Quadrupolar mechanism for vibrational sum frequency generation at air/liquid interfaces: Theory and experiment

Abstract

Vibrational sum frequency generation (VSFG) spectroscopy has been widely utilized to investigate various interfaces through molecular vibration. VSFG is usually attributed to the breakdown of the inversion symmetry at the interface within the electric dipole approximation. Although the electric dipole approximation is a very good approximation in the isotropic bulk, its validity is questionable at the interface because a large electric field gradient exists in the thin interface region. Thus, the quadrupole contribution may become significant at the interface. Here, we discuss the quadrupole contribution in VSFG from theoretical and experimental viewpoints. We describe a theory as to how the quadrupole contribution appears in experimental VSFG spectra by deriving the vibrational selection rule and discussing the interface selectivity. With this theoretical framework, we examine the mechanism of VSFG at the air/benzene and air/decane interfaces. The accurate determination of the vibrational frequencies realized by heterodyne-detected VSFG spectroscopy reveals that VSFG at the air/benzene interface arises from the quadrupolar mechanism. This means that the observation of VSFG does not imply a molecular alignment so that interfacial benzene molecules may be randomly oriented. Meanwhile, at the air/decane interface, it is concluded that the VSFG signal arises from the ordinary dipolar mechanism. This implies that decane molecules are aligned in a preferential direction at the interface despite their low polarity. This study demonstrates the importance of examining the mechanism of VSFG before discussing the interfacial structure based on VSFG spectra. A strategy to distinguish different mechanisms is also proposed. The present study further shows that the quadrupolar mechanism, if properly taken into consideration, enables us to obtain information about interfacial molecules beyond the restriction of the inversion symmetry breaking required by the dipolar mechanism, through the large electric field gradient localized at the interface.