The Spectrum Calculator and using Algebraic Expressions

It is sometimes useful to compare spectra from unknown samples with spectra from established databases such as the Beamson and Briggs “The XPS of Polymer Database”. Typically, an unknown polymer may be a blend of a number of spectra from the database and so some means of constructing spectra from the fundamental units can assist with the comparison. To aid the recognition of such blends, CasaXPS offers a mechanism for creating spectra from algebraic expressions defined in terms of other spectra.

Figure 1

Figure 1 shows valence band spectra from a set of polymers all copied into a single Experiment Frame. The VAMAS blocks within a VAMAS file (and hence within the Experiment Frame) are indexed 0 to 7. To specify a given spectrum with a file, a string of the form “vb<int>” is used. These identification strings can be displayed on the data using the display option shown in Figure 2.

Figure 2

To construct a new spectra from these data, the identification strings are used to define an algebraic expression of the form, “(vb0 + vb1 + vb2) / 3.0”. Expressions of this format can be entered into a dialog window activated from the Calculator property page (Figure 3).

Figure 3

Enter a valid algebraic expression into the Edit Box on the dialog window and press the OK pushbutton. A new Experiment Frame is created and the resultant spectrum appears as a new VAMAS block in the Experiment Frame.

If the spectra are recorded using different acquisition dwell-time etc, then the spectra used in the calculation may need to be normalised before the new spectrum is created. The Normalise options on the Calculator property page offer a means of normalising the data with respect to CPS or Counts per bin. Once the new spectrum has been created, one means of comparing the new spectrum to the data from an unknown sample is to load the new spectrum into the Calculator using the Set Operand pushbutton (Figure 3). The operand in the Calculator is displayed for as long as the Calculator page is the active property page on the Spectrum Processing window.