Most experiments in chemical reaction dynamics involve preparing the reactants and probing the products; they do not detect what happens in the intervening transition state region.
One of the few exceptions is anion photoelectron spectroscopy. Under favourable circumstances, the photoelectron spectrum of a stable ABC- anion can provide a vibrationally-resolved picture of the dynamics of the corresponding neutral reaction, A + BC to AB + C, that can be interpreted with the aid of the Franck-Condon principle. When the anion equilibrium geometry lies close to the neutral transition state, as it does in the case of FH2-, measuring the anion photoelectron spectrum is therefore tantamount to performing transition state spectroscopy:
This figure compares the experimental photoelectron spectrum of para-FH2- with an early theoretical simulation from our research group. The photoelectron spectrum was measured by Neumark and coworkers, and the simulation was performed using an ab initio F + H2 potential energy surface constructed by Stark and Werner. No other F + H2 potential energy surface available at the time was found to give nearly such good agreement with the experimental spectrum.
The interpretation of the spectrum is illustrated below. The FH2- anion has a linear equilibrium geometry (as a result of the F-...H2 ion-induced dipole interaction), but the neutral FH2 transition state geometry is bent (as a result of the F(2P3/2)...H2 quadrupole-quadrupole interaction). The coordinate that experiences the largest change in potential energy on photodetachment is therefore a bending coordinate, which explains why the spectrum is dominated by a bending (or H2 hindered-rotational) progression:
Note also that the peaks in the spectrum have a finite width because they occur in the continuum (the FH2 transition state species formed in the photodetachment decays to F + H2 and HF + H), and that owing to the symmetry of the nuclear spin wavefunction only even j hindered-rotational levels are seen in the pFH2- photoelectron spectrum.
This study of the FH2- photoelectron spectrum was originally published in J. Chem. Phys. 99, 6345 (1993) and in Science 262, 1852 (1993); more recent developments are discussed in Chem. Phys. Lett. 256, 465 (1996) and reviewed in J. Chem. Soc. Faraday Trans. 93, 673 (1997).