Two-Dimensional Infrared Spectroscopy of Supercooled Water

Abstract

We present two-dimensional infrared (2D IR) spectra of the OD stretch vibration of isotope diluted water (HOD/H2O) from ambient conditions (293 K) down to the metastable supercooled regime (260 K). We observe that spectral diffusion slows down from 700 fs to 2.6 ps as we lower the temperature. A comparison between measurements performed at the magic angle with those at parallel polarization shows that the 2D IR line shape is affected by the frequency-dependent anisotropy decay in the case of parallel polarization, altering the extracted correlation decay. A fit within the framework of an Arrhenius law reveals an activation energy of Ea = 6.2 ± 0.2 kcal/mol and a pre-exponential factor of 1/A = 0.02 ± 0.01 fs. Alternatively, a power law fit results in an exponent γ = 2.2 and a singularity temperature Ts = 221 K. We tentatively conclude that the power law provides the better physical picture to describe the dynamics of liquid water around the freezing point. 2D IR spectroscopy of supercooled water AbstractWe present 2D IR spectra of the OD stretch vibration of isotope diluted water (HOD/H 2 O) from ambient conditions (293 K) down to the metastable supercooled regime (260 K). We observe that spectral diffusion slows down from 700 fs to 2.6 ps as we lower the temperature.Comparison between measurements performed at magic angle with those at parallel polarization shows that the 2D IR lineshape is affected by the frequency dependent anisotropy decay in the case of parallel polarization, altering the extracted correlation decay. A fit within the framework of an Arrhenius law reveals an activation energy of E a = 6.2 ± 0.2 kcal/mol and a pre-exponential factor of 1/A = 0.02 ± 0.01 fs. Alternatively, a power law fit results in an exponent γ = 2.2 and a singularity temperature T s = 221 K. We tentatively conclude that the power law provides the better physical picture to describe the dynamics of liquid water around the freezing point.

2D IR Spectroscopy with Phase-Locked Pulse Pairs from a Birefringent Delay Line

“…The tilt of the 2D IR signal is a measure of the correlation between pump and probe vibrational frequencies. Consistent with the literature [30,31], this correlation is lost on a picosecond timescale, and this dynamics can be related to the fluctuations of the hydrogen bond network of water. The small tilt observed here can be explained by the low temporal resolution of the set-up (≈ 200 fs).…”

Two-Dimensional Infrared Spectroscopy of Isotope-Diluted Low Density Amorphous Ice

“…1), but the width of the distribution of hydrogen bond strengths as well as the amount of inhomogeneous broadening is comparable. The latter is deduced from the tilt of the 2D IR lineshape, which reflects the correlation of pump-and probe frequencies, and which has been observed also for liquid water at early enough population times [28][29][30][31][32]. However, water molecules are highly mobile in liquid water and exchange hydrogen bond partners on a 1 ps timescale.…”

“…Spectral diffusion is a main mechanism for such lineshape changes in liquid water [28][29][30][31][32]), but the glassy state assures that there is no rearrangement of hydrogen bonds on the timescale of the experiment.…”

“…Nonlinear vibrational spectroscopy, such as two dimensional infrared spectroscopy (2D IR) [26][27][28][29][30][31][32][33], is an excellent tool to monitor the local structure and dynamical properties of the hydrogen bonds in water. These experiments make use of the fact that the vibrational frequency of the OH stretch vibration is highly correlated with the OO distance of a pair of hydrogen bonded water molecules [34][35][36][37][38].…”

Two-dimensional infrared spectroscopy of isotope-diluted ice Ih

“…2 by the green square in the t 2 = 200 fs of OH of HOD/D 2 O), apart from which the 2D IR lineshapes are hardly tilted along the diagonal in neither of the isotopomers. Such a tilt, which is observed for example for liquid water, [59][60][61][62] would reflect a correlation between pump and probe frequency, i.e., would reflect an inhomogeneity in hydrogen bond strengths. Apparently, proton disorder in ice Ih affects the hydrogen bond strength of the vibrational chromophores too little to cause appreciable inhomogeneous broadening.…”