Limitations on quantum ladder climbing in complex systems

The use of cesium Rydberg atoms as a test system for quantum ladder climbing with chirped laser pulses is described. Quantum ladders can be used in the coherent control of chemical reactions; for instance, by exciting a specific molecular bond by climbing its vibrational ladder. A Rydberg atom is an easy test system because of the large transition dipole moments between Rydberg states. The Rydberg states are easy to access and to analyze. Picosecond far-infrared laser pulses around a wavelength of 50 mm, with peak intensities of 10⁹ W/cm², from the free-electron laser for infrared experiments (FELIX) at Nieuwegein, The Netherlands, were used. It is found that the complexity of the system puts severe limits on the allowed intensities that are used to drive the ladder. At high intensity the ladder is distorted due to coupling with states outside the ladder. This distortion frustrates efficient climbing.