Fig. 1. Pressure control system for multi-pass gas cell

Fig. 2. Proportional valve control circuit

Fig. 3. Equal-amplitude oscillation of pressure of multi pass cell

Fig. 4. Pressure control system

Fig. 5. Pressure control curve

Fig. 6. Pressure control data (60 Torr)

Fig. 7. Relation between ¹³CO₂ absorption spectrum and pressure

Fig. 8. Relation between ¹²CO₂ absorption spectrum and pressure

Fig. 9. Absorption lines of ¹³CO₂ and ¹²CO₂ at pressure of 0.26 atm

Fig. 10. CO₂ isotope abundance measurement data