Second-order non-linear optical technique

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asynunetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more common but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

Second-order non-linear optical techniques have been applied for monitoring the adsorption of soluble surfactants at the air/water interface and to characterize the resulting (sub)monolayers ). For Langmuir monolayers SHG allows the observ-... 

Spectroscopic studies of liquid interfaces provide important information about the composition and structure of the interfacial region. Early work was mainly carried out at the solid liquid interface and involved techniques such as neutron and X-ray diffraction, and reflection FTIR spectroscopy. More recently, powerful techniques have been developed to study the liquid liquid and liquid gas interfaces. These studies are especially important because of their relevance to biological systems such as cell membranes. The techniques described here are second-harmonic generation (SHG) and vibrational sum frequency spectroscopy (VSFS). They are both second-order non-linear optical techniques which are specific to the interfacial region. Since the second-order effects involve signals of low intensity, they rely on high-power lasers. 

Sum frequency generation is a second-order non-linear optical technique that has unique advantages for probing the vibrational spectrum of molecules adsorbed at a surface. The vibrational SFG process occurs when two laser beams, one in the visible spectral region and one in the infrared spectral region, are incident on the sample so that a third beam at the sum frequency of the incident beams is emitted, as shown in (1). 

The real and imaginary parts of third-order non-linear optical susceptibilities of 10 M solutions of Fe2(CO)6 (/t3-E3P) CpCr(GO)2 (E = S or Se) and the well-known precursor clusters Fe3(CO)9(/i3-E)2 in toluene were measured using Z-scan and ARINS techniques, respectively. The mixed metal clusters display nearly three times the second molar hyperpolarizability values of the corresponding Fes precursors. The results suggest a rich potential of mixed metal, mixed non-metal class of clusters as materials exhibiting large non-linearity. 

An elegant alternative which provides an intrinsic surface specificity is provided by non-linear optical reflection techniques based on a second-order effect (Fig. 1). Second-harmonic-generation (SHG) and sum-frequency-generation (SFG) spectroscopy contributed significantly to our current understanding of liquid-air, solid-liquid and liquid-liquid interfaces. Many fundamental insights on the structure of liquid-air interfaces are based on SHG and SFG experiments which are discussed in the next section. 

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