Tag: photochemistry

SLAC researchers ‘watch’ molecules steered by laser light

SLAC researchers ‘watch’ molecules steered by laser light

Using the powerful LCLS X-ray laser, they directly imaged for the first time how molecules rearranged during a chemical reaction controlled by light.

Key takeaways:

  • For the first time, researchers have imaged a molecule undergoing a coherently controlled chemical reaction – a reaction steered with pulses of laser light.
  • Powerful X-ray pulses from the LCLS are routinely used to visualize how a molecule’s atoms rearrange in real time during a reaction initiated by a laser pulse, but here, with the help of a novel analysis method, the researchers imaged a reaction that was controlled by a third pulse.
  • This approach could help researchers understand and eventually control light-activated reactions in photochemistry, catalysis, and light-responsive materials. 

Since the 1980s, researchers have sought to use laser light to control chemical reactions relevant to photochemistry, catalysis and light-responsive materials. But this technique, known as coherent control, has a blind spot: There hasn’t been a way to directly see the molecules in these reactions as their structures rearrange. 

Now, researchers at the Department of Energy’s SLAC National Accelerator Laboratory have imaged a coherently controlled chemical reaction for the first time. Their work, published in Physical Review A, uses ultrafast X-rays from the Linac Coherent Light Source (LCLS) to show in real time how atoms move in a molecule that was excited and manipulated with laser light. 

“There are many challenges with controlling chemical reactions, but seeing is believing,” said study lead author Tom Hopper, assistant professor at the University of Central Florida who was a postdoc at SLAC at the time of the study. “If you can see something directly, it opens up a new level of control.”

Illuminating a blind spot

What makes coherent control so tricky is that the molecule being manipulated with laser light will eventually deviate from the desired pathway. If researchers can see the molecule’s structural evolution in real time, they can start to put together a picture of when and how this happens, which may help them figure out how to prevent it.

One of the simplest and most successful methods of coherent control is the “pump-dump scheme.” This involves hitting a molecule with two laser pulses: A “pump” pulse first excites the molecule, initiating the reaction, followed by a “dump” pulse that nudges the reaction down a certain pathway.

Read more on the SLAC website

Image: This illustration shows a pump–dump–probe sequence of ultrafast laser and X-ray pulses used to control and image a chemical reaction. The pump laser pulse excites molecules within a sample and initiates a chemical reaction. It is followed by a second laser pulse, the dump pulse, that nudges the reaction down a certain pathway. Finally, an X-ray probe pulse traverses the sample at varying stages of the chemical reaction. The scattered X-rays create diffraction patterns on a detector (at right). The changing patterns contain information about the molecular structure and how it evolves during the reaction. 

Credit:  Greg Stewart/SLAC National Accelerator Laboratory

Looking for photochemistry inside particles

Looking for photochemistry inside particles

At the Swiss Light Source (SLS), a new photochemical reaction cell was developed for the X-ray microscope at the PolLux beamline. This allowed the researchers to mimic sunlight mediated chemical reactions in airborne particles we normally inhale. Utilizing the new reaction cell, the X-ray microscope was used to image the interior of particles for the chemistry that produced a high concentration of persistent carbon centered radicals (CCR) and reactive oxygen species (ROS), which are harmful compounds when inhaled and can cause damage in the respiratory tract. Two main factors were 1) a very high particle viscosity that effectively locks the CCRs in a glass-like state and 2) oxygen deficiency, or anoxia, to prevent smaller ROS to be formed with a shorter lifetime that easily diffuse out of the particle before inhalation. When relative humidity in air is <60%, particles can become highly viscous or even glass-like, which drastically reduces the mobility of all molecules. Although sunlight induced radical formation is likely to be unhindered, high viscosity would instead inhibit molecular diffusion and block oxygen from accessing the particle interior. This leads to preservation of large amounts of radicals. Amazingly, this may apply to all organic light absorbing atmospheric compounds making radical abundance and persistence an unforeseen issue until now.

Particles composed of citric acid and iron were investigated as a model for iron containing organic particles. About 1 in 20 airborne particles contain iron in urban areas at a significant concentration as identified by previous studies. The oxidation state of iron was mapped across individual particles using X-ray spectromicroscopy to reveal where photochemical reactions, oxidation and molecular diffusion took place inside. Oxidation and formation of ROS took place rapidly, but surprisingly, only near the particle surfaces, i.e. an oxidized reaction front extending only hundreds of nanometers was directly observed. This was entirely due to the rapid depletion of oxygen in the particle due to slow molecular transport and fast reaction cycling. In addition to X-ray microscopy, the researchers used an electrodynamic balance (collaboration with ETHZ) and a coated wall flow tube reactor to study these radical forming particles and constrain the overall reactive cycle and the production and release of radicals to air.

Read more on the PSI website

Image: A chemical scheme and X-ray image showing particles oxidized only near their surface. Light in iron-organic particles start a cycle of oxidizing reactions (purple text) forming carbon centered radicals (yellow text) and reactive oxygen species (red text). We directly imaged oxidation happening only near the particle surfaces indicated by the brighter colour in micrometer and submicrometer viscous particles in the right image.

Credit: PSI