Adding Column to Existing Multirow Table

What I'd like to do is add a first column to the table below. This new column would have the title "Element" and would have only three rows (one for every two rows in the "Upper Level" column) titled "Argon", "Krypton" and "Xenon".

I believe this would make the table nicer, but I'm still new to the \multirow and \multicolumn so I've not been able to adjust things to make this work. Please see the code below and an image of the generated table.

Please let me know if you need anything clarified.

\begin{table}[t!]
    \caption{Comparing experiment to literature for branching ratios of two strongest transitions to metastable state for Ar, Kr and Xe.}
    \centering
        \begin{tabular}{|c|c|c|c|c|c|c|}
            \hline
            \textbf{Upper Level}
            & \textbf{Lower Level}
            & \textbf{$\mathbf{\lambda_{ij}}$ (nm)} 
            & \textbf{$\mathbf{A_{ij} (10^{7} s^{-1})}$}
            & \textbf{$\mathbf{\beta_{exp}}$ (\%)} 
            & \textbf{$\mathbf{\beta_{th}}$ (\%)}
            & \textbf{$\mathbf{\beta_{rec}}$ (\%)}
            \\
            \hline
            \multirow{3}[8]{1in}{\centering Ar - 2p$^{6}$ (2$\times$188\,nm)} 
            & 1s$_{2}$ & 922.5 & 0.59-0.63 & 16 & 16 & 26 \bigstrut\\\cline{2-7}
            & 1s$_{4}$ & 800.6 & 0.48-0.51 & 13 & 12 & 25 \bigstrut\\\cline{2-7}
            & 1s$_{5}$ & 763.5 & 2.62-2.96 & 71 & 72 & 49 \bigstrut\\
            \hline
            \multirow{3}[8]{1in}{\centering Ar - 2p$^{8}$ (2$\times$188\,nm)} 
            & 1s$_{2}$ & 978.5 & 0.11-0.17 & 3 & 5 & 7 \bigstrut\\\cline{2-7}
            & 1s$_{4}$ & 842.5 & 2.11-2.52 & 66 & 67 & 51 \bigstrut\\\cline{2-7}
            & 1s$_{5}$ & 801.5 & 1.00-1.04 & 31 & 28 & 43 \bigstrut\\
            \hline
            \multirow{3}[8]{1in}{\centering Kr - 2p$^{6}$ (2$\times$215\,nm)} 
            & 1s$_{2}$ & 1373.9 & 0.02-0.03 & 0 & 1 & 7 \bigstrut\\\cline{2-7}
            & 1s$_{4}$ & 819.0 & 0.9-1.1 & 26 & 24 & 22 \bigstrut\\\cline{2-7}
            & 1s$_{5}$ & 760.2 & 2.6-3.1 & 74 & 75 & 71 \bigstrut\\
            \hline
            \multirow{3}[8]{1in}{\centering Kr - 2p$^{8}$ (2$\times$217\,nm)} 
            & 1s$_{2}$ & 1547.4 & 0.003-0.008 & 0 & 0 & 1 \bigstrut\\\cline{2-7}
            & 1s$_{4}$ & 877.7 & 1.5-2.6 & 68 & 68 & 69 \bigstrut\\\cline{2-7}
            & 1s$_{5}$ & 810.4 & 0.7-1.1 & 32 & 32 & 30 \bigstrut\\
            \hline
            \multirow{3}[8]{1in}{\centering Xe - 2p$^{6}$ (2$\times$252\,nm)} 
            & 1s$_{2}$ & 4932.4 & 0.002 & 0 & 0 & 0 \bigstrut\\\cline{2-7}
            & 1s$_{4}$ & 895.5 & 1.06 & 21 & 30 & 9 \bigstrut\\\cline{2-7}
            & 1s$_{5}$ & 823.4 & 2.49 & 79 & 70 & 91 \bigstrut\\
            \hline
            \multirow{3}[8]{1in}{\centering Xe - 2p$^{9}$ (2$\times$256\,nm)} 
            & 1s$_{2}$ & 10698.1 & 0 & 0 & 0 & 0 \bigstrut\\\cline{2-7}
            & 1s$_{4}$ & 992.6 & 1.96 & 49 & 65 & 71 \bigstrut\\\cline{2-7}
            & 1s$_{5}$ & 904.8 & 1.07 & 51 & 35 & 29 \bigstrut\\
            \hline
        \end{tabular}
    \label{tbl:choosingTransition}
    \end{table}

Table at this point

Here is a potential solution. It uses a rotated parbox that spawns along the rows.

