TY - JOUR

T1 - Nonperturbative approach to non-condon effects

T2 - Must a nonadiabatic transition always occur at the potential surface crossing?

AU - Goldstein, Robert F.

AU - Franzen, Stefan

AU - Bialek, William

PY - 1993

Y1 - 1993

N2 - We present a generating function method for the computation of non-Condon effects in the context of nonadiabatic multiphonon transition theory. The method is nonperturbative in the electron-phonon coupling and nonperturbative in the dependence of the transition matrix element on nuclear coordinates, but it does assume the overall transition rate is small enough to be treated perturbatively. We do not assume the transition operator is given by the Born-Oppenheimer breakdown operator but rather consider more general forms of the transition matrix element V(x), where x represents nuclear coordinates. In particular, for narrow forms of V(x) such as a delta function, the optimum position (that position of V(x) which produces the maximal transition rate) is temperature dependent and may not coincide with the transition state. This calls into question the classical notion that a transition occurs via a thermal fluctuation to the potential surface crossing. The classical significance of the transition state does appear valid, however, at low temperature and large electron-phonon coupling. Furthermore, when V(x) is broad and the non-Condon effects are weak, the transition rate can be expressed in the Condon approximation with suitably scaled parameters. Since this scaling is temperature dependent, there may be an apparent change in the reorganization energy and in the enthalpy and entropy of activation. When many vibrational modes couple to the transition, there are circumstances under which a single promoting mode can dominate the vibrational effects. In the context of optical line shapes, this means there are circumstances under which a weakly coupled vibrational mode can cause the wings of a line to assume a strictly exponential, rather than Gaussian, shape.

AB - We present a generating function method for the computation of non-Condon effects in the context of nonadiabatic multiphonon transition theory. The method is nonperturbative in the electron-phonon coupling and nonperturbative in the dependence of the transition matrix element on nuclear coordinates, but it does assume the overall transition rate is small enough to be treated perturbatively. We do not assume the transition operator is given by the Born-Oppenheimer breakdown operator but rather consider more general forms of the transition matrix element V(x), where x represents nuclear coordinates. In particular, for narrow forms of V(x) such as a delta function, the optimum position (that position of V(x) which produces the maximal transition rate) is temperature dependent and may not coincide with the transition state. This calls into question the classical notion that a transition occurs via a thermal fluctuation to the potential surface crossing. The classical significance of the transition state does appear valid, however, at low temperature and large electron-phonon coupling. Furthermore, when V(x) is broad and the non-Condon effects are weak, the transition rate can be expressed in the Condon approximation with suitably scaled parameters. Since this scaling is temperature dependent, there may be an apparent change in the reorganization energy and in the enthalpy and entropy of activation. When many vibrational modes couple to the transition, there are circumstances under which a single promoting mode can dominate the vibrational effects. In the context of optical line shapes, this means there are circumstances under which a weakly coupled vibrational mode can cause the wings of a line to assume a strictly exponential, rather than Gaussian, shape.

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U2 - 10.1021/j100145a009

DO - 10.1021/j100145a009

M3 - Article

AN - SCOPUS:0012933910

SN - 0022-3654

VL - 97

SP - 11168

EP - 11174

JO - Journal of physical chemistry

JF - Journal of physical chemistry

IS - 43

ER -