Research

August 1996

August 31, 1996

Choosing a Model and Appropriate Transition Dipole Moments for Time-Dependent Calculations of Intervalence Electronic Transitions

Posted by David Talaga

J Phys Chem 1996 V100 P8712

Intervalence electron transfer spectra in mixed-valence mols. are frequently modeled by an interacting pair of adiabatic potential energy surfaces.  The presence or absence of a double min. in the lower surface is correlated with trapped or delocalized charges, resp.  In the time-dependent picture of the spectroscopy, calcns. are conveniently carried out in a diabatic basis.  The choice of a diabatic basis for a given adiabatic potential surface is not unique.  The appropriateness of a given representation depends on the phys. model that is chosen to represent the system.  Three diabatic models are presented that give the same adiabatic potential surface.  The 1st model represents charge transfer between 2 sites, the 2nd represents a transition between bonding and antibonding MOs, and the 3rd represents a nonbonding to nonbonding transition.  Each of these models gives rise to a different calcd. absorption spectrum even though they arise from the same adiabatic picture.  An important consideration after a model is chosen is the selection of the transition dipole moment.  The symmetry of the transition dipole moment was derived and discussed for each of the models for the different polarization directions of the incident light, and the symmetry depends on the choice of the model.  The Condon approxn. corresponds to different polarization directions in the different models.  The explicit relations and interconnections between the 3 models and the adiabatic model are derived.

August 1, 1996

Copper Fluoride Luminescence during UV Photofragmentation of Bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)copper(II) in the Gas Phase

Posted by David Talaga

Gas phase 308 and 350-370 nm photolysis of bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)copper(II), Cu(hfac)2, produces CuF as well as copper atoms and dimers. These metal-contg. fragments, identified by luminescence spectroscopy, are studied under a variety of gas phase conditions ranging from 1 bar in a static chamber to 10-4 mbar in a collision-free mol. beam. Copper atom and dimer luminescence is obsd. at the higher pressures, whereas at low pressures (total pressure no greater than the vapor pressure of the sample) exclusively CuF emission is obsd. The a, A (w = 0, 1, 2), B, and C excited states at 681.0, 567.6, and 505.1, and 491.7 nm are obsd. The 3P0- component of the A state is obsd. for the first time. The CuF luminescence obeys a quadratic power law with 308 nm excitation. The partitioning of excess energy into fragment degrees of freedom is detd. from the intensities of the emission lines. The vibrational and rotational temps. of the CuF fragment are in excess of 1700 K. Mechanisms of CuF formation, comparisons with the free ligand and with other volatile copper complexes, and the implications for laser-assisted chem. vapor deposition are discussed.