The heart of our experiment lies in the tandem photofragmentation mass spectrometer. Electrospray ionization (ESI) or other soft ionization techniques will be used to gently isolate solution-phase species of interest into the gas phase. The ability to manipulate isolated ions and to carefully control their local chemical environment using a diverse array of techniques has turned mass spectrometery into a power analytical tool for such complex analysis as proteomics, metabolomics, and drug discovery (including the on-going research here at WashU by Prof. Gross and WashU’s Biomedical Mass Spectrometry Resource, and by Prof. Patti). Manipulations will be made to the isolated starting reactants, such as controlled clustering with solvent molecules like water, H/D isotopic exchange, or pre‑activation through collisions or laser interaction. Critically, such manipulations will allow us to systematically alter the local chemical environment in a well-controlled fashion leading to important chemical insight into what factors most strongly influence the observed reaction dynamics.
Once prepared, the ions will be collisionally cooled in an ion trap attached to a temperature‑controlled helium cryostat. For most experiments, the trap will be operated near 10 K resulting in the relaxation of the chemical structures into their lowest energy arrangements. By removing the bulk solvent using ESI and quenching the ion’s internal energy in the cryogenic trap, the measured optical spectra will contain well‑resolved, sharp transitions that will allow for the unambiguous assignment of the spectral features and thus the underlying structures and inter- and intramolecular interactions which give rise to those spectra.
The cold ions are next separated in the first stage of the tandem time-of-flight mass spectrometer. The composition-selected species of interest will be intersected with either IR or UV‑Vis lasers resulting in photofragmentation when the laser frequency is on-resonance with an underlying vibrational (IR) or electronic (UV-Vis) transition. The photofragment is separated from the parent ion using a second mass spectrometry stage utilizing a reflectron. The amount of photofragment generated is measured as a function of laser frequency to obtain an “action” detected absorption spectrum.
Several types of laser systems will be used to unravel the multidimensional potential energy surface (PES) of the reactive processes under study, illustrated by the cartoon schematic shown here. First, high‑resolution IR/UV‑Vis sources pumped by Nd:YAG lasers (7 ns pulse duration, <5 cm-1 bandwidth) can sensitively map out the structural landscape of the starting reactant complexes. Since reactive molecular groups are extremely sensitive to their local environment, high-resolution IR vibrational spectroscopy of isolated ions has proven to be an invaluable method to reveal not only molecular structures, but how the delicate interplay between intra- and intermolecular interactions dictate chemical properties in a site‑specific fashion. High‑resolution, tunable UV‑Vis spectroscopy will be used to explore the electronic component of the reactive PES. Since many catalytic processes involve concerted motions of electrons and nuclei, characterizing changes in the electronic energy levels as a function of the local chemical environment is needed to establish the degree and nature of coupling between the key electronic and nuclear degrees of freedom.
The most crucial and novel component of our research program lies in the integration of ultrafast spectroscopic methods to (1) initiate reactions in the composition-selected species, (2) capture and interrogate short‑lived reaction intermediates, and (3) characterize the time-evolving shape of the PES. To accomplish these goals, we will use an ultrafast (30 fs) Ti:Sapphire laser to pump optical systems which will convert the 800 nm output into broadly tunable ultrafast pulses spanning from the IR to the UV. A single UV‑Vis pulse will be used to initiate the reaction and the reactive process will be monitored with ultrafast (fs-ps) time resolution using a two-pulse IR pump-probe scheme. Ultrafast IR pump-probe pairs allow access to higher-lying vibrational states not accessible with the ns high-resolution systems above, giving us the ability to map the time-evolving shape of the PES with molecular detail. Further, the ultrafast IR pump-probe scheme has the ability to reveal “cross peaks” between different vibrational modes which signal direct coupling between molecular groups or chemical transformations over the course of the reaction. Crucially, the time evolution of the well‑resolved IR transitions and cross peaks of isolated systems will allow us to capture and unambiguously identify and characterize the structures, dynamics, and chemical behavior of critical reaction intermediates and transition-state species throughout the entire course of the reaction.
