# Non-Uniqueness of Parameters Extracted from Resonant Second-Order Nonlinear Optical Spectroscopies

## TL;DR

This paper demonstrates that parameter extraction from second-order nonlinear optical spectroscopies often yields multiple equivalent solutions, highlighting the need for careful analysis to identify physically meaningful parameters.

## Contribution

It introduces an algorithm to calculate all equivalent parameter sets and discusses the implications of non-uniqueness in spectral data analysis.

## Key findings

- Multiple equivalent parameter sets can fit the same spectral data.
- Existence of 'ghost resonances' complicates data interpretation.
- A systematic procedure helps identify the most physically relevant parameters.

## Abstract

Experimental data from second-order nonlinear optical spectroscopies (SFG, DFG, and SHG) provide parameters relevant to the physical chemistry of interfaces and thin films. We show that there are in general 2 N or 2 N-1 equivalent sets of parameters to fit an experimental curve comprising N resonant features, of vibrational or electronic origin for example. We provide the algorithm to calculate these sets, among which the most appropriate has to be selected. The main consequences deal with the existence of "ghost resonances", the need of a critical analysis of fit results, and the procedure to search for better sets of parameters coherent with applied constraints.

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Source: https://tomesphere.com/paper/1905.06029