h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

T1 and T2 in NMR are both effectively thermal relaxation, there is no reemission of photons.
T1 is spin-lattice relaxation - in other words it is the random fluctuations of the induced magnetism with the surroounding environment that cause a relaxation along the z axis.
T2 is spin-spin relaxation so that nearby spins in the same molecule contribute to the loss of coherency in the xy plane.
Don't ask me about the maths, that is, as you say, very tough in NMR.

Because NMR is in the radio wave region of the em spectrum, spontaneous emission is regarded as effectively zero because the wavelegnth is so long. Radio waves can be 1500 metres long or more. With no spontaneous emission and almost equal populations in the two (basic) energy levels it can be very easy to saturate signals and see nothing. This is also useful sometimes but also leads to the relative insensitivity of NMR as compared to UV or IR spectroscopy.

I agree with what you've saying about NMR transitions. I didn't come across, or I don't remember where in the equations I was looking at, the absolute frequency determined the rate/probability of emission. But I'll ask some NMR jocks I know about it, see what they have to say.

NMR is basically a radio-frequency absorption spectroscopy, right? Large, fixed magnetic field, sweep the RF frequency, measure absorption of the RF, right?

Indeedy Although sweeping the RF field is no longer done really, pulse NMR followed by fourier transform of the resultinf Free Induction Decay is the norm. Much more powerful as you can do so much with pulse sequences.

Way back in post 7, Orcus says:
"If the top energy level is more highly populated that the bottom (quite difficult to achieve -but possible in some special systems)"

I just want to point out that it's not that difficult to achieve. It's how a diode works and a long time ago now (over 40 years) researchers working with the semiconductor GaAs showed how you could build a laser.
In it's basic form it's just an GaAs p-n junction diode. When biased into it's conducting stated electrons and holes have shuffled across the bandgap and you will get random photons being emitted as the electron-hole pairs recombine (why LEDs work).
Make the junction the right size and stick a mirror on one end and a partially mirrored surface at the other and the photons have a bit more trouble being released from the material. Now if one of the photons that is being reflected inside the diode happens across an electron-hole pair that are close but not quite near enough to spontaneously combine then it can give them the nudge needed for recombination and another photon is released (the stimulated emission).
The exact hows, whys, wherefores and so on is where you get into the quantum stuff that as an electronic engineer I didn't need to learn.

Now I hope I managed to get that right, it was a long time ago that all that stuff was taught to me

I was trying to skirt around how lasers work as it's not really relevant to the question.

Of course it's not *that* difficult to achieve.

However in a two level system it's *impossible* to achieve by thermal or other excitation as the best you'll ever get is an equal population in both energy levels. You need a third energy level (or more) and an efficient crossing process to the third energy level to achieve a population inversion and so laser action.
(there is the excimer and exciplex where there are two levels but the lower level is unbound in these).

What I was trying to point out was that only certain substances under specific conditions allow a population inversion and hence a laser. In other words you need to jump through hoops to do it in the general scheme of things.
I think your post there illustrates quite nicely that achieving a population inversion require some complicated stuff to be going on

I think you're both right. Lasers are pretty ubiquitous, but then again, we can't make anything lase - we don't have lasers of any color we want, and we can't just scale up the power...