This project is mainly theoretical in nature and involves the use of electronic structure calculations combined with Franck-Condon Factor calculations to simulate and hence assign, electronic spectra and photoelectron spectra of small reactive intermediates.
The main objective is to compute potential energy surfaces of the states involved to obtain reliable excitation or ionization energies, force constants and vibrational constants of the participating states. This basic information will then be used to compute the Franck-Condon envelope of a photoelectron or electronic band. Reliability of the ab initio molecular orbital calculations is vital in determining the value of such simulations. In practice, if reliable ab initio electronic structure calculations are performed, a vibrationally resolved photoelectron band of a reactive intermediate can be combined with the results of such calculations via an iterative Franck-Condon approach (IFCA) to deduce reliable cationic (or neutral) state geometries and assign the structure in the vibrationally resolved band
The computational approach uses a method, based on the harmonic oscillator approximation for calculating Franck-Condon Factors (FCFs) with the use of Cartesian co-ordinates. This method has been extended by fitting the potential of each state to an anharmonic oscillator expression, by fitting each ab initio potential to a power series in products of the internal co-ordinates. The anharmonic terms in this potential can be considered as a perturbation to a harmonic oscillator potential and the required vibrational overlap integrals are expressed in terms of analytical expressions involving the overlap integrals of harmonic wavefunctions in normal coordinates. The anharmonic Franck-Condon programme has recently been written and tested, and an initial study with this method on the photoelectron spectrum of ClO2 shows that use of this code is essential if extended vibrational series are observed. Related studies on triatomics containing hydrogen atoms show that the anharmonic code is also needed if vibrations involving hydrogen atoms are excited.
The next steps involve testing of the method on selected triatomic reactive species where suitable electronic and PE spectra are available, extension of the method to allow bent-to-linear transitions to be considered and extension of the method to tetratomic and larger molecules.