Ytdyklly

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usepackage[UKenglish]{babel}

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usepackage{hyperref}

usepackage[utf8]{inputenc}



usepackage{graphicx}

usepackage{chngcntr}

counterwithin{figure}{section}



usepackage[labelfont=bf]{caption}

captionsetup{font=small}



usepackage[font=small]{subcaption}



usepackage[export]{adjustbox}



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setlength{droptitle}{-2cm}

predate{}

date{}

postdate{}

begin{document}

begin{figure}[H]

centering

includegraphics[width=textwidth]{example-image}

caption{Theoretical model of erbium excitation by sensitization through 

organic ligand, occuring as follows: 

begin{enumerate}

item Organic ligand (singlet) groundstate, 'S$_{0}$', is excited to the 1st 

singlet excited state, 'S$_{1}$' by the absorbtion of a photon.

item In some cases the ligand de-excites, emitting a photon, flourescing, 

in some cases energy transfers from the 1st singlet excited state to the 

erbium, subsaquently:

begin{enumerate}

item The erbium de-excites, by non-radiative transition, to a lower excited 

(4f orbital) state.

item By induced electric dipole transition, the erbium de-excites, emitting 

the desired 1.5$mu$m wavelength emission.

end{enumerate}

item In other cases the organic ligand transitions from 1st excited singlet 

state to 1st excited triplet state, 'T$_{1}$', by some inter-system-crossing 

mechanism (ISC) where a photon may be emitted, (with a long decay time) 

phosphorescing, or the energy transfers from the 1st excited triplet state 

to the erbium.

item The erbium de-excites, by non-radiative transition, to a lower excited 

(4f orbital) state.

item By induced electric dipole transition, the erbium de-excites, emitting 

the desired 1.5$mu$m wavelength emission.

end{enumerate}}



label{fig:1.2}

end{figure}







end{document}

caption isn't really intended for such long texts but if you suppress the check for it being one line (nooneline option) and provide a shorter caption for the list of figures, then it works

Note however with the sample image that I used, it's too large for the page

latex warns

 Overfull vbox (79.45732pt too high) has occurred while output is active

You could reduce the text or use a smaller font. or scale the image more to make more room or ....

documentclass[12pt]{article}





usepackage{cite}



usepackage[UKenglish]{babel}

%usepackage{fixltx2e}

usepackage{hyperref}

usepackage[utf8]{inputenc}



usepackage{graphicx}

usepackage{chngcntr}

counterwithin{figure}{section}



usepackage[labelfont=bf,nooneline]{caption}

captionsetup{font=small}



usepackage[font=small]{subcaption}



usepackage[export]{adjustbox}



usepackage{float}



usepackage{titling}

setlength{droptitle}{-2cm}

predate{}

date{}

postdate{}

begin{document}

begin{figure}[H]

centering

includegraphics[width=textwidth]{example-image}

caption[short caption for the list of figures]{Theoretical model of erbium excitation by sensitization through 

organic ligand, occuring as follows: 

begin{enumerate}

item Organic ligand (singlet) groundstate, 'S$_{0}$', is excited to the 1st 

singlet excited state, 'S$_{1}$' by the absorbtion of a photon.

item In some cases the ligand de-excites, emitting a photon, flourescing, 

in some cases energy transfers from the 1st singlet excited state to the 

erbium, subsaquently:

begin{enumerate}

item The erbium de-excites, by non-radiative transition, to a lower excited 

(4f orbital) state.

item By induced electric dipole transition, the erbium de-excites, emitting 

the desired 1.5$mu$m wavelength emission.

end{enumerate}

item In other cases the organic ligand transitions from 1st excited singlet 

state to 1st excited triplet state, 'T$_{1}$', by some inter-system-crossing 

mechanism (ISC) where a photon may be emitted, (with a long decay time) 

phosphorescing, or the energy transfers from the 1st excited triplet state 

to the erbium.

item The erbium de-excites, by non-radiative transition, to a lower excited 

(4f orbital) state.

item By induced electric dipole transition, the erbium de-excites, emitting 

the desired 1.5$mu$m wavelength emission.

end{enumerate}}



label{fig:1.2}

end{figure}







end{document}