On the 45th Anniversary of the Samara Branch of the Lebedev Physical Institute of the Russian Academy of Sciences

Simulating the laser cooling of CaO⁺ molecular ions

S. O. Tuchin^a, A. A. Pershin^a, I. O. Antonov<sup>ab

a</sup> Samara Branch of P. N. Lebedev Physical Institute, Russian Academy of Sciences
^b Samara National Research University

Abstract: A kinetic model has been developed for the optical cooling of the rotational levels of the CaO⁺ (X²Π, υ = 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, υ = 8) by tunable broadband laser radiation with a frequency cutoff option and overlapping the bandwidth of spectral transition, (B²Π, υ″ = 8) ← (X²Π, υ = 0), with the ground state rotational quantum number values, J > J_m; (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, υ = 0, J = J_m. 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⁺, υ′ = 1) → (B²Π, υ″ = 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 < J_m. At a spectral cutoff of 33256 cm^-1 (J_m = 0.5), the population of the X²Π state (υ = 0, J = 3.5) is eight times higher than the thermal state (T = 300 K).

Keywords: molecular ions, laser cooling, electron excited states, kinetic model, CaO⁺.

Received: 16.05.2025
Revised: 11.07.2025
Accepted: 19.07.2025

 English version: Quantum Electronics, 2025, 52:suppl. 6, S672–S678