Coherent states and related quantizations for unbounded motions
  We build coherent states (CS) for unbounded motions along two different procedures. In the first one we adapt the Malkin-Manko construction for quadratic Hamiltonians to the motion of a particle in a linear potential. A generalization to arbitrary potentials is discussed. The second one extends to continuous spectrum previous constructions of action-angle coherent states in view of a consistent energy quantization. 
V. G. Bagrov, J.-P. Gazeau, D. M. Gitman and A. D. Levin
quant-ph