A One-Afternoon Experiment: The Making of "Time Resolved Studies of Ultracold Ionizing Collisions" (part 1)

As I said in the introduction to the previous post, this was the first paper on which I was the lead author, and it may be my favorite paper of my career to date. I had a terrific time with it, and it led to enough good stories that I'm going to split the making-of part into two posts.

The experiment itself was based on an earlier paper by Phil Gould at UConn. Phil was a post-doc at NIST back in the day, and used to visit our group fairly regularly. On one of these visits, he stopped by the xenon lab, and gave me a pre-print of their time-resolved collision paper, saying "You guys really ought to try this." It's another terrific example of the high level of collegiality in the AMO community.

I read the paper, and thought it was pretty cool, but it seemd like they had covered most of the bases. I was working on other things at the time, so I didn't do anything with it for several weeks.

Then there was one day when I was trying to do something that would require the maximum possible power from one of our Ti:Sapph lasers, and the laser just wouldn't cooperate. Fixing it would require basically an entire day to strip the laser down and rebuild it, and I didn't feel like doing that, so I said "You know what, I'm going to try that thing Phil suggested. It'll take an afternoon to get some results, and it'll make a good graph for my thesis. Then I'll fix the laser, and get back to the real project.

I did, in fact, get a signal that afternoon. The problem was, the signal had two peaks, when I was expecting one.

We had had a number of electronic problems with the system in the past, so my first thought was that it was some sort of echo signal, and I spent a day and a half tracing wires and looking for ground loops to no avail. We were totally stumped, especially because four microseconds is an awfully long time for a noise echo, and then Steve Rolston realized that it wasn't a four-microsecond delay, it was a factor of the square root of two, and what we were seeing was the molecular ion signal due to associative ionization.

Once we figured that out, we were off, and we spent three months getting the rest of the data and writing the paper. My co-authors gave me grief about this the whole time-- when we were taking data, Scott Bergeson would look over at me every hour or so and say "A one-afternoon experiment, huh?"

This was also the experiment where we used the "biological lock" (Scott's term). We needed to measure the collision signal for lots of different laser detunings, over a wide range of frequencies. It's a little tricky to make an electronic system to do this, but we didn't need all that much precision, so we just split off some light and looked at the difference between the trap laser frequency (which was stabilized electronically) and the control laser frequency by beating them together and measuring the difference frequency with a spectrum analyzer.

That was enough to get the laser where we needed it, but then the control laser tended to drift over the 5-10 minute experimental runs. So we would have somebody stand next to the laser control box, looking at the spectrum analyzer, and when the frequency started to move away from the point we wanted, they would use the manual tuning knob to bring it back. It was a little tedious, but we took turns doing it, and it was faster than spending a week or two getting a good stabilization system working.

Scott did point out, though, that this probably constituted the most expensive laser locking system in the world, given what an NRC post-doc gets paid.

As I said, this was probably my favorite of the experiments that I did at NIST, because it was mine from start to finish. It was my off-hand idea to do the experiment in the first place, I made the decisions about what to measure, and I did almost all of the analysis myself.

This was also one of the first times I felt like a Real Physicist, because the analysis I was doing wasn't trivial. I needed to figure out what the various bits of the signal meant before I could extract information from them, and while there weren't any real "eureka!" moments, there were a lot of small epiphanies where things just clicked into place, and I realized what something meant, and how to use it to measure something interesting.

As I said, the whole thing was a blast, and is one of my fondest memories of grad school. Even the paper-writing, which I'll leave for a separate post.

More like this

I like the fact that somebody (in this case Steve) needed to understand that the delay wasn't 4us but a factor of \sqrt(2). That's a nice illustration of what happens a lot in experimental physics, when you have to look at something in a different way to understand it.

I remember being confused in our four-wave mixing experiment about what appeared to be too-perfect electromagnetically induced transparency (EIT). It took me a while to understand that we were looking at a gain process, which was closely related to EIT but needed to be understood in a different way to make sense.