3205is interesting to note that although the oxalyl dication is a minimum, the dimer of carbon monoxide, ethylenedione (O=C= C=O), is kinetically and thermodynamically unstable with respect to dissociation into 2 equiv of CO.'* It is also interesting to note that CICO+ is directly formed by the ionization of oxalyl chloride (vide supra). The intermediately formed chlorooxalyl cation 3 loses C O readily. At the 3-21G* level of theory ion, 3 (C, symmetry employing standard bond lengths for the initial geometry) does not optimize to a minimum but rather dissociates into CICO+ and CO, verifying the experimental observation.In 1985 it was discovered that vaporization of graphite by laser irradiation produces a remarkably stable Ca cluster and to a lesser extent a stable C70 cluster as evidenced by mass spectrometry.* Kroto, Heath, OBrien, Curl, and Smalley proposed the structure for the 60-carbon cluster to be a truncated icosahedron composed of 32 faces of which 12 are pentagonal and 20 are hexagonal, a structure analogous to a soccerball and reminiscent of the geodesic domes of Buckminster Fuller. Thus, c 6 0 is commonly referred to as "buckminsterfullerene". The structural support for c 6 0 as well as the related cluster CT0 comes from a variety of experimental and theoretical ~t u d i e s .~-~ ( I ) Considered Novel Aromatic Systems. 11. Part IO: Bausch, J. W.; Gregory, P. S.; Olah. G. A.; Prakash, G. K. S.; Schleyer, P. v. R.; Segal, G. A. J. Am. Chem. Soc. 1989, I l l , 3633. (2) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. E. Nature (London) 1985,318, 162. Also see: Rohlfing, E. A.; Cox, D. M.; Kaldor, A.Recently the crystal structures of the alkalides K+(C222)K-(I), Rb'(C222)Rb-(11) and Rb+(18C6)Rb-(111) have been reported.' The alkali-metal anions form dimers in I and I1 and

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Absolute cross sections for Li photodetachment have been measured near the Li(22P)+as channel threshold. The energy dependence deviates rapidly from normal Wigner-cusp behavior on each side of threshold. From an analysis of the total cross sections in terms of a two-channel scattering theory that includes the effects of a virtual state, a parametric expression has been derived which accurately describes the energy dependence. A least-squares fit of the data yields the position of the virtual state (55+10 meV below 22P), the phase shift at the Li(22P) +as threshold (1. 4+0.1 rad), and the electron affinity of Li (6&7.3 +0.7 meV).The threshold behavior of scattering and excitation cross sections has been a subject of extensive theoretical and experimental studies for more than three decades in the fields of atomic, nuclear, and elementary-particle physics. However, only with the recent development of high-resolution laser-photodetachment methods have experimental studies begun to disclose the strong effects that neighboring resonances can have on the threshold behavior. It is therefore of experimental usefulness as well as theoretical importance to understand, describe, and predict these effects. Similar to the resonance phenomena in other short-range potential scattering systems (atom-atom, neutron-nucleus, etc.), ' electron scattering and photodetachment resonances are of three types. In the electron-atom (molecule) systems they are caused by the short-range attractive (correlation energy) forces that commonly exist in the collision complex at energies near the thresholds of excited-state channels. If the correlation potential is sufficiently strong that the compound state is "bound" below the continuum of the excited state channel, it is a "bound, " "closed-channel, " or "Feshbach" resonance. If the attractive potential is not quite strong enough to form. a bound resonance, a "virtual state" can exist for an s-wave channel, or a "shape" resonance can exist if l~1. The scattering phase shift changes rapidly and passes through (n+ 2)rr as the total energy passes through the levels of bound and shape resonances, but it does not quite reach (n+ T)n. in the case of a virtual state.In his treatment Wigner3 showed that in the neighborhood of a threshold the cross section is, apart from a constant, the same function of energy regardless of the excitation mechanism so long as the long-range interaction of the product particles is the same. Thus, photodetachment threshold behavior is the same as that of electron scattering for the same final state, and studies by photodetachment have the advantages of high-energy resolution and the definition of the final-state spin and angular momentum.Dramatic interference effects due to Feshbach resonances were first observed by Patterson et al. 4 in the photodetachment of K, Cs, and Rb just below the thresholds of the first excited neutral product channels. Lee applied multichannel reaction-matrix theory to develop a ten-parameter expression which was capable of fitting the Cs result...

He" photodetachment cross sections near the He(2 3 P) threshold have been measured and analyzed. The data were found to deviate rapidly from the Wigner threshold law because of the strong influence of the nearby 4 P e shape resonance. A modified formula has been derived, which accounts for this effect. It successfully fits the data not only in the threshold region but over the resonance itself. The threshold data yield an electron affinity for He(2 3 S) of 77.5 ±0.8 meV, in agreement with the accurate calculations of Bunge and Bunge.