Search

We develop a model to probe the relative orientation of two second-order polarizable daunomycin molecules that are intercalated into a DNA duplex using optical second-harmonic (SH) generation. The SH field generated by the daunomycin molecules interfere with each other. Because the interference depends on the relative orientation of the daunomycin molecules, we can control the interference by changing the number of base pairs separating them. The relative orientation changes as the number of base pairs separating them, multiplied by 36°, which is the twist angle between neighboring base pairs. In this paper, we derive a set of relationships between the relative angle of the molecules and the nonlinear susceptibility elements, and we calculate the SH field generated by the DNA/molecular-pair complex attached to an isotropic dielectric sphere. Calculations reveal that the SH intensity varies periodically with the relative orientation of the two chromophores in the plane perpendicular to the helical axis. The predicted periodicity is in close agreement with experimental results. Structural changes induced by foreign molecules binding to DNA will change the relative orientation of the two chromophores and thereby change the SH interference pattern. We discuss the potential of this SH interference method in providing a new way to probe structural changes induced by the formation of biomolecule complexes. An important feature of the method is that it is label-free, that is, the binding molecule, in this case, daunomycin, is not tagged. more...

The complementary interface-sensitive methods of sum fre- quency generation (SFG) and surface tension have been used to investigate surfactants at the air/water interface. SFG yields information on the density and orientation of surfactants at the interface, whereas surface tension yields information on the surface excess of surfactants, which, however, is not their interfacial density. The rapid rise in the SFG signal to its maximum value at 1 mM, which remained essentially constant thereafter, was attributed to the formation of large surfactant domains. Above 1 mM, the domains, all of which have the same density, dominated the SFG signal. As a consequence, the SFG signalremainedessentiallyconstant.Thesurfaceexcesshadanequallyrapidrise to its maximum value at 1 mM, which remained constant until reaching a concentration of 3 mM, where it dropped to a zero value, indicating that a full surfactant monolayer had been formed. This coincides with the formation of bulk micelles, which is commonly referred to as the critical micelle concentration. The orientation of the surfactant carbonyl chromophore was obtained from polarization measure- ments of the SFG signal and showed a small change at and above a 1 mM concentration. The SFG results and the surface tension results, though inherently different, were found to be consistent with each other. more...

We present a novel method for obtaining the orientational distribution of molecules at the air/water interface by using a circularly polarized pump pulse that is incident normal to the interface to excite interfacial molecules to their excited electronic states. The second harmonic generation intensities of two linearly polarization combinations (45� -in/ S-out and P-in/P-out) were measured. The ratio of their intensities measured before and after the pump pulse was then used to obtain the mean orientational angle and the orientational distribution utilizing a Gaussian distribution. The mean orientational angle and the orientational distribution of the organic dye molecule, coumarin 314, at the air/water interface were found to be 55 ( 2� with respect to the surface normal and a full width at half-maximum of 16 ( 3� . more...

Competitive adsorption between coumarin 314 (C314) and acetonitrile at the air–water interface was studied using second harmonic generation, taking advantage of both the surface specificity and molecule selectivity of the technique. The surface concentration of C314 was selectively probed at the air–aqueous interface. As acetonitrile was added to the solution it was observed that acetonitrile displaces C314 at the interface. The experimental results are well-represented by a competitive Frumkin–Fowler–Guggenheim (cFFG) adsorption model. more...

The interface-selective second harmonic generation (SHG) nonlinear optical spectroscopy and the surface tension method have been used to measure the effect of electrolyte on the adsorption isotherm of 4-hydroxyacetophenone (4-HA) at the air−solution interface. It was observed that high NaCl concentration favors adsorption of organic molecules to the air−solution interface. Measurements of the polarization of the SHG field showed that the orientation of the organic molecule at the air−solution interface is close to that in the neat air−water interface and does not vary with organic concentration. The free energy of adsorption of the molecule to the air−solution interface becomes significantly more negative with an increase in salt concentration, and good agreement was found between the results from the two different techniques. We conclude that the increased affinity of 4-HA for the air−water interface in the presence of NaCl is due to the salting-out effect. more...

