Explosion Dynamics of Methane Clusters Irradiated by 38 nm XUV Laser Pulses

Abstract

We have studied the explosion dynamics of methane clusters irradiated by intense, femtosecond, 38 nm (32.6 eV) XUV laser pulses. The ion time-of-flight spectrum measured with a Wiley-McLarentype time-of-flight spectrometer reveals undissociated molecular CH + 4 ions, fragments which are missing hydrogen atoms due to the breakage of one or more C-H bonds (CH + 3 , CH + 2 and CH + ) and the recombination product CH + 5 . Also visible on the time-of-flight traces are atomic and molecular hydrogen ions (H + and H + 2 ), carbon ions, and larger hydrocarbons such as C2H + 2 and C2H + 3 . No doubly-charged parent ions (CH 2+ 4 ) were detected. The time-of-flight results show that total and relative ion yields depend strongly on cluster size. The absolute yields of CH + 5 and H + scale linearly with the yields of the other generated fragments up to a cluster size of N = 70, 000 molecules, then begin to decrease, whereas the yields of the CH + n (n = 1 − 4) fragments plateau at this cluster size. The behavior of H + may be understood through the electron recombination rate, which depends on the electron temperature and the cluster average charge. Moreover, the CH + 5 behavior is explained by the depletion of both CH + 4 and H + via electron-ion recombination in the expanding nanoplasma.