Quantitative\nSignal Analysis of Sum-Frequency Scattering\nExperiments from Aerosol Surfaces

Droplet\ninterfaces are instrumental in processes of biology, engineering,\nproduction, and environmental systems. The chemical and physical properties\nof heterogeneous interfaces are known to be different from those of\ntheir underlying bulk phases, and different again when considering\nthe curved surface of submicron aerosol droplets. The recently developed\ntechnique of vibrational sum-frequency scattering (VSFS) spectroscopy\nfrom airborne particles has emerged as an interface-specific method\nfor the in situ analysis of this unique system. While the technique\nhas shown promise in debut works, a quantitative analysis of the VSFS\nsystem has not yet been performed. Here we provide a comprehensive\nanalysis of a VSFS spectrometer with reference to the well-documented\nplanar analog. We decompose the VSFS signal into coherent and incoherent\nas well as resonant and nonresonant components as a function of incident\npulse delay time. We then quantify and compare resonant and nonresonant\nVSFS and VSFG experimental data using the same laser and detection\nsystems. Using the air/water interface as a guide, we show that the\nresonant and nonresonant contributions to the SF responses are comparable\nfor the two systems by extracting second-order susceptibilities and\nhyperpolarizabilities, and using them to estimate single-particle\nsusceptibilities. A quantitative analysis of the signal detection\nsystems for the scattering and planar geometries is made, and conversion\nefficiencies for VSFG, VSFS, and other nonlinear scattering experiments\nare compared. Lastly, the possibility of a low-repetition (1 kHz)\nVSFS spectrometer is considered, determining that it may be possible\nwith modern laser technology but is inevitably less efficient than\na high-repetition (100 kHz) system. Though this multistep analysis\nwe obtain a better understanding of the components of the VSFS signal\nfrom aerosol particles, further validate the feasibility of the experiments,\nand provide insight to those wishing to conduct similar experiments\nand how they may be improved.