Orientational Motions of Vibrational Chromophores in Molecules at the Air/Water Interface with Time-Resolved Sum Frequency Generation

Abstract

The first time-resolved experiments in which interfacial molecules are pumped to excited electronic states and probed by vibrational sum frequency generation (SFG) are reported. This method was used to measure the out-of-plane rotation dynamics, i.e. time dependent changes in the polar angle, of a vibrational chromophore of an interfacial molecule. The chromophore is the carbonyl group, the rotation observed is that of the sCdO bond axis, with respect to the interfacial normal, and the interfacial molecule is coumarin 314 (C314) at the air/water interface. The orientational relaxation time was found to be 220 ( 20 ps, which is much faster than the orientational relaxation time of the permanent dipole moment axis of C314 at the same interface, as obtained from pump-second harmonic probe experiments. Possible effects on the rotation of the sCdO bond axis due to the carbonyl group hydrogen bonding with interfacial water are discussed. From the measured equilibrium orientation of the permanent dipole moment axis and the carbonyl axis, and knowledge of their relative orientation in the molecule, the absolute orientation of C314 at the air/water interface is obtained.

Adding a dimension to the infrared spectra of interfaces using heterodyne detected 2D sum-frequency generation (HD 2D SFG) spectroscopy

“…1F. The spectrum shown has t 1 ¼ 0, which makes it equivalent to a pump-probe spectrum (27,28). This spectrum contains both negative and positive peaks, which are due to the ν ¼ 0 → 1 and ν ¼ 1 → 2 transitions, respectively, as discussed above.…”

“…Recently, landmark 2D SFG spectra were reported (25,26). Those experiments, along with related pump-probe SFG experiments (27)(28)(29), proved that the signal strengths are sufficient for performing higherorder nonlinear spectroscopy at interfaces. However, one cannot interpret those spectra in the same manner as a 2D IR spectrum because the spectra are distorted by a complicated convolution of signals, as we discuss below.…”

Anisotropy in the dielectric spectrum of hydration water and its relation to water dynamics

“…25 Experimentally, resolving the anisotropic absorption spectrum and, therefore, a direct verification of our results will be possible by anisotropic dielectric relaxation spectroscopy on oriented lipid bilayer stacks 4,26 or surface-sensitive measurement techniques such as time-resolved pump-probe second harmonic 27 or sum frequency generation on single planar interfaces. 28 Alternatively, one could use oriented probe molecules in order to study the anisotropic absorption at interfaces. 29…”

Molecular rotation in 3 dimensions at an air/water interface using femtosecond time resolved sum frequency generation

“…[25][26][27][28][29][30] SFG is a powerful method to investigate structural dynamics, vibrational energy relaxation, thermal transport, and other interfacial processes. 19,[31][32][33][34][35][36][37] In recent work, we have used visible-pump-time-resolved vibrational SFG experiments to study rotational dynamics of a single chemical group of a molecule at the air/water interface using the visible-pump-SFG probe technique. 32 In the work reported here, we exploit this technique to observe the rotational dynamics of a molecule in 3 dimensions by measuring the rotational dynamics of two chromophores of a selected molecule which are not collinearly oriented on the molecule.…”