The dynamics of molecular solvation at the air/water interface has been monitored with femtosecond time-resolved pump-sum frequency generation (TR-SFG), a technique that has been shown to be feasible in the study of ultrafast rotional motions. In the work reported here, the solvation process was monitored by femtosecond photoexcitation of interfacial coumarin 314 (C314) molecules. In these experiments, the SFG signal is brought into a vibrational resonance with the carbonyl symmetric stretch of C314 by tuning the IR pulse to the carbonyl frequency by using a pump-TR-SFG probe. Two solvation time constants were obtained, 230 ± 40 fs and 2.17 ± 0.3 ps. These results are the same within experimental error as those measured in time-resolved second-harmonic generation (TR-SHG) experiments. This suggests that the solvent response is due to solvation-induced shifts of the electronic-state energies in the SFG hyperpolarizability and not significantly to solvation effects on the energy of the carbonyl vibration no... more...

How is interfacial water organized beneath an acetonitrile monolayer at the air/acetonitrile aqueous solution interface? The method of vibrational sum frequency generation was used to address these issues. It was found that the different water structures expressed at different vibrational spectral regions, i.e. the “ice-like”, the “liquid-like”, and the non-hydrogen bonded “free” OD, responded differently as the acetonitrile bulk concentration was increased. At the bulk acetonitrile concentration that was sufficient to form an acetonitrile monolayer and at higher bulk concentrations, it was observed that there are no longer vibrational resonances that correspond to the oriented collective water vibrations, i.e. the “ice-like” and “liquid-like” structures of water as observed at the air/water interface. The story was different in the free OD vibrational region where a red-shifted resonance was found at acetonitrile concentrations sufficient to form an acetonitrile monolayer. Speculations on the origin of t... more...

The in-plane rotational dynamics of the molecular probe coumarin 314 (C314) at the negatively charged surfactant sodium dodecyl sulfate (SDS)/aqueous interface was measured using femtosecond time-r... more...

The first time-resolved experiments in which interfacial molecules are pumped to excited electronic states and probed by vibrational sum frequency generation (SFG) are reported. This method was used to measure the out-of-plane rotation dynamics, i.e. time dependent changes in the polar angle, of a vibrational chromophore of an interfacial molecule. The chromophore is the carbonyl group, the rotation observed is that of the -C=O bond axis, with respect to the interfacial normal, and the interfacial molecule is coumarin 314 (C314) at the air/water interface. The orientational relaxation time was found to be 220+/-20 ps, which is much faster than the orientational relaxation time of the permanent dipole moment axis of C314 at the same interface, as obtained from pump-second harmonic probe experiments. Possible effects on the rotation of the -C=O bond axis due to the carbonyl group hydrogen bonding with interfacial water are discussed. From the measured equilibrium orientation of the permanent dipole moment axis and the carbonyl axis, and knowledge of their relative orientation in the molecule, the absolute orientation of C314 at the air/water interface is obtained. more...

The effect of charge on the rotational dynamics of the molecular probe coumarin 314 (C314) at air/water interfaces covered with the negatively charged surfactant sodium dodecyl sulfate (SDS) was investigated using femtosecond time-resolved second harmonic spectroscopy. The out-of-plane orientational time constant at the highest SDS surface coverage of 100 A2 per molecule is 383 +/- 9 ps. The rotational dynamics is slower than at the air/water interface where the out-of-plane reorientational time constant is 336 +/- 6 ps. At the air/water interface the rotational dynamics is over three times slower than the bulk orientational diffusion time of 100 ps. The relatively small effect of the surfactant charge density on the C314 rotation time constant is surprising, considering the marked dependence of the C314 orientation, spectra, and surfactant phase diagram on surfactant density. more...

A method to determine the absolute orientation of molecules at liquid interfaces by sum frequency generation (SFG) is reported. It is based on measurements of the orientations of two nonparallel vibrationally active chromophores in the molecule of interest combined with a rotation matrix formulation to obtain the absolute molecular orientation. We chose m-tolunitrile, a planar molecule adsorbed to the air/water interface, as a proof-of-method experiment. Quantitative analysis of different polarization sum frequency intensities facilitate unique peak assignments of the methyl and nitrile groups of m-tolunitrile. The SFG analysis of the measurement yields a nitrile group tilting at 53 degrees to the surface normal, and the C3 axis of the methyl group is almost upright at 23 degrees with respect to the surface normal. Using a rotation matrix formulation, we found that the angle between the surface plane and the m-tolunitrile molecular plane is 70 degrees. more...

Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (102) crystallographic surfaces of corundum (alpha-Al2O3) single-crystal substrates. The pH value of the point of zero charge (pH(pzc)) for each surface was determined by monitoring the SH response during three consecutive pH titrations conducted with 1, 10, and 100 mM NaNO3 carbonate-free aqueous solutions. The crossing point of the three titration curves, which corresponds to the pH(pzc), occurs at pH 4.1 +/- 0.4 for the (0001) surface and pH 5.2 +/- 0.4 for the (102) surface. SHG measurements that were recorded as a function of NaNO3 concentration at fixed pH values were found to corroborate the pH(pzc) values identified in the pH titrations. A comparison of the SHG results with surface protonation constants calculated using a simple electrostatic model suggests that surface relaxation and bonding changes resulting from surface hydration do not account for differences between experimental observations and model predictions. The measured pH(pzc) values for the alpha-Al2O3 single-crystal surfaces are significantly more acidic than published values for Al-(hydr)oxide particles which typically range from pH 8 to 10. This discrepancy suggests that the charging behavior of Al-(hydr)oxide particles is determined by surface sites associated with defects assuming that differences in surface acidity reflect differences in the coordination environment and local structure of the potential-determining surface groups. more...

The effect of a positively charged dodecyl trimethylammonium bromide (DTAB) surfactant on the aqueous solvation of coumarin 314 (C314), a solvatochromic molecule, at the air/water interface was investigated. Steady-state and femtosecond time-resolved second harmonic generation (SHG) spectroscopy was employed as a surface-selective tool to characterize the solvation of C314. The molecular orientation and the static and dynamic solvation of the probe C314 at the positively charged surfactant interface were found to be significantly different from that at the previously studied neat air/water interface and at a negatively charged surfactant air/water interface. Steady-state SHG spectra show a red shift and therefore increased local polarity at the positively charged interface (λmax = 436 nm) compared to the negatively charged interfaces (λmax = 431−432 nm) and the surfactant-free air/water interface (λmax = 423 nm), though less polar than bulk water (448 nm). Two diffusive dynamical time scales τ1 and τ2 of ... more...

The transport rate of an organic cation, malachite green (MG), across a unilamellar bilayer (∼105 nm) of the dioleoylphosphatidylglycerol (DOPG) liposome has been studied by the SHG technique. This is the first time to our knowledge that the effects of anions on molecular cation transport have been observed. Our studies show four results. First, in the presence of sodium chloride (NaCl) or sodium bromide (NaBr), the time constant for transport of MG across the DOPG bilayer increases with the increase in the concentration of the counterion (i.e., Cl- or Br-). Second, with the organic electrolytes, sodium citrate (NaCitrate) and sodium ethanesulfonate (NaEtSO3), the transport rate is independent of the concentration of the counterion (i.e., Citrate- and EtSO3-). Third, at the same counterion concentration, the transport rate depends on the species of the counterion used. The rate of MG transport is the slowest with Cl-, faster with Br-, and the fastest with the two organic counterions, Citrate- and EtSO3-. ... more...

Time-resolved second harmonic generation spectroscopy of a solvatochromic dye coumarin 314 was used to investigate how the solvation dynamics at the air/water interface are affected by lipid surfactants with anionic carboxylate and sulfate headgroups. The surfactants were chosen to mimic common functional groups and the negative charge of biological aqueous interfaces such as cell membranes and folded proteins. The diffusive component of the solvation response, associated with rearrangement of the water hydrogen bond network, exhibits a biexponential decay. The two time scales change as a function of the surface charge density, compared to the bare air/water interface solvation dynamics, τ1 = 250 ± 50 fs and τ2 = 2.0 ± 0.4 ps, which are similar to the bulk water response, τ1 = 130−250 fs and τ2 = 0.6−1.2 ps. The faster τ1 component is unaffected at low to intermediate sulfate surface coverage (270 ± 50 fs at 500 A2/molecule and 225 ± 25 fs at 250 A2/molecule) but becomes slower at high surface coverage (1... more...

