Validation of broadband infrared normalization in sum-frequency generation vibrational spectroscopy through simultaneous chiral terms on α-quartz crystal

Abstract

Sum-frequency generation vibrational spectroscopy (SFG-VS) has been widely used for characterizing various interfaces. However, obtaining SFG signals with a high signal-to-noise ratio can be challenging for certain interfaces, such as those involving powder particles, which scatter the SFG light and make it difficult to obtain accurate spectra. To address these challenges, we developed a new approach using a z-cut α-quartz crystal as the substrate loaded with a very small amount of powder sample. This approach not only amplifies the SFG signal from particles through the interference of the electric field from the quartz crystal, but also allows for phase reference and normalization of the broadband infrared SFG spectrum. By distinguishing the different polarizations of the SFG light, we were able to separate and simultaneously collect the achiral and chiral SFG signals. We used the chiral SFG signal to normalize the achiral SFG intensity, thereby avoiding any potential changes to the interface caused by loading substances onto the quartz, as well as coincidence differences resulting from the instability of light at different moments. We demonstrated our method by measuring the adsorption of CH3OD on a quartz substrate loaded with MoC nanoparticles. Our approach produced a high signal-to-noise ratio SFG spectrum, regardless of the interface situation.

General Normalizing Approach for Broadband Infrared Sum-Frequency Generation Spectroscopy via a Transmitted Signal in α-Quartz

“…In the cross-term 2l c χ I,Im (2) χ B (2) , the surface molecular resonance term is amplified by the interference with the nonresonant term of bulk quartz, which can improve the detection sensitivity. 23 However, in the transmitted SFG signal, the |l c χ B (2) | 2 term is significantly larger for the transmitted SFG signal due to its longer coherence length l c , which is about 22 times the reflected SFG term. Hence, |l c χ B…”

“…It is worth noting that we have previously reported a normalization method through the chiral SFG signal of z-cut α-quartz. 23 This approach that is normalized by the chiral SFG of quartz not only amplifies SFG signals through electric field interference within the quartz crystal but also facilitates phase referencing and normalization of broadband infrared SFG spectra. However, the previous method is not suitable for chiral molecules since both chiral and achiral resonance SFG can be generated from bulk quartz.…”

“…The α-quartz crystal, a nonlinear crystal with D 3 symmetry, can contribute to the SFG signal through its bulk. ,, When the sample is placed on the z-cut quartz surface, considering the dipolar terms, both the reflected and transmitted SFG signals consist of contributions from the quartz’s bulk and the adsorbed molecule layer on the surface, , that is χ eff false( 2 false) = χ I false( 2 false) + i χ normalB ( 2 ) false| normalΔ k z false| where the χ I (2) term of the surface molecule is in resonance, which includes the real χ I,Re (2) and imaginary i χ I,Im (2) terms, and χ B (2) is real, which represents the bulk susceptibility of quartz. The absolute value of the denominator in eq yields the factor l c = 1/|Δ k z |, often defined as the coherence length, calculated by Δ k z = | k SFG,z – k IR,z – k VIS,z |. , As illustrated in Figure a, the sign of k SFG,z is opposite the incident light for the reflected SFG, while it is the same as the transmitted SFG.…”

“…Consequently, the intensity of the SFG signal, I SFG , from the sample on the quartz surface is proportional to |χ eff (2) | 2 and is represented as I SFG ∝ false| χ eff false( 2 false) false| 2 = false| χ I , Re false( 2 false) + i χ I , I normalm false( 2 false) + l c i χ B false( 2 false) false| 2 = false| l c · χ B false( 2 false) false| 2 + 2 l c χ I , I normalm false( 2 false) χ B false( 2 false) + false| χ I false( 2 false) false| 2 Usually, | l c χ B (2) | 2 ≫ |χ I (2) | 2 in the reflected SFG signal, allowing us to ignore the small contribution of the |χ I (2) | 2 term in comparison to the other two terms. In the cross-term 2 l c χ I,Im (2) χ B (2) , the surface molecular resonance term is amplified by the interference with the nonresonant term of bulk quartz, which can improve the detection sensitivity . However, in the transmitted SFG signal, the | l c χ B (2) | 2 term is significantly larger for the transmitted SFG signal due to its longer coherence length l c , which is about 22 times the reflected SFG term.…”

“…In our earlier work, we introduced a novel normalization approach employing the simultaneous acquisition of the SFG vibrational spectrum of a sample and a reference SFG spectrum for normalization at the same sample position. This method has been effectively applied in broadband IR SFG vibrational spectroscopy 23 and the doubly resonant SFG spectroscopy. 24 The essence of this method lies in the discrepancies in chirality SFG responses between z-cut αquartz and molecules.…”