60th Annual Report on Research 2015

I. Overview

The goal of this project is to use newly developed sources of femtosecond deep ultraviolet laser pulses to study mechanisms of elementary chemical reactions. In recent years, our group has developed methods for generating sub-100-fs ultraviolet laser pulses at 200 nm using nonlinear processes in noble gases. Light at 200 nm is required to initiate ring opening reactions in cyclic monoalkenes such as cyclohexene and norbornene (see Figure 1). Studies of such elementary reactions may yield insights into the mechanisms at work in combustion processes.

Figure 1. Dominant reaction pathways for norbornene (top) and cyclohexene (bottom).  Dynamics are initiated by photoexciting the equilibrium system at 200 nm. Sub-picosecond relaxation to the ground state by way of a sequence of conical intersections. At the conical intersections, the systems are in the intermediate states indicated in the middle panel. Vibrational cooling in the ground electronic state stabilizes the final products shown on the right; these dynamics span the picosecond to 10's of picoseconds time scales.

II. Progress to Date

IIA. Ultrafast Dynamics in Norbornene and Cylcohexene

The experiments conducted in this project aim to understand the ring opening mechanisms in cylcohexene and norbornene (see Figure 1). To this end, femtosecond pump-probe experiments have been conducted with 200-nm laser pulses. Femtosecond laser spectroscopies are challenged by undesired photoionization processes and dispersion management  in this wavelength range. Photoionization processes must be suppressed by keeping the laser fluence very low, which makes the signals small. In addition, the polarizability response of the solvent dominates the signal when the pump and probe pulse are overlapped, thereby obscuring information about internal conversion at pulse width limited delay times (less than 200 fs in our setup). For this reason, extremely short lasers pulses must be employed to resolve the internal conversion dynamics in these systems.

We have succeeded in detecting the vibrational cooling dynamics that take place subsequent to internal conversion in both cyclohexene and norbornene. The transient absorption data shown in Figure 2 are consistent with a significantly faster vibrational cooling/product stabilization process in norbornene. The mechanism responsible for this difference in behaviors is unclear at this point. Dynamics at delay times less than 200 fs are hidden by the response of the cyclohexane solvent. It appears that the internal conversion process may be slightly faster in norbornene; however, we intend to repeat these experiment with better time resolution in the fall of 2015.

Figure 2. Transient absorption data obtained with 200-nm pump and probe pulses for (a) cyclohexene and (b) norbornene in cyclohexane solvent. (c) The comparison of fits obtained for the two systems suggests that internal conversion-induced vibrational cooling is faster in norbornene than it is in cyclohexene. It appears that the internal conversion processes take place within the first 0.5 ps following excitation. Future work will attempt to improve the time resolution of this apparatus and obtain better information about the internal conversion mechanisms.

IIB. Photodissociation of Triiodide

The setup we developed for generating the 200-nm laser pulses also yields high-energy 267-nm laser pulses. These 267-nm pulses were used to study the photodissociation reaction of triiodide with a new experimental method, two-dimensional resonance Raman (2DRR) spectroscopy. 2DRR spectroscopy yields insights into chemical reactions that cannot obtained using conventional time-resolved approahes. With acknowledgment of ACS PRF support, two applications of 2DRR spectroscopy were published in the Journal of Chemical Physics in 2014-2015. One of these articles was received the honor of an Editor's choice for 2014 (Molesky et al. J. Chem. Phys. 141, 114202, (2014)). Figure 3. Summary of 2DRR experiments conducted on triiodide (from Molesky et al. J. Chem. Phys,143, 124202 (2015)). This figure shows that vibrational motions of the reactant and product can be detected in the two dimensions of the spectrum by varying the pulse configuration: (a) both dimensions represent triioidide; (b) both dimensions represent diiodide; (c) motions of triiodide and diiodide are detected in separate dimensions. The 2DRR method can also be used to obtain unique insights into ring opening reactions.

  When photoexcited in the UV spectral range, triiodide dissociates into diiodide and iodine. The reaction is faster than the 300-fs period of the bond stretching motions in both triiodide and diiodide. Therefore, the diidodide product exhibits coherent vibrational motions. Information about the geometry changes that transform the reactant to the photoproduct can be derived from oscillatory components of transient absorption signals. Relaxation dynamics for these systems in solution were studied by several groups in the 1990's. We have shown that 2DRR spectroscopy can be used to explore correlations between triiodide and diiodide. Figure 2 shows that particular 2DRR pulse configurations can be used to view correlations between coherent motions of the reactant and product in ultrafast chemical reactions. For example, our analysis of the spectrum presented in Figure 3c suggests that a nonequilibrium bond length of displacement of 0.1 Å in triiodide alters the vibrational wavenumber of the diiodide product by 7 cm^-1. The 2DRR approach holds great potential for studies of ring-opening reactions.

III. Impact of ACS-PRF Support on Personnel Support of ACS-PRF has facilitated the development of 2DRR spectroscopy. Our work may inspire other groups to implement this experimental method. The significance of this development was recognized by the editors' of the Journal of Chemical Physics in 2014. Such recognition will have a positive impact on the careers of the PI and students. Two graduate students and one undergraduate worked on this project. All students developed technical skills and co-authored journal articles.

IV. Future Directions In summary, we have successfully resolved the vibrational cooling dynamics that take place subsequent to internal conversion in both cyclohexene and norbornene. Differences in the vibrational cooling rates reflect structure-dependent redistribution of vibrational quanta. The internal conversion processes are slightly faster than our 100-fs time resolution. We plan to repeat these experiments with sub -50-fs time resolution in the fall of 2015. In addition, we will use the newly developed 2DRR method described in Section IIB to explore correlations between reactants and products in elementary ring-opening reactions.