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Stellar population studies in the infrared (IR) wavelength range have twomain advantages with respect to the optical regime: they probe differentpopulations, because most of the light in the IR comes from redder andgenerally older stars, and they allow us to see through dust because IR lightis less affected by extinction. Unfortunately, IR modeling work was halted bythe lack of adequate stellar libraries, but this has changed in the recentyears. Our project investigates the sensitivity of various spectral features inthe 1--5\,$\mu$m wavelength range to the physical properties of stars($T_{eff}$, [Fe/H], log g and aims to objectively define spectral indices thatcan characterize the age and metallicity of unresolved stellar populations. Weimplemented a method that uses derivatives of the indices as functions of$T_{eff}$, [Fe/H] or log g across the entire available wavelength range toreveal the most sensitive indices to these parameters and the ranges in whichthese indices work. Here, we complement the previous work in the I and K bands,reporting a new system of 14, 12, 22, and 12 indices for Y, J, H, and Latmospheric windows, respectively, and describe their behavior. Our analysisindicates that features sensitive to the effective temperature are present andmeasurable in all the investigated atmospheric windows at the spectralresolution and in the metallicity range of the IRTF library for asignal-to-noise ratio greater than 20-30. The surface gravity is morechallenging and only indices in the H and J windows are best suited for this.The metallicity range of the stars with available spectra is too narrow tosearch for suitable diagnostics. For the spectra of unresolved galaxies, thedefined indices are valuable tools in tracing the properties of the stars inthe IR-dominant stellar populations.

Infrared Telescope Facility (IRTF) spectral library II: New indices in Y, J, H, and L atmospheric windows