Periodic orbits and molecular photoabsorption

A method for the semiclassical evaluation of molecular photoabsorption rates in terms of contributions of periodic orbits is developed and applied to a collinear model of CO₂. It is shown that for a satisfactory quantitative description effects have to be taken into account which originate from a finite extension of the Franck-Condon transition region in comparison with the extension of Fresnel zones of the contributing periodic orbits.