Ionic and vibrational properties of an ultra-low ionization potential molecule: Tetrakis(dimethylamino)ethylene

Threshold ionization spectra of nozzle-jet cooled tetrakis(dimethylamino) ethylene (TDAE) were measured with high-resolution electron and laser ionization techniques, and Raman spectra of the molecule at room temperature and under liquid nitrogen were recorded with laser excitation. The TDAE ion signal shows a gradual increase at the onset of ionization, and the upper bounds of the adiabatic ionization potential (IP) measured from the electron ionization and laser ionization are 5.3 ± 0.2 and 5.20 ± 0.05 eV, respectively. In combination with the experimental measurements, density functional theory calculations were used to predict the adiabatic and vertical IPs and vibrational frequencies. The predicted adiabatic IP (5.2 eV) and CC stretching frequency (1622 cm^-1) are in excellent agreement with the measured values. The adiabatic IP is about 0.6 eV lower than the vertical IP (5.8 eV). The large difference between the adiabatic and vertical IPs arise from the significant geometry change upon ionization and is consistent with the experimental observation of the slowly rising ion signal at the ionization onset of the molecule in both the electron ionization and photoionization experiments. Raman spectroscopy of TDAE at room temperature and at liquid nitrogen (Raman under nitrogen, RUN) is reported in an attempt at examining higher energy conformers.