Recent advancements in technology have enabled researchers to produce extremely brief ion pulses, lasting under 500 picoseconds. This innovation can be utilized to observe materials or even visualize chemical reactions in real time.
To capture a fast-moving subject in a photograph, a camera must have a very short exposure time. This concept holds true across various fields of physics; for instance, incredibly brief laser pulses are employed to witness the actions occurring within atoms.
Ion pulses are also beneficial in solving complex questions in physics. A novel technique has been developed to produce highly intense, minuscule pulses of charged particles that can be fired at a surface with precise control in the future. This technique will allow for the examination of rapid processes occurring on that surface. For example, chemical reactions can be analyzed in real time as they unfold.
Typically, only the aftermath is visible
“Ion beams have been in use for many years—to analyze materials and to clean or alter surface properties,” explains Prof. Richard Wilhelm from the Institute of Applied Physics at TU Wien. “However, in the conventional approach, we can only observe the final outcome: ions are directed toward a surface, and we analyze how the material has been modified post-impact. The challenge until now has been creating such fleeting ion pulses that can trace the sequence of the impact.”
The ion pulses crafted in TU Wien’s lab last less than 500 picoseconds. A picosecond is a millionth of a millionth of a second, a duration that is nearly unfathomable for humans. In this brief window, light only traverses 15 centimeters. Although this duration is considerably longer than the quickest laser pulses, which last in the attosecond range, it still provides a suitable timeframe for surface analysis.
Lasers create electrons, electrons create ions
To produce these remarkably brief and intense ion pulses, a multi-layered process was necessary. Initially, a laser pulse strikes a cathode, causing it to release electrons. These electrons are then accelerated toward a stainless steel target. “Specific atoms like hydrogen and oxygen tend to gather on the stainless steel surface,” remarks Richard Wilhelm. “When the electrons collide with these atoms, some are ejected and sent flying off.”
Among these ejected atoms, some remain neutral while others become ionized. By utilizing electric fields, researchers can choose which atoms to utilize further and direct them with great accuracy as short ion pulses aimed at the target surface for analysis.
“As this process starts with a laser pulse, we can determine exactly when the ion pulse is created and when it strikes a surface,” Richard Wilhelm explains. “This allows us to assess the surface with incoming ions at varying intervals while a specific laser-induced chemical reaction occurs. We collect diverse signals that depict the reaction’s progression on a picosecond scale.”
Adaptable new technology
Previously, only the simplest ions, such as protons, were employed for this process. However, this same technique could also be applied to create ion pulses from other elements, like carbon or oxygen. “‘The ions we generate depend on the atoms we place onto the stainless steel surface contacted by the electrons,” Richard Wilhelm notes. It’s possible to also create pulses of neutral atoms or negatively charged ions.
This breakthrough was made possible thanks to the START Prize awarded to Richard Wilhelm by the Austrian Science Fund FWF in 2019, which enabled exploratory and complex research endeavors deemed risky. “Well-supported START grants are essential for transforming daring ideas into reality,” remarks Wilhelm. Plans are already underway to shorten the duration of ion pulses even more by utilizing specially designed alternating electromagnetic fields to regulate the speed of the initial ions and the subsequent ions in the pulse.
“We have established a promising and strikingly efficient method for probing ultrafast processes that were previously inaccessible,” Richard Wilhelm asserts. This technique can be integrated with existing ultrafast electron microscopy technologies, offering insights into various dimensions of the physics and chemistry associated with surfaces.
To photograph a rapidly moving object, a camera with a very brief exposure time is essential. This principle is universally applicable in physics; for instance, extremely short laser pulses are utilized to visualize atomic processes.
Moreover, ion pulses are crucial for unlocking answers to unresolved challenges in physics: a new approach has successfully generated highly intense, incredibly short pulses of charged particles, to be directed at a surface with extremely precise control in the future. This will permit the examination of quick processes occurring on that surface, enabling real-time analysis of chemical reactions.
Typically, only the results are visible
“Ion beams have been utilized for many years—to investigate materials and to clean or alter material surfaces,” highlights Prof. Richard Wilhelm from TU Wien’s Institute of Applied Physics. “In the usual methods, we typically see only the final product: ions strike a surface, and we investigate how the material has been altered afterward. Previously, the primary issue was generating exceptionally brief ion pulses capable of tracking the impact’s timeline.”
The ion pulses produced in the TU Wien laboratory endure for less than 500 picoseconds. A picosecond is one-millionth of a millionth of a second—a span of time virtually beyond human comprehension.
The speed of light, while extremely fast, can travel only 15 centimeters in just 500 picoseconds. Although this duration is still millions of times longer than the most rapid laser pulses, which operate on an attosecond scale, it is suitable for surface analysis.
