Two-dimensional infrared spectroscopy of isotope-diluted ice Ih

We present experimental 2D IR spectra of isotope diluted ice Ih (i.e., the OH stretch mode of HOD in D(2)O and the OD stretch mode of HOD in H(2)O) at T = 80 K. The main spectral features are the extremely broad 1-2 excited state transition, much broader than the corresponding 0-1 groundstate transition, as well as the presence of quantum beats. We do not observe any inhomogeneous broadening that might be expected due to proton disorder in ice Ih. Complementary, we perform simulations in the framework of the Lippincott-Schroeder model, which qualitatively reproduce the experimental observations. We conclude that the origin of the observed line shape features is the coupling of the OH-vibrational coordinate with crystal phonons and explain the beatings as a coherent oscillation of the O···O hydrogen bond degree of freedom.     

Transient absorption of vibrationally excited ice Ih

The ultrafast dynamics of HDO:D2O ice Ih at 180 K is studied by midinfrared ultrafast pump-probe spectroscopy. The vibrational relaxation of HDO:D2O ice is observed to proceed via an intermediate state, which has a blueshifted absorption spectrum. Polarization resolved measurements reveal that the intermediate state is part of the intramolecular relaxation pathway of the HDO molecule. In addition, slow dynamics on a time scale of the order of 10-100 ps is observed, related to thermally induced collective reorganizations of the ice lattice. The transient absorption line shape is analyzed within a Lippincott-Schroeder model for the OH-stretch potential. This analysis identifies the main mechanism behind the strong spectral broadening of the v(OH)=1-->2 transition.     

Ultrafast vibrational dynamics of water at a charged interface revealed by two-dimensional heterodyne-detected vibrational sum frequency generation

Two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy is performed for an aqueous interface for the first time. The 2D HD-VSFG spectra in the OH stretch region are obtained from a positively charged surfactant∕water interface with isotopically diluted water (HOD∕D(2)O) to reveal the femtosecond vibrational dynamics of water at the charged interface. The 2D HD-VSFG spectrum is diagonally elongated immediately after photoexcitation, clearly demonstrating inhomogeneity in the interfacial water. This elongation almost disappears at 300 fs owing to the spectral diffusion. Interestingly, the 2D HD-VSFG spectrum at the 0 fs shows an oppositely asymmetric shape to the corresponding 2D IR spectrum in bulk water: The bandwidth of the bleach signal gets narrower when the pump wavenumber becomes higher. This suggests that the dynamics and mechanism of the hydrogen bond rearrangement at the charged interface are significantly different from those in bulk water.     

Multidimensional infrared spectroscopy of water. I. Vibrational dynamics in two-dimensional IR line shapes

In this and the following paper, we describe the ultrafast structural fluctuations and rearrangements of the hydrogen bonding network of water using two-dimensional (2D) infrared spectroscopy. 2D IR spectra covering all the relevant time scales of molecular dynamics of the hydrogen bonding network of water were studied for the OH stretching absorption of HOD in D2O. Time-dependent evolution of the 2D IR line shape serves as a spectroscopic observable that tracks how different hydrogen bonding environments interconvert while changes in spectral intensity result from vibrational relaxation and molecular reorientation of the OH dipole. For waiting times up to the vibrational lifetime of 700 fs, changes in the 2D line shape reflect the spectral evolution of OH oscillators induced by hydrogen bond dynamics. These dynamics, characterized through a set of 2D line shape analysis metrics, show a rapid 60 fs decay, an underdamped oscillation on a 130 fs time scale induced by hydrogen bond stretching, and a long time decay constant of 1.4 ps. 2D surfaces for waiting times larger than 700 fs are dominated by the effects of vibrational relaxation and the thermalization of this excess energy by the solvent bath. Our modeling based on fluctuations with Gaussian statistics is able to reproduce the changes in dispersed pump-probe and 2D IR spectra induced by these relaxation processes, but misses the asymmetry resulting from frequency-dependent spectral diffusion. The dynamical origin of this asymmetry is discussed in the companion paper.     

