Indirect versus direct photoionization with ultrashort pulses: interferences and time-resolved bond-length changes

The photoionization of NaI molecules with femtosecond laser pulses leads to photoelectron distributions which vary with the delay between a pump- and a probe-pulse. If the vibrational wave packet as prepared in the pump-transition is located in a region where the bonding character is ionic, the photoelectron, due to its localization on the iodine atom, may be ejected directly or be scattered from the Na+ ion. This leads to structures in the photoelectron spectrum which, in turn, reflect temporal bond-length changes.