  Exciton dissociation plays a crucial role in the performance of
optoelectronic devices based on two-dimensional (2D) transition metal
dichalcogenides (TMDs). In this work, we investigate the effect of an in-plane
electric field on the exciton resonance states in MX$_2$ (M = Mo, W; X = S, Se)
monolayers and few-layers using the complex coordinate rotation method and the
Lagrange-Laguerre polynomial expansion of the wave function. The exciton
properties are well described within the Mott-Wannier model incorporating the
nonlocal Keldysh potential. Our calculations reveal that an electric field
effectively dissociates excitons, with excited states being more easily
dissociated than the ground state. The critical field for exciton dissociation
is found to be smaller in WX$_2$ monolayers compared to MoX$_2$ monolayers due
to the smaller exciton reduced mass. Furthermore, the presence of a dielectric
substrate and an increase in the number of MX$_2$ layers enhance the exciton
susceptibility to the electric field, lowering the critical field for
dissociation. The dependence of exciton properties on the number of MX$_2$
layers can be well described by power functions. These findings provide
valuable insights for the design and optimization of high-performance
optoelectronic devices based on 2D TMDs.