Infrared and Raman line shapes of dilute HOD in liquid H2O and D2O from 10 to 90 degrees C

A combined electronic structure/molecular dynamics approach was used to calculate infrared and isotropic Raman spectra for the OH or OD stretches of dilute HOD in D2O or H2O, respectively. The quantities needed to compute the infrared and Raman spectra were obtained from density functional theory calculations performed on clusters, generated from liquid-state configurations, containing an HOD molecule along with 4-9 solvent water molecules. The frequency, transition dipole, and isotropic transition polarizability were each empirically related to the electric field due to the solvent along the OH (or OD) bond, calculated on the H (or D) atom of interest. The frequency and transition dipole moment of the OH (or OD) stretch of the HOD molecule were found to be very sensitive to its instantaneous solvent environment, as opposed to the isotropic transition polarizability, which was found to be relatively insensitive to environment. Infrared and isotropic Raman spectra were computed within a molecular dynamics simulation by using the empirical relationships and semiclassical expressions for the line shapes. The line shapes agree well with experiment over a temperature range from 10 to 90 degrees C.     

Wetting of mixed OHH(2)O layers on Pt(111)

We describe the effect of growth temperature and OHH(2)O composition on the wetting behavior of Pt(111). Changes to the desorption rate of ice films were measured and correlated to the film morphology using low energy electron diffraction and thermal desorption of chloroform to measure the area of multilayer ice and monolayer OHH(2)O exposed. Thin ice films roughen, forming bare (radical39 x radical39)R16 degrees water monolayer and ice clusters. The size of the clusters depends on growth temperature and determines their kinetic stability, with the desorption rate decreasing when larger clusters are formed by growth at high temperature. Continuous films of more than approximately 50 layers thick stabilize an ordered incommensurate ice film that does not dewet. OH coadsorption pins the first layer into registry with Pt, forming an ordered hexagonal (OH+H(2)O) structure with all the H atoms involved in hydrogen bonding. Although this layer has a similar honeycomb OH(x) skeleton to ice Ih, it is unable to reconstruct to match the bulk ice lattice parameter and does not form a stable wetting layer. Water aggregates to expose bare monolayer (OH+H(2)O), forming bulk ice crystallites whose size depend on preparation temperature. Increasing the proportion of water in the first layer provides free OH groups which stabilize the multilayer. The factors influencing multilayer wetting are discussed using density functional theory calculations to compare water adsorption on top of (OH+H(2)O) and on simple models for commensurate water structures. We show that both the (OH+H(2)O) structure and "H-down" water layers are poor proton acceptors, bonding to the first layer being enhanced by the presence of free OH groups. Formation of an ordered ice multilayer requires a water-metal interaction sufficient to wet the surface, but not so strong as to prevent the first layer relaxing to stabilize the interface between the metal and bulk ice.     

Dispersion of the Raman depolarization ratio of HDO in water and heavy water from 295 to 368 K, and from concentrated NaClO4D2OH2O

The dispersion of the Raman depolarization ratio rho(L) was measured for HDO in H(2)O and in D(2)O. rho(L) for the decoupled OD stretch displays a maximum at 2575 +/- 15 cm(-1) at 296 K and a minimum at 2675 +/- 15 cm(-1), in agreement with the isosbestic point 2570 +/- 10 cm(-1), and the enthalpy dispersion maximum, 2650-2675 cm(-1), respectively. However, three extrema were uncovered in rho(L) for the OH stretch of HDO in D(2)O, and their positions agree with the frequencies of a minimum and a maximum in the enthalpy dispersion and with the isosbestic frequency. The frequency of the rho(L) maximum (OH stretch) lies just above the frequency corresponding to the joint angle-frequency probability maximum. [Lawrence and Skinner, J. Chem. Phys. 118, 264 (2003)]. The low- and high-frequency minima in rho(L) (OH stretch), correspond, respectively, to very strong H-bonds, and extremely weak, long, bent H bonds. The frequencies of the maxima and minima in rho(L) for the decoupled OH and OD stretches are independent of temperature within experimental error between 295 and 368 K. rho(L) was also measured for the OD stretch from saturated NaClO(4) in D(2)OH(2)O; it displays a maximum at 2560 +/- 20 cm(-1) and a sharp minimum at 2650 +/- 5 cm(-1). The shape of the dispersion of (betaalpha)(2) approximately rho(L) for HDO in D(2)O was calculated with the aid of the molecular dynamics results of Lawrence and Skinner. beta(2) is the anisotropic polarizability and alpha is the isotropic polarizability. A maximum resulted in the calculated dispersion at 3400 +/- 10 cm(-1), in excellent agreement with the measured maximum of 3395 +/- 15 cm(-1). The H-bond angles decrease far below 180 degrees as the OH-stretching frequency increases to 3700 cm(-1) and above. Such small H-bond angles, and very large O-O distances, are tantamount to broken H-bonds and are thought to produce the minimum in rho(L) near 2650 cm(-1).     

