Semiclassical model for localization and vibrational dynamics in polyatomic

molecules

Alexander L. Burin

Quantum Coherent Properties of Spins – IIIMany thanks to Enrique del Barco, Stephen Hill

and Philip Stamp for inviting me

Mark Ratner & Alex Burin

Sarah TesarIgor Rubtsov

Semiclassical Model for Vibrational Dynamics in Polyatomic Molecules: Investigation of Internal Vibrational RelaxationAlexander L. Burin, Sarah L. Tesar, Valeriy M. Kasyanenko, Igor V. Rubtsov, and Grigory I. RubtsovJ. Phys. Chem. C, v. 114, pp 20510–20517 (2010)MARK RATNER FESTSCHRIFT

Motivationn-atomic molecule

possesses 3n-6 independent vibrational modes (harmonic approximation)

These modes are coupled by a weak anharmonic interaction

ProblemsEvolution of excited state. Would the molecule remember its

initial excitation?

What is lifetime of excited state?

What are energy relaxation pathways?

Significance: Quantum Computation

Significance: 2D Infrared Spectroscopy

OutlineLocalization (Stewart, McDonald)2DIR spectroscopy problems (Rubtsov)Summary of previous theoretical workProblemsSelf-consistent collision integral model Preliminary resultsComparison to experimentsConclusion; future plansAcknowledgement

Localization vs. thermalization

N<10 – localization N>>10 - delocalization

2D IR (AcPhCN, Rubtsov and coworkers)

Cross peak signal

h

Theoretical approachesLocal random matrix model (e. g. Bigwood, Gruebele,

Leitner, Wolynes). Replaces anharmonic interaction with random matrix elements . Gives reasonable prediction for localization transition using free parameter for interaction strength

Exact solution of Schrödinger equations on the restricted basis set of global harmonic states (e. g. Dreyer, Moran, Mukamel, 2003). Uses first principles anharmonic force constants, accurate enough in Density Fuctional Theory (Barone, 2005). Restricted to small molecules and low temperature (no more than 10000 states)

This work: Generalizes collision integral approach (Bagratashvili, Kuzmin, Letokhov , Stuchebrukhov, 1985). Determines environment effect self-consistently (Generalized Marcus-Levich-Jortner method)

Hamiltonian and Perturbation

Frequencies and interactions can be determined using first principle DFT method (Gaussian 09). The method works well for infrared absorption spectra (Barone, 05).

Model of anharmonic transitions

Driving force

Transition rates (Marcus 1955)

Definition of rate constant: reorganization energy

Definition of rate constant: preexponential factor

Non-adiabatic or environment controlled adiabatic regimes (Rips, Jortner, 1987)

Self-consistent definition of relaxation times: collision integral method

Application of theory to 1,4-acetylbenzonitrile (AcPhCN)

Localization transition,

Tg=129K, N(129)=30, consistent with Stewart and McDonald, 1982

Relaxation times at room temperature

The calculated relaxation times of the CN and CO stretches are 1.6 ps and 7.0 ps. Consistent with experimentally measured lifetimes in AcPhCN of 1.8 and 3.9 ps.

Energy transport at room temperature, CN stretch is excited at t=0, CO excitation energy is probed

Solvent has been treated in rate equation approximation, =50ps. Maximum shift is reached at t=16ps. Consistent with experimental estimate of 12 ps.

Summary and Future PlansNew self-consistent collision integral approach to

investigate internal vibrational relaxation in polyatomic molecules is proposed

Application of method to the representative AcPhCN molecule shows that this method predicts localization transition temperature, mode decay rates and internal kinetics consistently with the experiment

The modification of the method within the framework of small polaron transport theory and applications to other molecules are in progress

Acknowledgements

•Funding and Support• NSF Grant No. 0628092 • PITP and, personally,

Prof. Stamp for support of ab’s subbatical visit and ST’s visit