Ultrafast Hydrogen Migration by CEP-Locked Pulses

Very recently, the group of Prof. Yamanouchi, a world leading research group in the field of intense laser molecular science, has obtained experimental evidence indicating that hydrogen nuclei (protons) in light hydrocarbon molecules possibly undergo an extremely fast migration corresponding to displacements of ~0.5 nm within some 5 fs. The preliminary conclusions about this unprecedented speed of nuclear rearrangement are based on the examination of ionic debris emerging from a Coulomb-explosion type photo-dissociation triggered by a few-cycle intense laser field. The very notion that a heavy particle such as the proton, with the rest mass exceeding 1800 times that of the electron, is able to move so fast over a large distance may require a fundamental rethinking of the traditional approach to photo-chemistry based on the Born-Oppenheimer approximation. The dramatic findings by the Yamanouchi group cast a new light on the role of hydrogen bonds and possibly signify that, under the action of a strong field, a proton may tunnel through the potential barrier from one branch of the molecule to another rather than experience a slow classical motion on an order-of-magnitude slower time scale. In the framework of this joint Austro-Japanese project, the research group in Tokyo and the research group at the Vienna University of Technology, specializing in the development of phase-controlled intense laser pulse technology, set out to clarify the essential aspects of light-molecule interaction occurring within the extremely short time duration of around few femtoseconds via the investigation of ultrafast charge migration and hydrogen migration processes in hydrocarbon molecules (e.g. MeOH, EtOH) in ultrafast intense laser fields (~5 fs pulse duration, ~10-15 W/cm 2 peak intensity). Combining the coincidence momentum imaging (CMI) technique available in both partner laboratories with the carrier-envelope phase (CEP) stabilization technique of few-cycle intense laser fields, the partner groups aim to gain an unprecedented insight into the dynamics of chemical bond breaking and chemical bond creation/rearrangement within a molecule. CEP control of the light field is expected to be the key to a) gain attosecond time localization of the initial tunnel ionization step of the photo-dissociation reaction, b) serve as a tool to control the spatial localization of ultrafast (100 as - 5 fs) intramolecular charge migration triggered by ionization, c) control the non-adiabatic coupling of the bound valence electrons to the intense light field, d) shape the potential barrier of the proton tunneling (determined by the instantaneous external field strength + instantaneous intramolecular polarization wave caused by charge migration). The proposed CEP-controlled CMI experiments will be conducted both in Tokyo and Vienna initially using ultrafast Ti:sapphire laser amplifiers (carrier wavelength 800 nm) and in the second half of the project using a novel parametric source (wavelength 1.5 m) currently under development in Vienna. The longer-wavelength driver pulse will permit suppression of multi-photon excitations offering in addition the advantages of a longer optical cycle duration and higher ponderomotive energy.