Near-infrared vibrational second harmonic generation: a new nonlinear interfacial vibrational spectroscopy

Abstract

Determining the bond anharmonicity constant (χ_eν_e) and dissociation energy (D_e) at interfaces is challenging but important for understanding many fundamental physical and chemical processes. χ_eν_e and D_e can be examined by studying overtone transitions in the near-infrared region. We used near-infrared vibrational second harmonic generation (NIR-vSHG), a new nonlinear spectroscopic technique, to study the overtone of the buried near-surface free OH at muscovite mica surfaces in air, and the CH stretch in chloroform and acetonitrile at alumina interfaces. NIR-vSHG offers distinct advantages over mid-infrared vibrational sum frequency generation (MIR-vSFG) spectroscopy, such as elimination of the need for temporal and spatial overlapping of focused laser beams, Fresnel factor correction, and time-consuming technical training. In contrast to MIR-vSFG, NIR-vSHG provides enhanced optical accessibility by avoiding substrate absorption in the MIR range and simplifies spectral analysis by minimizing possible interference from Fermi resonances and Fresnel factor correction in surface-specific studies. This work yields important bond dissociation energy information that can shed light on interfacial bond activation, and establishes a new tool for detecting the vibrational characteristics of interfacial environments.