  We theoretically analyze a state-of-the-art experimental method based on a
combination of electron spin resonance and scanning tunneling microscopy
(ESR-STM), to directly probe the spin fluctuations in the Kondo effect. The
Kondo impurity is exchange coupled to the probe spin, and the ESR-STM setup
detects the small level shifts in the probe spin induced by the spin
fluctuations of the Kondo impurity. We use the open quantum system approach by
regarding the probe spin as the "system" and the Kondo impurity spin as the
fluctuating "bath" to evaluate the resonance line shifts in terms of the
dynamic spin susceptibility of the Kondo impurity. We consider various common
adatoms on surfaces as possible probe spins and estimate the corresponding
level shifts. It is found that the sensitivity is most pronounced for the probe
spins with transverse magnetic anisotropy.
