Re: microwave, RF heating

[John Denker <jsd@MONMOUTH.COM>]

At 01:44 PM 5/29/00 -0500, brian whatcott wrote:
> The ruling explanation for [at least one kind of] microwave absorption
>  is bond rotation - rather than molecular 'hand clapping' or stretching.
> Can one on prior principle, deduce if bond rotation is hindered in
> a crystal lattice, I ask myself?

At 02:37 PM 5/29/00 -0500, Doug Craigen wrote:
> Microwave absorption spectral lines are mainly due to molecular
> rotation not vibrations (the latter are IR) and are typically 20-30
> GHz.  The microwave oven (2.5 GHz) is far below this - but the spectra
> lines are very smeared due to neighbor interactions so I can imagine
> that there is
> weak absorption due to rotations at 2.5 GHz.  Weak absorption is what we
> require so that the microwaves penetrate deeply.  It would be hard to
> cook food if exciting with a frequency near a spectral line.


I agree that we need to consider the effect of far-far-far off-resonance
transitions.  This dovetails nicely with last week's thread: all spectra
are continuous; tails matter.

Let's knock some numbers together.
        I = moment of inertia
        I = integral of r^2 dm, where r is distance and m is mass
        L = angular momentum = omega I
        L is quantized in units of hbar
hence
        I = hbar / 2 pi f, where f = frequency
and the challenge is to find I big enough so that f can be as small as 20
GHz.  This is just sorta almost possible if you imagine a hydrogen-bonded
H2O:OH complex where the oxygens are rotating around a common center,
separated by a few angstroms.  Maybe.

The rotation of such a critter would be greatly hindered by the crystal
lattice.  It would be pretty dodgy in the liquid, I suspect.

I'm not sure I 100% believe this picture, but the numbers are close enough
to reasonable that perhaps a refinement of this picture can be
found.  Water is funny stuff.

Such a picture (rotating complexes) predicts that the RF absorption should
have all sorts of strange dependence on temperature, density, dilution with
alcohol, et cetera.  The plot thickens!