\documentclass{standalone}
\usepackage{tikz}
\usepackage{siunitx}
\usepackage{mathtools,physics,bm,xcolor}
%\usetikzlibrary{arrows.meta}
%\tikzset{myarr/.style={
%        {Triangle[width=4pt, length=4pt]}-{Triangle[width=4pt, length=4pt]},
%}}
\usepackage{tikz}
    \usetikzlibrary{intersections}
\usepackage{pgfplots}
    \usepgfplotslibrary{fillbetween}
  \definecolor{darkgreen}{RGB}{0, 180, 0}
  
\begin{document}  

\begin{tikzpicture}[scale=2]

	% x axis
	\draw [->] (0,0) -- (0,4);
	\node [left] at (0,4) {Energy};
	% x axis
	\draw [->] (0,0) -- (4,0);
	\node [below] at (4,0) {Nuclear coordinates};
	% absorption 
	\draw [thick, blue, <->]  (1,1) -- (1,3);
	% emission 
	\draw [thick, red, <->]  (2,1.5) node[along]{$E^{fluo}$} -- (2,2.5);
	% adiabatic 
	\draw [thick, darkgreen, <->]  (2.5,1) node[right]{$E^\text{adia}$} -- (2.5,2.5);
	% 0-0 
	\draw [thick, blue, <->]  (3,1.1) node[along]{$E^{0-0}$} -- (3,2.7);

%  \node [right] at (2.65,-2) {$\theta$};
%  \draw [thick] (-2,-2.05) node[below]{\SI{70}{\degree}} -- (-2,-1.95);
%  \draw [thick] (0,-2.05) node[below]{\SI{90}{\degree}} -- (0,-1.95);
%  \draw [thick] (2,-2.05) node[below]{\SI{110}{\degree}} -- (2,-1.95);

  % Theta = 75 rectangular
%  \node[draw,circle,minimum size=85pt,opacity=0.3,thick] (a) at (-2,-1) {};
%  \node [above] at (-2,-0.2) {$D_{2h}$};
%  \draw [thick] (a.35) -- (a.145) -- (a.-145) -- (a.-35) -- (a.35);
%  \node[fill,draw,circle, label=above right:H, minimum size=3pt,inner sep=0pt] at (a.35) {};
%  \node[fill,draw,circle, label=below right:H, minimum size=3pt,inner sep=0pt] at (a.-35) {};
%  \node[fill,draw,circle, label=below left:H, minimum size=3pt,inner sep=0pt] at (a.-145) {};
%  \node[fill,draw,circle, label=above left:H, minimum size=3pt,inner sep=0pt] at (a.145) {};
%  \draw [dashed,thick] (-2,-1) -- (a.35);
%  \draw [dashed,thick] (-2,-1) -- (a.-35);
%  \draw [thick] (-1.8,-1) arc [start angle=0,end angle=35,radius=0.2];
%  \draw [thick] (-1.8,-1) arc [start angle=0,end angle=-35,radius=0.2];
%  \node [right] at (-1.8,-1) {$\theta$};
  
   % Theta = 90 square
%  \node[draw,circle,minimum size=85pt,opacity=0.3,thick] (b) at (0,-1) {};
%  \node [above] at (0,-0.2) {$D_{4h}$};
%  \draw [thick] (b.45) -- (b.135) -- (b.-135) -- (b.-45) -- (b.45);
%  \node[fill,draw,circle, label=above right:H, minimum size=3pt,inner sep=0pt] at (b.45) {};
%  \node[fill,draw,circle, label=below right:H, minimum size=3pt,inner sep=0pt] at (b.-45) {};
%  \node[fill,draw,circle, label=below left:H, minimum size=3pt,inner sep=0pt] at (b.-135) {};
%  \node[fill,draw,circle, label=above left:H, minimum size=3pt,inner sep=0pt] at (b.135) {};
%  \draw [thick,myarr,dashed] (b.-135) -- (b.45);
%  \node [above left] at (0,-1) {$d$};

   % Theta = 105 rectangular
%  \node[draw,circle,minimum size=85pt,opacity=0.3,thick] (c) at (2,-1) {};
%  \node [above] at (2,-0.2) {$D_{2h}$};
%  \draw [thick] (c.55) -- (c.125) -- (c.-125) -- (c.-55) -- (c.55);
%  \node[fill,draw,circle, label=above right:H, minimum size=3pt,inner sep=0pt] at (c.55) {};
%  \node[fill,draw,circle, label=below right:H, minimum size=3pt,inner sep=0pt] at (c.-55) {};
%  \node[fill,draw,circle, label=below left:H, minimum size=3pt,inner sep=0pt] at (c.-125) {};
%  \node[fill,draw,circle, label=above left:H, minimum size=3pt,inner sep=0pt] at (c.125) {};
%  \draw [dashed,thick] (2,-1) -- (c.55);
%  \draw [dashed,thick] (2,-1) -- (c.-55);
%  \draw [thick] (2.2,-1) arc [start angle=0,end angle=55,radius=0.2];
%  \draw [thick] (2.2,-1) arc [start angle=0,end angle=-55,radius=0.2];
%  \node [right] at (2.2,-1) {$\theta$};
  %%%%%%% Define Potential Function %%%%%%%
%    \pgfmathsetmacro{\DeGS}{1}
%    \pgfmathsetmacro{\RoGS}{1}
%    \pgfmathsetmacro{\alphaGS}{1}
%    \pgfmathsetmacro{\DeES}{1.2}
%    \pgfmathsetmacro{\RoES}{1.2}
%    \pgfmathsetmacro{\alphaES}{1.2}
%    \pgfmathdeclarefunction{GS}{1}{%
%        \pgfmathparse{%
%            \DeGS*((1-exp(-\alphaGS*(#1-\RoGS)))^2-1)%
%        }%
%    }%
%    \pgfmathdeclarefunction{ES}{1}{%
%        \pgfmathparse{%
%            \DeES*((1-exp(-\alphaES*(#1-\RoES)))^2-1)%
%        }%
%    }%
%%%%%%%% Energy Levels %%%%%%%
%    \pgfmathdeclarefunction{energyGS}{1}{%
%        \pgfmathparse{%
%            -\DeGS+(#1+.5) - (#1+.5)^2/(1*\DeGS)
%        }%
%    }%
%    \pgfmathdeclarefunction{energyES}{1}{%
%        \pgfmathparse{%
%            -\DeES+(#1+.5) - (#1+.5)^2/(1*\DeES)
%        }%
%    }%
%
%    \begin{axis}[
%            axis lines=none,
%            smooth,
%            no markers,
%            domain=0:4,
%            xmax=10,
%            ymax=10,
%            scale=1
%        ]
%        \addplot [black, samples=50, name path global=GSCurve] {GS(x)};
%        \addplot [black, samples=50, name path global=ESCurve] {ES(x)};
%    \end{axis}
\end{tikzpicture}

\end{document}