Veteran of the Physics Wars

Some interesting stuff to relate this time. My main task thus far has been to determine the decay-time of a photoluminescent liquid, which - believe it or not - is a fairly involved task.

Dr. Bickel and I have been developing a rapport, and we're working well together.

So. How does one go about finding the decay time (also known as the half-life or half-time) of a photoluminescent liquid?

First, we should talk a little about what a decay time is. In the sense that we're concerned with, it means the time it takes for the light produced by the liquid to reach half its maximum value. If you remember from my previous posts, when we're analyzing a light source, we use a PM Tube (photo-multiplier tube) to detect the light and translate it into a signal we can use. In most cases, we hook the PM tube up to an ammeter (a picoammeter in this case) and can read an absolute current coming from the tube. This tells us, in relative terms, how much light is being emitted by a source at a certain wavelength.*

To do this, we must first find the wavelength at which the light source (the liquid, in this case) emits the maximum light. Because we can't exceed 1μA on this PM tube (or risk damaging it) we first need to check that we won't exceed that value when we scan through the wavelengths. To do that, we simply turn our settings low, hook the PM tube into the picoammeter, and scan through the wavelengths we're interested in. In this case, we scanned from 3000Å - 8000ņ. When the current begins to climb too high (towards pegging the meter and potentially ruining your equipment) you simply adjust the scale upwards, and continue on. If the current starts to approach 1μA, we turn the voltage on the PM tube down, which will decrease the current. Anyway, when the maximum current is found, we run the strip chart across a narrowed-down range of wavelengths, and get a paper copy of the data.

Now we have the pertinent information. We know the maximum current is at a certain wavelength, and can now find the decay time for the source. To do that, we set the spectrograph to that wavelength (5300Å in this case) and then quickly place the source in the spectrograph, and run the strip chart. We find that the current takes its maximum value and then decreases rapidly towards an asymptote... which is the problem. We can't really define the decay time in absolute terms, since it's an asymptote, and everyone and their brother is going to weigh in and tell you that "No. It doesn't reach zero there, THIS is where it reaches zero." So we define it in terms of something everyone can agree on: the point where it reaches half the maximum value. Hence, "half-life, half-time," etc.

As I said, Dr. Bickel and I are getting along fine, and the college students who drop in have been very kind as well. Occasionally, Dr. Bickel will come by and we'll talk about scientific ethics, or the best way to do such-and-such a procedure. I'm learning a lot, and I couldn't have a better professor to work under.



*This is not an absolute measurement of the amount of light being emitted. The reading depends on the PM tube and the spectrograph. Different PM tubes are designed to work at different wavelengths - this is where that work function 'ϕ' comes into play - and different spectrographs will have an effect on the light reading as well.

†Angstroms. A unit of measure equal to 1/10th of a nanometer, i.e. 1x10^(−10)m.