# Is it possible to determine timescales of electron dynamics from the natural linewidth of an electronic transition?

A lot of work has been done recently on electron dynamics using attosecond pump-probe techniques; for instance in this paper. In this particular paper, the authors photoionized the neutral tetrapeptide $\mathrm{TrpLeu}_3$ to the cation and observed the repopulation from the HOMO-1 to the HOMO (highest-occupied molecular orbital) of the cation in a time-resolved manner.

Here's my question: Since Heisenberg's uncertainty principle guarantees that the natural linewidth of a energy measurement will be greater than or equal to the inverse of the state's lifetime times $\hbar$, i.e.

$\Delta E \geq \frac{\hbar}{\Delta t}$,

could you just measure the natural linewidth of the photons emitted by the transition between the transient state and the final state and deduce the lifetime of the transient state? Or would the broadening be too large for subfemtosecond states to accurately determine the natural linewidth? Or, would other effects such as Doppler broadening be too large to get a reasonable measurement of the natural linewidth?

Note: I'm a long way out of my field of specialty (computational chemistry) in this question, so I apologize in advance if this makes no sense whatsoever (please tell me why, though, in your answer if this is the case). Also, obviously this would only yield an upper bound for the lifetime; my question is more along the lines of if it would yield an upper bound worth obtaining.

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This is a standard technique for particle physics (where the lifetimes are short and the energy widths therefore large), but for relatively long lifetimes it becomes constrained by questions of energy resolution. I've seen a number of atto-second physics colloquia, but I have no grasp of their energy resolution. –  dmckee Dec 2 '10 at 1:44
@dmckee: Thanks for the informative comment. Could you point me to a journal article where this technique is used? I'd try and find it myself, but I'm not even sure what to look for, since I don't know the formal name of the technique. –  David Hollman Dec 2 '10 at 14:53
Uhm...Okay, this is kinda embarrassing: I'd have to go hunting. Because this is so common that (as far as I know) it doesn't have a name. Every particle physicist simple knows that lifetimes are found by measuring line widths... A grad school buddy of mine bashed over the $J/\Psi$ and $\Psi'$ peaks for Fermilab's e866 and rediscovering the lifetime was an early validation step for his analysis. –  dmckee Dec 2 '10 at 19:10