Fig. 4. Vibronic states and theoretical FIRMS intensity for toy model.

a States of toy model; note that the vibrational energy $\hslash \omega$ is a variable, and all values are considered in (b). Purely electronic transitions are shown in blue, purely vibrational transitions in green, cold vibronic inter-KD transitions in black, vibrationally hot vibronic transitions in red, and cold vibronic intra-KD transitions in dashed black; EPR transitions (not studied in this work) in orange. b Absorption intensity for cold intra-KD vibronic transitions (black dashed lines), cold inter-KD vibronic transitions (solid black lines), hot inter-KD vibronic transitions (red solid lines), and purely electronic (solid blue lines, obscured by the red ones) transitions, under irradiation from an IR source with uniform intensity and uniform vibronic coupling. Purely vibrational transitions are not shown. Constructed with model parameters $F$ = $G$ = 1 cm⁻¹, $A_{\mathrm{v}}$ = 10², $A_{\mathrm{e}}$ = 1, $\mathrm{\Delta }$ = 474 cm⁻¹ and $\delta$ = 2 cm⁻¹ (field ca. 2 T).