High frequency magnetic oscillations of the organic metal $\theta$-(ET)${}_4$ZnBr${}_4$(C${}_6$H${}_4$Cl${}_2$) in pulsed magnetic field of up to 81 T

De Haas-van Alphen oscillations of the organic metal $\theta$-(ET)${}_4$ZnBr${}_4$(C${}_6$H${}_4$Cl${}_2$) are studied in pulsed magnetic fields up to 81 T. The long decay time of the pulse allows determining reliable field-dependent amplitudes of Fourier components with frequencies up to several kiloteslas. The Fourier spectrum is in agreement with the model of a linear chain of coupled orbits. In this model, all the observed frequencies are linear combinations of the frequency linked to the basic orbit $\alpha$ and to the magnetic-breakdown orbit $\beta$.