FAQ9: PHI MultiPak Transmission Function Correction

PHI MultiPak binary files contain two coefficients labeled IntensityCalCoeff: a b. These coefficients relate to the formula for the transmission function

where E is the kinetic energy of the electrons collected by the detectors and PE is the Pass Energy of the analyzer.

The Intensity Calib option on the Spectrum Processing dialog window offers a means of both changing these coefficients and applying the transmission correction to the data. A transmission function appears in the VAMAS block as a second corresponding variable. That is to say, the first corresponding variable represents the spectrum intensities (ordinate index 0) at each of the analyzer energies using to measure the signal and for each of these spectral intensities a second entry appears corresponding to the transmission function value at the same analyzer energy. The VAMAS Ordinate field on the Intensity Calib property page allows the index for the transmission function to be altered from the default index of 1, however, as a rule CasaXPS converts transmission information into the default ordinate index and so this field should typically be left unaltered.

Apart from the NPL transmission correction procedure, it is believed that all other transmission correction mechanisms reference the recorded spectra to some reference mode of operation defined by the manufacturer of the instrument. After transmission correction, the intensities are calibrated so that the same sample measured with different pass energies results in the same intensity for a given line. The intensity, after transmission correction, is a relative value and therefore suitable for relative quantification rather than absolute measurements. The counts per bin no longer relate to the true measurement process and so error estimates based on Monte Carlo simulation are not valid for corrected spectra.

Transmission correction may be applied in two ways:

1. The spectrum itself is adjusted on a point by point basis, or
2. Intensities measured using the un-corrected data are adjusted when calculating the quantification tables.

CasaXPS offers both methods for transmission correction. The Intensity Calib option on the Spectrum Processing dialog window offers a means of adjusting the spectrum directly. If the PHI coefficients are altered using the options shown in Figure 1 and the Change Phi TF button is pressed, the corresponding variable holding the transmission values will be updated with new values. To see the results of the new transmission function it will be necessary to re-apply the transmission function to the raw spectrum. This may require undoing any processing operations involving the Intensity Calib option before pressing the Apply button on the property page shown in Figure 1.

Similarly, if a new transmission function is read from file (Figure 1: Add Transmission Function from File), the same considerations relating to the PHI transmission coefficients also apply.

Figure 1: Intensity Calibration Property Page.

The second method for transmission correction, where the integrated areas are corrected for transmission, is available on the Regions property page of the Quantification Parameters dialog window (Figure 2). Note that only one method should be used to quantify data. Either the data should be adjusted via the Intensity Calib option or the integrated areas adjusted, but not both.

When the chosen method for transmission correction is to adjust the integrated areas (by ticking the Automatic check box shown in Figure 2 and pressing the Update pushbutton), each time a quantification report is created the transmission correction to the areas will be recalculated. This means that changes to the transmission function made via the PHI transmission coefficients or reading a new transmission file on the Intensity Calib property page will take effect when a new quantification report is generated. Note that if no mean-free-path correction is required, the text-field next to the Automatic check box should be set to “0”. Otherwise the value for the mean-free-path correction should be entered in accordance with the expression for the intensity energy dependence seen in Figure 1.

Figure 2: Regions property page showing the flag used to enable transmission correction without altering the underlying spectral data.