Bibliographic review and new measurements of the infrared band strengths of pure molecules at 25 K: H2O, CO2, CO, CH4, NH3, CH3OH, HCOOH and H2CO

Infrared observations of the interstellar medium revealed the presence of several molecules in the solid phase such as H2O, CO2, CO, CH4, NH3, CH3OH, H2CO and HCOOH. Measurements of column densities and molecular abundances relative to water require the knowledge of infrared band strengths. We present a review of refractive indices at visible wavelengths, densities and infrared band strengths for all eight molecules. We also present new band strengths measured on icy films whose thicknesses have been determined using laser interference techniques. For CO2, CO, CH4 and NH3, our measurements are in agreement with previous determinations taking into account an uncertainty of about 20 per cent. For H2O ice films, the porosity and the density remain unreliable, leading to large uncertainties on the measured band strengths. Concerning amorphous CH3OH, H2CO and HCOOH, the densities and refractive indices are unknown leading to large uncertainties on the band strengths. However, we propose new values that are slightly different from previous determination. Our review and experimental work point out the most reliable band strengths for the eight studied molecules. For CH4, CH3OH, HCOOH and H2CO, the band strengths used to calculate abundances in the ices of interstellar medium seem to be inaccurate, leading to some doubts on the determined values.