Breakdown of the Single-Collision Condition in Gas-Phase High-Harmonic Generation

For high-harmonic generation (HHG) in atomic gases the liberated photoelectrons and their parent ions are usually treated as systems that are isolated from neighbouring atoms. Here, we show theoretically and experimentally that collisions with neighbouring atoms can have a significant impact on HHG, especially in the soft x-rays. These additional collisions negatively affect the HHG flux and the coherence of the emitted radiation. Using a simple model, we provide guidelines for the laser parameters needed to minimize these unwanted collisions in the case of phase-matched HHG. Those guidelines are essential for the development of high-flux multi-keV HHG sources.