Acta Phys. -Chim. Sin. ›› 2003, Vol. 19 ›› Issue (07): 661-665.doi: 10.3866/PKU.WHXB20030720

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Dissociation of Methane in Intense Femtosecond Laser Field

Tang Xiao-Ping;Wang Su-Fan;Gao Li-Rong;Wang Cao;Kong Fan-Ao   

  1. Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080
  • Received:2002-10-08 Revised:2003-03-03 Published:2003-07-15
  • Contact: Kong Fan-Ao

Abstract: Methane molecules were irradiated by a laser beam in the intensity range of 7.6 ×1013~1.4×1014 W•cm-2 (800 nm, 160 fs). A time of flight mass spectrometer was coupled to the laser system. The parent ions can be seen at low laser intensity of 7.6×1013 W•cm-2. When the laser intensity increases to 8 .0×1013 W•cm-2, CH3+ ions appear. The appearance of the CH2+, CH+, C+ ions are at the laser intensities of 1.0×1014 W•cm-2, 1.4×1014 W•cm-2 and 1.4×1014 W•cm-2, respectively. The facts show that dissociation of methane is a step-wise process. Coulomb explosion does not happen during the dissociation because no multi-electron ions are found in the mass spectra. Only H+ ion yield has anisotropic angular distribution when methane is irradiated by a linearly polarized laser. This fact implies that the chemical bonds in the molecule are pulled off by the laser field and that the H+ ion flies along the direction of the laser electronic field. The quasi-diatomic molecule model we proposed can explain the experimental results satisfactorily.

Key words: Femtosecond laser, Intense laser field, Methane, TOFMS, Dissociation