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P. Panwaria and A. Das
higher-order clusters as well as the indole…pyridine dimer and (indole) 2 …pyridine
trimer. Water complexes of indole as well as indole…pyridine are also observed in
the experiment.
Figure 4B displays electronic spectra of indole, indole…pyridine, and
(indole) 2 …pyridine measured in their respective mass channels using the 1C-R2PI
technique. The electronic spectrum of the indole…pyridine dimer shows two sets
of band with one group (A) appearing near the origin band of the indole monomer
while the other group (B) is quite away from the indole origin band. Interestingly, the
electronic spectrum measured in the (indole) 2 …pyridine trimer mass channel exactly
reproduces the bands marked by the species A observed in the indole…pyridine dimer
channel. Thus, the bands designated as the species A in the indole…pyridine dimer
spectrum (Fig. 4Bb) originate due to the S 1 ← S 0 transition of the (indole) 2 …pyridine
trimer, which fragments after the 2-photon ionization and appears in the mass channel
of the indole…pyridine dimer. The IR spectra measured by probing one of the electronic bands of each of the species A and B are distinctly different (Fig. 5A). A
Fig. 5 A IR spectra measured by probing the (a) 0 0
0 band of indole, (b) B 0
0 +30 cm −1 band of
indole…pyridine dimer, and (c) A 0
0 +27 cm −1 band of (indole) 2 …pyridine trimer in the N–H
stretching frequency region; B IR-UV hole-burning spectra recorded in the indole…pyridine dimer
mass channel by probing the vibrational bands at (a) 3411 cm −1 and (b) 3269 cm −1 . (c) R2PI
spectrum measured in the indole…pyridine dimer mass channel. Adapted with permission from
Ref. [115], copyright 2011, American Chemical Society