Mark Stocket
28, September 2009
Tell me about your project?
Well, there are clouds in space floating around particularly within the galaxy, there are huge clouds that
float around and they’re made of molecules. There are lots of kinds of different clouds; the ones we’re
studying in particular are called diffuse clouds because they’re diffuse and poofy.
The way you study these clouds is if there’s a star somewhere up there, and you’re looking at it, and
there’s one of these clouds between you and the star, some of the light from the star is absorbed by the
molecules in the clouds. These absorption features, a lot of it is normal stuff you’d expect to see in
space, like hydrogen and whatnot. But there’s something like well over 300 unidentified features in
these clouds and they are mysterious. People have been trying to figure out what has been causing
them for something like 90 years.
So since the early 1900s? We’ve been unable to identify these
What separates these clouds from Nebulae?
They are big and kind of thin clouds. Nebulae, you’re seeing light reflected off it, it’s so dense that you
don’t actually see light coming through, you see light bouncing off them. So these are a little different
What department do you work within?
Physics, I’m a graduate student
When did you start?
3 years ago, I’m starting my fourth year now, so 2006
If you could, run me through the procedure and why you use the SRC.
What we’re trying to do is take some molecules, what we want to do is get the spectrum of a bunch of
different molecules and see how they compare to the spectrum of these clouds and see if any features
match up with the unidentified features in the clouds. The problem is, since people have been working
on this for so long, the only things that are left that haven’t been tried, are hard to study.
In particular, we’re studying polycyclicaromatichydrocarbons, which are these big molecules t hat have
these hexagonal rings to them. This ring structure is important because it’s very stable. The interstellar
medium is an extreme place, lots of x-rays. So any large molecule that was not very stable was
obliterated. So that is sort of the prime candidate that’s causing this kind of feature, every small
molecule that’s easy to work with has been done and this is what’s left. The other side of the coin, for
being a very stable molecule, here on Earth they are very sooty, these things are dirt; they’re very
difficult go get into the gas phase. Out in space they are in the gas phase. But here on Earth, they form
clumps, they’re soot, you have to heat them up a lot to get them into the gas phase. But! You want to
study these molecules when they’re cold, because space is COLD, right. So there are some things
working against you. It’s important to study them cold because these large molecules, they have a lot of
different vibrational and rotational modes that they can go into if they are heated up. So the spectrum
between a room temp molecule and a 100 Kelvin molecule is night and day you wouldn’t recognize it. So
the goal is to somehow take these molecules that start out as soot, heat them up into the gas phase,
cool them down, don’t let them form clumps, and don’t let them stick to the wall of your vacuum
chamber, before you get a chance to shine your light on them. So we came up with this big stupid tube
that is supposed to do that.
The way it works is like a wind tunnel, there’s a little propeller that circulates a buffer gas of neon
around like a race track and that blows around.
What’s the purpose of the neon?
It’s supposed to cool the molecules. So there’s a little pod that heats up and is full of molecules and they
leak out of the pod, when they come out they’re hot, they get swept up by this flow of neon and they
collide with the neon atoms and they cool down.
So the neon doesn’t contaminate the spectra?
It’s in there, but neon is so understand that we can discern the neon noise from the spectra of our key
molecules.
What light frequencies do you use?
We use the visible, near UV, and the near IR pretty much from 250 nm-1000 nm.
Is that representative of the light frequencies these molecules are subjected to from starlight?
No, there is a lot more that they are subjected to. The interstellar spectrum is broad, here all of the
unidentified features here are within 400- 800nm so we have that region plus the bookends. We are
really just looking for these features.
We shine the full synchrotron continuum through our tube. It bounces back and forth any number of
times, then we take the light out then we put it in the monochrometer after it’s gone through the gas
sample. We don’t just do one wavelength at a time, we have a spectrograph that is broad, so we get half
the spectrum in one picture. It smears it all out. We have good resolution, very broad spectral coverage
all at once. Because this experiment is tricky and we have small quantities of this sample, we don’t want
to go individually through each wavelength
So your analysis afterward will weed out the frequencies you don’t want?
Right…The spectrometer can only look at a certain chunk at a time, so it looks like 500-800nm at once,
and then you can go through and look at any wavelength at a time.
As we all know, the scientific process doesn’t work perfectly, what were some of the troubleshoots?
Oh boy, there’s been all sorts of crazy problems. One in particular is getting the gas to circulate, you
have to have this propeller that blows this stuff around. To get the gas to move the speed we want, the
propeller has to rotate at something like 5000 rpm, in a vacuum, at 100 Kelvin, which is impossible, so
we had to make it possible, that took a lot of time and engineering.
Eventually, now we got it to work, and it blows it around as fast as we want. It took months of horrible
screeching noises and lots of cussing and stomping around to make it work.
The key problem that we have is that the molecules…what’s supposed to happen is when they come out
of the pod, they get swept up and they get blown away into the path of the light beam, they don’t come
out quite fast enough. The molecules as they come out, they stick back together as they cool off, sort of
like boiling water in a tea kettle.
The steam comes out of the spout, it’s coming out, you can see the steam, since you can see it, it’s not
water vapor, it’s water droplets, the water is re-condensing back, we have the same exact problem.
They are coming out of the pod in the gas phase, but they’re forming little clumps, which is not what we
wanted to study.
Is anyone else working on this project elsewhere?
The typical other groups are studying these same molecules is using a laser based technique and they
scan through one molecule at a time, it takes a lot longer. They spritz, a pulse of these molecules that
sprays out, when they do that they don’t have time to wander around and stick back together like they
do in our system. They don’t have the same clustering problems that we do, but they have a much
slower experiment.
The whole idea of our experiment was to do a whole lot of molecules really quickly and come up with a
data base of near matches and “maybes” and go in more carefully with these more sensitive laser
experiments and make a more firm identification. We want to rule out things that won’t work because it
takes so long with these laser experiments.
How did you come up with your list of molecules to test?
We started with a couple molecules that had already been done, with the laser based experiment, just
to have something to compare to. Then I went through and saw which molecules were available cheap,
not too toxic, had a high enough vapor pressure, that they’d be easy to work with and easy to vaporize.
The idea was once we did the first 4 or 5 molecules, we’d ask someone else what molecules they
wanted us to do. But, we never really got that far off the ground
We started with a couple molecules that had already been done, with the laser based experiment, just
to have something to compare to. Then I went through and saw which molecules were available cheap,
not too toxic, had a high enough vapor pressure, that they’d be easy to work with and easy to vaporize.
The idea was once we did the first 4 or 5 molecules, we’d ask someone else what molecules they
wanted us to do. But, we never really got that far off the ground
Why study this?
The total number of molecular species that have been identified in space in total is about 100, and there
are over 300 unidentified features in these clouds.
It’s a big category of things that we really do not understand about the interstellar medium. If we are
able to figure out what molecules are out there that show these absorption features, they would really
give a lot of information of the chemistry that’s taking place within these clouds. No one has ever been
there so no one knows what the environment is like.
If you can see the molecule out there you can ask, “How did it get there?” and also “How did it form?”
Would you say a lot of it is curiosity and understanding?
Mostly yeah.
What and When is the ideal end?
The idea was to have this as my Ph.D. project for me, which is supposed to be 5’ish years, if went really
well then a student could perhaps pick it up after me. It is supposed to be a survey where we do a
couple of molecules a week. There’s an infinite number of molecules you could imagine putting into this
tube, hopefully we will find some matches of these features, if so then we are creating a bunch of new
data sets.
People have interests in these molecules, they’re atmospheric contaminants and chemists like this
molecule because of its structure that I mentioned. So it is a survey that can go on as long as you want.