Dynamics Study on the Multi-Photon Dissociation of CS2+ to CS+

The dynamics on the multi-photon dissociation of CS2+ molecular ions to produce CS + ions has been investigated by measuring the CS + photofragment excitation（PHOFEX）spectrum in the wavelength range of 385～435 nm，where the CS2+ molecular ions were prepared purely by［3+1］multiphoton ionization of the neutral CS2molecules at 483.2 nm. With the ～60 ns delay，which is much more than the laser pulse width（～5 ns），between ionization laser and dissociation laser，the threshold wavelength of dissociation laser to produce CS+ fragment ion from CS2+ molecular ions was obviously observed in the PHOFEX spectrum. The adiabatic appearance potential of the CS+ was determined to be（5.852 ± 0.005）eV above the X 2Σg，3/2（0，0，0）level of CS2+. The product branching ratios，（CS+/S+），as measured from the PHOFEX spectra，increase from 0 to slightly larger than 1 in the wavenumber range of 47200～50400 cm-1 . The［1+1］dissociation mechanism to get to CS++S from CS2+ was discussed and preliminarily attributed to（i）CS2+（X 2Πg）→ CS2+（A2Πu）through one-photon excitation，（ii）CS2+（A2Πu）→ CS2+（X*）via internal conversion process due to the vibronic coupling between the A and X states，（iii）CS2+（X*）→ CS2+（B 2Σ+u）through the second photon excitation，and（iv）CS2+（B 2Σ+u）→CS +（X 2Σ+）+S（3P），because of the potential curve crossing with the repulsive 4Σ- state and/or the 2Σ- state correlated with the second dissociation limit. However，when the dissociation laser overlaps the ionization laser in time scale in the laser-molecule interaction zone，the appearance threshold is not available in the PHOFEX spectrum. This fact shows that there are other mixed three-photon paths of［1+1+1'］，［1+1'+1'］，and［1+1'+1］to produce CS+ fragment ion from CS2+ molecular ions besides the above［1+1］dissociation mechanism，that is，CS2+（X 2Πg）→ CS2+（A 2Πu）through one-photon excitation［1］of dissociation laser，CS2+（A 2Πu）→CS2+（X*）via internal conversion process due to the vibronic coupling between the A and X states，CS2+（X*）→ CS2+（B 2Σ +u）through the second photon excitation by dissociation laser［1］or ionization laser［1'］，and third photon excitation by ionization laser［1'］or dissociation laser［1］to reach the adiabatic appearance potential to produce CS+ with the dissociation laser wavelengths longer than 423. 89 nm，at which the［1+1］dissociation mechanism to get to CS+ is unavailable.