Observing the ammonia resonance


Andy G4JNT
 

A bit more from the Time Nuts Group
'''''''''''''''''''''''''''''''''''''''''''
As far as I recall there are two aspects to consider.
One is "low pressure" means vacuum pumps and pressure gauges calibrated
to ammonia (but any gauge and reasonable educated guess work will work
too), and if we want to make a permanent cell we need seal it... glass
or ceramic windows on the WG, plus all the rest to seal the in/out pipes
when we are happy with the ammonia pressure, otherwise it would be a
temporary experiment. Or leave the pumps and gas inlet running...

Second is that when the pressure is not too low absorption can be
intense but it is quite broad (thermal/collision broadening).
If we lower the pressure it will become sharper but less intense
or else we need more waveguide to get it deep enough.
Finding the good compromise is required hence the need to control the
ammonia pressure.

I don't remember anything in the 1Hz width... If my memory serves me
right I would say it was in the KHz region at half width, which should
be narrow enough to know where the center is within a few Hz...


Luis Cupido


On Mon, 20 Sept 2021 at 21:04, Robin Szemeti - G1YFG <robin@...> wrote:
A method of tracking the "dip" used in some other absorbtion resonance sensing systems (some HP rubidium vapour references I worked on at one point for example ) is to wobble the excitation frequency a little, say at 50Hz FM, with a deviation of a few Hz ... demodulate the output af the absorbtion cell in an AM demodulator ... if you are up the low side of the curve the demodulated 50Hz tone will be 180 degrees out of  phase with the driving signal  (as the frequency goes higher, more is absorbed, less output) .. if you are on the high side of the curve, the tone will be in phase with the driving signal ... this allows you to steer the centre frequency to the centre of the dip with a simple mixer circuit.

A further refinement to make sure you are centred on the dip can be to look at the 3rd harmonic of the demodulated signal. The FM modulated RF signal will be on the "cusp" of the absorption dip. This will produce a rise in the 3rd harmonic content of the AM demodulated signal, HP used a circuit to monitor the level of the 3rd harmonic and accurately keep the signal centred on the peak of the absorbtion band. I forget the precise details, but it was quite clever, and centred the RF within a few fractions of a Hertz on the rubidium absorbtion band.

On Mon, 20 Sept 2021 at 19:05, Andy G4JNT <andy.g4jnt@...> wrote:
On the Timenuts Group there's currently a discussion on, amongst other things, the ammonia resonance and it's possible use as a simpler frequency reference.
There is a narrow resonanance of the ammonia molecule at 23,870,129,007 Hz +/- 10 Hz  (ie just below our 24GHz band)

The resonance will be a Hz or so wide, and it ought to be possible to see this with relatively simple equipment.    First of all, get a length , several metres, of waveguide for 24GHz.   This needn't be 'proper' WG, just squashed water pipe will do to allow just the TE01 mode to propagate.    Fill this with ammonia (obvs. make the transitions gas tight)

Use an ADF5355 synth, and drive it's reference from a 48 bit DDS loike the AD9852.   Anything to be able to tune the 24GHz signal in sub-Hz steps, and to know what it is.   That chip will leak enough 4th harmonic from its VCO; launch this into the waveguide.   At the other end stick a 24GHz receiver tuned suitably.
Vary the input frequency a few Hz either side of the resonance and look for a dip.

One snag - if you use GPS locking, the residual variation of the GPSDO will cause it to drift across the resonance over a few tens of seconds.

Work out a way of feeding this dip back and controlling the synth reference in a locking loop and you have your own atomic frequency standard.   It's potentially  the same performance as cesium 

It seems in years gone by that caesium was possibly adopted instead of ammonia, simply because its resonance at 9.19GHz was easier to generate using ex-WW2 equipment that was 23GHz back in the 1940s when the first Cs standard was developed.


--
Robin Szemeti - G1YFG


Neil Smith G4DBN
 

Trying hard to recall some stuff from decades ago, but I think that when you excite the ammonia molecules, there will be many different states generated and you need to remove as many of those as possible using a gradiant field of some sort.  Pressure needs to be low enough to avoid collisions that would cause broadening, and you have to deal with the ammonia falling to bits, so some sort of purge/recharge approach needs to be considered, or a continuous very weak jet.

