Effective energy gap of semiconductors under irradiation with an ultrashort VUV laser pulse

We study theoretically the electronic excitation within semiconductors under irradiation with an ultrashort VUV laser pulse as provided by the new free-electron laser FLASH in Hamburg, Germany. Applying Monte Carlo technique we obtain the transient distribution of the excited electrons within solid silicon. We find the statistical nature of an effective energy gap for multiple electronic excitation, providing the fundamental understanding of the experimentally accessible pair creation energy measured as a long time limit. Considering photoabsorbtion, impact ionizations and Coster-Kroning transitions, we estimate the pair creation energy and give a general formula to calculate the effective energy gap for semiconductors.