Principles of Raman

Salvatore Amoruso

Lectures for the course of Atomic and Molecular Physics and Laser Spectroscopy

1 is based on Raman scattering Raman effect was first observed in 1928 and was used to investigate the vibrational states of many molecules in the 1930s.

Raman discovered that a very small fraction of the scattered light was present at wavelength different from the incident one (Raman shift), and got the Nobel prize in 1931.

Raman  Inelastic scattering

Initially, spectroscopic methods based on the phenomenon were used in research on the structure of relatively simple molecules.

However, the development of laser sources and new generations of monochromators and detectors has made possible the application of Raman spectroscopy to the solution of many problems of scientific and technological interest. Scattering We already studied the Rayleigh (elastic scattering) at the very beginning of the course. We discussed it as due to the re-emission of light from the molecular dipole induced by the incoming beam!  

Linear medium Pt   0 Et

Molecule induced electronic dipole     pt  q r t   E expik  r t

e e 0 0 

  pt  E 0 exp it

From the Lorentz model we got Rayleigh (elastic) scattering formula 1 I    4 s 4 Raman Scattering: classical description

Consider now molecular vibrations at   v pt  qere t  r0   cosvt v  Vibrating molecular dipole         v r  rr0 To make the formula easier       cos t       cos t 0 r v v 0 v v

   pt  Et     cos tE cost  0  v v 0    E  E   E cost v 0 cos  t v 0 cos   t 0 0 2 v 2 v Raman Scattering: classical description

2 2 4 2 2 4 2 2 4  2  E   E     E     I  p  0 0  v 0 v  v 0 v s 2 8 8

Rayleigh Raman Stokes Anti-Stokes Typically

Raman scattering << Rayleigh scattering -5 -6 -2 -3 Is(10 -10 ) Iinc Is  (10 -10 ) Iinc

        v r  rr0

Raman scattering depends on induced changes of polarizability, i.e. on the oscillation of the electronic charges in response to the incident field. Raman Scattering: classical description

 Very weak signal

 Linear response (Is,RIinc)

 Characteristic signature of the molecule vibrations

Laser Raman Spectroscopy Stokes vs anti-Stokes

4 Is Stokes  v    4 1  >> v) Is anti  Stokes   v 

Experimentally

Is(Stokes) > Is(anti-Stokes) Raman effect:Raman quanto IR absorption

Virtual states - mechanical description Raman effect: quanto-mechanical description

Molecule: electronic levels + vibrational levels

Interaction with the e.m. field

As for classical

Raman scattering depends on induced polarizability, i.e. on the oscillation of the electronic charges in response to the incident field.

Stokes anti-Stokes

N(n=1)=N exp(-hn /k T) N(n=0)=N0 0 v B

Is(Raman)  N(n)

4 Is Stokes n n v   hn v   4 exp  >> 1 Is anti  Stokes n n v   kBT  At high temperature of the sample the ratio can be used to measure the temperature

4 Is Stokes n n v   hn v   4 exp  Is anti  Stokes n n v   kBT  Example of a Raman spectrum: carbon tetracloride

CCl4 Example of a Raman spectrum: Benzene

992 cm-1

3062 cm-1

3046 cm-1 -1 -1

Intensity 1595 cm 1178 cm 606 cm-1 849 cm-1

Raman-shift [cm-1] Raman vs IR spectroscopy Both involve vibrational levels

IR: absorption of radiation

Raman: scattering of radiation

Some lines present in the IR spectrum are not present in Raman and viceversa Vibrational Modes Raman-shift = frequency of the normal vibration mode

3N-6 normal vibration modes

 a  a Induced polizability   Qk  Qk Qk ,0 Raman vs IR spectroscopy Selection rules

Raman Gross selection rule - The polarizability a must change during the vibration  a   a  a  a    Q Qk 0 k    0  Qk Q 0 Qk k  Qk Qk ,0

IR Gross selection rule - The molecule dipole momentum must change during the vibration          Q    Qk 0 k    0  Qk Q 0 k Qk  Qk Qk ,0 Transition probability Raman  a   a  1 T   Q     Q  Q   nn'   n' n Q    n'   n Q   Q Q0  Q Q0  2 

IR

      1 T   Q     Q  Q   nn'   n' n Q    n'   n Q   QQ0  QQ0  2 

Specific selection rule Gross selection rule for harmonic oscillator related to variation of dipole Dn=1 moment or polarizability Example: linear molecule CO2

  0 a changes Linear, symmetric molecule IR Raman active Example: CO2 The rule of mutual exclusion

It si not a pure coincidence that in the case of CO2 Raman active modes are IR inactive and vice versa. This is an example of the rule of mutual exclusion. In centrosymmetric molecules (i.e. those with a center of inversion symmetry) a vibrational mode may be either IR active or Raman active, but not both. Another example: symmetric and asymettric biatomic molecules

e.g. H2, N2, O2 e.g. CO Raman active modes Raman active Another example: symmetric and asymettric biatomic molecules

e.g. H2, N2, O2 e.g. CO IR active modes active IR Other molecules

For polyatomic molecules or asymmetric molecules there is need to check the Gross Selection Rule for any normal mode of vibration

THIS CAN BE VERY COMPLEX . . .

… and one should refer to the existing literature.

IR and Raman offers complementary techniques

Raman IR Weak signal intensity High background (absorption)

Low background (scattering) More bands . . . (once Rayleigh component is removed) . . . but Complex spectra Low Raman signal of water and glass (important for biological Infrared sources and detectors samples, etc..)