Ammonia is sort of pyramidal, with a Nitrogen atom sitting on top of three Hydrogens.  The Nitrogen can be in two states, so it either contributes to the dipole moment or the hydrogens, or opposes them, depending which "way up" the Nitrogen atom is.  The trick is to select molecules in the ground state and put them through an electric field which will push the two states in slightly different directions, so you can send the ones you want through a hole or a slit, then pass that into a cavity, where the molecules can then drop back to the lower-energy state when they are tickled with a sniff of 23.8 GHz, which then reinforces the oscillation and stimulates more molecules to flip.

I'll see if I can find the reference.  It was a very very long time ago. I recall a load of Hamiltonians and boards full of Dirac notation in a lecture in 1976 and thinking how cool that all sounded, but 24 GHz was a bit too esoteric for 18-year-old me adn I never made the connection with amateur radio stuff.

Thinking cap is on....

Neil G4DBN


Neil Smith G4DBN
 

OK, found it.  Should have known.... Feynmann Lectures on Physics, Section III, number 9, "The Ammonia Maser".

https://www.feynmanlectures.caltech.edu/III_09.html

It looks a whole lot less scary than I recall, but I got the explanation wrong, it is actually a full flip of the molecule in an electric field gradient, not the Nitrogen atom. the centres of mass stays where it is, but the molecule itself flips over.  If it is in the higher energy state in the electric field, it can be stimulated to emit a photon, and that is how a maser (or laser) works. There's a lot of fierce quantum notation and Kronecker deltas and stuff, but it's a lovely bit of exposition.  I wish I'd remembered this when I did an OU degree for a laugh, as one of the final courses was on quantum mechanics and it would have been fun to revisit that paper rather than some of the dry stuff we had to work on.

I see that Charles Townes, the author of the original Masers paper, preferred to define the M as "Molecular" rather than "Microwave", and the main use was for ultra low noise microwave amplifiers  rather than as clocks.

https://en.wikipedia.org/wiki/Maser#/media/File:Charles_Townes_and_first_maser.jpg

Sounds like a lot of fun to be had anyway.

Apologies for incorrect twaddle about the emission mechanism in the original post, I should research more before posting half-baked stuff.

I want a MASER now.

Neil G4DBN

On 20/09/2021 23:02, Neil Smith G4DBN wrote:
Trying hard to recall some stuff from decades ago, but I think that when you excite the ammonia molecules, there will be many different states generated and you need to remove as many of those as possible using a gradiant field of some sort.  Pressure needs to be low enough to avoid collisions that would cause broadening, and you have to deal with the ammonia falling to bits, so some sort of purge/recharge approach needs to be considered, or a continuous very weak jet.

Ammonia is sort of pyramidal, with a Nitrogen atom sitting on top of three Hydrogens.  The Nitrogen can be in two states, so it either contributes to the dipole moment or the hydrogens, or opposes them, depending which "way up" the Nitrogen atom is.  The trick is to select molecules in the ground state and put them through an electric field which will push the two states in slightly different directions, so you can send the ones you want through a hole or a slit, then pass that into a cavity, where the molecules can then drop back to the lower-energy state when they are tickled with a sniff of 23.8 GHz, which then reinforces the oscillation and stimulates more molecules to flip.

I'll see if I can find the reference.  It was a very very long time ago. I recall a load of Hamiltonians and boards full of Dirac notation in a lecture in 1976 and thinking how cool that all sounded, but 24 GHz was a bit too esoteric for 18-year-old me adn I never made the connection with amateur radio stuff.


Robin Szemeti - G1YFG
 

Have a look at page 32 of the HP 5065A rubidium manual, it describes the lock process quite nicely, a similar method could be used for amonia if one was so inclined.


We used to maintain a few of these at the BBC for various reasons, mostly to do with remote stations and staying "in sync" with london.


On Mon, 20 Sept 2021 at 23:21, Neil Smith G4DBN <neil@...> wrote:
OK, found it.  Should have known.... Feynmann Lectures on Physics,
Section III, number 9, "The Ammonia Maser".

https://www.feynmanlectures.caltech.edu/III_09.html

It looks a whole lot less scary than I recall, but I got the explanation
wrong, it is actually a full flip of the molecule in an electric field
gradient, not the Nitrogen atom. the centres of mass stays where it is,
but the molecule itself flips over.  If it is in the higher energy state
in the electric field, it can be stimulated to emit a photon, and that
is how a maser (or laser) works. There's a lot of fierce quantum
notation and Kronecker deltas and stuff, but it's a lovely bit of
exposition.  I wish I'd remembered this when I did an OU degree for a
laugh, as one of the final courses was on quantum mechanics and it would
have been fun to revisit that paper rather than some of the dry stuff we
had to work on.

