  Formamide (NH2CHO) and methylamine (CH3NH2) are known to be the most abundant
amine-containing molecules in many astrophysical environments. The presence of
these molecules in the gas phase may result from thermal desorption of
interstellar ices. The aim of this work is to determine the values of the
desorption energies of formamide and methylamine from analogues of interstellar
dust grain surfaces and to understand their interaction with water ice. TPD
experiments of formamide and methylamine ices were performed in the
submonolayer and monolayer regimes on graphite (HOPG) and non-porous amorphous
solid water ice surfaces at 40-240 K. The desorption energy distributions of
these two molecules were calculated from TPD measurements using a set of
independent Polanyi-Wigner equations. The maximum of the desorption of
formamide from both graphite and ASW ice surfaces occurs at 176 K after the
desorption of H2O molecules, whereas the desorption profile of methylamine
depends strongly on the substrate. Solid methylamine starts to desorb below 100
K from the graphite surface. Its desorption from the water ice surface occurs
after 120 K and stops during the water ice sublimation around 150 K. It
continues to desorb from the graphite surface at temperatures higher than 160
K. More than 95 % of NH2CHO diffuses through the water ice surface towards the
graphitic substrate and is released into the gas phase with a desorption energy
distribution (7460-9380 K), which is measured with the best-fit pre-exponential
factor A=10^18 s-1. However, the desorption energy distribution of methylamine
from the np-ASW ice surface (3850-8420 K) is measured with the best-fit
pre-exponential factor A=10^12 s-1. A fraction of solid methylamine monolayer
of roughly 0.15 diffuses through the water ice surface towards the HOPG
substrate. This small amount desorbs later with higher binding energies
(5050-8420 K).
