What is a transition state?
Chemical reactions are always accompanied by the breaking and formation of chemical bonds. Amidst these bond rearrangements, the system exists in an intermediate state with a specific configuration, known as a high-energy activated complex. This critical state from the reactant channel to the product channel is termed the transition state.
The lifetime of a transition state is exceedingly brief, typically on the scale of femtoseconds (trillionths of a second), posing significant challenges for their observation and characterization in experiments. Since transition state is a key concept in chemistry for understanding chemical kinetics and reaction dynamics, due to its fleeting nature, the direct observation of transition states in chemical reactions has long been one of the "holy grails" in the fields of chemistry.
How to probe the transition state?
The motivation to better understand the transition state has spurred the development of various experimental techniques, including the crossed molecular-beam technique, femtosecond laser technology, and the anion photodetachment method. Each method boasts distinct advantages and limitations. The pioneering work for the crossed molecular-beam method was carried out by Dudley Herschbach, Yuan T. Lee, whose groundbreaking research on reaction dynamics earned them the shared 1986 Nobel Prize in Chemistry. This method involves the collision of two beams of reactant molecules at high velocities, mimicking the collision conditions found in actual chemical reactions. Through observation of the distribution and velocity of reaction products, researchers can deduce the kinetic mechanisms of the reaction and indirectly glean insights into the reaction transition state. However, in these experiments, the transition state is not directly probed.
Femtosecond laser technology detects transition states by utilizing ultra-short pulses of femtosecond laser. Ahmed H. Zewail was awarded the Nobel Prize in Chemistry in 1999 for his pioneering work in utilizing femtosecond laser technology to study chemical reactions. This technology relies on the pump-probe method, involving exciting reactant molecules into high-energy excited states, thereby triggering the formation of chemical reaction transition states. By adjusting the frequency and intensity of the laser, one can control the vibration and electronic structure of reactant molecules, influencing the dynamics of the reaction process. However, this method has limitations to specific excited states due to the selection rules of optic transitions.
Another complementary and more direct experimental method to characterize the transition state is the anion photodetachment method. Photodetachment of the anion projects its vibrational wavefunction vertically onto the neutral PES by ejecting the electron. If a stable anionic complex exists with a geometry similar to the transition state of the corresponding neutral reaction, the vibrationally resolved photoelectron spectrum contains detailed information of dynamics near the transition state. Recently, Neumark group from University of California Berkeley have demonstrated several exquisite experiments using the cryogenic slow electron velocity-map imaging method (Cryo-SEVI) to probe resonances spanning the transition-state region for F + H2, F + CH3OH, and F + NH3.
Limitations of direct anion photodetachment experiments.
However, the direct anion photodetachment experiments is always limited by the Frank-Condon principle in the process of molecular electronic transition, and can only observe quantum states in regions with large overlapping integrals of negative ions and neutral transition state wave functions. Therefore, only partial transition state information can be obtained for a few systems in these experiments, and difficult for regions with weak Frank-Condon. For example, high-lying Feshbach resonances with an excited HF stretching mode (vHF = 2-4) were recently identified in the transition-state region of the F + NH3 → HF + NH2 reaction through photo-detaching FNH3− anions, but the direct photodetachment failed to observe the lower-lying vHF = 0,1 resonances and bound states due apparently to negligible Franck-Condon factors. (as shown by the blue arrow in Fig. 1).
Fig. 1: Energy diagram for direct photodetachment and autodetachment via a dipole-bound state (DBS) of the FNH3− anion to the neutral F + NH3 → HF + NH2 reactive PES.
A novel approach via anionic dipole-bound state.
Here, we precent a new approach to probe the regions of reactive potential energy surfaces that are out of the Franck-Condon-active areas based on the anionic dipole-bound state (as shown by the red arrow in Fig. 1). Unlike direct photodetachment, our detection depends on the autodetachment of a dipole-bound state (DBS) of the FNH3− anion, which proceeds with an indirect photodetachment mechanism. By tuning the photon energy, the FNH3− anion in the ground state can be resonantly excited to the DBS. The energy of the DBS is higher than the ground state energy of the neutral transition state structure, it will quickly autodetach due to the vibronic coupling, resulting in the ejection of the DBS electron. The weak transitions in the direct photodetachment can be significantly enhanced through resonances via a DBS. So, we identified so-far unreported quantum states near the reaction-complex region of the prototypical F + NH3 → HF + NH2 reaction. By combing high-resolution photoelectron spectroscopy with high-level quantum dynamical computations, we unveiled a new series of vibrational Feshbach resonances and bound states with vHF = 0-1 along the F + NH3 reaction path, which allowed an accurate experimental determination of the electron affinity of the FNH3 complex.
This study demonstrates the usefulness of a photoexcitation tool to probe regions of reactive potential energy surfaces that are out of the Franck-Condon-active areas, but might bear interesting features mediating the dynamics of a chemical reaction. Moreover, the existence of a DBS state also enables the pump-probe type experiment to observe ultrafast dynamics, which may greatly enrich the understanding of chemical reaction dynamics in the future.