ISSN:
0020-7608
Keywords:
Computational Chemistry and Molecular Modeling
;
Atomic, Molecular and Optical Physics
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The matrix element for two-photon absorption is calculated as a residue of the general quadratic response function for a system subjected to an external time-dependent field. The matrix element is correct through first-order in electron correlation, and includes contributions in addition to those considered in previous calculations which were based on a truncated form of the Random Phase Approximation (RPA). Equations of motion are derived from the spectral representation of quadratic response, leading to equivalent expressions for two-photon absorption in the dipole length, mixed, and dipole velocity formulations. Because the transition moments evaluated in the three formalisms must be equivalent in a complete basis, the calculated results provide a measure of the adequacy of the basis set representation.As a test case, the X1σg+ (v = 0) → (E, F)1Σg+ (v = 6) two-photon process of H2 is considered. It is first demonstrated that agreement between length and velocity forms of the two-photon transition moment is obtained only through the consideration of all contributing terms. The vibrationally-averaged transition moment is decomposed into individual state contributions. It is found that the B1Σu+ and C1Πu-coupled matrix elements are comparable at the RPA level to values obtained at the near-full CI limit, and, although there is some disparity as to the relative weight of the remaining states, the total transition moment in the two formalisms are in good agreement. Using the transition moment obtained at the excitation frequency 1/2ωf, the integrated two-photon excitation cross-section is calculated. The value is in excellent agreement with the CI result and lies near the experimental error bars of the absorption cross-section determined from a single-beam measurement.
Additional Material:
5 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/qua.560382449
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