Nonlocal Electromagnetic Response/Collision-Induced Spectra

Katharine Hunt

University Distinguished Professor

17A CEM

517-353-1152


Research webpage


Primary Research Area

Theoretical and Computational (Th)

Other Area(s) of Interest

Biological (Bi)

Chemical Physics (CP)

Material (Ma)

Physical (Ph)

Research

(Research Description PDF)

Quantum systems in time-dependent fields We are deriving new results for quantum systems that are perturbed by time-dependent electromagnetic fields, in cases where the adiabatic theorem does not hold. In their text Quantum Mechanics, Landau and Lifshitz showed that the excited-state coefficients in the wave function can be separated into adiabatic and non-adiabatic terms. Ordinarily, one would expect to find cross-terms between the adiabatic and non-adiabatic coefficients, when the expectation value of the Hamiltonian is computed. However, Anirban Mandal and I proved that the cross-terms vanish identically. The energy separates completely into adiabatic and non-adiabatic terms. The adiabatic term accounts for the adjustment of the ground state to the perturbation—without transitions—while the non-adiabatic term gives the energy change due to transitions to excited states. Subsequently, we proved that the power absorbed by a molecule from an applied field is equal to the time-derivative of the non-adiabatic term in the energy.

The standard Hamiltonian for a molecule in an electromagnetic field includes an arbitrary gauge potential, which arises when the vector and scalar potentials of the applied field are altered by a gauge transformation, leaving the electric and magnetic fields themselves unchanged. Physically meaningful quantities must be gauge-invariant, so the expectation value of the standard molecular Hamiltonian cannot be interpreted as the energy of a molecule in an applied field—a problem recognized in the mid-1950’s by Kramers. Anirban Mandal and I analyzed the full Hamiltonian for the molecule and the electromagnetic field, and showed that the standard Hamiltonian for the field contains a gauge-dependent term that exactly cancels with the gauge-dependent term in the molecular Hamiltonian. This opens a route to determine the energy of a molecule in a field, in a gauge-invariant way.

Currently, we are investigating the differences between our results for the probability of transitions to excited states for systems in laser fields vs. transition probabilities obtained from Fermi’s “golden rule.” We are looking at donor/acceptor complexes and three-state model systems, where there is rapid decay from the initially excited state.

Collision-induced spectroscopic processes Spectroscopic processes that are forbidden for single molecules may be observed in dense gases and liquids, due to electronic charge redistribution that occurs during molecular collisions. Our recent work has focused on collision-induced absorption of infrared radiation by samples containing H2 or H2/He mixtures in stellar atmospheres, and on collision-induced absorption by oxygen and nitrogen in Earth’s atmosphere. We evaluate the total dipole moments as functions of the bond lengths, intermolecular distances, and orientation angles, and then express the results in the spherical-tensor form needed for spectroscopic line shape calculations. Line-shape calculations based on our results have been carried out by our collaborators, Lothar Frommhold and Martin Abel (University of Texas, Austin) and Tijs Karman, Gerrit Groenenboom, and Ad van der Avoird (Radboud University, Nijmegen, Netherlands).

Manipulation of labeled biomolecules with light – We are also working on a theoretical analysis of the dynamics of fluorescently labeled protein molecules in laser fields. The theory of optical manipulation of molecules is well established for small molecules where the induced dipole forces predominate, and for very large molecules where the net forces associated with Mie scattering predominate. Fluorescently labeled proteins fall into an intermediate size range, where neither of the limiting cases applies and new theory is needed. This project involves collaboration with members of Bob Cukier’s research group, who provide expertise in molecular dynamics simulations. Our current work focuses on leucine zipper proteins interacting with fluorescently labeled DNA strands.

Collision-induced binary absorption spectrum of N2 gas at 78 K, calculated with our ab initio dipole moments. Contributions to the spectrum are separated based on the angular momentum of each N2 molecule, the vector sum of their angular momenta, and the angular momentum of relative motion of the molecules [from T. Karman, E. Miliordos, K. L. C. Hunt, G. C. Groenenboom, and A. van der Avoird, J. Chem. Phys. 2015, 142, 084306].

