Solving the "Magic Angle" Challenge in Determining Molecular Orientation at Interfaces

TL;DR

This paper presents a new spectroscopic method to accurately measure molecular orientation heterogeneity at interfaces, overcoming the longstanding 'magic angle' ambiguity and enabling detailed analysis of molecular distributions.

Contribution

The authors developed a heterodyne two-dimensional sum frequency generation spectroscopy technique to determine both mean tilt angle and orientational distribution of molecules at interfaces, solving the 'magic angle' problem.

Findings

Successfully measured the tilt angle of CO2 reduction catalysts at 53 degrees.

Determined the orientational distribution width to be 5 degrees.

Demonstrated the method's applicability to various energy, catalytic, and biological interfaces.

Abstract

We introduce a novel method to determine the orientation heterogeneity (mean tilt angle and orientational distribution) of molecules at interfaces using heterodyne two-dimensional sum frequency generation spectroscopy. By doing so, we not only have solved the long-standing "magic angle" challenge, i.e. the measurement of molecular orientation by assuming a narrow orientational distribution results in ambiguities, but we also are able to determine the orientational distribution, which is otherwise difficult to measure. We applied our new method to a CO2 reduction catalyst/gold interface and found that the catalysts formed a monolayer with a mean tilt angle between the quasi-C3 symmetric axis of the catalysts and the surface normal of 53 deg, with 5 deg orientational distribution. Although applied to a specific system, this method is a general way to determine the orientation heterogeneity of an ensemble-averaged molecular interface, which can potentially be applied to a wide-range of energy material, catalytic and biological interfaces.