Ultrafast preparation and CARS monitoring of ground state dynamics

Summary form only given. The ground electronic state is where most chemical reactions take place, and therefore, the ability to prepare molecules in highly excited, nonthermal states is an excellent starting point for controlling chemical reactions. In order to prepare a molecule in a significantly non-thermal way, the preparation must be completed on a time scale short compared to the energy redistribution time, of the order of a picosecond. Moreover, the Franck-Condon factor dictates very low probability to directly excite a high vibrational state when starting from a molecule in its lowest states, due to minimal wavefunction overlaps between the states. Further experimental complications occur because electronic ground states of molecules do not normally fluoresce or ionize, so that their direct detection is very difficult. We present a method for preparing molecules in arbitrarily high vibrational excited states in the electronic ground states, and provide a built in diagnostics method for detecting these states. In this method, termed (TD)CARS for Two-Dimensional Time-Delayed CARS we prepare a wavepacket in the electronic excited state by means of a short pump pulse. This wavepacket is allowed to propagate in the excited state, and a second, delayed pulse brings it down to the desired ground state wavepacket. The center frequency and delay of the second pulse are optimized by a procedure, which was developed and is discussed. The generated ground state wavepacket is monitored by completing a CARS sequence, thus providing a direct signal from the oscillating high vibrational wavepacket.