Emission of electrons from rare gas clusters after irradiation with intense VUV pulses of wavelength 100 nm and 32 nm

Kinetic Boltzmann equations are used to describe electron emission spectra obtained after irradiation of noble-gas clusters with intense vacuum ultraviolet (VUV) radiation from a free-electron-laser (FEL). The experimental photoelectron spectra give a complementary and more detailed view of nonlinear processes within atoms and clusters in an intense laser field compared to mass spectroscopy data. Results from our model obtained in this study confirm the experimental and theoretical findings on the differing ionization scenarios at longer (100 nm) and shorter (32 nm) VUV radiation wavelengths. At the wavelength of 100 nm the thermoelectronic electron emission dominates the emission spectra. This indicates the plasma formation and the inverse bremsstrahlung (IB) heating of electrons inside the plasma. This effect is clearly visible for xenon (with the fitted temperature of 6–7 eV), and less visible for argon (with the fitted temperature of 2–3 eV). The two-photon-ionization rate for argon that initiates the cluster ionization, is much lower than the single-photoionization rate for xenon. Also, more of the photoelectrons created within an argon cluster are able to leave it, as they are more energetic than those released from a xenon cluster. Therefore, the IB heating of plasma electrons in argon is less efficient than in xenon, as the density of the electrons remaining within the cluster is lower.

At a wavelength of 32 nm the dominant ionization mechanism identified from the electron spectra of argon clusters is the direct multistep ionization. The signature of the thermalization of electrons is also observed. However, as the heating of electrons due to the inverse bremsstrahlung process is weak at these radiation wavelengths and pulse fluences, the increase of the electron temperature with the pulse intensity is mainly due to the increasing photoionization rate within the irradiated sample.