PNNL

Abstract

The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. By using a high-resolution single mode infrared-optical parametric oscillator laser to prepare CH3I in single (J,K) rotational levels of the v1 (symmetric C–H stretching) =1 vibrational state, we have obtained rovibrationally resolved infrared–vacuum ultraviolet–pulsed field ionization–photoelectron (IR-VUV-PFI-PE) spectra of the CH3I?X˜ 2E3/2 ;v1 +=1;J+ ,P+) band, where (J,K) and (J+ ,P+) represent the respective rotational quantum numbers of CH3I and CH3I?. The IR-VUV-PFI-PE spectra observed for K=0 and 1 are found to have nearly identical structures. The IR-VUV-PFI-PE spectra for (J,K)=(5,0) and (7, 0) are also consistent with the previous J-selected IR-VUV-PFI-PE measurements. The analysis of these spectra indicates that the photoionization cross section of CH3I depends strongly on |?J?|=|J?-J| but not on J and K. This observation lends strong support for the major assumption adopted for the semiempirical simulation scheme, which has been used for the simulation of the origin bands observed in VUV-PFI-PE study of polyatomic molecules. Using the state-to-state photoionization cross sections determined in this IR-VUV study, we have obtained excellent simulation of the VUV-PFI-PE origin band of CH3I+(X˜ 2E3/2), yielding more precise IE(CH3I)=76 930.7±0.5 cm-1 and v1 +=2937.8±0.2 cm-1.