We have demonstrated that second-harmonic generation (SHG) can be used to investigate the in situ transfer of molecules between colloidal microparticles. The transfer kinetics of organic cations, malachite green (MG) initially adsorbed on to spherical polystyrene sulfate (PSS) microparticles, to disk-shaped clay particles in aqueous solution was obtained. The surface population of MG on the PSS particles during the transfer process can be measured in real time. In the present studies, the adsorption free energy and the number of adsorption sites were found from the adsorption isotherm measured by the SHG method (Wang, H.; Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 4446). The kinetics of transfer was observed to depend linearly on the particle density of clay. The observed transfer was biexponential with decay times of 17 ( 1 s and 217 ( 4 s for the case of a clay particle density of 3 10 9 /cm 3 . This method was further applied to study the transfer of MG between PSS microparticles. At a PSS density of 4 10 8 /cm 3 , the decay time was 40 s. Because of the coherent nature of the SHG signal, the transfer kinetics can be obtained in situ by the SHG method. A kinetic model is used to describe the transfer process from which the desorption and adsorption rate constants of MG from and onto PSS particles were obtained. more...

The static absorption and ultrafast solvation dynamics of a tricarbocyanine dye IR144 at the silica/acetonitrile and the silica/butanol interfaces were investigated by using steady-state second harmonic generation (SHG) spectroscopy and pump-probe time-resolved SHG (TRSHG) experiments. At both interfaces, the peak wavelength for the S0 f S1 transition of adsorbed IR144 is significantly red-shifted with respect to the maximum absorption wavelength in the corresponding bulk solvent. Because IR144 is negatively solvatochromic, this shift is indicative of a smaller polarity at the interface. A theoretical model of TRSHG is discussed which allows extraction of the solvation dynamics correlation function from the time dependence of the TRSHG signal measured in the pump-probe experiment at a chosen probe wavelength. The time constant for the diffusive component of solvation at the silica/acetonitrile interface was found to be 1.05 ( 0.14 ps. It is somewhat shorter, although of the same order of magnitude, than the time constant for the diffusive solvation in bulk acetonitrile, 2.23 ps. At the silica/butanol interface, a time constant of 1.20 ( 0.15 ps was extracted, whereas in the bulk butanol, the corresponding diffusive solvation is biexponential with times of 3.5 and 33 ps. more...

Effect of the negatively charged carboxylate group of the stearic acid surfactant CH3(CH2)16COO- on the solvation at the surfactant air/water interface was investigated. A direct comparison is made... more...

The electronic excited-state solvation dynamics of coumarin 314 (C314) adsorbed at the air/water interface was investigated by femtosecond time-resolved second-harmonic generation (TRSHG). The second-harmonic spectrum of C314 at the air/water interface was determined using a tunable optical parametric generator, giving a spectral peak at 419 ± 3 nm. The solvatochromic shift is in good agreement with the recently proposed interfacial polarity scale, which predicts that the empirical polarity parameter of the interface is the arithmetic mean of the two bulk phases. The time-resolved second-harmonic response to the excited-state solvation was measured at two fundamental wavelengths (840 and 900 nm) and with two pump polarizations (S and P). The results indicate that the static spectral peaks of the S- and P-pumped distributions are offset ∼5 nm, while the dynamic interfacial solvation proceeds with two time constants, τ1 = ∼200 fs and τ2 = 1.2 ps, which are independent of C314 interfacial orientation. The sl... more...

Solvation dynamics at an air−water interface covered with a Langmuir monolayer of a neutral surfactant stearic acid CH3(CH2)16COOH was measured using femtosecond pump−probe spectroscopy of a molecu... more...

The surface density and electrolyte concentration dependences of the pKa of CH3(CH2)21NH3+ at the air/water interface are obtained using the method of second harmonic generation. If it is assumed that the amphiphiles are uniformly distributed at the interface, the application of the Gouy−Chapman model yields pKa values ranging from 10.2 to 8.2 as the interface density varies from 100 to 22 A2/molecule. By accounting for the inhomogeneity of the interface, i.e., there are amphiphiles islands of “liquid phase” density immersed in a “gas phase” of very low-density amphiphiles, a single pKa of 10.1 ± 0.2 is obtained at all interface densities. The acidity of the long chain amine is enhanced by 3 times at the air/water interface compared with the bulk aqueous solution. more...

