  Time-resolved optical emission spectroscopic measurements of a plasma
generated by irradiating a Cr target using 60 picosecond (ps) and 300 ps laser
pulses is carried out to investigate the variation in the linewidth
($\delta\lambda$) of emission from neutrals and ions for increasing ambient
pressures. Measurements ranging from 10$^{-6}$ Torr to 10$^2$ Torr show a
distinctly different variation in the $\delta\lambda$ of neutrals (Cr I)
compared to that of singly ionized Cr (Cr II), for both irradiations.
$\delta\lambda$ increases monotonously with pressure for Cr II, but an
oscillation is evident at intermediate pressures for Cr I. This oscillation
does not depend on the laser pulse widths used. In spite of the differences in
the plasma formation mechanisms, it is experimentally found that there is an
optimum intermediate background pressure for which $\delta\lambda$ of neutrals
drops to a minimum. Importantly, these results underline the fact that for
intermediate pressures, the usual practice of calculating the plasma number
density from the $\delta\lambda$ of neutrals needs to be judiciously done, to
avoid reaching inaccurate conclusions.
