  Bound quasiparticles, negatively charged trions and neutral excitons, are
associated with the direct optical transitions at the K-points of the Brillouin
zone for monolayer MoS$_2$. The change in the carrier concentration,
surrounding dielectric constant and defect concentration can modulate the
photoluminescence and Raman spectra. Here we show that exposing the monolayer
MoS$_2$ in air to a modest laser intensity for a brief period of time enhances
simultaneously the photoluminescence (PL) intensity associated with both trions
and excitons, together with $\sim$ 3 to 5 times increase of the Raman intensity
of first and second order modes. The simultaneous increase of PL from trions
and excitons cannot be understood based only on known-scenario of depletion of
electron concentration in MoS$_2$ by adsorption of O$_2$ and H$_2$O molecules.
This is explained by laser induced healing of defect states resulting in
reduction of non-radiative Auger processes. This laser healing is corroborated
by an observed increase of intensity of both the first order and second order
2LA(M) Raman modes by a factor of $\sim$ 3 to 5. The A$_{1g}$ mode hardens by
$\sim$ 1.4 cm$^{-1}$ whereas the E$^1_{2g}$ mode softens by $\sim$ 1 cm$^{-1}$.
The second order 2LA(M) Raman mode at $\sim$ 440 cm$^{-1}$ shows an increase in
wavenumber by $\sim$ 8 cm$^{-1}$ with laser exposure. These changes are a
combined effect of change in electron concentrations and oxygen-induced lattice
displacements.
