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The Engelking group studies ions and radicals. The main focus of this effort concentrates on reactive intermediates. Organic species, such as the methoxy radical CH3O, and the methylnitrene radical, CH3N, have been the object of several spectroscopic studies. Inorganic radicals also interest our group, such as phosphinidine PH and CF.

Chemical species are picked for study because they have unusual internal molecular dynamics or because they have significant, unanswered questions about their structure or dynamics. Many are important in process chemistry, combustion chemistry, or atmospheric chemistry. For example, both the methoxy and methylnitrene radicals have doubly degenerate electronic states that are split by a distortion of the molecular geometry (Jahn-Teller effect). The methylnitrene radical had been undetected until the work of our group, and many believed that alkyl nitrenes were unstable, and hence unobservable, in spite of some contrary theoretical predictions.

The PH and CF radicals both have high-spin and low-spin electronic states; by observing the weak, spin-orbit forbidden, electronic transitions in these radicals, the relative energies of the high-spin and low-spin states could be established.

Ions are also of interest to our group. One set of studies explored the attraction between the electron and the neutral, but dipolar, OH and NH radicals produced when either OH- or NH- had their electrons removed by light. There are two limiting cases for this long-ranged electron-dipole interaction. In one extreme, the positive end of the dipole stops rotating freely, and is locked onto the negative electron, attracting it back to the molecule. At the other extreme, the dipole rotates end over end so rapidly that the electron "sees" only a net zero, average electric field. A simple theory, developed here, predicts that the actual situation is intermediate between these two extremes: it predicts the strength of the attraction, and it correctly predicts the effects that have been observed, also here, in the photodetachment spectrum.

Other ion-molecule studies involve the molecular analogues of "halo" states, which have been observed in nuclear physics. "Halo" states involve the binding of two or more particles at long range from a heavier core, thus giving rise to a "halo." These states may occur when an ion core attracts a neutral particle, such as an atom or molecule. Theoretically, binding of the ion and neutral can occur at long range, leading to molecular "halo" states, which have bond lengths between the ion and neutral many times that of an atomic diameter. Until recently, these had not been observed experimentally. An ion source developed in our group, the Corona Excited Supersonic Expansion, however, is capable of making copious amounts of these "halo" states for further study.

The Engelking group collaborates with John Hardwick to explore the vibrational quantum states of molecules in the regime where the motion would be classical chaotic. This effort aims to identify the "transition to chaos" in molecular spectra, and to put to the test some of the theories about this region where molecular spectra become "unassignable" by traditional spectroscopic methods.