Investigation of the hydrogen bonding in ice Ih by first-principles density function methods

It is a well recognized difficult task to simulate the vibrational dynamics of ices using the density functional theory (DFT), and there has thus been rather limited success in modelling the inelastic neutron scattering (INS) spectra for even the simplest structure of ice, ice Ih, particularly in the translational region below 400 cm(-1). The reason is partly due to the complex nature of hydrogen bonding (H-bond) among water-water molecules which require considerable improvement of the quantum mechanical simulation methods, and partly owing to the randomness of protons in ice structures which often requires simulation of large super-lattices. In this report, we present the first series of successful simulation results for ice Ih using DFT methods. On the basis of the recent advancement in the DFT programs, we have achieved for the first time theoretical outcomes that not only reproduce the rotational frequencies between 500 to 1200 cm(-1) for ice Ih, but also the two optic peaks at ～240 and 320 cm(-1) in the translational region of the INS spectra [J. C. Li, J. Chem. Phys 105, 6733 (1996)]. Besides, we have also investigated the impact of pairwise configurations of H(2)O molecules on the H-bond and found that different proton arrangements of pairwise H(2)O in the ice Ih crystal lattice could not alter the nature of H-bond as significantly as suggested in an early paper [J. C. Li and D. K. Ross, Nature (London) 365, 327 (1993)], i.e., reproducing the two experimental optic peaks do not need to invoke the two H-bonds as proposed in the previous model which led to considerable debates. The results of this work suggest that the observed optic peaks may be attributed to the coupling between the two bands of H-O stretching modes in H(2)O. The current computational work is expected to shed new light on the nature of the H-bonds in water, and in addition to offer a new approach towards probing the interaction between water and biomaterials for which H-bond is essential.     

Combined electronic structure/molecular dynamics approach for ultrafast infrared spectroscopy of dilute HOD in liquid H2O and D2O

We present a new approach that combines electronic structure methods and molecular dynamics simulations to investigate the infrared spectroscopy of condensed phase systems. This approach is applied to the OH stretch band of dilute HOD in liquid D2O and the OD stretch band of dilute HOD in liquid H2O for two commonly employed models of water, TIP4P and SPC/E. Ab initio OH and OD anharmonic transition frequencies are calculated for 100 HOD x (D2O)n and HOD x(H2O)n (n = 4-9) clusters randomly selected from liquid water simulations. A linear empirical relationship between the ab initio frequencies and the component of the electric field from the solvent along the bond of interest is developed. This relationship is used in a molecular dynamics simulation to compute frequency fluctuation time-correlation functions and infrared absorption line shapes. The normalized frequency fluctuation time-correlation functions are in good agreement with the results of previous theoretical approaches. Their long-time decay times are 0.5 ps for the TIP4P model and 0.9 ps for the SPC/E model, both of which appear to be somewhat too fast compared to recent experiments. The calculated line shapes are in good agreement with experiment, and improve upon the results of previous theoretical approaches. The methods presented are simple, and transferable to more complicated systems.     

Ultrafast vibrational energy relaxation of the water bridge

We report the energy relaxation of the OH stretch vibration of HDO molecules contained in an HDO:D(2)O water bridge using femtosecond mid-infrared pump-probe spectroscopy. We found that the vibrational lifetime is shorter (~630 ± 50 fs) than for HDO molecules in bulk HDO:D(2)O (~740 ± 40 fs). In contrast, the thermalization dynamics following the vibrational relaxation are much slower (~1.5 ± 0.4 ps) than in bulk HDO:D(2)O (~250 ± 90 fs). These differences in energy relaxation dynamics strongly indicate that the water bridge and bulk water differ on a molecular scale.     

