Single-photon spectroscopy of singlet sulfur atoms and the autoionization lifetime measurements of the superexcited singlet states

摘要

Single-photon excitation spectra from the lowest singlet ~1D_2 level of sulfur atoms were recordedwith a tunable vacuum ultraviolet (VUV) radiation source generated by frequency tripling in noble gases.The photolysis of CS_2 at 193 nm was used to produce the singlet S(~1D_2) sulfur atoms that were then excited to neutral superexcited states with the tunable VUV radiation.These superexcited states undergo autoionization into the first ionization continuum state of S~+(~4S_(3/2)~o)+e~-,which is not directly accessible from the S(~1D_2) state via an allowed transition.The excitation spectra were recorded by monitoring the S~+ signal in a velocity imaging apparatus while scanning the VUV excitation wavelength.Three new lines were observed in the spectra which have not been previously reported.The full widths at half maximum (FWHM) of each of the observed transitions weredetermined by fitting the profiles of each absorption resonances with the Fano formula.Autoionization lifetimes tau of these singlet superexcited states were obtained from FWHM using the Uncertainty Principle.Abnormal autoionization lifetimes were found for the 3s~23p~3(~2D~o)nd(~1D_2) and the 3s~23p~3(~2D~o)ns(~1D_2) Rydberg series,in which tau(5d) and tau(7S) are shorter than tau(4d) and tau(6s),respectively.This is contrary to the well-known scaling law of tau(n)propor.ton~3,which should be followed within a series unless there exist perturbations,from other series or new channels open up to which some members of the series can decay.Possible perturbations from the nearby triplet series are suspected for causing the broadening of the 5d and 7s levels.