We show that second harmonic (SH) spectroscopy, an intrinsically surface-selective technique, can be used to monitor protein (cytochrome c) adsorption to silica surfaces and negatively charged supported phospholipid bilayers. The origin of the SH signal is due to the effect of the adsorbed protein on the water molecules polarized near the charged interface. Although the protein does not contribute its own SH signal, its binding to the glass surface or the membrane reduces the polarization of the interfacial water molecules and this effect is proportional to the adsorbed protein concentration and can be monitored with high precision, in real time. The free energy of adsorption (ΔGads = −11.8 kcal/mol) of cytochrome c to glass was determined from an adsorption isotherm measurement of the SH signal as a function of the bulk protein concentration. A detection sensitivity of 0.1 pmol/cm2 of adsorbed cytochrome c protein is readily achieved. more...

This letter reports the application of second harmonic generation (SHG) to study the rotational dynamics of anisotropic microparticles. In this study, disk-shaped clay particles of montmorillonites with a diameter of 0.5 μm and a thickness of 0.01 μm suspended in liquid media were studied. Fluctuations in the SH signal observed upon irradiating the samples with the fundamental light are attributed to the rotational motion of the particles. The relaxation times obtained from the autocorrelation of the fluctuations are 0.024 ± 0.04 s for water, 0.25 ± 0.01 for glycerol/water (74:26 w/w), and 0.63 ± 0.01 s for glycerol/water (80:20 w/w). These values are of the same order of magnitudes for the relaxation times calculated using the Stokes−Einstein model for rotation of an oblate particle. more...

The ultrafast solvation dynamics of courmarin 314 (C314) molecules at the air/water interface has been shown to depend on their orientation with respect to the plane of the interface. A pump/second harmonic probe method using femtosecond laser pulses were used to measure the interface solvation dynamics. The pump light was S polarized, i.e., in the interface plane, or P polarized, i.e., in the plane perpendicular to the plane of the interface. The S pumped C314 molecules yielded a solvation time of 820 ± 60 fs, whereas P pumped yielded a solvation time of 1215 ± 90 fs. The interface inhomogeneous broadening on the solvation time scale is due to the differing orientations of C314 at the air/water interface. more...

The abundance in nature and the unique adsorptive and catalytic properties of clay particles make them a subject of fundamental and technological interest. Clay particles have long been recognized as important catalysts in petroleum-forming reactions and in the chemical evolution of certain organic compounds related to the origin of life. The large surface area make them excellent adsorbents and is the basis for their increasing use to remove organic and inorganic environmental pollutants. Industries also use them in a number of applications, e.g., decolorizing agents, carriers for insecticides, and detergent formulation. Second harmonic generation (SHG) is applied for the first time to study the surfaces of clay particles. An SHG signal was detected from an aqueous suspension of disk-shaped montmorillonite clay particles of 0.5 {micro}m diameter and 0.01 {micro}m thickness. The origin of the SHG signal is the polarization of water molecules by surface charges located in the edge region of the clay particles. On addition of the organic molecule 4-(2-pyridylazo)resorcinol (PR) to the clay aqueous suspension, a strong SHG signal originating from the PR molecules adsorbed onto the edge surfaces of the clay particles was obtained. The PR adsorbed on the basal planes do not contribute to the SHGmore » signal because of cancellation effects. These experimental results demonstrate the applicability of the SHG method for studying molecular adsorption and the electrostatic properties of clay particles.« less more...

The ultrafast orientational motions of coumarin 314 (C314) adsorbed at the air/water interface were investigated by time-resolved surface second harmonic generation (TRSHG). The theory and method of using TRSHG to detect both out-of-plane and in-plane orientational motions are discussed. The interfacial solute motions were found to be anisotropic, with differing out-of-plane and in-plane reorientation time constants. This report presents the first direct observation of in-plane orientational motion of a molecule (C314) at the air/water interface using TRSHG. The in-plane reorientation time constant is 600 ± 40 ps. The out-of-plane reorientation time constant is 350 ± 20 ps. The out-of-plane orientational motion of C314 is similar to the previous results on rhodamine 6G at the air/water interface which indicated increased interfacial friction compared with bulk aqueous solution. The surface reorientation times are 2−3 times slower than the bulk isotropic orientational diffusion time. more...

