Regge, T.; Introduction to Complex Orbital Momenta
Nuovo Cim. 14 (1959) 951;

Summary
In this paper the orbital momentum j, until now considered as an integer discrete parameter in the radial Schrödinger wave equations, is allowed to take complex values. The purpose of such an enlargement is not purely academic but opens new possibilities in discussing the connection between potentials and scattering amplitudes. In particular it is shown that under reasonable assumptions, fulfilled by most field theoretical potentials, the scattering amplitude at some fixed energy determines
the potential uniquely, when it exists. Moreover for special classes of potentials V(x), which are analytically continuable into a function V(z), z = x+iy, regular and suitable bounded in x > 0, the scattering amplitude has the remarkable property of being continuable for arbitrary negative and large cosine of the scattering angle and therefore for arbitrary large real and positive transmitted momentum. The range of validity of the dispersion relations is therefore much enlarged.

Related references See also E. P. Wigner and J. Neumann, Annals of Math. 59 (1954) 418;
M. N. Khuri, Phys. Rev. 107 (1957) 1148;
S. Gasiorowicz and H. P. Noyes, Nuovo Cim. 10 (1958) 78;
R. Jost and W. Kohn, Math. Phys. Medd. 27(9) (1953);
L. D. Faddeev, Soviet Physics Doklady 3 (1959) 747;
I. M. Gel'fand and Levitan, Amer. Math. Soc. Trans., Sec.2.1.250 (1955);