Laser-induced fluorescence based characterisation method for aggregation behaviour of rhodamine B (RhB) in water, ethanol, and propanol Seikh Mustafa Radiul a , Jugal Chowdhury b , Angana Goswami c , Simanta Hazarika a,* a Department of Physics, Gauhati University, Assam, India b Institute for Plasma Research, Gandhinagar, Gujarat, India c Department of Physics, Pandit Deendayal Upadhyaya Adarsha Mahavidyalaya, Dalgaon, Assam, India *Corresponding Author Email Id: simantahazarikagu@gmail.com #ORCID iD: https://orcid.org/0000-0003-1012-3066 Abstract: The aggregation behaviour of rhodamine B (RhB) dye has been studied with the change in the concentration of the RhB in water, ethanol and propanol using absorption and laser induced fluorescence spectroscopy. The dimer and monomer fluorescence emissions were observed simultaneously in all the solvents. The monomer to dimer fluorescence intensity ratio (     ), where   = Monomer fluorescence intensity,   = Dimer fluorescence intensity, has been calculated and found that at a certain concentration of RhB the magnitude of the ratio is different in different solvents. The ratio (     ) becomes equivalent to one for 1.5 gm/l, 3 gm/l and 4.2 gm/l, (gm/l: gram/litre) concentration of RhB in water, ethanol and propanol respectively. This concentration of RhB at which the ratio (     ) become unity is proposed as “critical concentration”. Above this critical concentration the dimeric fluorescence dominates and below which the monomeric fluorescence dominates. The parameter “critical concentration” provides a condition of equilibrium between the monomer and their aggregates. Moreover, in the solvent for which the magnitude of critical concentration is less the fluorophore molecule aggregates more easily and vice versa. The ratio (     ) and critical concentration quantify the aggregation process of the fluorophore in the solvent. Hence, the “critical concentration” value of the fluorophore (RhB) could be assumed as the characteristic parameter to study the aggregation behaviour of the fluorophore in solvents. The described method has the merit of use in the study of lasing ability and the wavelength tunability of the dye laser gain media as well as in the heavy metal sensing technology in water. This method may also be extended for other fluorophores and solvents.