ELSEVIER 21 March 1997 Chemical PhysicsLetters 267 (1997) 370-376 CHEMICAL PHYSICS LETTERS Resonant two-photon ionization spectroscopy of the 13-electron triatomic Si2 N Dale J. Brugh, Michael D. Morse Department of Chemistry, University of Utah, Salt Lake City, UT84112, USA Received 12 November1996;in f'malform2 January 1997 Abstract Resonant two-photon ionization spectra of 285i2N, 28'29Si2N, and 28"3°Si2N have been collected between 32000 cm- and 36000 cm- 1. A fit of the rotationally resolved spectrum of the most intense band of 2SSi2N at 34314 cm- l reveals the 2 + - transition to be Xu ~ X21-ig.I/2• A molecular orbital analysis is made to justify the assignment. The geometric structure is determined to be linear and centrosymmetric in both the ground and excited state with r~(Si-N) = 1.6395 + 0.0014 ,~ and r'(Si-N) = 1.6343 4- 0.0014/~. The ionization energy is determined to be less than 8.51 eV. 1. Introduction Twelve of the possible 100 X2Y molecules that can be formed from the second row atoms B, C, N, O, F, and their third row congeners possess 13 valence electrons. Of these, only C2N has been investigated in the gas phase with sufficient resolu- tion to observe the rotational structure and to unam- biguously assign a conformation to the molecule. In this case it was discovered that C2N exits as two isomers: CNC [1] and CCN [2]. Both species are clearly stable gas phase molecules. Subsequent cal- culations [3] have shown that these two isomers of C2N are very nearly isoenergetic. In fact, they are so close that no definite energetic ordering is yet possi- ble. Not long after the investigation of CNC and CCN, Zmbov and Margrave reported the discovery of Si 2N, another 13-electron species, in the gas phase follow- ing vaporization of pure Si inside a boron nitride Knudsen cell oven [4]. By the vanishing current method they determined the ionization potential of Si2N to be 9.3 +0.3 eV. They were also able to determine, by the third law method, the heat of atomization of this molecule to be 10.2_ 0.4 eV. Not until 1993 was there any further mention of Si2N in the literature. In that year Iraqi et al. [5] reported the identification of SiEN in the gas phase again. Their neutralization-reionization mass spec- trometry (NRMS) and collision induced dissociation (CID) studies indicated that, of the three possible isomers, the linear and centrosymmetric structure, SiNSi, was not produced in that experiment. Because the interpretation of their NRMS and CID results was largely based upon an assumed similarity be- tween SiEN and Si20 , this conclusion can be consid- ered tentative at best. Since this investigation, two theoretical studies of Si2N have been carried out. Boidyrev and Simons [6] have calculated the relative energies of four different ground state symmetries of Si2N and found that the 21-1gOsh structure lay lowest in energy with the C2v structure only 1574 cm -~ 0009-2614/97/$17.00 Copyright© 1997Elsevier Science B.V. All rights reserved. Pll S0009-261 4(97)001 08-5