We report the first direct observation of an interfacial charge-transfer complex using second-harmonic spectroscopy. The second-harmonic spectrum of catechol adsorbed on 0.4 micron-sized TiO2 (anatase) colloidal particles in aqueous suspension reveals a charge-transfer band centered at 2.72 eV (456 nm). In addition, the adsorption isotherm of catechol on the colloidal TiO2 suspension was obtained and gave an excellent fit to the Langmuir adsorption model. From this, we infer the free energy of the adsorption to be ΔG° = −6.8 kcal/mol. more...

A new noninvasive method for determining the surface electrostatic potential and surface charge density of microscopic particles using second harmonic generation (SHG) is described. The surface electrostatic properties of 1.05 μm polystyrene sulfate spheres in aqueous solution and that of 0.22 μm oil droplets in aqueous emulsions are obtained. Comparisons of the surface potentials obtained from SHG with the zeta potential obtained from electrophoresis are in excellent agreement with theoretical predictions. more...

A new application of second harmonic generation to selectively probe the surface of microscopic size centrosymmetric structures in centrosymmetric bulk media has been applied to polymer beads in aqueous solution and to oil droplets in an oil/water emulsion. The free energies of adsorption and the surface densities of organic molecules adsorbed from aqueous solution to 1 μm polystyrene bead surfaces and to 230 nm tetradecane oil droplet surfaces have been determined. The potential use of this method as a chemical and biochemical sensor without the need for a fluorescent or magnetic tag is noted. more...

The discovery of structural phase transitions in a series of alkanenitriles, acetonitrile (CH3CN), propionitrile (CH3CH2CN), nonadecanitrile [CD3(CH2)19CN], but not butyronitrile (CH3CH2CH2CN), at the air/water is described. Using sum frequency spectroscopy, the phase transition is manifested by an abrupt change in the orientation and an abrupt change in the vibrational frequency of the CN head group of the interface nitrile molecules. The competition between the distance dependent nitrile−nitrile dipolar interactions and the hydrogen bonding and solvation of the CN moieties is used to describe the phase transitions. A new application of second harmonic and sum frequency generation to study the interfaces of centrosymmetric microscopic particles is presented. Examples of this new use to the adsorption of molecules to a polystyrene microsphere/aqueous interface and the polarization of bulk water molecules by a charged microsphere of polystyrene sulfate are discussed. more...

A powerful approach to the study of interfaces has been developing rapidly in the past decade. It is based on the spectroscopic methods of second-harmonic (SHG) and sum-frequency generation (SFG). These nonlinear optical techniques, being spectroscopic, provide information at the most fundamental level. A microscopic description of equilibrium and dynamic interface processes requires knowledge of the molecules at the interface, their orientational structure, the energetics that drive chemical and physical processes, and the time scale of molecular motions and relaxation processes. The techniques of second-harmonic and sum-frequency generation have made it possible to selectively probe the chemistry, physics, and biology of gas/liquid, liquid/liquid, liquid/solid, gas/solid, and solid/solid interfaces at the molecular level. In this abbreviated article only liquid interfaces will be considered; gas/solid and solid/solid interfaces are not included. This restriction is necessary because of the enormous increase in SH and SF studies in recent years, which makes it extremely difficult to properly discuss the range of work being carried out around the world. Unfortunately not all of the fine work even in the area of liquid interfaces has been included because of both space and time limitations. A number of review articles are referred to which cover some ofmore » the research material not covered in this article. 99 refs.« less more...

The study of photochemical and photophysical processes at various liquid interfaces using second harmonic generation methods is described. Among the topics discussed are the dynamics of photoinduced structure changes, the transport of charge across an interface, the rotational motions of interfacial molecules, intermolecular energy transfer within the interface, interfacial photopolymerization, and photoprocesses at a semiconductor/liquid interface. more...

