Sungnam Park

Frequency-frequency correlation functions and apodization in two-dimensional infrared vibrational echo spectroscopy: A new approach

Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe structural dynamics under thermal equilibrium conditions on time scales ranging from femtoseconds to approximately 100 ps and longer. One of the important uses of 2D-IR spectroscopy is to monitor the dynamical evolution of a molecular system by reporting the time dependent frequency fluctuations of an ensemble of vibrational probes. The vibrational frequency-frequency correlation function (FFCF) is the connection between the experimental observables and the microscopic molecular dynamics and is thus the central object of interest in studying dynamics with 2D-IR vibrational echo spectroscopy. A new observable is presented that greatly simplifies the extraction of the FFCF from experimental data. The observable is the inverse of the center line slope (CLS) of the 2D spectrum. The CLS is the inverse of the slope of the line that connects the maxima of the peaks of a series of cuts through the 2D spectrum that are parallel to the frequency axis associated with the first electric field-matter interaction. The CLS varies from a maximum of 1 to 0 as spectral diffusion proceeds. It is shown analytically to second order in time that the CLS is the T(w) (time between pulses 2 and 3) dependent part of the FFCF. The procedure to extract the FFCF from the CLS is described, and it is shown that the T(w) independent homogeneous contribution to the FFCF can also be recovered to yield the full FFCF. The method is demonstrated by extracting FFCFs from families of calculated 2D-IR spectra and the linear absorption spectra produced from known FFCFs. Sources and magnitudes of errors in the procedure are quantified, and it is shown that in most circumstances, they are negligible. It is also demonstrated that the CLS is essentially unaffected by Fourier filtering methods (apodization), which can significantly increase the efficiency of data acquisition and spectral resolution, when the apodization is applied along the axis used for obtaining the CLS and is symmetrical about tau=0. The CLS is also unchanged by finite pulse durations that broaden 2D spectra.

Hydrogen bond dynamics in aqueous NaBr solutions

Hydrogen bond dynamics of water in NaBr solutions are studied by using ultrafast 2D IR vibrational echo spectroscopy and polarization-selective IR pump–probe experiments. The hydrogen bond structural dynamics are observed by measuring spectral diffusion of the OD stretching mode of dilute HOD in H2O in a series of high concentration aqueous NaBr solutions with 2D IR vibrational echo spectroscopy. The time evolution of the 2D IR spectra yields frequency–frequency correlation functions, which permit quantitative comparisons of the influence of NaBr concentration on the hydrogen bond dynamics. The results show that the global rearrangement of the hydrogen bond structure, which is represented by the slowest component of the spectral diffusion, slows, and its time constant increases from 1.7 to 4.8 ps as the NaBr concentration increases from pure water to ≈6 M NaBr. Orientational relaxation is analyzed with a wobbling-in-a-cone model describing restricted orientational diffusion that is followed by complete orientational randomization described as jump reorientation. The slowest component of the orientational relaxation increases from 2.6 ps (pure water) to 6.7 ps (≈6 M NaBr). Vibrational population relaxation of the OD stretch also slows significantly as the NaBr concentration increases.

Water dynamics--the effects of ions and nanoconfinement.

Hydrogen bond dynamics of water in highly concentrated NaBr salt solutions and reverse micelles are studied using ultrafast 2D-IR vibrational echo spectroscopy and polarization-selective IR pump-probe experiments performed on the OD hydroxyl stretch of dilute HOD in H(2)O. The vibrational echo experiments measure spectral diffusion, and the pump-probe experiments measure orientational relaxation. Both experimental observables are directly related to the structural dynamics of waters hydrogen bond network. The measurements performed on NaBr solutions as a function of concentration show that the hydrogen bond dynamics slow as the NaBr concentration increases. The most pronounced change is in the longest time scale dynamics which are related to the global rearrangement of the hydrogen bond structure. Complete hydrogen bond network randomization slows by a factor of approximately 3 in approximately 6 M NaBr solution compared to that in bulk water. The hydrogen bond dynamics of water in nanoscopically confined environments are studied by encapsulating water molecules in ionic head group (AOT) and nonionic head group (Igepal CO 520) reverse micelles. Water dynamics in the nanopools of AOT reverse micelles are studied as a function of size by observing orientational relaxation. Orientational relaxation dynamics deviate significantly from bulk water when the size of the reverse micelles is smaller than several nm and become nonexponential and slower as the size of the reverse micelles decreases. In the smallest reverse micelles, orientational relaxation (hydrogen bond structural randomization) is almost 20 times slower than that in bulk water. To determine if the changes in dynamics from bulk water are caused by the influence of the ionic head groups of AOT or the nanoconfinement, the water dynamics in 4 nm nanopools in AOT reverse micelles (ionic) and Igepal reverse micelles (nonionic) are compared. It is found that the water orientational relaxation in the 4 nm diameter nanopools of the two types of reverse micelles is almost identical, which indicates that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.

