Existence of weakly interacting OH bond at air/water interface

Sum rule comparison of narrowband and broadband sum frequency generation spectra and comparison with theory

Earlier sum frequency generation (SFG) experiments involve one infrared and one visible laser, and a measurement of the intensity of the response, yielding data on the surface sensitive properties of the sample. Recently, both the real and imaginary components of the susceptibility were measured in two different sets of experiments. In one set, a broadband infrared laser was used, permitting observations at very short times, while in another set the infrared laser was narrowband, permitting higher spectral resolution. The differences in the spectrum obtained by the two will be most evident in studying narrow absorption bands, e.g., the band due to dangling OH bonds at a water interface. The direct comparisons in the integrated amplitude (sum rule) of the imaginary part of the dangling OH bond region differ by a factor of 3. Due to variations in experimental setup and data processing, corrections were made for the quartz reference, Fresnel factors, and the incident visible laser wavelength. After the corrections, the agreement differs now by the factors of 1.1 within broadband and narrowband groups and the two groups now differ by a factor of 1.5. The 1.5 factor may arise from the extra heating of the more powerful broadband laser system on the water surface. The convolution from the narrowband SFG spectrum to the broadband SFG spectrum is also investigated and it does not affect the sum rule. Theory and narrowband experiments are compared using the sum rule and agree to a factor of 1.3 with no adjustable parameters.

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T₁ = 0.13 ps, which is slightly larger than that in the bulk (T₁ = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T₁ = 0.74 ps in the bulk decreases to T₁ = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Progress in phase-sensitive sum frequency generation spectroscopy

Sum frequency generation (SFG) spectroscopy is a unique and powerful tool for investigating surfaces and interfaces at the molecular level. Phase-sensitive SFG (PS-SFG) is an upgraded technique that can overcome the inherent drawbacks of conventional SFG. Here we review several methods of PS-SFG developed and reported in 1990-2020. We introduce how and by which group each PS-SFG method was designed and built in terms of interferometer implementation for optical heterodyne detection, with one exception of a recent numerical method that does not rely on interferometry. We also discuss how PS-SFG solved some typical problems for aqueous interfaces that were once left open by conventional SFG. These problems and their solutions are good examples to demonstrate why PS-SFG is essential. In addition, we briefly note a few terminology issues related with PS-SFG to avoid confusion.

Ab initio molecular dynamics study on energy relaxation path of hydrogen-bonded OH vibration in bulk water

The vibrational energy relaxation paths of hydrogen-bonded (H-bonded) OH excited in pure water and in isotopically diluted (deuterated) water are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The present study extends the previous NE-AIMD simulation for the energy relaxation of an excited free OH vibration at an air/water interface [T. Ishiyama, J. Chem. Phys. 154, 104708 (2021)] to the energy relaxation of an excited H-bonded OH vibration in bulk water. The present simulation shows that the excited OH vibration in pure water dissipates its energy on a timescale of 0.1 ps, whereas that in deuterated water relaxes on a timescale of 0.7 ps, consistent with the experimental observations. To decompose these relaxation energies into the components due to intramolecular and intermolecular couplings, constraints are introduced on the vibrational modes except for the target path in the NE-AIMD simulation. In the case of pure water, 80% of the total relaxation is attributed to the pathway due to the resonant intermolecular OH⋯OH stretch coupling, and the remaining 17% and 3% are attributed to intramolecular couplings with the bend overtone and with the conjugate OH stretch, respectively. This result strongly supports a significant role for the Förster transfer mechanism of pure water due to the intermolecular dipole-dipole interactions. In the case of deuterated water, on the other hand, 36% of the total relaxation is due to the intermolecular stretch coupling, and all the remaining 64% arises from coupling with the intramolecular bend overtone.

Energy relaxation path of excited free OH vibration at an air/water interface revealed by nonequilibrium ab initio molecular dynamics simulation

Nonequilibrium ab initio molecular dynamics (NE-AIMD) simulations are conducted at an air/water interface to elucidate the vibrational energy relaxation path of excited non-hydrogen-bonded (free) OH. A recent time-resolved vibrational sum frequency generation (TR-VSFG) spectroscopy experiment revealed that the relaxation time scales of free OH at the surface of pure water and isotopically diluted water are very similar to each other. In the present study, the dynamics of free OH excited at the surface of pure water and deuterated water are examined with an NE-AIMD simulation, which reproduces the experimentally observed features. The relaxation paths are examined by introducing constraints for the bonds and angles of water molecules relevant to specific vibrational modes in NE-AIMD simulations. In the case of free OH relaxation at the pure water surface, stretching vibrational coupling with the conjugate bond makes a significant contribution to the relaxation path. In the case of the isotopically diluted water surface, the bend (HOD)-stretching (OD) combination band couples with the free OH vibration, generating a relaxation rate similar to that in the pure water case. It is also found that the reorientation of the free OH bond contributes substantially to the relaxation of the free OH vibrational frequency component measured by TR-VSFG spectroscopy.