Condensed phase vs. Isolated gas phase spectra Solution phase A A A A A A W W W W W WW W W W W W W W W W W W: water A: sample ( nm) ( nm) Isolated gas phase (Supersonic jet) A A A A A A A S0S0 S1S

Supersonic expansion technique S0S0 S1S1 V=0 Dihedral angles (i) Conformational isomers Schematic potential energy diagram (ii) Non-covalently bonded complexes

LIF (Laser Induced Fluorescence) S 0,v=0  S1S1 monitor total fluorescence Fluorescence excitation spectroscopy

Can we perform spectroscopy in a mass spectrometer? Yes, we can perform high resolution electronic and vibrational spectroscopy in a mass spectrometer. How? Laser Desorption and REMPI (Resonantly Enhanced Multiphoton Ionization) Time of Flight Mass Spectrometry Supersonic jet expansion Ionization sourceMass analyzer Formation of non-covalently bonded clusters and high resolution spectroscopy ++ Jet-cooled Laser Desorption Time of Flight Mass spectrometer

Home-built Jet-cooled Laser Desorption REMPI (Resonantly Enhanced Multiphoton Ionization)Time OF Flight Mass Spectrometer Biomolecules (peptides) Low vapor pressure Decompose upon heating Vapor? Reduce the time-scale for heating dT/dt = K/sec 1000 K temp jump in a 10 ns laser pulse Desorption without fragmentation To bring the molecules in vapor phase (i)Thermal heating (ii)Laser desorption Computation: M05-2X, M06-2X, DFT-D level of calculation using Gaussian 09, Gamess programs Experimental Set up and computational methods

Spectroscopic techniques 1C-R2PI (one color Resonant two photon ionization) Mass selected Electronic spectrum RIDIRS (Resonant ion dip Infrared spectroscopy) IR spectrum S0S0 v = 0 S1S1 D0D0 h 2 (UV) tuned h 2 (UV) h 1 (IR) fixed 100 ns IR-UV hole-burning spectroscopy Discriminate different conformers UV-UV hole-burning spectroscopy

Time of Flight mass spectrum of complexes of indole and pyridine

Electronic spectra of complexes of indole and pyridine S0S0 S1S cm -1 S0S0 S1S1 IndolePyridine cm -1 S0S0 S1S cm -1 S0S0 S1S1 Indole…pyridine dimer (Indole) 2 …pyridine trimer cm -1 (35240) D0D0 D0D0 D0D0 D0D0 h (34969) (35164) One-color R2PI spectra

IR spectra of species A and B RIDIRS

Single conformer or multiple conformers of dimer and trimer? Are all these peaks due to transitions of one or multiple conformers of the trimer? Are all these peaks due to transitions of one or multiple conformers of the dimer?

IR-UV holeburning spectra of A and B UV spectrum (IR off) UV spectrum (IR fixed at 3411 cm -1 band of the trimer) UV spectrum (IR fixed at 3281 cm -1 band of the trimer) UV spectrum IR fixed at 3269 cm -1 band of the dimer Conclusion: One conformer of the dimer and one conformer of the trimer are observed.

Determination of the structures of the observed dimer and trimer Comparison of experimental and theoretical IR spectra (ind) 2.py-1 Indole