0 50000 1e+05 1.5e+05 2e+05 t 0 0.2 0.4 0.6 0.8 1 population FIG. 3: Population transfer in the three-level λ -system, achieved using an optical frequency comb as in Eq.(1) with the field parameters detuned off resonance with the frequencies of the λ -system. The detuning δ is equal to ω 31 / 2. The carrier frequency is ω$_{L}$ =5.4, the modulation frequency is Ω=4.4, the Rabi frequency is Ω$_{R}$=1, and the modulation amplitude is Φ 0 = 4. The λ -system transition frequencies are ω 21 =4.9, ω 32 =5.9 (in the unites of frequency ω 31 = 70 T Hz ). Full population transfer to the final, cold state, (green), occurs within 42 pulses. During this time, population of the initial Feshbach state, (black), reduces to zero and the excited state, (red), gets substantially populated during the transitional time. The population is reversed by the next 42 pulses. Fourier transform of the pulse train in Eq.(1). The Fourier transform reads Here, J$_{n}$ (Φ 0 ) is the Bessel function of the order n and Φ 0 is the modulation index. When multiplied by exp ( − 1 / 2( ω$_{L}$ + n Ω − ω ) $^{2}$τ $^{2}$), it determines the shape of the power spectrum of the optical frequency comb. Depending on the value of Φ 0 , the power spectrum has different number of maxima as it is seen in Fig.5 for Φ 0 = 3 , 4 , 5 , 8. The increase in modulation index brings additional, intense peaks of modes into spectrum and broadens it. These maxima are located at different frequencies for different values of Φ 0 affecting the population dynamics in the λ -system. The power spectrum of the pulse train with the modulation amplitude Φ 0 =4 has three maxima, the highest one is at ω = 4 . 9 (which is in resonance with the ω 21 ), making the pulse train with Φ 0 =4, an optimal one for coherent accumulation of the population in the final state of the λ -system. It provides full population transfer to the ground electronic and rovibrational state and, thus, cooling the internal degrees of freedom 7 E ( ω ) = ( E 0 τ ) / 2Σ$_{n}$ J$_{n}$ (Φ 0 ) exp ( − 1 / 2( ω$_{L}$ + n Ω − ω ) $^{2}$τ $^{2}$) · Σ$_{k}$ exp ( iωkT ) . (2)