We present an exact Liouvillian spectral reconstruction for a finite-temperature bosonic mode, including eigenvalues, physical eigenoperators, and expansion coefficients for arbitrary initial conditions. The eigenvalues are independent of the thermal occupation, whereas the physical eigenoperators retain the reservoir-temperature dependence, with the thermal stationary state corresponding to the zero Liouvillian mode. Photon-number statistics and two-time correlations connect the spectral structure with relaxation, fluctuations, and temporal coherence by revealing the Liouvillian sectors selected by different observables. The zero-temperature dynamics are recovered in the limit n¯→0.