Ultrafast Dynamics of Hydrogen Bond Exchange in Aqueous Ionic Solutions

The structural and dynamical properties of aqueous ionic solutions influence a wide range of natural and biological processes. In these solutions, water has the opportunity to form hydrogen bonds with other water molecules and anions. Knowing the time scale with which these configurations interconvert represents a key factor to understanding the influence of molecular scale heterogeneity on chemical events in aqueous ionic solutions. We have used ultrafast IR spectroscopy and Car-Parrinello molecular dynamics (CPMD) simulations to investigate the hydrogen bond (H-bond) structural dynamics in aqueous 6 M sodium perchlorate (NaClO4) solution. We have measured the H-bond exchange dynamics between spectrally distinct water-water and water-anion H-bond configurations with 2DIR spectroscopy and the orientational relaxation dynamics of water molecules in different H-bond configurations with polarization-selective IR pump-probe experiments. The experimental H-bond exchange time correlates strongly with the experimental orientational relaxation time of water molecules. This agrees with prior observations in water and aqueous halide solutions, and has been interpreted within the context of an orientational jump model for the H-bond exchange. The CPMD simulations performed on aqueous 6 M NaClO4 solution clearly demonstrate that water molecules organize into two radially and angularly distinct structural subshells within the first solvation shell of the perchlorate anion, with one subshell possessing the majority of the water molecules that donate H-bonds to perchlorate anions and the other subshell possessing predominantly water molecules that donate two H-bonds to other water molecules. Due to the high ionic concentration used in the simulations, essentially all water molecules reside in the first ionic solvation shells. The CPMD simulations also demonstrate that the molecular exchange between these two structurally distinct subshells proceeds more slowly than the H-bond exchange between the two spectrally distinct H-bond configurations. We interpret this to indicate that orientational motions predominantly dictate the rate of H-bond exchange, while translational diffusion must occur to complete the molecular exchange between the two structurally distinct subshells around the perchlorate anions. The 2DIR measurements observe the H-bond exchange between the two spectrally distinct H-bond configurations, but the lifetime of the hydroxyl stretch precludes the observation of the slower molecular exchange. Our 2DIR experiments and CPMD simulations demonstrate that orientational motions predominantly equilibrate water molecules within their local solvation subshells, but the full molecular equilibration within the first solvation shell around the perchlorate anion necessitates translational motion.

Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation

We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.

Generation of optical vector beams with a diffractive optical element interferometer

We present a novel approach to generating radially and azimuthally polarized vector beams that utilize an interferometer constructed from two identical diffractive optical elements. The measured polarization properties of four vector beam states and their phase relationships are in good agreement with theoretical expectations. This interferometer is passively phase stable and robust, making it suitable for linear and nonlinear optical (superresolution) microscopy.

Synthesis of Monoclinic Potassium Niobate Nanowires That Are Stable at Room Temperature

We report the synthesis of KNbO(3) nanowires (NWs) with a monoclinic phase, a phase not observed in bulk KNbO(3) materials. The monoclinic NWs can be synthesized via a hydrothermal method using metallic Nb as a precursor. The NWs are metastable, and thermal treatment at ∼450 °C changed the monoclinic phase into the orthorhombic phase, which is the most stable phase of KNbO(3) at room temperature. Furthermore, we fabricated energy-harvesting nanogenerators by vertically aligning the NWs on SrTiO(3) substrates. The monoclinic NWs showed significantly better energy conversion characteristics than orthorhombic NWs. Moreover, the frequency-doubling efficiency of the monoclinic NWs was ∼3 times higher than that of orthorhombic NWs. This work may contribute to the synthesis of materials with new crystalline structures and hence improve the properties of the materials for various applications.

