OCR Text |
Show nm is the S1-S0 origin. Under cold molecular beam conditions the vibronic bands of the two isomers can he dl ~ tangui s hcd readily: this would he nearly imposs ib1c at room temperature. The large shift in the S I So origins between 1.2 and 1.4 OCB (275.92 and 279.67 nm. respectively) arises from the dependence of excited state electronic structure on the position of the CI substituents on the benzene ring. The S 1 state is more stabilized and hence lower in energy for the 1,4 isomer. The parent ion has an electronic structure quite similar to the S 1 excited state and thus the I.P. of the 1,4 isomer is also lower than that of 1.2 OCB. Fig. 6 also makes a point concerning the relative sensitivity of REMPI to 1,2 and 1.4 OCB. Based on the listed purity of the 1.2 OCB (98.5%) the maximum 1.4 OCB impurity is 1.5%. Assuming that the vapor pressures are approximately equal at room temperature the fact that the strongest 1.2 and 1,4 DCB bands in Fig. 6 are roughly equal implies that the REMPI technique is very much more sensitive toward the 1,4 isomer. Such a difference in sensitivity can be caused by dramatically different transition probabilities in either the SI-SO absorption (AI) or the ionization step (A2). We have also performed both jet-cooled LIF and REMPI spectroscopy on a series of mono- and dichloronaphthalenes. We were successful in obtaining an LIF spectrum for 2-chloronaphthalene (2 CN) in the presence of some naphthalene impurity. As in the jet-cooled REMPI spectra of the DCB's this spectrum exhibits sharp vibronic bands and it is trivial to distinguish 2 CN from naphthalene. Attempts were made to take LIF spectra of the dichloronaphthalenes (DCN's), but they were unsuccessful. In naphthalene the SI excited state has a fluorescence lifetime ('tf) of -300 ns and a quantum yield of fluorescence (<I»f) of -0.24,12 however in chloronaphthalene 'tf drops to -30 ns and <l>f decreases to -0.007.13 The cause of this dramatic decrease in lifetime and quantum yield is an increased rate of intersystem crossing to the triplet state as discussed in the Sec. I. The rate of intersystem crossing increases by -100 in chloronaphthalene relative to naphthalene.14 If we extrapolate this trend to the DCN's it is clear why the LIF experiments failed. The quantum yield is simply too small and the lifetime too short (this makes it difficult to distinguish the fluorescence from scattered laser light using boxcar gating techniques). The REMPI technique can often be used in cases where LIF fails because the ionization rate out of S I can be made competitive with the rate of intersystem crossing to the triplet manifold. This is especially true in two-color REMPI where the ionizing laser intensity, and hence the ionization rate. can be made very large. Indeed we have been successful in obtaining jet-cooled REMPI spectra for a whole series of DCN's. Fig. 7 shows an example of such a spectrum taken for a 1,2/1.4/1,8 DCN mixture. The labelled bands are the S I-SO origins and it is the origin region of the spectrum that provides the highest isomeric selectivity because it is the least spectrally congested. The DCN's exhibit more complicated REMPI spectra than do the DCB's because the fonner have a greater number of vibrational modes and exhibit a denser vibronic structure. Note also that the S I-SO spectra for the DCN's are at much longer wavelength than those of the OCB's, a general trend that is caused by the lowering in excited-state energy upon increase in the size of the 1t bonding framework. As Fig. 7 demonstrates the different DCN isomers in this mixture 12 |