From Lasers to Electrons to Ions
To create incredibly brief and intense ion pulses, a complex multi-step process was engineered: It begins with a laser pulse directed at a cathode, which then releases electrons. These electrons are accelerated before striking a stainless steel target. “Certain atoms, such as hydrogen and oxygen, tend to accumulate on the stainless steel surface,” explains Richard Wilhelm. “When these electrons strike the layer of atoms, some are expelled and escape into space.”
Among the atoms that escape, some remain electrically neutral while others become ionized. To selectively choose which atoms to utilize, electric fields can direct them as short ion pulses aimed precisely at the targeted surface for analysis.
“Since this entire process is initiated by a laser pulse, we can accurately control the timing of the ion pulse generation and its impact on the surface,” Wilhelm states. “This allows us to analyze the surface interactions at various time intervals during a chemical reaction activated by a laser. Different signals reveal the evolution of the reaction within a picosecond timeframe.”
Innovative and Versatile Technology
Until now, only the simplest ions, specifically protons, were used for this method. However, it can also be adapted to generate other ion types like carbon or oxygen ions. “‘It all hinges on the specific atoms we attach to the stainless steel target that receives the electrons, which can be controlled accurately,” notes Richard Wilhelm. Additionally, it’s possible to create pulses of neutral atoms or even negatively charged ions.
There are plans in motion to further decrease the duration of the ion pulses. Achieving this would involve using specially designed alternating electromagnetic fields to slightly slow down the initial ions in the pulse while simultaneously speeding up those that follow.
“We’ve created a promising and highly efficient method for studying ultrashort processes whose dynamics were previously unobservable,” states Richard Wilhelm. This technique can be integrated with current ultrafast electron microscopy methods, offering a deeper understanding of the physics and chemistry involved in surface phenomena.
To capture rapidly occurring events, you need an extremely fast camera with a short exposure time. The same principle applies in physics: for instance, ultrashort laser pulses are utilized to visualize atomic interactions.
In addition to laser pulses, charged particle ion pulses can bring clarity to unresolved physics questions. A novel technique now enables the generation of extremely short, powerful ion pulses that can be precisely aimed at surfaces. This innovation facilitates the examination of swift processes on these surfaces, allowing for the analysis of chemical reactions even while they are happening.
Typically, Only the End Result is Observed
“Ion beams have long been employed for various purposes, including analyzing materials and modifying or cleaning their surfaces,” says Prof. Richard Wilhelm from the Institute of Applied Physics at TU Wien. “However, traditionally, we only see the final outcomes: ions hit a surface, and then we examine the resultant changes in the material. The real challenge until now has been to produce ion pulses short enough to monitor the actual impact as it progresses.”
The ion pulses created at TU Wien’s lab last less than 500 picoseconds. A picosecond is a millionth of a millionth of a second—a duration so brief it’s hard to comprehend. In comparison, light travels just 15 centimeters in 500 picoseconds. Nevertheless, this time frame is still millions of times longer than the shortest laser pulses available, which operate on the attosecond scale. Yet, it falls within an optimal range for surface analysis.
From Lasers to Electrons to Ions
To produce such swift ion pulses with great intensity, an intricate multi-step procedure had to be developed: A laser pulse is directed at a cathode, prompting the emission of electrons. These electrons are then accelerated towards a stainless steel target. Wilhelm highlights, “Atoms like hydrogen and oxygen tend to gather on the stainless steel surface. When electrons strike this atom layer, some of these atoms are ejected and move away.”
Among the atoms that drift away, some remain neutral, while others are ionized. Electric fields can be utilized to specify which ones to work with next, directing these atoms precisely as ion pulses towards the intended surface.
“Because the process starts with a laser pulse, we are capable of very precise control over when to generate the ion pulse and when it impacts the surface,” Wilhelm continues. “This allows for probing the surface with incoming ions at various moments during a specific laser-based chemical reaction, yielding different signals that visualize the reaction’s dynamics on a picosecond scale.”
Innovative and Versatile Technology
The method has previously focused mainly on protons, the simplest ions. However, it is adaptable for generating other types of ions, including carbon or oxygen. “‘The choice simply depends on the atoms we deliberately apply to the stainless steel layer being bombarded by electrons,” explains Wilhelm. It’s also possible to generate pulses of neutral atoms or negatively charged ions.
Plans are already underway to further reduce the duration of ion pulses. Specialized alternating electromagnetic fields can achieve this by slowing down some of the initial ions in the pulse while accelerating the ones that follow.
Wilhelm remarks, “We’ve developed an exciting and remarkably efficient technique for exploring ultrashort processes that were previously inaccessible. This technique can be merged with existing ultrafast electron microscopy technologies to unveil different aspects of surface physics and chemistry.”
