High yield, single crystal ice via the Bridgman method.

The surface chemistry of ice and of water is an important topic of study, especially given the role of ice and water in shaping the environment. Although snow, granular, and polycrystalline ice are often used in research, there are applications where large surface areas of a known crystallographic plane are required. For example, fundamental spectroscopy or scattering studies rely on large area samples of known crystalline orientation. In addition, due to its slower dynamics and decreased number of molecular configurations, ice can be viewed as a reduced complexity model for the complex hydrogen bonding environment found at the surface and within the bulk of liquid water. In our studies using Sum Frequency Generation (SFG) vibrational spectroscopy, we have shown that each crystalline face has a unique spectral signature and therefore a unique chemistry and chemical activity. A reliable, reproducible, high performance method of producing large single crystal samples is needed to support this surface chemistry research. The design, construction, and use of a computer-controlled, ice-growth machine based on the Stockbarger modified Bridgeman technique is described. The instrument reliably produces relatively large single crystals that are optically flawless (that is, no visible flaws when viewed in a crossed polarizer), and in very high yield. Success rates of 95% are typical. Such performance has not been observed in the literature.

Hydrogen bonding in the hexagonal ice surface.

A recently developed technique in sum frequency generation spectroscopy, polarization angle null (or PAN-SFG), is applied to two orientations of the prism face of hexagonal ice. It is found that the vibrational modes of the surface are similar in different faces. As in the basal face, the prism face of ice contains five dominant resonances: 3096, 3146, 3205, 3253, and 3386 cm(-1). On the basal face, the reddest resonance occurs at 3098 cm(-1); within the bandwidth, the same as the prism face. On both the prism and basal faces, this mode contains a significant quadrupole component and is assigned to the bilayer stitching hydrogen bonds. The bluest of the resonances, 3386 cm(-1), occurs slightly blue-shifted at 3393 cm(-1) in the basal face. The prism face has two orientations: one with the optic or c axis in the input plane (the plane formed by the surface normal and the interrogating beam propagation) and one with the c axis perpendicular to the input plane. The 3386 cm(-1) mode has significant intensity only with the c axis in the input plane. On the basis of these orientation characteristics, the 3386 cm(-1) mode is assigned to double-donor molecules in either the top half bilayer or in the lower half bilayer. On the basis of frequency considerations, it is assigned to double-donor molecules in the top half bilayer. These are water molecules containing a nonbonded lone pair. In addition to identification of the components of the broad hydrogen-bonded region, PAN-SFG measures the tangential vs longitudinal content of the vibrational modes. In accord with previous suggestions, the lower frequency modes are predominantly tangential, whereas the higher frequency modes are mainly longitudinal. On the prism face, the 3386 cm(-1) mode is entirely longitudinal.

Multiplexed polarization spectroscopy: measuring surface hyperpolarizability orientation.

Infrared-visible sum frequency generation (SFG) has seen increasing usage as a surface probe, particularly for liquid interfaces since they are amenable to few alternate probes. Interpreting the SFG data to arrive at a molecular-level configuration on the surface, however, remains a challenge. This paper reports a technique for analyzing and interpreting SFG data--called polarization-angle null or PAN-SFG. PAN-SFG enables ready identification of the ratio of the surface tangential and longitudinal hyperpolarizabilities--the hyperpolarizability direction--as well as the phase relationship between these components separated from the optical factors due to the substrate and experimental geometry. Separation of the surface optical factors results in an immediate connection between the null angle and the surface species polarization. If the Raman polarizability is also known, then PAN-SFG analysis, like the previously reported null techniques, provides a very accurate orientation. In addition, the reported polarization-angle, phase-shift analysis enables facile separation of the nonresonant background polarization from that of the resonant signal. Beyond orientation, PAN-SFG can be used to deconvolute overlapping resonances and identify components beyond a dipole response. This paper reports PAN-SFG for two systems providing deeper insight into both. An acetonitrile-water mixture was previously reported to undergo a phase transition at 7 mol %, attributed to a sudden change in orientation. PAN-SFG demonstrates that acetonitrile generates a classic dipole response and provides compelling evidence that the acetonitrile configuration remains constant as a function of concentration. An alternate model for the phase transition is presented. Like many aqueous systems, the SFG spectrum of the hydrogen-bonded region of ice consists of broad and overlapping features; features previously identified with PAN-SFG. Here PAN-SFG analysis is used to show that the reddest of these, the feature at 3098 cm(-1), contains a significant quadrupole contribution that grows as the temperature is lowered. The quadrupole and its temperature dependence are used to assign the 3098 cm(-1) feature to bilayer-stitching-hydrogen bonds. This is the first definitive assignment in the hydrogen-bonded region of water.

