A time-resovled ultrasensitive detection technique of probing cooperative vibronic spectra of rare earth ions is described. Coordination of terbium ion with arginine in low concentration solutions is studied and the coordination number variation of water molecules with pH value in these solutions is determined quantitatively with this technique. It is concluded that arginine can coordinate with terbium ion when pH>1, with the pH value of the solutions increasing, the coordination number of arginine increases.