Laser induced fluorescence experiments indicate change of spectral line profiles of atoms and molecules compared to the free system when embedded under liquid helium. Based on relativistic density functional approach a model has been proposed here to estimate the width of the spectral lines embedded under liquid helium in which the central atom resides within a cluster of helium atoms. The model assumes an overall floppy motion of the liquid helium atoms due to finite temperature and subsequent determination of the width from the ab initio estimated potential energy diagram. Calculated width of the resonance lines of Rb and Cs show good agreement with available laser spectroscopic data.