Tuesday, 2 December 2014

what is a femtosecond, anyway?

I have started a new job which will involve running a Laser-ablation ICP-MS. As a result I have been doing a fair bit of reading on the subject, and, in the process learning (and re-learning) a fair bit of stuff.  Today’s reading (Li et al. 2013*) included a sentence that said “Compared to nanosecond-laser ablation systems, the femtosecond-laser ablation systems provide significantly improved analytical performance…” (and it went on to list what some of those improvements were.  However, it was the first half of the sentence that caused me to come to a full stop and consult the internet for help—I didn’t have any idea what “femtosecond” means.  Clearly it is a unit of time, but how big is it, and is it bigger or smaller than a nanosecond?

Luckily, these days the answer is easy to obtain, checking any list of SI units and their prefixes will reveal that femtoseconds are much shorter than nanoseconds (and therefore faster is better when it comes to analytical performance of laser-ablation ICP-MS systems). The sequence of gradually smaller and smaller divisions in the SI system is: deci, centi, mili, micro, nano, pico, femto, atto, zepto, and yocto.  While I have probably seen this list many times since I was introduced to the SI system as a child, prior to today only the first five were familiar to me.  I already knew that there were 10 centimetres in every decimetre, and 10 millimetres in every centimetre.  I had memorized the fact that there are 1000 micrometres in a millimetre when first I started doing petrology, since it is fairly common to measure minerals in thin sections using micrometers.

However, while I knew the word “nano” meant “even smaller than micro”, that was as much as I knew—I hadn’t bothered to look up that it meant 10-9, which means that there are 1000 of them in every micrometer. The even smaller units I didn’t know at all. Femto is not just a little smaller than nano, it is two steps down the scale smaller: 10-15 means that there are 1,000,000 femtometers in a nanometer.  Or, if we switch back to the seconds that appeared in the sentence that triggered this diversion:

One nanosecond is to one second as one second is to 31.710 years.
One femtosecond is to a second as a second is to about 31.7 million years.

So it isn’t just faster, it is really, really, really lots faster.  No wonder one gets different results with femtosecond-laser ablation than one does with nanosecond-laser ablation.

*Li XC, Fan HR, Santosh M, Hu FF, Yang KF, Lan TG (2013) Hydrothermal alteration associated with Mesozoic granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong Gold Province, China. Ore Geology Reviews 53:403-421

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