A simple new technique for separating sequence band lines in a sealed-off longitudinal d .c . discharge CO 2 laser is described . Non-Littrow mounting of the grating allows lasing to be obtained on sequence bands without use being made of an in-cavity hot cell . As an example, the oscillation spectra on regular and sequence bands of two isotopic species are presented .The sequence bands (00°2-[10°1, 02°1] 1 , 11 transitions) make it possible to increase the number of CO 2 laser lines, and are of great interest in solving various problems of atmospheric monitoring, optical pumping of FIR lasers [1,2], and laser media diagnostics [3]. Owing to the anharmonicity of the CO 2 molecule, the sequence line frequencies lie between the frequencies of the regular transitions . The sequence line gains are usually -30 to 40 per cent of those for the lines of the regular band . Therefore, sequence transitions do not appear in the radiation of CO 2 lasers that use only an autocollimation grating for discriminating the lines of regular transitions [4] .In order to obtain oscillation on the sequence band, the method of suppressing lasing on the regular band 00°1-[10°0, 02°0] 1 , 11 by inserting into the cavity a cell with heated (-350'Q CO 2 is usually employed [5,6] . The use of heated CO 2 for suppressing all transition of the regular band is very efficient in obtaining a large number of sequence-band lines . However, this technique has two drawbacks . Firstly, the fact that CO 2 is a weak 10µm absorber requires that the in-cavity cell length be approximately one-third of the discharge length . This leads to an increase of the cavity length, which is inconvenient for many applications . Secondly, cell windows introduce an additional loss into the cavity . To reduce the cavity length, Reid and Siemsen [4] used a short in-cavity cell containing a stronger absorber (NH 3) for suppressing oscillation on the regular lines . By employing this technique they obtained lasing on five sequence lines .In the present paper the possibilities are considered of CO 2 laser tuning on the sequence-band lines by using non-Littrow (non-autocollimation) mounting of the grating .With non-autocollimation mounting of the grating, radiation is doubly diffracted by it during one round-trip, which leads to the fact that the angular dispersion of this type of cavity is equal to three times the dispersion of the grating .A higher spectral resolution may be achieved by using so-called `grazing incidence' on the grating . Grazing incidence geometry is successfully employed in high-gain systems such as dye lasers [7] and high-pressure TE CO 2 lasers [8] for line-narrowing oscillation . However with such geometry an increase in the angle of incidence leads to a sharp decrease in the grating efficiency [8]. The gain on the sequence band of CO 2 in a sealed-off longitudinal discharge laser is typically
