Vibrational Dynamics of Interfacial Water by Free Induction Decay Sum Frequency Generation (FID-SFG) at the Al₂O₃(1120)/H₂O Interface

Abstract

The dephasing dynamics of a vibrational coherence may reveal the interactions of chemical functional groups with their environment. To investigate this process at a surface, we employ free induction decay sum frequency generation (FID-SFG) to measure the time that it takes for free OH stretch oscillators at the charged (pH ≈ 13, KOH) interface of alumina/water (Al2O3/H2O) to lose their collective coherence. By employing noncollinear optical parametric amplification (NOPA) technology and nonlinear vibrational spectroscopy, we showed that the single free OH peak actually corresponds to two distinct oscillators oriented opposite to each other and measured the total dephasing time, T2, of the free OH stretch modes at the Al2O3/H2O interface with a sub-40 fs temporal resolution. Our results suggested that the free OH oscillators associated with interfacial water dephase on the time scale of 89.4 ± 6.9 fs, whereas the homogeneous dephasing of interfacial alumina hydroxyls is an order of magnitude slower.

Revisiting the growth mechanism of atomic layer deposition of Al2O3: A vibrational sum-frequency generation study

“…The interpretation of the small differences in shape and area of the -OH signal at 150 and 250 C is not straightforward and beyond the scope of this work. 42 Replacing TMA with a fully deuterated molecule [D-TMA, Al(CD 3 ) 3 ] should lead to a shift of the stretch mode of the -CH 3 groups by a factor of $ ffiffi ffi 2 p toward lower wavenumbers. Figure 10 shows the signal of the C-D stretch mode of the -CD 3 groups which was detected at 2125 cm À1 which is indeed shifted by a factor of $ ffiffi ffi 2 p compared to stretch mode of -CH 3 at 2890 cm À1 .…”

Probing the Mineral–Water Interface with Nonlinear Optical Spectroscopy

“…For the (11 0) interface in contact with water, the frequency of the dangling OH group of Al 2 OH has been reported to be similar to that of the (0001) interface [44] . A free induction decay study [45] on the (1120) surface provides indications that this free‐OH stretch mode might consist of two modes centered at 3644, assigned to aluminum hydroxy groups, and 3679 cm −1 , attributed to a free‐OH stretch of interfacial water, with dephasing times of about 90 and 900 fs, respectively. The SFG response of the interfacial water molecules at about 3200 and 3400 cm −1 at this (11 0) interface is sensitive to the pH value, with a minimum in the SFG intensity around pH 6.7 [44] .…”

Specifics about Specific Ion Adsorption from Heterodyne-Detected Second Harmonic Generation

“…[18][19][20] Likewise, nonlinear optical measurements have revealed that halide and alkali ions influence silanol group dissociation at silica/water interfaces, [21][22][23] a result that perhaps is linked to reports that certain alkali chlorides at high ionic strengths enhance silica dissolution rates. 18,[24][25][26][27][28] Perturbations to the hydrogen-bonded network Page 3 of interfacial water molecules 29 and a considerable lengthening of the T2 relaxation time of interfacial water molecules with certain ions at elevated ionic strength [30][31][32] have been reported as well. We now investigate how a series of alkali halides interact with the fused silica/water interface at concentrations ranging from 10 µM to 0.1 M at circumneutral pH.…”