Practical Electron Microscopy and Database

The principle of generating a pulsed electron beam using an RF-cavity chopped-beam in electron microscopes (EMs) is based on a technique that modulates a continuous electron beam into discrete pulses. This is achieved by passing the electron beam through an RF (radio frequency) cavity, which selectively allows electrons to pass through in short bursts:

• Continuous Electron Beam:
  • The electron beam originates from a continuous source, such as a conventional field emission or thermionic emission electron gun. Normally, this beam is continuous, meaning electrons are emitted and directed towards the sample without temporal interruption.

• Radio Frequency (RF) Cavity:
  • The RF cavity is an electromagnetic resonator that generates oscillating electric fields at a specific radio frequency. When a continuous electron beam passes through this cavity, the oscillating electric fields act as a modulating barrier.
  • The oscillating fields in the cavity periodically deflect or allow the electron beam to pass. Only during specific intervals in the oscillation cycle, where the field strength allows electrons to pass unimpeded, do the electrons continue traveling down the beam path.
  • These intervals result in electron bunches (or packets) that emerge from the RF cavity, effectively chopping the continuous beam into discrete pulses.

• Chopping the Beam:
  • The RF frequency (typically in the gigahertz (GHz) range) determines how often the electron beam is chopped, or pulsed. The frequency controls the time interval between the electron packets. For example, a frequency of 5 GHz means the beam is chopped into pulses every 200 picoseconds.
  • The duration of the pulses (i.e., how long the beam is allowed to pass through the RF cavity) is controlled by the duty cycle of the cavity. A short duty cycle results in narrow electron pulses, while a longer duty cycle gives broader pulses.

• Electron Bunching:
  • When the electrons pass through the RF cavity at the right moment (corresponding to the oscillation phase where the field allows them through), they form bunched groups of electrons. These bunches are what constitute the pulsed electron beam.
  • The number of electrons per pulse can vary based on the initial beam intensity and the RF cavity’s configuration. However, in each pulse, the electrons are temporally concentrated into a tight group rather than spread out continuously.

• Controlling Pulse Characteristics:
  • The pulse repetition rate (how frequently the pulses occur) is set by the RF frequency. This allows for very high repetition rates, often in the range of gigahertz, which are suitable for fast measurements in electron microscopy.
  • The pulse width (duration of the pulse) is determined by the interaction between the electron beam and the oscillating field in the RF cavity. Tighter pulse widths (shorter durations) can be achieved by careful tuning of the duty cycle and RF cavity parameters.

• Applications:
  • The pulsed beam produced by the RF cavity is useful in time-resolved experiments where precise timing of electron interactions is critical.
  • This approach can also be used to mitigate radiation damage in sensitive specimens by spacing out electron doses, allowing relaxation time between pulses, similar to the concept of the laser-based pulsed electron beam.
  • RF-chopped beams are also used in ultrafast electron microscopy and dynamic studies, where short bursts of electrons are required to capture transient states of a material.

• Temporal Control: Electron pulses are precisely timed based on the RF frequency.
• Minimization of Damage: By pulsing the beam, less continuous exposure to the sample can be achieved, which is particularly useful for delicate materials.
• High Repetition Rates: The GHz range of RF frequencies allows for high repetition rates, making this method useful for fast, repetitive measurements.