$(k, a)$-generalized Fourier transform with negative $a$

The $(k, a)$-generalized Fourier transform $\mathscr{F}_{k, a}$ introduced by Ben Sa\"id--Kobayashi--{\O}rsted is a deformation family of the classical Fourier transform with a Dunkl parameter $k$ and a parameter $a > 0$ that interpolates minimal representations of two different simple Lie groups. In the present paper, we focus on the case $a < 0$. As a main result, we find a unitary transform that intertwines the known case $a > 0$ and the new case $a < 0$.