Ultrafast electronic relaxation in the S1 state of azulene and some of its derivatives

✍ Scribed by P. Wirth; S. Schneider; F. Dörr

• Publisher: Elsevier Science
• Year: 1976
• Tongue: English
• Weight: 472 KB
• Volume: 42
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(76)80658-6

✦ Synopsis

A mode-locked dye laser is used to generate the S+J TPF pattern of azulene and some of its derivatives. Since none of the patterns shows a Iarger width at half maximum than the one obtained with dimethyl-POPOP, a lifetime of less than 1 ps has to be postulated for 'the excited vlbronic levels Sr,, of these compounds.

1. introduction

Among all molecules, azulene seems to be the most preferable one for studies with picosecond laser pulses. Firstly, due to its low energy gap E(S1--So), internal conversion seems to play the dominant role in the energy relaxation of the first excited singlet state [l] placing its lifetime in the region of picosecends. Furthermore, azuIene exhibits, due to the unusual large energy gap E(S2-S,), fluorescence from the second excited state. Since the quantum yield for this S2 --f So fluorescence is decently high, the transient 5, -+ S, absorption can be measured in a convenient way by monitoring the emission of the final state S2 [2,3]. Previous measurements of the Iifctime of azulene in the S, state have given values-of 7.5 ps when excited into the u = 4 level [2] and 8 ps* when excited into the vibrational ground state [3] . Unfortunately, no attention was paid in neither experiment to the contrast ratio of peak to background when the twophoton fluorescence pattern (TPF pattern) was used to determine the pulse length. Bradley et al. [4] , however, have shown by model calculations that the halfwidth of the TPF pattern can be used to determine the pulse length only in those cases, where the contrast ratio is at Ieast 2.0. The aim of this investigation * This value is corrected by a factor of 2 as discussed in section 2.