Stimulated Raman scattering

1
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 stagesoscillation and amplification.

• Dye molecule has broad absorption and
  fluorescence (emission) bands.

Rhodamine B
3
How a dye laser works

• Vibrational and rotational levels are a virtual
  continuum.
• At RT most molecules in v0 of S0
• Absorption follows the Franck-Condon principle.

• Absorption to some v level in S1.
• Vibrational relaxation to v0.
• Lasing occurs from v0 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
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
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
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
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 lt 0.01cm-1. (very narrow)
9
Rotationally resolved spectra
n-propyl benzene
LIF excitation spectrum
Rotational band contours
Calculated conformer structures
10
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 v0 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
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.