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Since they became available to users in 2009 [1], hard x-ray free-electron lasers (XFELs) have been transformational tools for many scientific disciplines. These kilometer-sized machines generate x rays with unprecedented brightness at energies above 1 kilo-electron-volt and in pulses as short as a few femtoseconds. It is not always entirely clear how these bright pulses interact with targets. In the case of nuclear physics, the expectation was that XFEL light would predominantly interact with nuclei through direct photoexciation. But a new theoretical study of excited nuclear states has revealed that, surprisingly, XFEL-nuclear interactions are dominated by a secondary mechanism. As described in Physical Review Letters [2], this indirect photoexcitation mechanism, which involves electron capture from the laser-induced plasma, is over a million times more efficient. The results may be interesting for future energy storage applications, in which excited nuclei perform as nuclear batteries.

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