Facile collection of two-dimensional electronic spectra using femtosecond pulse-shaping technology

This letter reports a straightforward means of collecting two-dimensional electronic (2D-E) spectra using optical tools common to many research groups involved in ultrafast spectroscopy and quantum control. In our method a femtosecond pulse shaper is used to generate a pair of phase stable collinear laser pulses which are then incident on a gas or liquid sample. The pulse pair is followed by an ultrashort probe pulse that is spectrally resolved. The delay between the collinear pulses is incremented using phase and amplitude shaping and a 2D-E spectrum is generated following Fourier transformation. The partially collinear beam geometry results in perfectly phased absorptive spectra without phase twist. Our approach is much simpler to implement than standard non-collinear beam geometries, which are challenging to phase stabilize and require complicated calibrations. Using pulse shaping, many new experiments are now also possible in both 2D-E spectroscopy and coherent control.open798

MICROWAVE SPECTRA OF THE O2_2-HF COMPLEX

Author Institution: Department of Chemistry, University of Minnesota, Minneapolis,; MN 55455Microwave spectra have been recorded for the complex O$_2$-HF. Spectra were readily located based on results of previous infrared work.} \underline{\textbf{117}}(2), 693 (2002).} but yield spectroscopic constants of somewhat higher accuracy. The observed transitions show well resolved structure arising from the $^1$H and $^1$$^9$F nuclear spins. Magnetic super-hyperfine structure due to the interaction of the proton and fluorine nuclei with the spin magnetic moment of O$_2$ appear to be of comparable magnitude to the HF spin-spin interaction in at least some of the observed transitions. Progress on the simultaneous analysis of these hyperfine and super-hyperfine effects will be reported

MICROWAVE SPECTRA OF O2_2--HF AND O2_2--DF AND GLOBAL FITTING WITH IR DATA: INSIGHT INTO THE NATURE OF HYPERFINE INTERACTIONS

Author Institution: Department of Chemistry, University of Minnesota, Minneapolis, MN 55455Understanding intermolecular interactions involving open shell systems is important to both combustion and atmospheric research. In this talk, we present microwave spectra of O$_2$--HF and O$_2$--DF, and analysis of their hyperfine structure. Last year, spectra were reported} Symposium on Molecular Spectroscopy} {\bf TE08} (2006)} for six different pure rotational transitions of O_2--HF , though at the time, the analysis of the hyperfine structure was still incomplete. A simultaneous fit of microwave and infrared} {\bf117}, 693(2002)} data was also described. In this talk, we report a complete analysis of the hyperfine structure for O_2$--HF and a new global fit including microwave and infrared frequencies. New assignments for the$F_1$quantum numbers, together with complete assignment of$F$quantum numbers has allowed all observed transitions of O$_2$--HF to be fully analyzed with confidence. Calculated spectral intensities are also consistent with experimental observation. The Fermi contact parameters for the two nuclei are found to have opposite signs, consistent with a simple model based on spin polarization. Progress on analysis of magnetic and nuclear quadrupole hyperfine structure in O$_2$--DF and global fit with IR data will be reported. The derived hyperfine parameters unambiguously establish the correspondence between the magnetic hyperfine constants and the two nuclei of the H(D)F

Super-Resolution Structured Pump–Probe Microscopy

Imaging excited-state dynamics in complex materials provides an important avenue for understanding the role played by micro- and nanoscale structural and compositional heterogeneity in the performance of active electronic and optoelectronic devices. Here, we develop structured pump–probe microscopy (SPPM), a subdiffraction-limited, time-resolved microscopy that incorporates a focused diffraction-limited probe field together with a spatially modulated pump excitation field. We apply the technique to study free carrier dynamics in silicon nanowires, demonstrating far-field ultrafast time-resolved spectroscopy with a 114 nm point-spread function while remaining in the perturbative excitation regime. We predict SPPM will provide an accessible approach for characterizing the ultrafast dynamics of subwavelength regions in complex materials systems

Probing Excited State Delocalization and Charge Separation in Hierarchical Perylene Diimide Materials with Time-Resolved Broadband Microscopy

Using a variety of steady state and time-resolved microscopies, this work directly compares the excited state dynamics of two distinct morphologies of a hierarchical perylene diimide material: a kinetically trapped 1D mesoscale aggregate produced with a redox-assisted self-assembly process, and a thin film produced via conventional solution-processing techniques. Although the constituent monomer is identical for both materials, linear dichroism studies indicate that the kinetically trapped structures possess significantly higher long-range order than the conventional thin film. A comparison of the two systems with broadband pump–probe microscopy reveals distinct differences in their excited state dynamics. In the kinetically trapped structures, polarization-resolved kinetics, as well as a picosecond redshift of the ground state bleach provide evidence for rapid excited state delocalization, which is absent in the thin film. A comparison of transient spectra collected at 1 μs indicates the presence of long-lived charge separated states in redox treated samples, but not in the thin film. These results provide direct evidence that control of the supramolecular assembly process can be leveraged to affect the long-range order of derived PDI materials, thus enabling increased yield and lifetime of charge separated states for light harvesting applications. Furthermore, these results highlight the need for microscale broadband probes of organic materials to accurately capture the influence of local morphology on excited state functionality

MICROWAVE OBSERVATION OF THE OH-H2_2O RADICAL COMPLEX

Author Institution: Department of Chemistry, University of Minnesota, Minneapolis,; MN 55455; Department of Chemistry, Amherst College, Amherst, MA 01002-5000The radical complex OH-H$_2$O has been observed by rotational spectroscopy. Spectra for $^1$$^6$OH-$^1$$^6$OH$_2$O and $^1$$^8$OH-$^1$$^8$OH$_2$ have been analyzed using a two-state model which accounts for nuclear motion on both the $^2$A$^\prime$ and $^2$A$^{\prime\prime}$ potential surfaces. Partial quenching of the OH orbital angular momentum dramatically affects the rotational spectra, and the $^2$A$^\prime$-$^2$A$^{\prime\prime}$ energy separation, $\rho$, is determined to be -146.50744(42) \wn. The ground state of the complex has approximately 86{\%} $^2$A$^\prime$ character and the vibrationally averaged OH-OH$_2$ hydrogen bond distance is 1.952 \AA. The magnetic hyperfine constants for the OH proton in the complex are significantly altered from monomer values

Ultrafast Excited-State Transport and Decay Dynamics in Cesium Lead Mixed Halide Perovskites

While significant research efforts directed toward characterizing the excited-state dynamics of lead halide perovskites have enabled promising advances in photovoltaic, light-emitting diode, and laser technologies, a detailed correlation between composition and functionality in this promising class of materials remains unestablished. We use pump–probe microscopy to characterize both transport and relaxation dynamics in individual crystals of CsPbI₂Br, a mixed halide, all-inorganic analogue to the well-studied organic–inorganic hybrid perovskites. In contrast to the methylammonium lead tri-iodide perovskite, we find excited-state dynamics that decay primarily via first-order and Auger mechanisms. By global fitting of power-dependent kinetics collected from individual domains, we find a range of Auger rate constants between 3.3 × 10^–30 and 1.5 × 10^–28 cm⁶/s, with negligible contributions from second-order (bimolecular) processes. Direct imaging of the excited-state spatial evolution reveals an average diffusion constant of 0.27 cm²/s, a value that is nearly an order of magnitude smaller than that of single-crystal, organic–inorganic analogues