“…In these spectroscopic studies, the TS is reached via photon excitation from a specific precursor, usually a van der Waals complex between the two reactants [7,[16][17][18][19][20][21][22][23][24][25][26][27] or a stable negative ion [6,[28][29][30][31][32][33]. Using van der Waals complexes as precursors, one alternative developed by Wittig and coworkers [7,26,27] consists in photodissociating one of the reactants ejecting one fragment towards the second partner at restricted geometry conditions, imposed by the structure of the initial van der Waals precursor. In a second possibility, one of the partners within the complex is promoted to excited electronic states where the reaction takes place, as first done by Soep and co-workers [16][17][18][19][20], and lately applied by Polanyi and co-workers [21][22][23] and González-Ureña and co-workers [24,25].…”