Alkali metal atom keeps receiving attention on basic physics study and applications. Studying of ultracold alkali metal spectroscopy has greatly push forward the progress of quantum mechanisms. Alkali metal atoms are used as most precise time base e.g. rubidium atomic clock, and frequency standard source [1]. The ultrafine split of alkali metal atom levels is sensitive to electric field and magnetic field, known as Stark and Zeeman effect, thus they are used as field meters [[2], [3], [4]]. Alkali metal atom serves as light source, e.g. diode pumped alkali laser [5], stimulated electron Raman scattering infrared source [6] and continuous wave FWM blue light source [[7], [8], [9], [10]].

Alkali-rare gas excimer can bring out spectral broadband transitions, and they are widely studied model of diatom molecular spectroscopy [11,12]. Recently, such excimer is used as laser medium, which is well known excimer pumped alkali laser. Many lasing wavelengths has been realized. E.g. Na–Xe 589 nm [13], Cs–Ar 852 nm [14], Cs–Ar/Kr/Xe 852 nm [15], Cs–Kr 894 nm [16].

There is some successful trial of using alkali metal atoms as electromagnetic wave sensors, e.g. microwave sensors [[17], [18], [19], [20], [21]], frequency up-conversion terahertz sensors [22], FWM infrared sensors [23]. These systems work on some certain atom lines, showing narrowband characteristics.

In this work, Cs–Xe excimer is used to study broadband fluorescence frequency upconversion since it has strong inter-atom interaction [24,25]. As a demonstration, by excimer transition, 550–552 nm excitation light is converted to 541 nm fluorescence, which is broadband in comparison with Cs atom situation. The conversion show linear characteristics, when the excitation energy decrease, the broadband converting property keeps itself. Lower Cs–Xe gas temperature can greatly reduce the energy-pooling base thus increase the sensitivity.