I see that Charles Townes, the author of the original Masers paper,
preferred to define the M as "Molecular" rather than "Microwave", and
the main use was for ultra low noise microwave amplifiers  rather than
as clocks.

https://en.wikipedia.org/wiki/Maser#/media/File:Charles_Townes_and_first_maser.jpg

Sounds like a lot of fun to be had anyway.

Apologies for incorrect twaddle about the emission mechanism in the
original post, I should research more before posting half-baked stuff.

I want a MASER now.

Neil G4DBN


On 20/09/2021 23:02, Neil Smith G4DBN wrote:
> Trying hard to recall some stuff from decades ago, but I think that
> when you excite the ammonia molecules, there will be many different
> states generated and you need to remove as many of those as possible
> using a gradiant field of some sort.  Pressure needs to be low enough
> to avoid collisions that would cause broadening, and you have to deal
> with the ammonia falling to bits, so some sort of purge/recharge
> approach needs to be considered, or a continuous very weak jet.
>
> Ammonia is sort of pyramidal, with a Nitrogen atom sitting on top of
> three Hydrogens.  The Nitrogen can be in two states, so it either
> contributes to the dipole moment or the hydrogens, or opposes them,
> depending which "way up" the Nitrogen atom is.  The trick is to select
> molecules in the ground state and put them through an electric field
> which will push the two states in slightly different directions, so
> you can send the ones you want through a hole or a slit, then pass
> that into a cavity, where the molecules can then drop back to the
> lower-energy state when they are tickled with a sniff of 23.8 GHz,
> which then reinforces the oscillation and stimulates more molecules to
> flip.
>
> I'll see if I can find the reference.  It was a very very long time
> ago. I recall a load of Hamiltonians and boards full of Dirac notation
> in a lecture in 1976 and thinking how cool that all sounded, but 24
> GHz was a bit too esoteric for 18-year-old me adn I never made the
> connection with amateur radio stuff.







--
Robin Szemeti - G1YFG


Neil Smith G4DBN
 

I have an Rb reference here that works that way, using a change in the light transmission of the vapour at resonance for detection by wobbling the source and looking for the second harmonic of the modulation which will peak when the central frequency is bang on the absorption line.  I guess we could use one of the 23 GHz Wavetek boards as a detector, but would there be a difference in the ground states without the presence of an electric field?  Bit too late at night for quantum.

I still want a maser.  I am SO cheesed off that I sold my vacuum pump because I couldn't imagine a use for it. Ha!

Neil G4DBN

On 20/09/2021 23:50, Robin Szemeti - G1YFG wrote:
Have a look at page 32 of the HP 5065A rubidium manual, it describes the lock process quite nicely, a similar method could be used for amonia if one was so inclined.


We used to maintain a few of these at the BBC for various reasons, mostly to do with remote stations and staying "in sync" with london.

On Mon, 20 Sept 2021 at 23:21, Neil Smith G4DBN <neil@...> wrote:
OK, found it.  Should have known.... Feynmann Lectures on Physics,
Section III, number 9, "The Ammonia Maser".

https://www.feynmanlectures.caltech.edu/III_09.html

It looks a whole lot less scary than I recall, but I got the explanation
wrong, it is actually a full flip of the molecule in an electric field
gradient, not the Nitrogen atom. the centres of mass stays where it is,
but the molecule itself flips over.  If it is in the higher energy state
in the electric field, it can be stimulated to emit a photon, and that
is how a maser (or laser) works. There's a lot of fierce quantum
notation and Kronecker deltas and stuff, but it's a lovely bit of
exposition.  I wish I'd remembered this when I did an OU degree for a
laugh, as one of the final courses was on quantum mechanics and it would
have been fun to revisit that paper rather than some of the dry stuff we
had to work on.

I see that Charles Townes, the author of the original Masers paper,
preferred to define the M as "Molecular" rather than "Microwave", and
the main use was for ultra low noise microwave amplifiers  rather than
as clocks.

https://en.wikipedia.org/wiki/Maser#/media/File:Charles_Townes_and_first_maser.jpg

Sounds like a lot of fun to be had anyway.

Apologies for incorrect twaddle about the emission mechanism in the
original post, I should research more before posting half-baked stuff.

I want a MASER now.

