CC BY 3.0 UnportedWeisshaupt, JannickJuvé, VincentHoltz, MarcelWoerner, MichaelElsaesser, Thomas2022-07-222022-07-222015https://oa.tib.eu/renate/handle/123456789/9769https://doi.org/10.34657/8807The interaction of intense femtosecond pulses with metals allows for generating ultrashort hard x-rays. In contrast to plasma theories, tunneling from the target into vacuum is introduced as electron generation step, followed by vacuum acceleration in the laser field and re-entrance into the target to generate characteristic x-rays and Bremsstrahlung. For negligible space charge in vacuum, the Kα flux is proportional to the incident intensity and the wavelength squared, suggesting a strong enhancement of the x-ray flux by mid-infrared driving pulses. This prediction is in quantitative agreement with experiments on femtosecond Cu Kα generation.enghttps://creativecommons.org/licenses/by/3.0/530500Electromagnetic pulsePlasma theoryCharacteristic x raysElectron generationHard X-ray generationIncident intensityQuantitative agreementUltra-short light pulseUltrashort hard x-raysVacuum accelerationX raysArticle