum of autoionization and predissociation pathways, based on the risetime of predissociation products. The observation of dissociation products from the short-lived states shows that the predissociation decay pathways compete with autoionization pathways on an ultrafast timescale.

Monitoring the Photocleaving Dynamics of Colloidal MicroRNA-Functionalized Gold Nanoparticles Using Second Harmonic Generation

Photoactivated drug delivery systems using gold nanoparticles provide the promise of spatiotemporal control of delivery that is crucial for applications ranging from regenerative medicine to cancer therapy. In this study, we use second harmonic generation (SHG) spectroscopy to monitor the light-activated controlled release of oligonucleotides from the surface of colloidal gold nanoparticles. MicroRNA is functionalized to spherical gold nanoparticles using a nitrobenzyl linker that undergoes photocleaving upon ultraviolet irradiation. The SHG signal generated from the colloidal nanoparticle sample is shown to be a sensitive probe for monitoring the photocleaving dynamics in real time. The photocleaving irradiation wavelength is scanned to show maximum efficiency on resonance at 365 nm, and the kinetics are investigated at varying irradiation powers to demonstrate that the nitrobenzyl photocleaving is a one-photon process. Additional characterization methods including electrophoretic mobility measurements, extinction spectroscopy, and fluorimetry are used to verify the SHG results, leading to a better understanding of the photocleaving dynamics for this model oligonucleotide therapeutic delivery system.

Photoelectron angular distributions and cross section ratios of two-color two-photon above threshold ionization of argon.

Anisotropy parameters and cross section ratios of two-color two-photon above threshold ionization sidebands from argon are measured using photoelectron velocity map imaging with the selected 13th or 15th high-order harmonics in a perturbative 800 nm dressing field. A new data analysis technique determines accurate anisotropy parameters of the photoelectron angular distributions for each sideband by subtracting a sequence of percentages of the single-photon ionization background from the above threshold ionization signal to correct for the angular averaging of overlapping photoelectron energies. The results provide a fundamental test of theoretical predictions based on second-order perturbation theory with a one-electron model and the soft-photon approximation and show agreement with theory for the cross section ratios. However, discrepancies between the theoretically predicted and experimentally determined photoelectron angular distributions demonstrate the need for a more comprehensive theoretical description of two-color two-photon above threshold ionization.

Excited-state dynamics of size-dependent colloidal TiO2-Au nanocomposites

The ultrafast excited-state dynamics of size-dependent TiO2-Au nanocomposites synthesized by reducing gold nanoclusters to the surface of colloidal TiO2 nanoparticles are studied using pump-probe transient absorption spectroscopy with 400 nm excitation pulses. The results show that the relaxation processes of the plasmon depletion band, which are described by electron-phonon and phonon-phonon scattering lifetimes, are independent of the gold nanocluster shell size surrounding the TiO2 nanoparticle core. The dynamics corresponding to interfacial electron transfer between the gold nanoclusters and the TiO2 bandgap are observed to spectrally overlap with the gold interband transition signal, and the electron transfer lifetimes are shown to significantly decrease as the nanocluster shell size increases. Additionally, size-dependent periodic oscillations are observed and are attributed to acoustic phonons of a porous shell composed of aggregated gold nanoclusters around the TiO2 core, with frequencies that decrease and damping times that remain constant as the nanocluster shell size increases. These results are important for the development of improved catalytic nanomaterial applications.

Photoelectron angular distributions from autoionizing 4s14p66p1 states in atomic krypton probed with femtosecond time resolution

Photoelectron angular distributions (PADs) are obtained for a pair of 4s(1)4p(6)6p(1) (a singlet and a triplet) autoionizing states in atomic krypton. A high-order harmonic pulse is used to excite the pair of states and a time-delayed 801 nm ionization pulse probes the PADs to the final 4s(1)4p(6) continuum with femtosecond time resolution. The ejected electrons are detected with velocity map imaging to retrieve the time-resolved photoelectron spectrum and PADs. The PAD for the triplet state is inherently separable by virtue of its longer autoionization lifetime. Measuring the total signal over time allows for the PADs to be extracted for both the singlet state and the triplet state. Anisotropy parameters for the triplet state are measured to be β(2)=0.55 ± 0.17 and β(4)=-0.01 ± 0.10, while the singlet state yields β(2)=2.19 ± 0.18 and β(4)=1.84 ± 0.14. For the singlet state, the ratio of radial transition dipole matrix elements, X, of outgoing S to D partial waves and total phase shift difference between these waves, Δ, are determined to be X=0.56 ± 0.08 and Δ=2.19 ± 0.11 rad. The continuum quantum defect difference between the S and D electron partial waves is determined to be -0.15 ± 0.03 for the singlet state. Based on previous analyses, the triplet state is expected to have anisotropy parameters independent of electron kinetic energy and equal to β(2)=5∕7 and β(4)=-12∕7. Deviations from the predicted values are thought to be a result of state mixing by spin-orbit and configuration interactions in the intermediate and final states; theoretical calculations are required to quantify these effects.

Time-resolved photoelectron angular distributions and cross-section ratios of two-colour two-photon above threshold ionization of helium

Time-resolved anisotropy parameters and cross-section ratios of the positive and negative sidebands from two-colour two-photon above threshold ionization of helium atoms are measured using photoelectron velocity map imaging with the selected 19th high-order harmonic at 29.1 eV in an 810 nm perturbative dressing field. The intensities of both the sidebands and the single-photon ionization depletion follow a Gaussian correlation function where the photoelectron angular distributions and cross-section ratios of the sidebands do not change as a function of the temporal delay between the extreme ultraviolet and infrared pulses. The experimental results are compared with theoretical predictions using the soft-photon approximation, showing poor agreement, and analytical expressions are derived using second-order perturbation theory to determine the relative magnitudes of the resulting S and D partial waves of the above threshold ionization features.