Singly, doubly, triply, and quadruply charged krypton ions are formed by multiphoton absorption processes in krypton atoms. They are induced by a 50-psec laser pulse at 1.064 μm in the ${10}^{13}$-${10}^{14}$-W-${\mathrm{cm}}^{-2\mathrm{}}$ intensity range. The percentage ratio between the numbers of doubly and singly ionized Kr atoms is 10% at 8×${10}^{13}$ W ${\mathrm{cm}}^{-2\mathrm{}}$. It is shown that ${\mathrm{Kr}}^{2+}$ ions result from a direct 33-photon absorption process. At 1.5×${10}^{13}$ W ${\mathrm{cm}}^{-2\mathrm{}}$, the 33-photon absorption rate is only about 60 times less than the 13-photon absorption rate which gives singly charged ions.

• Received 12 March 1982

DOI:https://doi.org/10.1103/PhysRevLett.48.1814