Possibility of resonant three-photon isotope-selective excitation of vibrational states with v = 3 of the UF$_6$ molecule mode $\nu_3$ by radiation from two pulsed IR lasers in a gas-dynamically cooled molecular flow

We consider the possibility of resonant isotope-selective population of states with v = 3 of the vibrational mode $\nu_3$ of UF$_6$ molecules as a result of three-photon bichromatic excitation of the molecules by IR radiation from two pulsed CF$_4$ lasers and two pulsed para-H$_2$ lasers. We propose specific schemes and analyse the possibility of exciting the $^{238}$UF$_6$ and $^{235}$UF$_6$ molecules into states with v = 3 of the vibrational mode $\nu_3$ using two CF$_4$ lasers (or para-H$_2$ lasers) generating in the region of 16 $\mu$m at frequencies that are significantly (by $\sim$3.5 – 17 cm$^{-1}$) detuned from the linear absorption bands at the frequencies of $\nu_3$ vibration of these molecules in a gas-dynamically cooled molecular flow. However, the sum of the doubled lasing frequency 2$\nu_{L1}$ of the first laser and the frequency $\nu_{L2}$ of the second laser is equal to a frequency of the transition v = 0 $\to$ v = 3 of the vibrational mode $\nu_3$ of the UF$_6$ molecules. When both laser pulses coincide in time, the possibility of selective excitation of UF$_6$ molecules from the ground vibrational state with v = 0 to the state with v = 3 of the vibrational mode $\nu_3$ is realised. The proposed schemes for the excitation of UF$_6$ molecules to states with v = 3 are compared with the previously implemented schemes for the effective excitation of states with v = 3 of the vibrational mode $\nu_3$(F$_1$) and states with v = 2 of the vibrational mode $\nu_3$ (A$_1$) of SF$_6$ molecules by three- and two-frequency radiation of pulsed CO$_2$ lasers, respectively.