\subsection{Actinometry}
\label{aber}

Given a radiation source and a sample, the incident photon flux density ($q^{0,V}_{n,p}$) is the amount of incident photons per time interval ($q^{0}_{n,p}$) and  divided by the volume of the sample:

\begin{equation}
\label{eq:qv}
q^{0,V}_{n,p}=\left( \frac{q^{0}_{n,p}}{V}\right)
\end{equation}

Aberchrome 540 (Aberchromics Ltd.) was used as an actinometer for the measurements of $q^{0,V}_{n,p}$ from Rayonet RPR lamp (as an artificial source), at the excitation wavelength. The method for the determination of $q^{0,V}_{n,p}$ has been described in detail elsewhere \citep{Schuster_1992, Kuhn}.