Vibrationally Resolved Photoionization Delays in the Water Molecule

We have implemented a theoretical approach to provide time- and vibrationally resolved photoelectron spectra and ionization time delays of polyatomic molecules as those expected from current high energy resolution reconstruction of attosecond beatings by interference of two-photon transitions setups. Its application to the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi mathvariant="normal">H</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> <a:mi mathvariant="normal">O</a:mi> </a:mrow> </a:math> molecule reveals that two-photon ionization delays extracted from the calculated photoelectron spectra do not vary monotonically with photon energy and can increase or decrease along the vibrational progressions. The difference between one- and two-photon ionization delays is substantially larger when the molecular cation is left in the symmetric stretching mode (up to 35 as, i.e., comparable to the actual ionization delays) rather than in the bending mode, thus showing a strong vibrational selectivity in the laser-photoelectron interaction. Hence, the variation of calculated two-photon delays with the final vibrational state does not simply reflect the energy dependence of the Coulomb phases, as usually assumed in a structureless electronic continuum, but also the changes in the molecular geometry associated with the different vibrational modes. As vibrational excitation accompanying the emission of a photoelectron is the rule rather than the exception in polyatomic molecules, the present Letter shows the importance of considering nuclear motion for a correct interpretation of ionization time delays.