Recent experimental and theoretical studies of electron solvation in liquid water have led to a detailed kinetic picture of this process. The electron solvation dynamics can be viewed as an excited state relaxation process between different electronic energy levels of the solvated electron. Simulations have suggested the importance of direct relaxation from upper excited electronic states as well as from the lowest excited electronic state (the wet electron state) to the ground electronic state. Using this as a model to analyze the experimental data, it is found that the quantum yield of wet electron formation cannot be less than 0.5 and that the peak of the wet electron absorption spectrum shifts to the IR with a decrease of wet electron formation quantum yield, with limiting values for the peak of 1.1−1.5 eV. more...

We have performed femtosecond studies of electron solvation in a series of neat linear alcohols. The measured formation time of the solvated electron is consistent with previous studies using solutes. The major character of electron solvation, i.e. the two-state character, is evidenced by the finding of an approximate isosbestic wavelength for methanol at 750 nm. Isotope effects for CH{sub 3}OH, CH{sub 3}OD, and CD{sub 3}OD as well as chain length effects in slowing the solvation dynamics are attributed to hydroxyl librational and translational motions of the alcohol molecules that form the electron cavity. A slower process was also observed, which is attributed to the cooling of the solvated electron as it `vibrationally` relaxes in the ground electron state. In addition to electrons, an ultrafast transient at early times is clearly resolved, this transient species is attributed to either the neutral excited state or the cation of the alcohol. 31 refs., 4 figs. more...

IR + visible sum frequency generation (SFG) has been used to obtain the vibrational spectra and orientations for both the nitrile (CN) head group and the methyl (CD[sub 3]) tail of the insoluble long-chain amphiphile CD[sub 3](CH[sub 2])[sub 19]CN at the air/water interface. The results show that the orientations of the head group and the terminal group vary with amphiphile surface density, but in markedly different ways. It is found at the amphiphile density, which corresponds to the phase transition from the gas/liquid coexistence region to the liquid region, that the hydrogen bonds of the CN head groups to water are broken, the water is squeezed out of the monolayer, and the orientation of the head group changes sharply. The possibility that the rupturing of the hydrogen bonds and reorientation of the CN head groups may be the trigger for the phase transition is discussed. In addition to the SFG study of the amphiphile at the air/water interface, a method was discovered that enables SFG to remove the ambiguity in the ratio of the two Raman polarizability elements for a symmetric stretch. 30 refs., 6 figs. more...

The electronic relaxation and photodetachment dynamics of an electron originating from an aqueous iodide ion have been studied using femtosecond absorption spectroscopy. The results indicate the importance of an upper excited charge transfer to solvent (CTTS) state, of odd parity, in the potential well of the solvated ion. The upper excited CTTS state, which is accessed by two-photon excitation, relaxes in more...

The authors have shown how the fluctuations in the signal from surface second harmonic generation can be utilized for the study of a heterogeneous surface such as palmitic acid (C{sub 15}H{sub 31}COOH) spread on the air/water interface, under conditions of gas-liquid coexistence. The authors report observations of time-correlated fluctuations in the SH signal, with decay constant of approximately 6 s. This is attributed to motions of the liquidlike clusters of palmitic acid. If the motion is diffusive, a diffusion constant of about 10{sup {minus}8} cm{sup 2}/s is estimated for these clusters. more...

The picosecond and femtosecond decays observed in earlier laser experiments on neat alkanes are consistent with the Onsager model of geminate recombination as well as previous experimental studies using {gamma}-ray sources. Recently, however, there have been femto-second dynamic studies of neat alkanes in which a different interpretation of the kinetic data has been proposed. The authors believe this interpretation to be incorrect, and they present data on both linear and branched alkanes. 21 refs., 3 figs. more...

The effects of penta- and hexadecanoic acids on the population and orientational structure of p-nitrophenol, at the airlwater interface at a bulk pH of 2, have been studied by using the technique of second-harmonic generation. The results indicate that, with increasing fatty acid at the interface, (1) the population of p-nitrophenol at the airlwater interface first increases and then decreases, (2) the long axis of p-nitrophenol is oriented increasingly toward the surface normal, and (3) the behavior of p-nitrophenol is not sensitive to the monolayer liquid-expanded-liquid-condensed phase transition. This indicates that the p-nitrophenol is adsorbed near the polar group of the fatty acid and thus is a good probe of the lipid head group region. more...