Cation vibrational spectra of indole and indole-argon van der Waals complex. A zero kinetic energy photoelectron study

Zero kinetic energy (ZEKE) photoelectron spectra of indole obtained via nine vibrational levels of the S-1((1)L(b)) manifold (namely, 0(0), 42(2), 41(2), 29(1), 39(1)42(1), 28(1), 27(1), 26(1), and 37(2)) are reported. The adiabatic ionization energy of indole has been determined to be I-a = 62592 +/- 4 cm(-1) (7.7604 +/- 0.0005 eV). Cation fundamental vibrational modes up to 807 cm(-1) above the Do origin have been determined on the basis of the Delta upsilon = 0 propensity. The presence of the Delta upsilon = 1 excitation for the out-of-plane modes indicates the presence of a significant vibronic interaction in the ground state cation. For the van der Waals (vdW) complex of indole with Ar, it has been found that the red shift in the S-1 origin amounts to 27 cm(-1) upon the complex formation, whereas the decrease in I-a is 88 cm(-1) with respect to indole. The ZEKE D-0-S-1 O-0(0) photoelectron band of indole-Ar shows a low-frequency progression due to a vdW bending mode whose frequency has been found to be 13 cm(-1). The assignments of low-frequency fundamental vibrational modes for the neutral indole have been reexamined by using the 6-311G** basis set.