Transient excited singlet state absorption in the laser dye α-NPO

Abstract

Transient excited singlet state absorption spectrum of the laser dye, α-NPO (8 × 10-5 M/liter in toluene) was recorded in the 4500-6200 Å region with an optical delay of ~ 3 ns between the pump and the probe beams. Nine maxima and three shoulders are observed for the first time in the spectrum. A shoulder at 21090 cm-1 and three maxima at 20,530, 18,685, and 17,980 cm-1 are tentatively attributed to transitions from the vibrational levels at 26,740, 27,350, 29,170, and 29,885 cm-1 belonging to the first excited singlet electronic state S1, respectively, to a common upper level at 47,855 ± 25 cm-1 while three other maxima at 20,035, 19,525, and 17,570 cm-1 and another shoulder at 16,945 cm-1 are attributed, respectively, to transitions from the same four vibrational levels of S1 to another common upper level at 46,805 ± 70 cm-1. Transitions from the different vibrational levels of S1 to three different upper vibrational levels at 47,645 ± 30, 43,130, and 48,760 cm-1 could similarly be assigned to the remaining three maxima at 20,320, 18,445, and 16,390 cm-1 and the shoulder at 22,020 cm-1. Absorption from the higher vibrational levels of the excited state S1 may be understood on the basis of the fact that the pump and probe beams, each of ~ 8-ns pulse width, interact simultaneously with the molecules for ~ 5 ns.