Dn shift independent of inc, the Characteristic signature of lines are narrower species Possibility to work in UV-Vis- Near IR region Applicable to gas, liquid and (good sources and detectors) solid samples Characteristic signature of species Can also access range not available for IR (e.g. 100-700 cm-1) Lattice vibrations

It’s a salt RAMAN SPECTROMETER filter Spectral light source selection and recording sample

Removal of Rayleigh scattering can be accomplished by approprieate filters or spectral selection apparatus (double or triple spectrograph)

Usually Sokes emission is recorded Notch filter @ 633 nm Intense monochromatic light source provides higher signal and narrow lines LASER Which excitation wavelength is worth using ?

Argon laser: 514 nm and 488 nm Nd:Yag laser: 1064 nm, 527 nm, 355 nm, 263 nm He:Ne: 632.5 nm Diode lasers: 976, 830, 780, . . . , . . . nm

1 I    4  s v 4

UV better than VIS better than IR

263 nm vs 1064 nm Gain a factor 256 BUT 527 nm vs 1064 nm Gain a factor 16 BUT Drawback Fluorescence Fluorescence can hinder the Raman signal or complicate the analysis Molecular damage Absorption can lead to damage of the sample

Opportunity Resonant Raman Raman scattering excited close to a real state can enhance the signal by order of magnitude Fluorescence Raman spectra of a dye molecule

Scattering is simultaneous to excitation

Fluorescence occurs on longer time scale

Appropriate gating could allow separating the two signals Another example Poly (9 vinylcarbazol) is a polymer used for OLED fabrication Resonant Raman Higher sensitivity (enhancement up to 106) absorption Raman Resonant Raman Resonant Raman can be applied to excite specific parts of big molecules (as for instance in biological samples) used as cromophores or markers Resonant Raman vs Raman spectrum Para-ethyl phenols (PEP) in hexane (solvent)

| PEP - * solvent Appropriate selection of the excitation wavelength enhances the signal of the PEP only

Clearly seen in three spectral regions without solvent interference

Para-ethyl phenol

Present in beer and wine Resonant Raman of nanostructures – GaAs/AlAs Superlattices Resonant Raman of nanostructures – ZnSe nanowires EXPERIMENTAL FACTS SCATTERING PROBABILITIES AND GEOMETRY 3 P   1 cos2  Rayleigh 4

 3  2 PRaman    1 3  1 cos  4  8 

 is the .  For =0, same dependence P   0,180  2 P   90

Backscattering and transmission can allow gaining up to factor 2 in the probability of the scattering.

The actual intensity ratio depends on the molecule characteristics (e.g.  . Example: N2 molecule (=0.19 @=337.1 nm I  180 Raman  5104 IRayleigh  180 Other considerations Backscattering P  180 Raman 1.50 - need for access to one sample surface only, PRaman   90 useful for solids, thin films, etc.. - higher Raman scattering efficiency

90° collection PRaman   90 PRaman  180 1.34 Raman/Rayleigh ratio more favorable PRayleigh   90 PRayleigh  180 The scattered light is collected by a large condensing lens and sent to a monochromator and detector, typically an array or CCD Problem: we need to remove Rayleigh component and get high resolution BIG Raman Systems: double and triple monochromators

Double monochromator

Typical focal length  1 m Triple monochromator Intensity variation of scattered ligth for single, double and triple grating spectrometers. COMPACT Raman System: Notch filters and small monochromators

Notch filter @ 633 nm

Filters must be changed according to the excitation laser wavelength Analysis of pharmaceutical products transmission vs traditional Raman spectra SAMPLES

ONE APPLICATION TO ATMOSPHERIC SOUNDING THE RAMAN LIDAR (use intense pulsed laser sources)

NOT ENOUGH TIME FOR MANY DETAILS

JUST PRINCIPLES OF THE TECHNIQUE Backscattered signals due to elastic and anelastic (Raman) scattering Elastic (Mie, Rayleigh)  (molecules + aerosol) @ exc = 527 nm Anelastic (Raman)  only specific molecules  Raman,N2= 627 nm FROM Raman backscatter spectrum of the atmopsphere

inc=355 nm Schematic of a Raman lidar for H2O mixing ratio measurements

H2O mixing ratio measurements: an example

RAMAN MICROSCOPY Sketch of a Confocal Normal SOME EXAMPLES OF APPLICATIONS – Qualitative analysis

Example 1 - material crystalline phase identification: TiO2 Example 2 – modification induced by treatment Laser microfabrication

Target scanning 300 fs pulses at 527 nm 300 fs pulses at 263 nm Nanosecond pulses induce damage and modification of the sample.

Nanosecond laser

Femtosecond laser

Femtosecond pulses seems to preserve much better the properties of the sample. Example 3 – The fake «18th Century Carved Ivory Cat» …

CAT

PURE IVORY The differences in spectra allows identifying the CAT as made from (PMMA + polustirene) resine mixed with calcium carbonate.

PMMA is a synthetic polymer that did not exist in 18th century, as it was introduced in 1930 … and AMBER stones

Fake amber (cherry-tree resin)

Natural amber Methods to enhance the Raman signal: SERS and TERS SURFACE ENHANCED RAMAN SCATTERING Samples on nanostructured surfaces (typically Au, Ag, Cu,..) show strong enhancement (typ. 103-107) of the Raman signal Pyridine on Ag nanosphere at different distances from the metallic surface

The signal enhancement effect is progressively reduced as the distance of the molecule from Ag/substrate increases Explanation Plasmon resonance Au

Ag TIP ENHANCED RAMAN SCATTERING

Enancement of the light also increases at the nanometric tip of an AFM. Tip enhanced scattering utilizes this effect to enhance the local field and then the Raman signal intensity

The enhancement mechanism is not yet completely understood

Pump probe techniques: one pump pulse excites the sample and a probe pulse sounds the Raman scattering response.

With ultrashort laser pulses (ps-fs) the transient changes to the system can be studied A typical setup FROM