International Journal of Mass Spectrometry 304 (2011) 57–65 Contents lists available at ScienceDirect International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms Ionic and vibrational properties of an ultra-low ionization potential molecule: Tetrakis(dimethylamino)ethylene Nasrin Mirsaleh-Kohan a,b,∗ , Wesley D. Robertson a,c , Jason Lambert a , R.N. Compton a,b , Serge A. Krasnokutski d , Dong-Sheng Yang d a Department of Physics, The University of Tennessee, Knoxville, TN 37996, United States b Department of Chemistry, The University of Tennessee, Knoxville, TN 37996, United States c Department of Physics, Emory University, Atlanta, GA 30322, United States d Department of Chemistry, University of Kentucky, Lexington, KY 4056, United States article info Article history: Received 4 November 2010 Received in revised form 8 April 2011 Accepted 8 April 2011 Available online 15 April 2011 Keywords: Tetrakis(dimethylamino)ethylene Low ionization potential Electron and photon ionization Raman frequencies Trochoidal electron monochromator (TEM) Nozzle-jet expansion abstract Threshold ionization spectra of nozzle-jet cooled tetrakis(dimethylamino)ethylene (TDAE) were mea- sured 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 C C stretching frequency (1622 cm -1 ) are in excellent agreement with the measured val- ues. 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The ionization potential (IP) of most organic compounds lies in the range from 7 to 12 eV. However, a previous photoionization study indicated that tetrakis(dimethylamino)ethylene (C 10 H 24 N 4 , TDAE) has a very low IP (≤5.36 eV) [1], which is comparable to that of the lithium atom (5.39 eV). Such a low IP molecule has attracted great interest over the past decade. Since TDAE readily gives up an electron, it has found many applications in various applied and research areas, such as plasma technology, semi-conductor indus- try, and electrospray mass spectrometry. In plasma research, a central interest is on the study of high-density, low-temperature plasmas (about 10 11–13 cm -3 ) at atmospheric pressure. The main difficulty of producing these plas- mas is a requirement of high power budget to initiate and sustain an air-plasma discharge. The high power budget can be reduced to some extent by the choice of a seed gas. Woodworth et al. [2] stud- ied the generation of a high-density plasma by using ultraviolet ∗ Corresponding author. E-mail address: nmirsale@utk.edu (N. Mirsaleh-Kohan). (UV) lasers to ionize low IP organic molecules. Scharer’s group [3,4] successfully produced plasmas with densities of about 10 13 cm -3 in TDAE vapor through a 193 nm one-photon ionization process. TDAE has also found important applications in particle physics and medical imaging. An alternative to expensive photomultiplier tubes in detecting UV photons is to employ photosensitive gases [5,6]. Detectors based on photosensitive gases can be employed to detect UV photons over large areas with a reasonable cost. In designing a detector based on these photosensitive gases, the absorption photon wavelength and absolute quantum efficiency are two important parameters. TDAE with its high vapor pres- sure (0.35 Torr at 20 ◦ C), large quantum efficiency, and broad spectrum of sensitivity has proved to be an ideal gas for the detec- tion of UV photons [7]. Another interest in TDAE arises from its ferromagnetic properties when it is doped into fullerene crys- tals, i.e., TDAE-C 60 . In 1991, Allemand et al. [8] recognized for the first time that TDAE-C 60 is a ferromagnetic material with a Curie transition temperature of 16 K. Later, Tanaka et al. [9] reported three magnetic phases for TDAE-C 60 with Curie transi- tion temperature of 25, 16, and 10 K. TDAE also has been chosen to reduce the barrier for the charge injection in electronic devices. It has been shown that a monolayer of TDAE deposited on a gold 1387-3806/$ – see front matter © 2011 Elsevier B.V. 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