152
D. K. Pandey et al.
Raman spectra when the change in quanta is more than one. The transition from
v = 0 to v = 2 is determined as the first overtone, while the transition from v = 0 to
v = 3 is defined for the second overtone. Therefore, the selection rules for overtones
is v = ±2, ±3, · · · ± n, which are observed at the higher wavenumbers. However,
rapid decrease in the probability of overtones occurs toward the higher order quanta.
Further, the combined transition from one quanta of vibration and another quanta
of different vibration leads to the new vibrational states, known as the combination
bands.
3 Raman Instrumentation
The first Raman microspectrophotometer was developed in the year of 1976 in
France, Europe [10]. The Raman spectrophotometer design has difficulty to enable
imaging measurement along with spectroscopic investigation. The former spectrophotometer was interfaced with an optical microscope which affected the instrument working on both single point examination and mapping. Raman spectrophotometer microscope is used for focusing a laser beam onto the surface of any
compound with the incorporation of short-wavelength lasers for the investigation of
large fragile objects or artifacts [11]. However, it can be dispersive or non-dispersive
depends upon the grating or prism generally used for the recorded spectra.
Raman spectra is presented as Raman intensity (arbitrary units)-vs-wavenumber
shift (cm
−1 ) [12], recorded in the range of 4000–10 cm
−1 [13]. Although, it is well
known that the 4000–400 cm
−1 range is significant for Raman analysis due to the
active normal modes of vibration of organic molecules occur in this range. The
quality of the Raman spectrum undeniably depending upon the design of the Raman
spectrophotometer along with the optical components. Therefore, it is important to
understand the role of components used in the Raman spectrometer (See schematic
Fig. 2).
3.1 Source
Before the discovery of the laser, the Raman effect was studied using the mercury arc
lamp as a light source (see Fig. 3a) having the 435.8 nm line of coiled low pressure
[12, 14, 15]. Then the invention of laser replaced mercury lamps as a source of
the incident radiation [12]. The availably of these laser sources with the range of
wavelengths along with stable and intense beam of radiation made them appropriate
for Raman scattering experiments. Later on, a wide range of lasers become very
popular like the argon-ion (488 and 514.5 nm), krypton ion (413.1 and 647.1 nm),
helium–neon (632.8 nm), near IR diode lasers (660–880 nm), neodymium–yttrium
aluminum garnet (Nd: YAG) and neodymium–yttrium ortho-vanadate (Nd: YVO 4 )
(1064 nm) and frequency-doubled Nd: YAG and Nd: YVO 4 diode lasers (532 nm)
D. K. Pandey et al.
Raman spectra when the change in quanta is more than one. The transition from
v = 0 to v = 2 is determined as the first overtone, while the transition from v = 0 to
v = 3 is defined for the second overtone. Therefore, the selection rules for overtones
is v = ±2, ±3, · · · ± n, which are observed at the higher wavenumbers. However,
rapid decrease in the probability of overtones occurs toward the higher order quanta.
Further, the combined transition from one quanta of vibration and another quanta
of different vibration leads to the new vibrational states, known as the combination
bands.
3 Raman Instrumentation
The first Raman microspectrophotometer was developed in the year of 1976 in
France, Europe [10]. The Raman spectrophotometer design has difficulty to enable
imaging measurement along with spectroscopic investigation. The former spectrophotometer was interfaced with an optical microscope which affected the instrument working on both single point examination and mapping. Raman spectrophotometer microscope is used for focusing a laser beam onto the surface of any
compound with the incorporation of short-wavelength lasers for the investigation of
large fragile objects or artifacts [11]. However, it can be dispersive or non-dispersive
depends upon the grating or prism generally used for the recorded spectra.
Raman spectra is presented as Raman intensity (arbitrary units)-vs-wavenumber
shift (cm
−1 ) [12], recorded in the range of 4000–10 cm
−1 [13]. Although, it is well
known that the 4000–400 cm
−1 range is significant for Raman analysis due to the
active normal modes of vibration of organic molecules occur in this range. The
quality of the Raman spectrum undeniably depending upon the design of the Raman
spectrophotometer along with the optical components. Therefore, it is important to
understand the role of components used in the Raman spectrometer (See schematic
Fig. 2).
3.1 Source
Before the discovery of the laser, the Raman effect was studied using the mercury arc
lamp as a light source (see Fig. 3a) having the 435.8 nm line of coiled low pressure
[12, 14, 15]. Then the invention of laser replaced mercury lamps as a source of
the incident radiation [12]. The availably of these laser sources with the range of
wavelengths along with stable and intense beam of radiation made them appropriate
for Raman scattering experiments. Later on, a wide range of lasers become very
popular like the argon-ion (488 and 514.5 nm), krypton ion (413.1 and 647.1 nm),
helium–neon (632.8 nm), near IR diode lasers (660–880 nm), neodymium–yttrium
aluminum garnet (Nd: YAG) and neodymium–yttrium ortho-vanadate (Nd: YVO 4 )
(1064 nm) and frequency-doubled Nd: YAG and Nd: YVO 4 diode lasers (532 nm)