Theoretical Physics

In my research we apply Quantum Mechanics to molecules experiencing some form of large amplitude motion. This is theoretical/computational physics - but is always related to experimental results.

1. “Floppy molecules” - large amplitude motion of nuclei.

Floppy molecules are molecules that have huge vibrational motions. Some molecules can isomerize from one form to another: for example the hydrogen atom in HCN can swing around and be bound to the nitrogen atom, forming CNH. Other molecules have internally spinning parts. For example SiC2 is triangular at equilibrium, but the C2 moiety can spin around like a pinwheel, while the silicon atom barely moves. There are also molecules which are bent at equilibrium, but which have the energy level structure of a linear molecule - these are called “quasilinear molecules.”

How do we learn how floppy molecules behave? One way is by using the “semirigid bender” (SRB) Hamiltonian with experimental data to find the potential energy function for the large-amplitude vibration and the trajectories of the atoms.

2. “MQDT” (Multichannel Quantum Defect Theory) - large amplitude motion of electrons (and possibly nuclei).

My work in this area has been a long-term collaboration with Dr. Ch. Jungen in Orsay, France. MQDT is exquisitely suited for the study of electronically excited molecular states. In such states the excited electron undergoes large amplitude excursions from the rest of the molecule. When the electron gets a long way out from the rest of the molecule, the approximations usually used in molecular physics (such as the Born Oppenheimer approximation) break down. MQDT bypasses the traditional approximations and even accounts for the infinite number of mutually interacting states that all molecules have.

MQDT is fantastic - it can describe electronically excited bound states while also being able to treat molecular ionization and dissociation - all in a unified form.

Recent Publications

SRB: “SiC2: A Molecular Pinwheel”, J. Chem. Phys. 100, 4110-4126 (1994), S. C. Ross, T. J. Butenhoff, E. A. Rohlfing, and C. McMichael Rohlfing,

"On The Low-lying CCN Bending Mode of the Close-to-Linear Molecule NCCNO", J. Phys. Chem. A 105, 10080-10088 (2001), Holger Lichau, Stephen C. Ross, Michael Lock, Sieghard Albert, Manfred Winnewisser, and Frank C. De Lucia.

MQDT: “Unified quantum-defect-theory treatment of molecular ionization and dissociation”, Phys. Rev. A, 55, R2503-R2506 (1997), Christian Jungen and Stephen C. Ross.

"MQDT Treatment of Triplet Gerade and Ungerade d-Symmetry Levels of H2 and its Isotopomers", Canadian Journal of Physics (Gerhard Herzberg Memorial Volume) 79, 561-588 (2001) S. C. Ross, Ch. Jungen and A. Matzkin.

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