Hi, this is a silly question Energy of a free particle is not quantized. Does it mean that it should have a continuous absorption spectrum?. When a cloud of free electrons in some type of plasmas is irradiated with light, theese "hot electrons" are accelerated as they absorb light. So, imagine water. We want to measure the IR spectrum of water, but instead of doing it in solution or in gas phase, we do it in a molecular beam (without collisions and interaccions), supposing that water molecules in the beam are completely free and cold would they absorb radiation in all the wavelengths, increasing the energy in traslation degrees??? It´s obvious I´m wrong, but where? Thanks in advance.