Monochromator calibration

The relationships between the positions of these devices and the monochromator wavelength $\lambda$ is given by

$$\lambda (\AA)\vert _{\mbox{stepper}} = {(\mbox{\tt SGM\_I}-\mbox{\tt ZOS\_I}) \over \mbox{\tt SPA\_I}}$$ (3.3)

$$\lambda (\AA)\vert _{\mbox{encoder}} = {(\mbox{\tt SGM\_I\_ENC}-\mbox{\tt ZOE\_I}) \over \mbox{\tt EPA\_I}}.$$ (3.4)

The parameters used in the calculation are as follows:

SPA_I: Steps Per Angstrom for the monochromator stepper motor. This number should remain constant once calibrated (unless the total deflection angle is changed significantly).

ZOS_I: Zero Order Steps, which is the stepper motor position when the grating is at zero order.

EPA_I: Encoder millimeters Per Angstrom for the Mitutoyo encoder.

ZOE_I: Zero Order Encoder, which is the encoder position in millimeters when the grating is at zero order.

These calibration factors are stored in the files /mnt/x1a/sgm_i.dat and /mnt/x1a/sgm_o.dat, respectively, which typically show something like

! Calibration as of Tue Aug 27 08:56:49 2002
steps_per_angstrom -504.2680
zero_order_steps 11308.78
encoder_per_angstrom -100.0
zero_order_encoder 20.

If a familiar peak is not at the expected energy, you may want to change the calibration. For starters, you can get a rough calibration (good to about 1%) by using a peak in the total electron yield in aluminum oxide at 541 eV. If you take a shutter photodiode scan, you should see this peak quite clearly (see Fig. 3.13).

Figure 3.13: Shutter photodiode spectrum showing the aluminum oxide total electron yield peak at 541 eV. You can use this peak to get a rough calibration of the monochromator.

Let's say you have taken a spectrum of absorption in CO$_{2}$ gas on the outboard branch (using a flow of 0.1 SCFH, for example). If you find that the second absorption peak is at 4.240 nm rather than the accepted value of 4.23534 nm, you should do the following to get the step position of the peak from the calibration in use at the moment:

IDL> sgm_convert,4.240,step,from='nm',to='step',/out
IDL> print,step
23529

This now gives you the step position of the peak which is supposed to be at 4.23534 nm or 292.74 eV. To change the calibrations (such as zero order position), use this information on measured step position versus accepted photon energy in eV in the routine

IDL> sgm_cal

and follow the instructions. In the above example, you would enter 23529 for the step position, and 292.74 eV for the energy. To read the calibration, you can do

IDL> read_sgm_cal,spa,zos,spe,zoe,/outboard
IDL> print,spa,zos,spe,zoe

Reference calibration energies are listed in Table 3.1, and COREX EELS spectra are shown as follows: CO$_{2}$ at the C (Fig. 3.14) and O (Fig. 3.15) edges, O$_{2}$ at the O edge (Fig. 3.17), N$_{2}$ at the N edge (Fig. 3.16), and H$_{2}$O at the O edge (Fig. 3.18). Some other reference spectra are found here.

Table 3.1: Calibration photon energies.

Molecule	Energy (eV)	Wavelength (nm)	Reference
CO$_{2}$ gas	290.74$\pm$0.39	4.26447	Fig. 3.14
	292.74	4.23534	Ma et al. [2]; Fig. 3.14
	294.96	4.20346	Ma et al. [2]; Fig. 3.14
	296.38	4.18332	Ma et al. [2]
	535.41$\pm$0.78	2.31571	Fig. 3.15
N$_{2}$ gas	401.11$\pm$0.39	3.09105	Fig. 3.16
	406.13	3.05285	Chen et al. [3]
	407.14	3.04527	Chen et al. [3]
O$_{2}$ gas	530.82$\pm$0.40	2.33573	Fig. 3.17
CaF$_{2}$ film	349.1	3.55157
	352.4	3.51831

Figure 3.14: CO$_{2}$ EELS spectrum at the C edge [4]. From the COREX database [5].

Figure 3.15: CO$_{2}$ EELS spectrum at the O edge [4]. From the COREX database [5].

Figure 3.16: N$_{2}$ EELS spectrum at the N edge [4]. From the COREX database [5].

Figure 3.17: O$_{2}$ EELS spectrum at the O edge (A. Hitchcock, unpublished). From the COREX database [5].

Figure 3.18: H$_{2}$O EELS spectrum at the O edge [6]. From the COREX database [5].