Stimulated Raman scattering

1. L AS L S E2 E1 Stimulated Raman scattering If high enough powered radiation is incident on the molecule, stimulated Anti-Stokes radiation can be generated. • The occurrence of Stokes emission populates E2. • This allows Anti-Stokes scattering to occur. The stimulated Raman scattering can be used to convert fixed frequency laser output, to other wavelengths. Non-linear effect, used in frequency doubling crystals in many dye laser systems.

2. How a dye laser works • Laser emission from solutions of large organic dyes. • 2 stagesoscillation and amplification. • Dye molecule has broad absorption and fluorescence (emission) bands. Rhodamine B

3. S1 LASING S0 How a dye laser works • Vibrational and rotational levels are a virtual continuum. • At RT, most molecules in v``=0 of S0 • Absorption follows the Franck-Condon principle. • Absorption to some v` level in S1. • Vibrational relaxation to v`=0. • Lasing occurs from v`=0 to some excited vibrational level in S0. • Population inversion occurs between different vibrational (vibronic) levels. Vibrational relaxation is energy transfer from dye molecule to the surrounding solvent.

4. Dye Fluorescence Diffraction Grating Pump laser Dye Cell  How a dye laser works Oscillator • Concentrated solution of dye. • Excited by fixed frequency pump laser. • Dye molecule fluorescence is collected, and dispersed using diffraction grating. • Radiation of the chosen wavelength allowed to exit the oscillator cavity

5. Amplified beam,  Oscillator beam,  Amp Cell Pump Laser How a dye laser works Amplifier • One or more cells containing slightly less concentrated dye solution. • Excited by pump laser beam. • Also radiation from oscillator. • Pump laser excites dye molecules. • Beam from oscillator stimulates emission from the excited dye molecules.

6. Tunable laser output  Pump Laser Beam splitter How a dye laser works • To tune wavelength, move diffraction grating in oscillator. • All computer controlled. • Change dyes to change wavelength region.

7. Rotational resolution Rotational spacings are very small  require a laser with a very narrow line width. • Many lasers operate in multi-mode fashion. • Modes active in the cavity satisfy the equation L = length of cavity  = wavelength n = integer Line profile of the laser output To reduce active modes, could reduce the length of the cavity, or…..

8. L D Laser Medium Etalon The use of etalons Etalon acts as a secondary cavity within the laser cavity Modes active in the cavity must satisfy resonance conditions for both the cavity and the etalon. Rotate etalon to select only a single mode. Get linewidths of < 0.01cm-1. (very narrow)

9. Rotationally resolved spectra n-propyl benzene LIF excitation spectrum Rotational band contours Calculated conformer structures

10. Ionisation continuum Ionisation continuum S1 S1 h h S0 S0 Ion dip Can use to obtain ground state vibrational frequencies. 2 laser process  ionisation and depletion lasers. • 1st process is multi-photon ionisation - measure ion current. • 2nd laser, infra-red - tune through vibrational levels in S0. • When molecule absorbs v``=0 depopulated. • Dip observed in the ion current.

11. Ion dip N-phenyl formamide Bands in the 3400-3600cm-1 region describe the N-H and O-H stretches

12. Ionisation continuum S1 h S0 Hole-burning Useful for untangling fluorescence or MPI spectra from different conformers. 2 laser process  interogation and depletion lasers. • Depletion laser saturates electronic transition. • Fluorescence or MPI experiment carried out simultaneously. • Vibronic bands due to depleted electronic transition disappear. Can use with ion-dip technique, missing transitions show up as a dip in the ion current.

13. Hole-burning Phenylalanine 6 conformer structures - A-E and X.