  A microscopic model for thermal excitation of vibrational ground state of a
molecule interacting with a condensed medium is developed. The master equation
for evolution of occupancies of the vibrational levels is derived. The rate
constant limiting the process of molecular thermal excitation is determined
analytically. It is shown that while this quantity is independent of
temperature at low temperatures coinciding with a rate speed limit for quantum
transitions due to uncertainty principle, at high temperatures it rather
linearly rises with temperature according to the Einstein's relation for
adiabatic transitions.
