I assume something like: incident, refracted or even evanescent waves acting upon molecules at the boundary, in a way that isn't simply thermal excitation.
What's neat, exciting even, is it could be a refrigerating effect: the evaporated molecules could be sorted by temperature, the higher-energy ones being thrown off while cooler stay behind; this is perfectly justifiable for any strength of effect up to the Carnot limit, i.e. the incident energy acts as power input and the liquid and gas phases are the thermal reservoirs.
Though it sounds like from their temperature measurements, it might be the opposite effect if anything. Or maybe it depends on wavelength, or orientation, or liquid characteristics.
But anything mechanistic they'll have to do a lot of research to figure out. The announcement doesn't seem to be very interesting, more of a heads-up really, but the underlying mechanics could prove fascinating.
it was not thought to happen for something as a simple body of water
You know, it might help if you defined new words. What in the world do you consider a "bizarre" chemical structure?
For that matter, what most scientists consider "bizarre"
is water's chemical structure (well, that exactly not so much, but the resulting thermodynamic properties--). So, now I really don't know what they're trying to say here.
Tim