In CasaXPS, the LA lineshape is a specialized asymmetric peak function used to fit XPS spectral features, particularly those from metallic or conductive samples that show pronounced asymmetry due to processes like electron energy loss or conduction band effects. The term “LA” stands for Lorentzian asymmetric. The lineshape is a mathematical modification of the standard Lorentzian (or Voigt) peak, designed to replicate the tailing commonly observed on the high binding energy side of metallic peaks.This lineshape can, of course, also be operated in symmetrical mode to describe standrad emissions.

The Lorentzian function can be described as in equation 1, where E is the peak position, and F is the FWHM. While equation 2 describes the LA modifier for the Lorentzian function used in CasaXPS, which is then modified using a Gaussian function of width m (between 0 and 499).

In CasaXPS the LA lineshape is used in the form:

LA(α, β, m)

Where α = reduction in tail spread to the high binding energy side

β = reduction in tail spread to the high binding energy side

m = width of Gaussian function used to convolute the Lorentzian curve

The LF lineshape is the same as the LA lineshape, but includes an additional parameter that controls the damping of the asymmetric tails (w). This value is controlled between 0 to 499, with the smaller the number the more extreme the damping.

LF(α, β, w, m)

While not necessarily physically meaningful, the LF lineshape gets around some of the practical limitations of fitting LA or DS lineshapes to a fixed, limited region of analysis.