Importance of interatomic contributions to molecular X-ray emission processes

The validity of the one-centre model which neglects the interatomic contributions to C K X-ray emission processes is examined using a simple model for which the transition moment elements are evaluated using single Slater-type LCAO functions and molecular orbital coefficients from CNDO/2 theory. The model is adequate to demonstrate simply the important feature that for molecules involving carbon atoms bonded to other first row atoms the two-centre terms can make a very significant contribution to the C K transition rates. In particular, for the CO, HCN and CO 2 molecules the total rates increased by 37, 29 and 52% respectively when interatomic contributions were included. Contributions to rates from 3p electronic on nearest-neighbour second-row atoms for the carbon atom are also significant. Relative rates are less sensitive than absolute rates to the inclusion of interatomic contributions.