Observation and theory of reorientation-induced spectral diffusion in polarization-selective 2D IR spectroscopy

Abstract

In nearly all applications of ultrafast multidimensional infrared spectroscopy, the spectral degrees of freedom (e.g., transition frequency) and the orientation of the transition dipole are assumed to be decoupled. We present experimental results which confirm that frequency fluctuations can be caused by rotational motion and observed under appropriate conditions. A theory of the frequency-frequency correlation function (FFCF) observable under various polarization conditions is introduced, and model calculations are found to reproduce the qualitative trends in FFCF rates. The FFCF determined with polarization-selective two-dimensional infrared (2D IR) spectroscopy is a direct reporter of the frequency-rotational coupling. For the solute methanol in a room temperature ionic liquid, the FFCF of the hydroxyl (O-D) stretch decays due to spectral diffusion with different rates depending on the polarization of the excitation pulses. The 2D IR vibrational echo pulse sequence consists of three excitation pulses that generate the vibrational echo, a fourth pulse. A faster FFCF decay is observed when the first two excitation pulses are polarized perpendicular to the third pulse and the echo, 〈XXY Y〉, than in the standard all parallel configuration, 〈XXXX〉, in which all four pulses have the same polarization. The 2D IR experiment with polarizations 〈XY XY〉 ("polarization grating" configuration) gives a FFCF that decays even more slowly than in the 〈XXXX〉 configuration. Polarization-selective 2D IR spectra of bulk water do not exhibit polarization-dependent FFCF decays; spectral diffusion is effectively decoupled from reorientation in the water system.

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

“…A theory for the effects of RISD on 2D-IR spectra was derived in the context of the Stark effect, 45,46 but the essential physical picture is the same even when the ori-gin of the vibrational frequency shifts is not a true Stark effect. Theoretical work has shown that both electrostatics beyond the dipole approximation and chemical interactions like charge transfer play important roles in determining the C − − − N stretching frequency.…”

“…The contributions of molecular rotation and structural fluctuations of the solvent can be quantitatively separated. [44][45][46] As a starting ansatz, we take the PW-FFCF to be a sum of two frequency fluctuation correlation functions,…”

“…Additionally, the time dependence of the RISD, R p (t), can be expressed solely in terms of the reorientation time for each polarization condition after the appropriate orientational averaging of the signals. 45,46 For ZZZZ , the result is…”

“…In this work, we combine polarization and temperature dependence to extract activation energies for RISD and SSD separately. Kramer et al explained that the polarization dependence of the FFCF is due to the coupling of rotational motion to spectral diffusion, 45,46 which extended the RISD concept developed for vibrational sum-frequencygeneration spectroscopy. 44 Temperature dependent dynamics naturally access activation energies, but, without separating the RISD and SSD, we could report as many as four separate activation energies -from FFCFs measured in ZZZZ , ZZXX , and magic-angle conditions as well as the rotational anisotropy itself.…”

“…At a single polarization condition, however, the frequency correlation function contains components of both spectral diffusion from rotations" RISD, and structural motions, SSD. 45,46 Performing 2D-IR investigations using both parallel ( ZZZZ ) and perpendicular ( ZZXX ) polarizations, a more complete picture of the independent structural and rotational activation energies of hydrogen bond switching in EAN Samples were prepared on two CaF 2 windows with a 25 µm teflon spacer in between the two windows in a brass cell holder. A few µL of the given solution was sandwiched between the two windows with the desired spacer.…”