zero filling 

https://doi.org/10.1351/goldbook.08304
Addition of zeros to a free induction decay or interferogram to decrease the frequency or wavenumber spacing between points in the Fourier-transformed spectrum.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 679 [Terms] [Paper]