Absolute determination of the frequency in an He$-$Ar laser $(\lambda = 2{.}39\mu)$

A determination was made of the frequency of an He$-$Ar laser emitting the $\lambda = 2{.}39\,\mu$ line as a result of the $3d \bigl[{1\atop2}\bigr]^0_0\to4p'\bigl[{1\atop2}\bigr]_1$ $(3d_6\to 2p_2)$ transition in Ar I. The measured frequency was synthesized by a metal$-$oxide$-$metal diode from the fourth harmonic of the frequency of a CO$_2$ laser corresponding to the $P$(24) emission line at $\lambda= 9{.}6\,\mu$ and the frequency of a microwave oscillator emitting in the range of 39 GHz. The synthesized frequency was $\nu_{{\mathrm{He-Ar}}(2{.}39\,\mu)} = 125\,053\,923 \pm 10$ MHz.