Laser Induced Fluorescence

1. Laser Induced Fluorescence Structural information about the ground and excited states of molecules. Excitation experiments  Excited state information Emission experiments  Ground state information A Jablonski diagram

2. Laser Induced Fluorescence Excitation process • Tune incident radiation. • When incident radiation matches a vibronic transition, radiation is absorbed. • The excited state fluoresces. • The total fluorescence is collected by a photomultiplier tube (PMT). Essentially, the observation of fluorescence, is used to infer the presence of a vibronic level.

3. Laser Induced Fluorescence Emission process • The laser is fixed at one of the excitation/absorption frequencies. • The emitted fluorescence is dispersed into its component wavelengths by a monochromator. • The spacing between the observed bands gives the spacing between the vibrational levels in the ground state. Need to do an excitation experiment first to determine the absorption frequencies.

4. Supersonic Jets Method of producing internally (vibrationally, rotationally) cold molecules. • Molecules emerge with a narrow spread of velocities. • Collisions partition vibrational and rotational energy into translational. • Effect is an increase in the translational velocity, u, of the gas. • Drop in density, reduces the localspeed of sound, a. The Mach number, M, is When M>1, beam is termed supersonic.

5. Supersonic Jets - Laser Desorption. How to get the molecules of interest into the supersonic jet? • If molecules are thermally stable or relatively volatile, • Thermally vaporise sample and mix with carrier gas in reservoir. • If sample is thermally labile or involatile, • Use laser desorption to vaporise sample. • Desorption affected by a pulsed CO2 laser, from a solid sample. • Sample molecules and carrier gas mix in a device called a “faceplate”. • The narrow channels promote collisional cooling. Faceplate

6. The Franck-Condon Principle Overlap of the wavefunctions in the initial and final states determine whether the transition will occur. Vibrational overlap integral • If S0 and S1 similar in shape • Biggest overlap between v’’=0 and v’=0. • Single band seen in the excitation spectrum.

7. The Franck-Condon Principle • When S0 and S1 different • Many levels in S1 have overlap with the v’’=0 wavefunction. • Several vibronic bands observed in the excitation spectrum. • Most intense band is that with greatest overlap. • Distribution is called the Franck-Condon envelope.

8. 4-hydroxyl biphenyl LIF excitation spectrum I In S0 =42 (electron diffraction) • Long vibronic progression  big change between S0 and S1 electronic states. • Constant spacing implies S1 state is a harmonic potential. • Spacing has low frequency (56cm-1) low frequency mode excited in S1. • Progression probably due to torsional motion.

9. Biphenyl • Analysis of the Franck-Condon envelope shows that the torsional angle changes by 42 between S0 and S1. • In S1 molecule is flat, =0 . • Can also use ab initio methods to model the torsional potentials. • Origin of the change in potential shapes is related to the shape of the HOMO and the LUMO. HOMO LUMO

10. Tyramine LIF excitation spectrum • Six bands observed. • Not vibronic structure, but due to six different molecular conformers. • Confirmed by power saturation experiments and “hole-burning” experiments. • Very narrow line-width lasers can resolve the bands to rotational resolution. • Can get rotational constants for each molecular conformer - this is hard!

11. Tyramine LIF emission spectra • Disperse the emission from each band in the excitation spectrum. • Each conformer has a slightly different pattern of vibrational bands in the ground state. • Different structures have different vibrational frequencies. Now have rotational and vibrational information about each conformer.

12. 1 2 6 5 3 4 Tyramine • Again, make recourse to ab initio methods. • Compare calculated vibrational and rotational frequencies with information gained from experiment. • Allows assignments of bands to different conformer structures. Interchange between conformer structures obtained by rotation of the tail segments.

13. Summary • Fluorescence is : • A sensitive probe of molecular structure in different electronic states. • A useful tool to study conformational behaviour in flexible molecules. • Applicable to both thermally stable and labile molecules. Next Week- What happens if the molecules are not fluorescent? Alternative absorption methods and the usefulness of ionisation.