+In contrast to Energy-resolved methods, Total Current (or Yield) methods measure the total current of secondary electrons whilst varying the primary electron energy. As the primary electron energy reaches the threshold for a particular mechanism of secondary electron scattering, the analysis of the total secondary electron current as a function of energy can give information about the threshold energies for processes of interest.
+
+Total Current methods are generally simpler to realise experimentally compared with Energy-resolved methods, as they do not require energy analysers, and current measurement may be performed external to the vacuum chamber, using a conventional low current ammeter. It is also simple to combine a Total Current methods with existing Energy-resolved methods.
+
+
+\subsection{Low Energy Total Current Spectroscopy}
+
+As the name suggests, low energy Total Current Specroscopy is based upon measurement of the total secondary electron current at low primary electron energies, typically in the range of $0-15$eV. At low primary energies, the secondary electron current is predominantly composed of inelastically reflected primary electrons which have lost energy in causing interband transitions.
+
+Figure \ref{tcs_simple.pdf} shows a simplified schematic for the Total Current Spectroscopy experiments conducted during this study. Electrons are produced via thermionic emission by heating a cathode. A series of electrodes are used to accelerate and focus a current $I_1$ onto the target. The energy of primary electrons is controlled by adjusting the power supply $U$, which determines the potential between the cathode and target. The transmitted current $I$ to flow through an ammeter external to the chamber. For a more detailed description of the experimental setup, Refer to Appendix B for a discussion of hardware to automate the measurement of $I$ and control of $U$. Refer to Appendix D for a discussion of the electron gun and its control circuit.
+
+\begin{center}
+ \includegraphics[scale=0.50]{figures/tcs_simple}
+ \captionof{figure}{Simplified Diagram of TCS Experiments}
+ \label{tcs_simple.pdf}
+\end{center}
+
+The goal of Total Current Spectroscopy is to measure variations in the secondary electron current, $I_2$. It can easily be demonstrated that this can be accomplished by measurement of $I$.
+
+In the following discussion, we will summarise the approach adopted by Komolov to relate measurement of $I(E_1)$ to characteristics of the sample under study \cite{komolov}.
+
+From the above, it is obvious that $I = I_1 - I_2$. Assuming that $I$ is a constant, independent of primary electron energy $E_1$, we define the Total Current Spectrum (TCS) as:
+\begin{align*}
+ S(E_1) &= \der{I}{E_1} = - \der{I_2}{E_1}
+\end{align*}
+This result also assumes that $I$ does not vary during the time taken to perform a measurement of $S(E_1)$ for a range of $E_1$ values. This is generally valid in the period after the cathode reaches thermal equilibrium.
+
+The energy of a single primary electron arriving at the sample is given by $E = e U + c$, where $e$ is the electron charge, $U$ is the potential difference between cathode and sample, and $c$ a constant including the contact potential between the cathode and sample.
+In reality, the cathode emits electrons with a distribution of energies, which is further altered by the focusing properties of the electrodes; as a result, the energy of the incident primary electrons is described by a distribution $f(E - E_1)$ about the mean value $E_1$, with the maximum of the distribution at $E = E_1$.
+
+The primary electron current $I_1$ for a mean energy $E_1 = e U$ can be written as:
+\begin{align*}
+ I_1(E_1) &= e A \int_{0}^{\infty} f(E - E_1) dE
+\end{align*}
+Where $A$ is the surface area irradiated by the beam.
+
+Introducing the secondary emission coefficient $\sigma(E)$, which gives the probability for a primary electron of energy $E$ to give rise to a secondary electron, we can write the secondary electron current as:
+\begin{align*}
+ I_2(E_1) &= e A \int_{-E_1}^{\infty} \sigma(E_1)f( E - E_1) dE
+\end{align*}
+
+The total current $I$ may then be written as:
+\begin{align*}
+ I(E_1) &= e A \left[ \int_{0}^{\infty} f(E - E_1) dE - \int_{-E_1}^{\infty} \sigma(E_1)f( E - E_1) dE \right]
+\end{align*}
+
+Differentiating, using the fundamental theorem of calculus, we can determine the total current spectrum:
+\begin{align*}
+ S(E_1) = \der{I}{E_1} &= e A \left\{ [ 1 - \sigma(0)] f(-E_1) - \int_{0}^{\infty} f(E - E_1) \der{\sigma(E_1)}{E_1} dE \right\}
+\end{align*}
+
+The first term in the above expression is determined solely by the distribution of primary electrons $f$. This term will be maximised when $E_1 = 0$; meaning that $U$ is equal to the contact potential $c$ between the cathode and sample.
+
+The second term contains all dependence of $S(E_1)$ on characteristics of the sample. At the threshold for a particular process, the secondary emission efficiency $\sigma(E_1)$ is expected to undergo a sharp change. This results in a well defined maxima or minima in the derivative $\der{\sigma(E_1)}{E_1}$, which can be seen as a corresponding maxima or minima in the total current spectrum $S(E_1)$. From the convolving function $f(E - E_1)$, it can be seen that the distribution of primary electron energy determines the degree to which $\der{\sigma(E_1)}{E_1}$ may be resolved from measurement of $S(E_1)$.
+
+The total current spectrum $S(E_1) = \der{I}{E_1}$ can be obtained from measurement of $I(E_1)$ using a finite difference approximation. Often, the conventional ammeter and DC power supply in Figure \ref{tcs_simple.pdf} are replaced with a lock-in amplifier and AC power supply, as in Komolov's description \cite{komolov}. Lock-in amplifier techniques have the advantage of measuring $S(E)$ directly. The lock-in amplifier also eliminates unwanted sources of noise. For this study, the lock-in amplifier approach was inpractical due to the limitations on available equipment. For future studies, it is suggested that the lock-in amplifier approach be adopted.
+
+
+\subsubsection{The Secondary Emission Coefficient}
+
+$\sigma(E)$ can be written as the sum of two components, representing the probability for secondary electrons arrising due to elastic reflections or any mechanism involving primary electron energy loss.