Isotope effects in ice Ih: a path-integral simulation

Ice Ih has been studied by path-integral molecular dynamics simulations, using the effective q-TIP4P/F potential model for flexible water. This has allowed us to analyze finite-temperature quantum effects in this solid phase from 25 to 300 K at ambient pressure. Among these effects we find a negative thermal expansion of ice at low temperatures, which does not appear in classical molecular dynamics simulations. The compressibility derived from volume fluctuations gives results in line with experimental data. We have analyzed isotope effects in ice Ih by considering normal, heavy, and tritiated water. In particular, we studied the effect of changing the isotopic mass of hydrogen on the kinetic energy and atomic delocalization in the crystal as well as on structural properties such as interatomic distances and molar volume. For D(2)O ice Ih at 100 K we obtained a decrease in molar volume and intramolecular O-H distance of 0.6% and 0.4%, respectively, as compared to H(2)O ice.     

Excited-state intermolecular proton transfer of firefly luciferin III. Proton transfer to a mild base

Steady-state and time-resolved techniques were employed to study the excited-state proton transfer (ESPT) from d-luciferin, the natural substrate of the firefly luciferase, to the mild acetate base in aqueous solutions. We found that in 1 M aqueous solutions of acetate or higher, a proton transfer (PT) process to the acetate takes place within 30 ps in both H(2)O and D(2)O solutions. The time-resolved emission signal is composed of three components. We found that the short-time component decay time is 300 and 600 fs in H(2)O and D(2)O, respectively. This component is attributed either to a PT process via the shortest water bridged complex available, ROH··H(2)O··Ac(-), or to PT taking place within a contact ion pair. The second time component of 2000 and 3000 fs for H(2)O and D(2)O, respectively, is attributed to ROH* acetate complex, whose proton wire is longer by one water molecule. The decay rate of the third, long-time component is proportional to the acetate concentration. We attribute it to the diffusion-assisted reaction as well as to PT process to the solvent.     

Azide-water intermolecular coupling measured by two-color two-dimensional infrared spectroscopy

We utilize two-color two-dimensional infrared spectroscopy to measure the intermolecular coupling between azide ions and their surrounding water molecules in order to gain information about the nature of hydrogen bonding of water to ions. Our findings indicate that the main spectral contribution to the intermolecular cross-peak comes from population transfer between the asymmetric stretch vibration of azide and the OD-stretch vibration of D(2)O. The azide-bound D(2)O bleach/stimulated emission signal, which is spectrally much narrower than its linear absorption spectrum, shows that the experiment is selective to solvation shell water molecules for population times up to ~500 fs. The waters around the ion are present in an electrostatically better defined environment. Afterwards, ~1 ps, the sample thermalizes and selectivity is lost. On the other hand, the excited state absorption signal of the azide-bound D(2)O is much broader. The asymmetry in spectral width between bleach/stimulated emission versus excited absorption has been observed in very much the same way for isotope-diluted ice Ih, where it has been attributed to the anharmonicity of the OD potential.     

Energy relaxation dynamics of the hydration complex of hydroxide

We use polarization-resolved mid-infrared pump-probe spectroscopy to study the dynamics of the hydration shells of hydroxide ions (OH(-)). We excite the OH stretch vibrations of H(2)O molecules solvating the OH(-) ion and observe that this excitation decays with a relaxation time constant T(1) of 200 fs. This relaxation is followed by a thermalization process that becomes slower with increasing concentration of OH(-). The prethermalized state is observed to be anisotropic, showing that the energy of the excited OH stretch vibrations is dissipated within the hydration complex. The anisotropy of the prethermalized state decays both as a result of the reorientation of the OH(-) hydration complex and heat diffusion from the excited complexes to unexcited complexes. Modeling the anisotropy data at different concentrations allows for an accurate estimate of the number of water molecules in the hydration shell of OH(-), the reorientation dynamics of the OH(-) hydration complex, and the molecular-scale heat diffusivity.     

Vibrational relaxation of CN stretch of pseudo-halide anions (OCN-, SCN-, and SeCN-) in polar solvents

