The rapid development of amplified femtosecond lasers has brought many new tools and opportunities into molecular sciences. Most obvious is the ability to measure ultrafast molecular processes: “Time”. The ability to observe atomic motions during a molecular transformation depends greatly upon the choice of final state which is used to project these dynamics onto. Additionally, these transformations often involve the non-adiabatic flow of vibrational energy and charge within the molecule, requiring techniques sensitive to both electronic and vibrational dynamics and their coupling. From our own work, Time-Resolved Photoelectron Spectroscopy (TRPES), will be used to illustrate one approach to these problems [1]. Examples from current topics in molecular dynamics will be given. The ‘other side of the coin’ of ultrashort pulses is “Phase”. This suggests immediately that ultrashort pulses will be natural tools for the coherent optical phase control of quantum processes. One variant of quantum control is based upon the interference between resonant spectroscopic transitions. Another variant, optimal control using strong, shaped laser pulses is based upon strong non-resonant field ionization/fragmentation of polyatomic molecules. We have developed a new intermediate field strength variant, based upon the non-resonant Dynamic Stark Effect, which is non-perturbative but non-ionizing [2]. We illustrate this approach to Quantum Control with our method of ‘Switched Wavepackets’ and show how it can be used, as one example, to create field-free molecular axis alignment. Finally, the last aspect of femtosecond laser pulses is “Intensity”. The behaviour of matter in strong (ionizing) non-resonant laser fields brings out both new physics and chemistry. To date, much of this work has been in atomic physics. However, most of these models do not apply to molecular systems, which have both extended geometries, leading to a Nonadiabatic Multi-Electron ionization [3], and many valence electrons, leading to strong Dynamic Polarization effects which greatly suppress ionization [4]. Examples and illustrations will be given from the unusual behaviour of polyatomic molecules and transition metal clusters in strong, non-resonant laser fields. References: [1] Nature 401,52,(1999); Annu.Rev.Phys.Chem. 54, 89 (2003) [2] Physical Review Letters 90, 223001 (2003) [3] Physical Review Letters 86, 51 (2001) [4] (submitted to Physical Review Letters)
Institut Steacie des sciences moléculaires -CNRC