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 (S(1)). Surprisingly, with M = Mo and W we also find a fundamental of an antisymmetric (b(2) 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 S(1). 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.

Excited electronic states of MnO4−: Challenges for wavefunction and density functional response theories

“…We have previously shown how the supposedly simple TiO 2 molecule is challenging to describe [3]. The challenges when trying to accurately study the electronic excited states of transition metal complexes present a hurdle on the path to fully understanding the rich and varied reactive photochemistry that many transition metal complexes display, with more cases being reported regularly, see for example [4][5][6][7][8][9][10][11]. There are also a number of wellknown complexes that despite their modest size and simple structure have been classed as 'tough' examples when trying to fully understand the nature of their ground and electronic excited states due to often poor agreement between many theoretical methods and experiment [12].…”

Enhancement of strong-field multiple ionization in the vicinity of the conical intersection in 1,3-cyclohexadiene ring opening

“…Figure 2 indicates that double ionization of the UV--excited CHD/HT system peaks at about 150 fs time delay. Previous experiments and theoretical models that have investigated the S 0 /S 1 conical intersection find that the wavepacket launched by the UV pulse passes through a conical intersection at about this same time, approximately 140 fs after the photoexcitation [3,14,19]. This may indicate that when the S 1 and S 0 states become nearly degenerate, the double ionization rate is strongly enhanced.…”

“…The molecule has been studied extensively [3--13] (for a recent review see [14] and the references therein), and is suitable for testing new experimental techniques. Ion fragment time--of--flight (TOF) mass spectra of the parent molecule CHD and the product HT differ significantly when short intense 800 nm radiation is used for fragmentation [3,4,15]. The HT isomer is characterized by a more violent fragmentation that leads to an increase of smaller--mass fragments, particularly H + , compared to that of CHD.…”

“…The HT isomer is characterized by a more violent fragmentation that leads to an increase of smaller--mass fragments, particularly H + , compared to that of CHD. This qualitative difference between the CHD and HT fragmentation TOF mass spectra has been used in time--resolved experiments to monitor the CHD ring opening [3,4].…”

“…Such time--resolved fragmentation experiments have been used to follow wavepacket motion [2] and study unimolecular reactions [3,4].…”