We are studying collisional energy transfer in our group because it is of prime importance for a variety of chemical reaction systems, e.g., in combustion processes or atmospheric chemistry. As illustrated on the right side for a prototypical unimolecular reaction, collisions activate and deactivate reactands or stabilize products.

The central questions to be answered are:

- How do molecules behave in collisions with a bath gas M?
- How fast is the energy transfer happening?
- How do the energy distributions evolve in time?

We have developed a unique experiment from which we can obtain details of the energy transfer process so far not achievable. A short explanation of the method follows:

The KCSI method is a "direct" method for investigating energy transfer in highly vibrationally excited ground state molecules. The hot molecules are generated by excitation with a pulsed nanosecond laser (wavelength Lambda0) and (typically) "internal conversion" to the ground electronic state S₀.

The deactivation process (indicated by the distributions on the left side) is then monitored by an energy selective two photon ionization process (Lambda1 + Lambda2) via a resonant intermediate state S_n (right side).

In the intermediate state there is a competition between ionization through Lambda2 and an energy dependent loss process with the rate coefficient k(E) which increases with energy. Ionization is then only possible from the narrow yellow region in S_n.

Lambda1 projects this region down to the ground electronic state, which results in a so-called "observation window": By the kinetic competition in S_n the ionization is controlled and only molecules in a selected energy region in S₀ (the long yellow stripe) can be ionized. The window can be shifted by varying Lambda1.

The experimental setup is shown in the picture on the right. The pump and probe wavelengths are generated by two different laser systems (e.g., YAG pumped dye laser and excimer pumped dye laser) and are directed into the ionization cell, the central part of the experiment. The typical total pressure in the cell is about 10 mbar. The KCSI ions are detected by a capacitor arrangement connected to highly sensitive amplification electronics.

Several modifications of this scheme (here Lambda1 = Lambda2) have been applied by us to different molecules.

Systems studied so far are, among others, toluene or azulene colliding with a variety of bath gases, e.g., helium, nitrogen and n-heptane.

To evaluate our KCSI signals we use a master equation approach not shown here in detail. The central quantity we obtain is the collisional transition probability P(E',E). Examples for toluene are shown on the right side.

The KCSI method is the only method so far which can obtain complete P(E',E) distributions to fully describe the energy transfer of highly vibrationally excited molecules.

For the first time, we have established a specific form for
P(E',E) which can describe all our experimental results obtained so far - an exponential form with a parametric exponent in the argument.

Having fully characterized P(E',E), we are able to evaluate all quantities of specific interest in modeling, e.g., combustion processes, specifically the first and second moments of energy transfer.

If you want to know more details please contact us. Additional information is also available in the references given below.