Recent experiments indicate that nonradiative transitions not only differ for various single vibronic levels but are also strongly dependent on the rotational states involved. Thus the rotational selection rules should be considered for these rotational transitions. Rotational matrix elements and selection rules are presented for internal conversion between two triplet or two singlet states and for intersystem crossing between a singlet and a triplet for polyatomic molecules in Hund's case b. The selection rules are, in general, different for each process and depend on the nature of the angular momentum coupling as well as the interaction operator.

The excimer laser was borrowed from the San Francisco Laser Center, supported by the National Science Foundation under Grant No. CHE79-16250 awarded to the University of California at Berkeley in collaboration with Stanford University.

We present here a detailed theoretical development of two-photon absorption (TPA) in the gas phase including rotational effects. A general formula for the transition probabilities between rovibronic states is derived in the rigid rotor approximation. Similarly to Raman theory the intensity of the rotational lines can be attributed to three independent contributions, which derive from the isotropic, antisymmetric and anisotropic part of the TPA-tensor. Each of these contributions carries a different geometrical factor so that they may be distinguished by polarization experiments. It follows directly from the theory that different rotational branches of one and the same band may differ in their polarization behaviour. Until recently, the latter effect, although central to symmetry assignments, has been overlooked. Application to the asymmetric rotor is discussed in detail and two-photon rotational band contour calculations for naphthalene- h 8 are presented.

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A technique for measuring ’’effective’’ absorption coefficients is described. It circumvents deviations from Beer’s law caused when the excitation source bandwidth is larger than the absorber bandwidth. The technique employs a fluorescence cell placed after an absorption cell to selectively monitor absorption in the center region of the source line. Model calculations relating the fluorescence intensity to source and absorber line shapes indicate that this method should yield linear Beer’s law plots for moderate values of k0Nl and α, where α is the ratio of the source bandwidth to the absorber bandwidth. This technique has been applied to a number of single rotational levels in the 410 transition of the H2CO ? 1A2←? 1A1 system using pulsed, tunable dye laser excitation. The effective absorption coefficients determined experimentally have been compared to the theoretically calculated absorption coefficients.