Ligand Exchange Dynamics in Aqueous Solution Studied with 2DIR Spectroscopy

We have used time-resolved multidimensional vibrational spectroscopy, generally termed 2DIR spectroscopy, to study the equilibrium dynamics of ligand exchange in an aqueous solution containing 3.4 M Mg(ClO(4))(2) and 1.2 M NaSCN. The sensitivity of the CN stretching frequency of thiocyanate (SCN(-)) to contact ion pair formation with Mg(2+) ions generates distinct spectroscopic signatures for the MgNCS(+) contact ion pair and the free SCN(-). We have utilized 2DIR spectroscopy to successfully resolve the interconversion between these thiocyanate configurations and measured the MgNCS(+) contact ion pair dissociation time constant to be 52 +/- 10 ps. We attribute the observed dynamics to perchlorate-thiocyanate anion exchange in the first solvation shell of the Mg(2+) cation. Magnesium ions in this concentrated ionic solution will be coordinated by water molecules, as well as perchlorate and thiocyanate ions. While prior studies have observed microsecond residence times for water ligands in the first coordination sphere of Mg(2+), our study represents the first experimental observation of anion exchange in the first solvent shell of the Mg(2+) cation. We have also used orientational relaxation and spectral diffusion dynamics to quantify the dynamical distinctions between the free anion and the anion in the contact ion pair.

Ion-pairing dynamics of Li + and SCN − in dimethylformamide solution: Chemical exchange two-dimensional infrared spectroscopy

Ultrafast two-dimensional infrared (2DIR) spectroscopy has been proven to be an exceptionally useful method to study chemical exchange processes between different vibrational chromophores under thermal equilibria. Here, we present experimental results on the thermal equilibrium ion pairing dynamics of Li(+) and SCN(-) ions in N,N-dimethylformamide. Li(+) and SCN(-) ions can form a contact ion pair (CIP). Varying the relative concentration of Li(+) in solution, we could control the equilibrium CIP and free SCN(-) concentrations. Since the CN stretch frequency of Li-SCN CIP is blue-shifted by about 16 cm(-1) from that of free SCN(-) ion, the CN stretch IR spectrum is a doublet. The temperature-dependent IR absorption spectra reveal that the CIP formation is an endothermic (0.57 kJ∕mol) process and the CIP state has larger entropy by 3.12 J∕(K mol) than the free ion states. Since the two ionic configurations are spectrally distinguishable, this salt solution is ideally suited for nonlinear IR spectroscopic investigations to study ion pair association and dissociation dynamics. Using polarization-controlled IR pump-probe methods, we first measured the lifetimes and orientational relaxation times of these two forms of ionic configurations. The vibrational population relaxation times of both the free ion and CIP are about 32 ps. However, the orientational relaxation time of the CIP, which is ∼47 ps, is significantly longer than that of the free SCN(-), which is ∼7.7 ps. This clearly indicates that the effective moment of inertia of the CIP is much larger than that of the free SCN(-). Then, using chemical exchange 2DIR spectroscopy and analyzing the diagonal peak and cross-peak amplitude changes with increasing the waiting time, we determined the contact ion pair association and dissociation time constants that are found to be 165 and 190 ps, respectively. The results presented and discussed in this paper are believed to be important, not only because the ion-pairing dynamics is one of the most fundamental physical chemistry problems but also because such molecular ion-ion interactions are of critical importance in understanding Hofmeister effects on protein stability.

Dynamics around solutes and solute–solvent complexes in mixed solvents

Ultrafast 2D-IR vibrational echo experiments, IR pump-probe experiments, and FT-IR spectroscopy of the hydroxyl stretch of phenol-OD in three solvents, CCl4, mesitylene (1, 3, 5 trimethylbenzene), and the mixed solvent of mesitylene and CCl4 (0.83 mole fraction CCl4), are used to study solute-solvent dynamics via observation of spectral diffusion. Phenol forms a complex with Mesitylene. In the mesitylene solution, there is only complexed phenol; in the CCl4 solution, there is only uncomplexed phenol; and in the mixed solvent, both phenol species are present. Dynamics of the free phenol in CCl4 or the mixed solvent are very similar, and dynamics of the complex in mesitylene and in the mixed solvent are very similar. However, there are differences in the slowest time scale dynamics between the pure solvents and the mixed solvents. The mixed solvent produces slower dynamics that are attributed to first solvent shell solvent composition variations. The composition variations require a longer time to randomize than is required in the pure solvents, where only density variations occur. The experimental results and recent MD simulations indicate that the solvent structure around the solute may be different from the mixed solvents mole fraction.