Selected Publications

Gauge-invariant expectation values of the energy of a molecule in an electromagnetic field, Anirban Mandal and Katharine L. C. Hunt, J. Chem. Phys. 2016, 144, 044109.

Non-adiabatic current densities, transitions, and power absorbed by a molecule in a time-dependent electromagnetic field, Anirban Mandal and Katharine L. C. Hunt, J. Chem. Phys. 2015, 143, 034102.

Quantum mechanical calculation of the collision-induced absorption spectra of N2-N2 with anisotropic interactions, Tijs Karman, Evangelos Miliordos, Katharine L. C. Hunt, Gerrit C. Groenenboom, and Ad van der Avoird, J. Chem. Phys. 2015, 142, 084306.

Ground and excited states of vanadium hydroxide isomers and their cations, VOH0,+ and HVO0,+, Evangelos Miliordos, James F. Harrison, and Katharine L. C. Hunt, J. Chem. Phys. 2013, 138, 114305.

Adiabatic and nonadiabatic contributions to the energy of a system subject to a time-dependent perturbation, Anirban Mandal and Katharine L. C. Hunt, J. Chem. Phys. 2012, 137, 164109.

Interaction-induced dipoles of hydrogen molecules colliding with helium atoms:  A new ab initio dipole surface for high-temperature applications, Xiaoping Li, Anirban Mandal, Evangelos Miliordos, and Katharine L. C. Hunt, J. Chem. Phys. 2012, 136, 044320.

Infrared absorption by collisional H2-He complexes at temperatures up to 9,000 K and frequencies from 0 to 20,000 cm–1, Martin Abel, Lothar Frommhold, Xiaoping Li, and Katharine L. C. Hunt, J. Chem. Phys. 2012, 136, 044319.

News

Awards/Honors

2001 Senior Faculty Award Sigma Xi Honor Society (Michigan State University)
2000 - 2002 Member Midwest Chemistry Chairs’ Group
1999 Panelist, SBIR Program, Chemistry Division National Science Foundation
1998 - 2003 Selection Committee Member (appointed by British Consul General) Marshall Scholarship Program, Midwest Region
1998 Member National Science Foundation (Committee of Visitors (triennial review of division), Chemistry Division)
1998 - 2002 Chairperson Michigan State University (Department of Chemistry)
1997 Panelist, SBIR Program, Chemistry Division National Science Foundation
1995 - 1997 Councilor for Physical Chemistry American Chemical Society
1993 Panelist, Chemistry 3 (Physical Chemistry), Panel to recommend funding for proposals from scientists in the former Soviet Union International Science Foundation
1992 Panelist, Graduate Research Traineeship Program, Panel to evaluate departmental proposals, Chemistry Division National Science Foundation
1992 Member Midwest Theoretical Chemistry Conference XXV (Conference Organizing Committee)
1992 - 2004 University Distinguished Professor Michigan State University
1991 Junior Faculty Award Sigma Xi Honor Society (Michigan State University)
1991 Panelist, Presidential Young Investigator Award Review Panel, Chemistry Division National Science Foundation
1986 - 1987 Member American Chemical Society (Michigan State University Local Section)
1982 Session Chairperson Fifteenth Annual Midwest Theoretical Chemistry Conference
1978 Ph.D. University of Cambridge, England
1978 - 1979 NSF National Needs Postdoctoral Fellowship National Science Foundation
1975 - 1978 Danforth Fellowship
1975 Bachelor of Science Michigan State University
1975 - 1978 Marshall Scholarship
1971 - 1975 National Merit Scholar
1971 - 1975 Alumni Distinguished Scholarship Michigan State University (Alumni Association)
Board of Trustees Award Michigan State University
Phi Beta Kappa Phi Beta Kappa
Phi Kappa Phi Phi Kappa Phi
Chairperson Michigan State University (Department of Chemistry)