Superatom spectroscopy and the electronic state correlation between elements and isoelectronic molecular counterparts

Samuel J. Peppernicka, K.D. Dasitha Gunaratneb, and A.W. Castleman Jr.b,c,1

aDepartment of Materials Science and Engineering, bDepartment of Chemistry, and cDepartment of Physics, 104 Chemistry Research Building, Pennsylvania State University, University Park, PA 16802.

Edited by R. Stephen Berry, The University of Chicago, Chicago, Illinois, and approved December 2, 2009 (received for review October 5, 2009)

Detailed in the present investigation are results pertaining to the diagram is usually employed to demonstrate the electronic state photoelectron spectroscopy of negatively charged atomic and transposition from the separated limit to the molecular their isoelectronic molecular counterparts. Experiments utilizing frame as the equilibrium bond distance (re) approaches its opti- the photoelectron imaging technique are performed on the nega- mal value (3). This same analogy can be carried into the present tive ions of the group 10 noble metal block (i.e. Ni−,Pd−, and Pt−)of discussion, where the isoelectronic element, Ni, is fixed at one the periodic table at a energy of 2.33 eV (532 nm). The ac- extreme that represents the point where re ¼ 0 and the molecular cessible electronic transitions, term energies, and orbital angular electronic state superposition of TiO collapses to the atomic lim- momentum components of the bound electronic states in the it. As a chemical bond is formed between Ti and O, the re reaches are then compared with photoelectron images collected for isoelec- a point where the diatomic (or superatom) is created, and the tronic early transition metal heterogeneous diatomic , electronic states comprising it are superimposed from the isoelec- − M-X (M ¼ Ti; Zr; W; X ¼ O or C). A superposition principle connect- tronic atomic derivative. In conducting the experiments herein, ing the spectroscopy between the atomic and molecular species is we implement the technique of photoelectron imaging to observed, wherein the electronic structure of the diatomic is substantiate these conjectures, because apart from obtaining observed to mimic that present in the isoelectronic atom. The CHEMISTRY − − − the energy eigenvalues associated with occupied orbitals in an molecular ions studied in this work, TiO , ZrO , and WC can then atom or , the acquired photoelectron angular distribu- be interpreted as possessing superatomic electronic structures rem- tion (PAD) permits a glimpse into the quantized space appor- iniscent of the isoelectronic elements appearing on the periodic tioned to the prior to its release into the continuum. table, thereby quantifying the superatom concept. The PAD is further characterized by the anisotropy β parameter (see SI Text), which provides a quantitative means to connect the ∣ ∣ ∣ cluster anions photoelectron imaging superatoms observed partial wave signatures with the parent atomic or ∣ angular distributions transition metal molecular orbitals from which electron photodetachment occurs. The Cooper–Zare formula (4) provides a quantitative descrip- lchemy, the medieval practice that endeavored to transform tion of the β parameter evolution for partial wave emission from Amundane elements into the rarer, exotic variety has found bound electronic states of definite orbital angular momentum. many analogies in the modern physical sciences ranging from However, in molecular systems, where the orbital angular mo- gas-phase clusters (1) to surface science (2). Particularly intrigu- mentum is not necessarily a good quantum number, applying ing was a report (2) demonstrating that a tungsten carbide surface Δl ¼1 to