\documentclass{article}
\usepackage{array,multirow,graphicx}
\usepackage{bigstrut}
\begin{document}

\newcommand{\rotatemultirow}[1]{\parbox[t]{2mm}{\multirow{6}{*}{\rotatebox[origin=l]{90}{#1}}}}



\begin{table}[t!]
    \caption{Comparing experiment to literature for branching ratios of two strongest transitions to metastable state for Ar, Kr and Xe.}
    \centering
        \begin{tabular}{|c|c|c|c|c|c|c|c|}
            \cline{2-8}
          \multicolumn{1}{c|}{}
          &\textbf{Upper Level}
            & \textbf{Lower Level}
            & \textbf{$\mathbf{\lambda_{ij}}$ (nm)} 
            & \textbf{$\mathbf{A_{ij} (10^{7} s^{-1})}$}
            & \textbf{$\mathbf{\beta_{exp}}$ (\%)} 
            & \textbf{$\mathbf{\beta_{th}}$ (\%)}
            & \textbf{$\mathbf{\beta_{rec}}$ (\%)}
            \\
           \hline
          \rotatemultirow{Argon\hspace{1cm}}
          &\multirow{3}[8]{1in}{\centering Ar - 2p$^{6}$ (2$\times$188\,nm)} 
             & 1s$_{2}$ & 922.5 & 0.59-0.63 & 16 & 16 & 26 \bigstrut\\\cline{3-8}
          &  & 1s$_{4}$ & 800.6 & 0.48-0.51 & 13 & 12 & 25 \bigstrut\\\cline{3-8}
          &  & 1s$_{5}$ & 763.5 & 2.62-2.96 & 71 & 72 & 49 \bigstrut\\
              \cline{2-8}
           & \multirow{3}[8]{1in}{\centering Ar - 2p$^{8}$ (2$\times$188\,nm)} 
             & 1s$_{2}$ & 978.5 & 0.11-0.17 & 3 & 5 & 7 \bigstrut\\\cline{3-8}
          &  & 1s$_{4}$ & 842.5 & 2.11-2.52 & 66 & 67 & 51 \bigstrut\\\cline{3-8}
          &  & 1s$_{5}$ & 801.5 & 1.00-1.04 & 31 & 28 & 43 \bigstrut\\
          \hline
          \rotatemultirow{Krypton\hspace{1cm}}
          &\multirow{3}[8]{1in}{\centering Kr - 2p$^{6}$ (2$\times$215\,nm)} 
             & 1s$_{2}$ & 1373.9 & 0.02-0.03 & 0 & 1 & 7 \bigstrut\\\cline{3-8}
          &  & 1s$_{4}$ & 819.0 & 0.9-1.1 & 26 & 24 & 22 \bigstrut\\\cline{3-8}
          &  & 1s$_{5}$ & 760.2 & 2.6-3.1 & 74 & 75 & 71 \bigstrut\\
           \cline{2-8}
          &\multirow{3}[8]{1in}{\centering Kr - 2p$^{8}$ (2$\times$217\,nm)} 
             & 1s$_{2}$ & 1547.4 & 0.003-0.008 & 0 & 0 & 1 \bigstrut\\\cline{3-8}
          &  & 1s$_{4}$ & 877.7 & 1.5-2.6 & 68 & 68 & 69 \bigstrut\\\cline{3-8}
          &  & 1s$_{5}$ & 810.4 & 0.7-1.1 & 32 & 32 & 30 \bigstrut\\
          \hline
          \rotatemultirow{Xenon\hspace{1cm}}
          &\multirow{3}[8]{1in}{\centering Xe - 2p$^{6}$ (2$\times$252\,nm)} 
             & 1s$_{2}$ & 4932.4 & 0.002 & 0 & 0 & 0 \bigstrut\\\cline{3-8}
          &  & 1s$_{4}$ & 895.5 & 1.06 & 21 & 30 & 9 \bigstrut\\\cline{3-8}
          &  & 1s$_{5}$ & 823.4 & 2.49 & 79 & 70 & 91 \bigstrut\\
           \cline{2-8}
          &\multirow{3}[8]{1in}{\centering Xe - 2p$^{9}$ (2$\times$256\,nm)} 
             & 1s$_{2}$ & 10698.1 & 0 & 0 & 0 & 0 \bigstrut\\\cline{3-8}
          &  & 1s$_{4}$ & 992.6 & 1.96 & 49 & 65 & 71 \bigstrut\\\cline{3-8}
          &  & 1s$_{5}$ & 904.8 & 1.07 & 51 & 35 & 29 \bigstrut\\
            \hline
        \end{tabular}
    \label{tbl:choosingTransition}
    \end{table}
\end{document}

Unfortunately, I did not succeed to automatically center vertically the names, and I used a hspace to finely adjust their vertical position.