Photodissociation of group-6 hexacarbonyls: observation of coherent oscillations in an antisymmetric (pseudorotation) vibration in Mo(CO)5 and W(CO)5

Abstract

On dissociation of M(CO)₆, M = Cr, Mo and W, by a femtosecond UV laser (<270 to 360 nm), pronounced coherent oscillations are observed in the pentacarbonyl products on probing by long-wavelength (810 nm) ionization in the gas phase. They are vibrations in the ground state, driven by the slope from a conical intersection on relaxation from the initially formed excited state (S₁). Surprisingly, with M = Mo and W we also find a fundamental of an antisymmetric (b₂ in C_4v) vibration. From positive and negative displacements along such a coordinate one would expect the same signal, so that there should be only overtones. Vibrational selection rules are therefore considered for time-resolved spectroscopy. The reason for the symmetry breaking is suggested to result from the fact that the phase in superposition of wave functions is established by the pump process and this phase is conserved in probing, independently of the probe delay. An antisymmetric fundamental can be observed, if there is a small tunneling splitting in a state involved in the probe process. The observations also imply some conclusions on the dissociation and relaxation processes and the potentials: with longer wavelengths, the wave packet enters on the same surface but from a different direction to S₁. Only a very minor fraction of the available energy appears as coherent oscillation. There is no equipartition at the end, and a second CO is cleaved off in few picoseconds, even if there is only very little excess energy. Triplets do not contribute, even in the tungsten system and at longest wavelengths. The dissociation mechanism involves passage of the wave packet from all initial states over an avoided crossing to a repulsive ligand-field surface. It predicts that in some other molecules, the barrier caused thereby is larger and for long photolysis wavelength the lifetime is long enough for intersystem crossing to take place; it also predicts wavelength dependences in these cases. It is again emphasized that there is no vertical internal conversion; instead, the molecule is controlled by slopes and intersections of potentials. Also lifetimes can be considered as a control parameter in photochemistry.