Sum-frequency generation: polarization surface spectroscopy analysis of the vibrational surface modes on the basal face of ice I(h).

In recent years, sum-frequency generation (SFG) has been used to investigate numerous interfaces including aqueous interfaces. A longstanding challenge to interpretation of the SFG results, along with the related aqueous-solution infrared and Raman spectra, is a lack of connection between features in the broad hydrogen-bonded region and molecular-level interactions or configurations. This paper reports results of a newly developed polarization analysis of the generated sum-frequency signal as a function of wavelength both to deconvolute spectral resonances and to characterize the dynamic polarization associated with the resonances. Operationally, the polarization angle of the generated sum frequency is determined by identifying the null angle. The technique is hence termed polarization-angle null analysis or PAN. PAN applied to ice is very powerful; it reveals that the hydrogen-bonded region of the basal face of ice I(h) contains at least five oscillators, each with a distinct polarization. The dynamic polarizability of the longest wavelength oscillator is nearly entirely transverse (perpendicular to the surface normal, i.e., in the surface plane); in contrast, the shortest wavelength oscillator is almost entirely longitudinal (along the surface normal).

The single-crystal, basal face of ice I(h) investigated with sum frequency generation.

Sum frequency generation spectroscopy has been used to investigate the hydrogen-bonded region of single-crystal, hexagonal ice in the temperature range of 113-178 K. The temperature and polarization dependences of the signal are used in conjunction with a recent theoretical model to suggest an interpretation of the bluest and reddest of the hydrogen-bonded peaks. The reddest feature is associated with strong hydrogen bonding; the dynamic polarizability of this feature is primarily parallel to the surface. It is assigned to a cooperative motion among the companion to the free-OH and four-coordinate oscillators hydrogen bonded to dangling lone-pair molecules on the surface. The bluest hydrogen-bonded feature is similarly assigned to a cooperative motion of the OH stretch of dangling lone-pair molecules and of four-coordinate molecules in the lower half bilayer that are hydrogen bonded to free-OH molecules. Reconstruction induced strain is present at as low as 113 K. These results provide a richer picture of the ice surface than has heretofore been possible.

Comparative study of acetic acid, methanol, and water adsorbed on anatase TiO2 probed by sum frequency generation spectroscopy.

Sum frequency generation (SFG) vibrational spectroscopy is used to investigate the surface adsorption of three probe molecules-acetic acid, methanol, and water--on a film composed of nanoscale anatase TiO(2) particles. On the TiO(2) surface, only one adsorption mode, chemisorption, is observed for acetic acid. This is evidenced by one sharp SFG peak in the C-H region, which is stable with time and robust both to evacuation and to the addition of water. A Langmuir constant of (9.21 +/- 0.71) x 10(3) is determined from the adsorption isotherm. In the case of methanol adsorption, however, there are two adsorption modes, molecular physisorption and dissociative chemisorption. The corresponding SFG signals are stable with time but diminished with addition of water. Changes in the SFG features for methanol and for the methoxy species with addition of water and subsequent evacuation provide the first experimental proof of reversible hydroxylation and dehydroxylation at the TiO(2) surface. For water adsorption, only one mode, physisorption, is observed on the hydroxylated TiO(2) surface. The water adlayer is mobile, as is evidenced by variation of the water H-bonded SFG signal with time. Competitive adsorption among the three molecular probes is clearly resolved by in situ SFG measurements. The adsorption strength follows the order acetic acid (strongest), methanol, water (weakest). The adsorption order as well as the difference in response of methanol versus acetic acid adsorption to addition of water has direct implications for understanding TiO(2) photocatalysis as well as the surface modifications involved in TiO(2) photoelectrochemical solar cells and processes in TiO(2) nanomaterial synthesis and assembly.

Direct observation of competitive adsorption between methanol and water on TiO2: an in situ sum-frequency generation study.

The competitive adsorption between water and methanol, as well as reversible hydroxylation/dehydroxylation on the TiO2 surface, is unambiguously resolved for the first time by an in situ sum-frequency generation study.

Diffusional fluorescence quenching of aromatic hydrocarbons.

The quenching of the fluorescence of five aromatic hydrocarbons by three halogenated organics and by molecular oxygen has been measured. Both fluorescence intensity and fluorescence lifetime measurements have been employed to validate results and interpretation; linear Stern-Volmer analyses are shown to apply throughout. The fluorescence quenching rate constant of molecular oxygen for the five aromatic hydrocarbons is essentially equivalent to the diffusion rate constant independent of the fluorophore excitation energy. The halogenated organic-fluorophore rate constants vary by a factor of 965 and are shown to correlate roughly with the energy difference between the quencher and fluorophore excited electronic states in accord with a standard model of quantum two-level mixing. The value of the coupling interaction energy is approximately 2500 cm(-1).