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 E 2. This allows Anti-Stokes scattering to occur. L L S AS E1E1 E2E2 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.

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

Vibrational and rotational levels are a virtual continuum. At RT, most molecules in v``=0 of S 0 Absorption follows the Franck-Condon principle. How a dye laser works S0S0 S1S1 LASING Absorption to some v` level in S 1. Vibrational relaxation to v`=0. Lasing occurs from v`=0 to some excited vibrational level in S 0. Population inversion occurs between different vibrational (vibronic) levels. Vibrational relaxation is energy transfer from dye molecule to the surrounding solvent.

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. Dye Cell Pump laser Diffraction Grating Dye Fluorescence Radiation of the chosen wavelength allowed to exit the oscillator cavity

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 Oscillator beam, Amplified beam, Amp Cell Pump laser excites dye molecules. Beam from oscillator stimulates emission from the excited dye molecules.

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

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…..

The use of etalons D L Laser Medium Etalon 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) Etalon acts as a secondary cavity within the laser cavity

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

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 S 0. Ionisation continuum S0S0 S1S1 Ionisation continuum S0S0 S1S1 hh hh When molecule absorbs v``=0 depopulated. Dip observed in the ion current.

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

Hole-burning Useful for untangling fluorescence or MPI spectra from different conformers. 2 laser process  interogation and depletion lasers. Ionisation continuum S0S0 S1S1 hh 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.

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