Femtosecond Isomerization Dynamics in the Ethylene Molecule

The ethylene molecule plays a fundamental and prototypical role for the understanding of photo-isomerizaton processes and particularly for ultrafast energy conversion through nonadiabatic transitions and state crossing via conical intersections. We have developed a high power femtosecond laser based pump-probe system to study femtosecond isomerization dynamics in various model molecules. By focusing 25-mJ laser pulses into a 5-cm-long xenon-filled gas cell, we can deliver about 10$^9$ photons per harmonic per pulse onto a target gas, with the photons ranging in energy from 8 to 40 eV. In this talk I will present the results of our studies of the dynamics in the excited ethylene cation (C$_2$H$_4^+$) using a high intensity high harmonic source. The dynamics in the excited ethylene cation leads, among other channels, to isomerization to the ethyledene configuration (CH$_3$CH$^+$), which is predicted to be a transient configuration for electronic relaxation. With an intense femtosecond EUV pulse as pump, and a NIR (near infra-red) pulse as probe, we measure a time scale of $45\pm10$ fs for formation of the transient ethylidene configuration (lifetime of $60\pm15$ fs ) through detection of the NIR-induced fragmentation to CH$_3^+$ and CH$^+$. Also, a H$_2$-stretch transient configuration (believed to succeed ethylidene), yielding H$_2^+$, is found to be populated after $100\pm10$ fs. These studies were also extended to excited state dynamics in the neutral ethylene using a recently developed split mirror technique enabling XUV pump - XUV probe capability. In order to achieve this we optimized our high harmonic system for high power in order to produce a very intense source of high harmonics that allows multiphoton (XUV) absorption by a single molecule. In particular, we were able to measure two-photon double-ionization of Ethelyne and argon and three-photon double ionization of neon.