Ultrafast fiber-optic electron gun to reveal atomic motions during transition states
Triggered by ultrafast laser pulses, ultrabright electron pulses and a streak camera will help to observe and capture atomic motions in real time.
One of the most enduring “Holy Grail” experiments in science has been attempts to directly observe atomic motions during structural changes. This prospect underpins the entire field of chemistry because a chemical process occurs during a transition state -- the point of no return separating the reactant configuration from the product configuration.
What does that transition state look like and, given the enormous number of different possible nuclear configurations, how does a system even find a way to make it happen?
Researchers at the Max Planck Institute for the Structure and Dynamics of Matter (Munich, Germany) are reporting ultrabright electron sources with sufficient brightness to literally light up atomic motions in real time at a time scale of 100 fs, making these sources particularly relevant to chemistry because atomic motions occur in that window of time.1 The electron source is based on a 100-μm-diameter multimode optical fiber with one end coated with a 30-nm-thick gold layer to form a back-illuminated photocathode. The photocathode was triggered by laser pulses with a 257 nm wavelength, ∼180 fs pulse duration, and 10 kHz repetition rate.
The low-energy (1-2 kV) time-resolved electron diffraction concept uses the fiber optics for miniaturization and the ability to stretch the electron pulse and applies streak camera technology to potentially obtain subpicosecond temporal resolution -- a difficult feat within the low-electron energy regime.
"The first atomic movies use a stroboscopic approach akin to an old 8 mm camera, frame by frame, in which a laser excitation pulse triggers the structure, then an electron pulse is used to light up the atomic positions," says Dwayne Miller, one of the researchers. "We believed that a streak camera could get a whole movie in one shot within the window defined by the deliberately stretched electron pulse. It solves the problem of low electron numbers and greatly improves image quality."
Of the myriad possible nuclear configurations, the group discovered that the system collapses to just a few key modes that direct chemistry and that a reduction in dimensionality that occurs in the transition state or barrier-crossing region can be inferred. “We see it directly with the first atomic movies of ring closing, electron transfer, and bond breaking,” says Miller.
1. Chiwon Lee et al., Applied Physics Letters (2018); doi: 10.1063/1.5039737; http://aip.scitation.org/doi/full/10.1063/1.5039737