Neil G4DBN


On 20/09/2021 23:02, Neil Smith G4DBN wrote:
> Trying hard to recall some stuff from decades ago, but I think that
> when you excite the ammonia molecules, there will be many different
> states generated and you need to remove as many of those as possible
> using a gradiant field of some sort.  Pressure needs to be low enough
> to avoid collisions that would cause broadening, and you have to deal
> with the ammonia falling to bits, so some sort of purge/recharge
> approach needs to be considered, or a continuous very weak jet.
>
> Ammonia is sort of pyramidal, with a Nitrogen atom sitting on top of
> three Hydrogens.  The Nitrogen can be in two states, so it either
> contributes to the dipole moment or the hydrogens, or opposes them,
> depending which "way up" the Nitrogen atom is.  The trick is to select
> molecules in the ground state and put them through an electric field
> which will push the two states in slightly different directions, so
> you can send the ones you want through a hole or a slit, then pass
> that into a cavity, where the molecules can then drop back to the
> lower-energy state when they are tickled with a sniff of 23.8 GHz,
> which then reinforces the oscillation and stimulates more molecules to
> flip.
>
> I'll see if I can find the reference.  It was a very very long time
> ago. I recall a load of Hamiltonians and boards full of Dirac notation
> in a lecture in 1976 and thinking how cool that all sounded, but 24
> GHz was a bit too esoteric for 18-year-old me adn I never made the
> connection with amateur radio stuff.







--
Robin Szemeti - G1YFG
-- 
Neil
http://g4dbn.uk


alwyn.seeds1
 

Dear Robin,

One of the BBC’s Rubidium standards used to sit at Droitwich for the long wave transmitter; its frequency wanderings, such as they were, being documented by NPL in monthly bulletins.

So far as I can measure, Droitwich seems now to be GPS locked matching GPS to within the short term drift of the OCXO in my GPS standard.

There was, however, an incident a few years ago, where locking was lost allowing easily measurable error to build up. A comment on another forum and the error went away the following week.

Regards,

Alwyn


_____________________________________________________

Alwyn Seeds, Director
SynOptika Ltd.,
114 Beaufort Street,
London,
SW3 6BU,
England.


SynOptika Ltd., Registered in England and Wales: No. 04606737
Registered Office: 114 Beaufort Street, London, SW3 6BU, United Kingdom.
_____________________________________________________


Robin Szemeti - G1YFG
 

Indeed it was! I never worked on that particular reference, but I saw it from time to time on my visits to Droitwich. At one point, I used to maintain some of the circuits between pebble mill and Droitwitch and would build equalising circuits and temperature compensation circuits etc.

We used a similar Rubidium at Pebble Mill to "lock" the building pulse generators to. We would perform a syncronisation process at around 6am to the reference signal coming up one of the feeds from London, then we would free-run for the remainder of the day, but remain very closely aligned to London's timing. This enabled us to insert one of our studios into the feed without any disturbance to the outgoing signal.   If we were not perfectly aligned as we made the changeover, TV's across the midlands would have frame-roll. We had to sync lock at 6am before the rest of the building was active, as a disturbance to the pulse distribution during the day would cause havoc with tape machines and any other systems that were in use at the time.

We used to check our Rubidium by observing the phase of our 4.433 MHz colour subcarrier to the London subcarrier and adjusting the Rubi to minimise drift throughout the day. We could usually remain within a few degrees.

I remember having to perform open heart surgery on it when the excitation coil around the RF driven Rubidium lamp went open circuit ...


On Tue, 21 Sept 2021 at 16:59, alwyn.seeds1 <a.seeds@...> wrote:
Dear Robin,

One of the BBC’s Rubidium standards used to sit at Droitwich for the long wave transmitter; its frequency wanderings, such as they were, being documented by NPL in monthly bulletins.

So far as I can measure, Droitwich seems now to be GPS locked matching GPS to within the short term drift of the OCXO in my GPS standard.

There was, however, an incident a few years ago, where locking was lost allowing easily measurable error to build up. A comment on another forum and the error went away the following week.

Regards,

Alwyn


_____________________________________________________

Alwyn Seeds, Director
SynOptika Ltd.,
114 Beaufort Street,
London,
SW3 6BU,
England.


SynOptika Ltd., Registered in England and Wales: No. 04606737
Registered Office: 114 Beaufort Street, London, SW3 6BU, United Kingdom.
_____________________________________________________


--
Robin Szemeti - G1YFG