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-McLaren-type time-of-flight spectrometer reveals undissociated molecular CH4+ ions, fragments which are missing hydrogen atoms due to the breakage of one or more C-H bonds (CH3+, CH2+ \ and\ CH+) and the recombination product CH5+. Also visible on the time-of-flight traces are atomic and molecular hydrogen ions (H+ and\ H2+), carbon ions, and larger hydrocarbons such as C2 H2+ and C2H3+. No doubly-charged parent ions (CH42+) 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 CHn+(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 CH5+ behavior is explained by the depletion of both CH4+ and H+ via electron-ion recombination in the expanding nanoplasma.