Abstract
A kinetic model has been developed for the optical cooling of the rotational levels of the CaO+ (X2Π, \({v}\) = 0) molecular ion ground state. This was achieved by placing the molecular ion in an ion trap and subjecting it to laser and thermal radiation. The model includes (1) excitation of the electron level (B2Π, \({v}\) = 8) by tunable broadband laser radiation with a frequency cutoff option and overlapping the bandwidth of spectral transition, (B2Π, \({v}{{'' }}\) = 8) ← (X2Π, \({v}\) = 0), with the ground state rotational quantum number values, J > Jm; (2) interaction with the thermal radiation from the medium; and (3) spontaneous radiative relaxation of excited states. The laser cutoff frequency coincided with the frequency of transition from the state of (X2Π, \({v}\) = 0, J = Jm. The simulation includes 50 rotational levels for three electron states of X2Π, A2Σ+, and B2Π, 42 vibrational levels for X2Π and A2Σ+, and 9 vibrational levels for B2Π. The levels of the lower electron excited state were dispersed by inducing the radiation of the second laser at the transition (A2Σ+, \({v}{{'}}\) = 1) → (В2Π, \({v}{{'' }}\) = 8). The rates of radiation transitions between states were determined based on calculated values of Einstein coefficients and flux densities of laser and thermal radiation. It is shown that the rotational level populations are depleted by laser radiation in tens of milliseconds, simultaneously with a significant accumulation of the population of “dark” levels with J < Jm. At a spectral cutoff of 33256 cm–1 (Jm = 0.5), the population of the X2Π state ( \({v}\) = 0, J = 3.5) is eight times higher than the thermal state (T = 300 K).
