Photoelectron spectroscopy and electronic structure of clusters of the group V elements. III. Tetramers: The 2T2 and 2A1 excited states of P+4, As+4, and Sb+4

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Abstract
Methods employing high resolution HeI (584 Å) photoelectron spectroscopy have been applied to the tetrameric clusters of the group V elements, to resolve details of vibronic and spin–orbit structure in the first three electronic states of P+4, As+4, and Sb+4. Measured spacings of distinct vibrational progressions in the ν1 mode for the 2A1 states of P+4 and As+4, yield vibrational frequencies of 577 (5) cm−1 for P+4 and 350 (6) cm−1 for As+4. Franck–Condon factor calculations suggest bond length changes for the ions in the 2A1 states of 0.054 (3) Å for P+4 and 0.060 (3) Å for As+4. Strong Jahn–Teller distortions in the ν2(e) vibrational mode dominate the structure of the 2E ground states of the tetrameric ions. Both Jahn–Teller and spin–orbit effects appear in the spectra of the 2T2 states of the tetrameric ions, with the spin–orbit effect being dominant in Sb+4 and the Jahn–Teller effect dominant in P+4. Vibrational structure is resolved in the P+4 spectrum, and the ν3(t2) mode is found to be the one principally active in the Jahn–Teller coupling. A classical metal-droplet model is found to fit well with trends in the IPs of the clusters as a function of size.