Synopsis: Molecular snapshots with femtosecond x rays

Early results from the Linac Coherent Light Source expose the inner dynamics of molecules during ionization with ultrashort x-ray pulses.

Imaging the internal dynamics of excited atoms and molecules with high spatial resolution requires fast, energetic pulses of x rays. As Matthias Hoener and colleagues from an international collaboration report in Physical Review Letters, initial results on how molecules absorb high-energy, femtosecond x-ray pulses are now emerging from the Linac Coherent Light Source (LCLS), which began operation last year at SLAC National Accelerator Laboratory in California, US.

Hoener et al. used the LCLS free-electron laser to produce x-ray pulses with wavelength $1.1\phantom{\rule{0.333em}{0ex}}\text{nm}$ ($1100\phantom{\rule{0.333em}{0ex}}\text{eV}$ photon energy) and pulse widths from $280$ femtoseconds down to $4$ femtoseconds. The team directed the pulses at puffs of nitrogen (${\text{N}}_{2}$) gas and analyzed the resulting ionized molecules with a time-of-flight mass spectrometer.

All the x-ray pulses delivered the same total amount of energy to the nitrogen gas, but Hoener et al. observed that the spectrum of ions depended on the pulse width: Long duration pulses completely stripped the electrons from the nitrogen atoms, but shorter pulses were not able to produce the fully ionized state. Hoener et al. argue that for shorter pulses, the cycle of an outer-shell electron falling into the emptied core-shell (Auger decay) cannot occur quickly enough, limiting the achievable ionization.

The experiments reveal the fundamental dynamics of how molecules absorb high-intensity, hard x rays and bear directly on what may be possible in future single-shot studies of chemical structure and dynamics—one of the most important applications of free-electron lasers. – David Voss

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