The vibrational relaxation dynamics of pseudo-halide anions XCN- (X = O, S, Se) in polar solvents were studied to understand the effect of charge on solute-to-solvent intermolecular energy transfer (IET) and solvent assisted intramolecular vibrational relaxation (IVR) pathways. The T1 relaxation times of the CN stretch in these anions were measured by IR pump/IR probe spectroscopy, in which the 0-1 transition was excited, and the 0-1 and 1-2 transitions were monitored to follow the recovery of the ground state and decay of the excited state. For these anions in five solvents, H2O, D2O, CH3OH, CH3CN, and (CH3)2SO, relaxation rates followed the trend of OCN- > SCN- > SeCN-. For these anions and isotopes of SCN-, the relaxation rate was a factor of a few (2.5-10) higher in H2O than in D2O. To further probe the solvent isotope effect, the relaxation rates of S12C14N-, S13C14N-, and S12C15N- in deuterated methanols (CH3OH, CH3OD, CH3OH, CD3OD) were compared. Relaxation rate was found to be affected by the change of solvent vibrational band at the CN- stretching mode (CD3 symmetric stretch) and lower frequency regions, suggesting the presence of both direct IET and solvent assisted IVR relaxation pathways. The possible relaxation pathways and mechanisms for the observed trends in solute and solvent dependence were discussed.     

Multidimensional infrared spectroscopy of water. II. Hydrogen bond switching dynamics

We use multidimensional infrared spectroscopy of the OH stretch of HOD in D2O to measure the interconversion of different hydrogen bonding environments. The OH stretching frequency distinguishes hydrogen bonded (HB) and non-hydrogen-bonded (NHB) configurations by their absorption on the low (red) and high (blue) sides of the line shape. Measured asymmetries in the two dimensional infrared OH line shapes are manifestations of the fundamentally different spectral relaxations of HB and NHB. HB oscillators exhibit coherent oscillations within the hydrogen-bonded free energy well before undergoing activated barrier crossing, resulting in the exchange of hydrogen bonded partners. Conversely, NHB oscillators rapidly return to HB frequencies within 150 fs. These results support a picture where NHB configurations are only visited transiently during large fluctuations about a hydrogen bond or during the switching of hydrogen bonding partners. The results are not consistent with the presence of entropically stabilized dangling hydrogen bonds or a conceptual picture of water as a mixture of environments with varying hydrogen bond strength separated by barriers >kT.     

Molecular dynamics simulations of D2O ice photodesorption

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D(2)O in an amorphous D(2)O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H(2)O, the main processes after UV photodissociation are trapping and desorption of either fragments or D(2)O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D(2)O molecule photodesorption. D(2)O desorption takes places either by direct desorption of a recombined D(2)O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D(2)O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D(2)O in the top four monolayers and are compared quantitatively with those for H(2)O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5-2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D(2)O photodesorption probability is larger than that of H(2)O (by about a factor of 1.4-2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D(2)O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D(2)O ice, because the D atom formed after D(2)O photodissociation has a larger momentum than photogenerated H atoms from H(2)O, and D transfers momentum more easily to D(2)O than H to H(2)O. The total (OD + D(2)O) yield has been compared with experiments and the total (OH + H(2)O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D(2)O ice than when we compare with calculated yields for H(2)O ice.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). II. Velocity map imaging studies

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ? CH(3)O (CD(2)OH ? CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).     

Time-domain calculations of the polarized Raman spectra, the transient infrared absorption anisotropy, and the extent of delocalization of the OH stretching mode of liquid water

The polarized Raman spectrum and the time dependence of the transient infrared (TRIR) absorption anisotropy are calculated for the OH stretching mode of liquid water (neat liquid H2O) by using time-domain formulations, which include the effects of both the diagonal frequency modulations (of individual oscillators) induced by the interactions between the dipole derivatives and the intermolecular electric field, and the off-diagonal (intermolecular) vibrational coupling described by the transition dipole coupling (TDC) mechanism. The IR spectrum of neat liquid H2O and the TRIR anisotropy of a liquid mixture of H2O/HDO/D2O are also calculated. It is shown that the calculated features of these optical signals, including the temperature dependence of the polarized Raman and IR spectra, are in reasonable agreement with the experimental results, indicating that the frequency separation between the isotropic and anisotropic components of the polarized Raman spectrum and the rapid decay (approximately 0.1 ps) of the TRIR anisotropy of the OH stretching mode of neat liquid H2O are mainly controlled by the resonant intermolecular vibrational coupling described by the TDC mechanism. Comparing with the time evolution of vibrational excitations, it is suggested that the TRIR anisotropy decays in the time needed for the initially localized vibrational excitations to delocalize over a few oscillators. It is also shown that the enhancement of the dipole derivatives by the interactions with surrounding molecules is an important factor in generating the spectral profiles of the OH stretching Raman band. The time-domain behavior of the molecular motions that affect the spectroscopic features is discussed.