Peak separation methods for inverse photoelectron spectra: Comparing second derivative, curve fitting, and deconvolution analyses

Inverse photoelectron spectroscopy (IPES) is a powerful technique for probing the unoccupied electronic states of materials. It can be regarded as the inversion process of photoelectron spectroscopy (PES), which examines the occupied states. Recently developed low-energy inverse photoelectron spectroscopy (LEIPS) can significantly advance the study of unoccupied states, owing to minimal sample damage and suppressed dark counts compared to conventional IPES. However, the instrumental resolution remains at 0.2 eV, which is one order of magnitude lower than that of PES. Spectral broadening caused by the low instrumental resolution often results in overlapping peaks. Therefore, peak separation is crucial in the analysis of LEIPS spectra. In this study, we compared three peak separation methods-second derivative, curve fitting, and deconvolution. These methods were applied to modeled and experimental LEIPS spectra of the lowest unoccupied molecular orbital-derived band of pentacene, which consists of two splitting peaks due to the two inequivalent molecules in the unit cell. We systematically and quantitatively evaluated the performance of each method in terms of analysis parameters and discussed its robustness to noise as well as its peak separation capabilities as a function of peak energy spacing and intensity ratio. This work offers a practical framework for peak separation in LEIPS, with extensions to PES and a wide range of spectroscopies.