Gas-phase HeI ultraviolet photoelectron spectra (UPS) of 9-H carbazole and N-substituted derivatives are reported in this paper. The property and the eigenvalus of each related molecular orbital obtained from the calculations are well consistent with the character of spectra bands and the changing rules of I~v between carbazole and N-substituted carbazole. In addition to the assignment of the spectra, some useful conclusions were obtained: comparing the N-alkylated derivatives with the parent carbazole, the π orbitals' character on the carbazole ring have no change; only the related ionization potentials (IPs) shift to lower energy, this is, the selective destabilitation of the π orbitals. Increasing the carbon atomnumber in the n-alkyl has little effect on the π orbitals' character and their IPs, nor does the substitution site of the alkyls. The selective destabilization has main effect on the π orbitals which largely located on the nitrogen atom.

Key words: IMIDAZOLE, HELIUM, GEOMETRICAL ISOMERISM, MOLECULAR ORBIT