|(e) SJ spectroscopy of small molecules:
i. Fluorescence Spectroscopy of N-methyl Pyrrole: N-methyl pyrrole is an interesting molecule from the internal rotor point of view. However, the spectroscopic studies of the internal rotor transitions in the first excited state were hampered so far because the first excited state is one photon forbidden 3s Rydberg state. Recently weak internal rotor transitions were reported that belonged to this forbidden state. Apparently the FC activity was induced due to the vibronic coupling with the nearby electronic states. With the aim of correctly assigning the absorption spectrum and understanding the appearance of the internal rotor transitions, the excitation and dispersed fluorescence (DF) spectra of NMP were recorded in the jet. The DF spectra provided a detailed look of the S1-S0 excitation inducing modes. A number of vibrational levels in the ground state were determined by means of the laser induced dispersed fluorescence spectra. The geometrical structure and the vibrational modes of NMP in the S0 electronic state were also calculated at the DFT and MP2 level using two basis sets, 6-311G** and D95++. The barrier for the methyl group rotation was calculated to be 45 cm-1.
ii. Spectroscopy of 1-Methyl-2-Pyridinimine: The heterocyclic compounds such as pyridine etc. have their first excited state (π,π*) mixed with the (n,π*) state. The relative ordering of the two states is very sensitive to the environments and the substituents leading to the complicated excitation spectrum. The S1-S0 excitation spectrum of one such compound, 1 methyl-2-pyridinimine, is quite interesting in the sense that it has two groups of low frequency transitions that are separated by ~1000 cm-1. The intensities of the individual transitions in each group monotonically increase with energy before the transitions end abruptly. Further, the lifetimes of the transitions in the two groups do not confirm to the observed trend i.e. shortening of the lifetimes with increased internal energy. The question being addressed was that whether two sets of transitions belong to two different electronic states or the spectrum is a manifestation of the FC activity in some mode within the same electronic state. The fluorescence spectra were recorded for several of the transitions observed in the excitation spectrum.
iii. Lifetime measurements of SO2 Clement bands: The spectroscopy of SO2 has been studied for over 70 years, but it has not been possible to get a proper understanding of the Clement’s bands, either in the context of the ro-vibronic assignments or their lifetimes. This is due to the fact that the Clement’s bands are a resultant of the mixing between the 1A2, 1B1 singlet states and to some extent also from the 3B1 excited state. A recent theoretical report predicts a conical intersection between the two singlet states at 3.86 eV above the ground state. Our interest in these bands was from the point of view of the Dissociative Electron Attachment from excited states of SO2. The actual extent of mixing of the states and their vibrational assignments are necessary to explain the fragmentation pattern and dynamics. We were interested, at least in part, in picking up the clues from the variation in the lifetimes of each of the transitions in these bands to see if they vary is any specific pattern that could be used to extract the coupling between various states. The jet cooled excitation spectrum was obtained in the region below the A band, i.e., 298 to 330 nm. A number of vibronic transitions were observed that do not show any specific pattern. Lifetime measurements were carried out on 17 peaks of which several peaks exhibit a double exponential decay; almost all peaks have at least one lifetime around 3 µs and the second lifetime is around 1 µs. Four vibronic transitions in the wavelength range 314 to 318 nm were picked for doing the lifetime measurements at higher resolution. Detailed analysis was concentrated on the 315.3 nm band as detailed rotationally resolved experimental data is available. Interestingly some of the transitions in this band exhibited quantum beats with 1 µs frequency. Though the beats have been reported in past in SO2 for some C(1B2)->X(1A1) transitions and a(3B1)-> X(1A1) transitions under the influence of a magnetic field, this is the first time that quantum beats are being observed in the Clement’s band. The magnitude of the frequency of the quantum beats observed in absence of any magnetic field suggests that there is a strong mixing between the rotational levels of the neighboring 1A2 and the 1B1 vibronic states. However, at this stage it is not possible to rule out coupling either with the triplet state or the ground electronic state. Attempt was made using the AsyrotWin program to fit the experimental data with ascertain the type of the excited state level for this transition.
iv. Ab intio Calculations and Fluorescence Spectroscopy of a Neurotransmitter: Serotonin is an important neurotransmitter that plays an important role in controlling various activities in human beings. Spectroscopic studies on this all important molecule are important from the point of view of various in vitro/vivo applications such as imaging, monitoring its concentrations, etc. using fluorescence spectroscopy. In solution phase the emission spectra have shown considerable red shift suggesting formation of oligomers. We have undertaken theoretical as well experimental studies on this molecule. Ab initio and DFT studies of serotonin showed that there are twenty-three stable conformers present in the gas phase. From the 23 conformers 15 stable structures of dimers were obtained. All the calculations were done at the HF and DFT level using basis sets 6-31+G* with Gaussian 98 program. Though some theoretical work has been done on conformational landscape of serotonin, there is no experimental evidence in the gas phase. We have recorded the fluorescence excitation spectra of jet-cooled Serotonin to get the information about the number of conformers present in the gas phase and also the dimer formation at higher concentration.