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Gas cells containing ammonia and methane at atmospheric pressure werecustom-made in our chemical laboratory. Low-resolution near-infraredspectroscopy of these gas cells were collected in our optical laboratory. Theyare compared with simulated spectra using the high-resolution transmissionmolecular absorption database (HITRAN) for temperatures of 300 K and 500 K, andwith near-infrared spectra of late-T and Y dwarfs, Jupiter and Saturn. Weselected for this investigation the spectral region between 1.5 and 1.7 microns(H-band) because it is covered by the Euclid red grism, it is particularlysensitive to the relative proportions of ammonia and methane opacity, and it isfree from strong contributions of other abundant molecules such as watervapour. The laboratory spectra showed that the ammonia and methane featuresthat are present in the simulations using the HITRAN database are incomplete.Using our laboratory spectra, we propose a modified version of the NH3-Hspectral ratio with expanded integration limits that increases the amplitude ofvariation of the index with respect to spectral type. Combinations of ourlaboratory spectra were used to find the best fits to the observed spectra withthe relative absorption ratio of ammonia to methane as a free parameter. Arelationship was found between the T$\rm_{eff}$ and the ratio of ammonia tomethane from spectral classes T5 to Y2 (1100 K -- 350 K), in qualitative fairlygood agreement with theoretical predictions for high gravity objects andtemperatures from 1100 K to 500 K. The ammonia to methane ratios in late-T andY dwarfs are similar to that of Jupiter, suggesting a similar chemicalcomposition. Simulations of the spectroscopic performance of Euclid suggestthat it will yield T$\rm_{eff}$ values and ratios of ammonia to methane forover 10$^3$ late T dwarfs in the whole wide survey.

Ammonia-methane ratios from H-band near-infrared spectra of late-T and Y dwarfs