Verma, R.S., and G.K. Joshi. “Electronic Spectral Parameters of the Saturated Solutions of Some Amino Acids and Some Bioactive N-Heterocyclic Compounds Doped With Pr(III) Ion”. Asian Journal of Chemistry, vol. 13, no. 2, Mar. 2010, pp. 704-8, https://asianpubs.org/index.php/ajchem/article/view/22653.