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

Determining the bond anharmonicity constant (χeν̃e) and dissociation energy (De) at interfaces is challenging but important for understanding many fundamental physical and chemical processes. χeν̃e and De 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.