Transition metal dichalcogenides exhibit remarkable optical properties due to the diverse number of strongly bound excitons, which can be fine-tuned by alloying. Despite a flurry of research activity in characterizing these excitons, a comprehensive and profound understanding of their behavior with temperature is lacking. Here, we report the rich spectrum of excitonic features within bulk van der Waals alloy ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_2$ and ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{Se}}_2$ single crystals through temperature-dependent reflectance spectroscopy and first-principles calculations. We observed Rydberg excitons and interlayer excitons in both the single crystals. Notably, we provide the first experimental evidence of highly energetic ${\mathrm{A}}^{\prime }$ and ${\mathrm{B}}^{\prime }$ excitons in ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_2$ at room temperature. The strong carrier-phonon scattering significantly broadens the ${\mathrm{A}}^{\prime }, {\mathrm{B}}^{\prime }$, and interlayer excitons at room temperature in bulk ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_2$ single crystal compared to its selenide. Our findings, supported by density functional theory and Bethe-Salpeter equation calculations, signify the crucial role of carrier-phonon interactions. These results open pathways for next-generation optoelectronic devices and quantum technologies operating at high temperature.

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• Excitons
• Single crystal materials
• Optical absorption spectroscopy