Resonant two-photon ionization-photoelectron spectroscopy of Cu 2: Autoionization dynamics and Cu2+ vibronic states

Abstract

Resonant two-photon ionization of gas phase Cu2 in a cold molecular beam in conjunction with time-of-flight photoelectron spectroscopy provides new vibronic state spectroscopic information for the dimer cation Cu2+. One color ionization via the 0-0, 1-0, and 2α-0 bands of Smalley's System V neutral Cu2 resonant states (J←X transition) accesses Cu2+ states in the range 0-1.4 eV. The electron kinetic energy measurements slightly refine the first adiabatic ionization energy of Cu2 to I1(Cu2) = 7.899 ± 0.007 eV. We observe two electronic states of Cu 2+ which we assign as X2Σ g+ and an excited 2II spin-orbit pair of sublevels with origins at Τ0(2II3/2) = 1.143 ± 0.002 eV and Τ0(2II1/2) = 1.256 ± 0.002 eV. The absence of spin-orbit splitting identifies the ground state 2Σ symmetry; the spin-orbit splitting of 898 ± 8 cm-1 identifies the excited states as 2II. Within X2Σg+ we observe a remarkably long vibrational progression, perhaps extending from ν = 0-80. The vibrational intervals determine the constants ωe = 188 ± 4 cm -1 and ωexe -0.75 ± 0.09 cm -1. The 2II vibrational intervals determine ωe = 244 ± 6 cm-1. The adiabatic bond dissociation energy of ground state Cu2+ is D 0(Cu+-Cu) = 1.84 ± 0.08 eV. The intensity pattern of the X2Xg+ vibrational bands exhibits multiple peaks whose positions and amplitudes are sensitive to the resonant J state vibrational level. For 0-0 excitation, we observe reproducible band intensity alternation. We present preliminary mass spectral and photoelectron data indicating that the cause of the highly non-Franck-Condon band intensities is excitation of long lived, dissociative autoionization states which undergo extensive nuclear motion on the time scale of electron ejection. We propose an autoionization mechanism that includes a description of the Cu2 J state and explains the observed phenomena invoking only one electron transition. © 1989 American Institute of Physics.