  It is showed on the basis of the multiple-quantum operator algebra space
formalism that ultra-broadband heteronuclear Hartmann-Hahn polarization
transfer could be achieved by the amplitude- and frequency-modulation
quasi-adiabatic excitation (90 degree) pulses, while it is usually difficult
for the adiabatic inversion pulses to achieve effectively broadband
Hartmann-Hahn transfer in a heteronuclear coupled two-spin system. The
adiabatic and quasi-adiabatic pulses have an important property that within
their activation bandwidth flip angle of the pulses is independent of the pulse
duration and the bandwidth increases as the pulse duration. This property is
importanr for construction of the heteronuclear Hartmann-Hahn transfer
sequences with the quasi-adiabatic 90 degree pulses. Theoretic analysis and
numerical simulation show that the heteronuclear Hartmann-Hahn transfer is
performed in the even-order multiple- quantum operator algebra subspace of the
two-spin system. The multiple-quantum operator algebra space formalism may give
a powerful guide to the construction of ultra- broadband heteronuclear
Hartmann-Hahn transfer sequences with the quasi-adiabatic 90 degree pulses.
