CHAPTER 1

Introduction

General:

1

When light interacts with matters, the photons which make up the light may be

absorbed or scattered, or may not interact with the material and may pass

straight through it. If the energy of the incident photon corresponds to the

energy gap between the ground state of a molecule and an excited state, the

photon may be absorbed and the molecule promoted to the higher energy

excited state. It is this change which is measured in absorption spectroscopy by

the detection of the loss of that energy of radiation from the light. It is also

scattered photon can be observed by collecting light at an angle to the incident

light beam.

A number of light-scattering phenomena which provide structural information

are now known and understood. The principal ones and those most used are

Rayleigh scattering, Raman scattering, hyper-Rayleigh scattering, hyper-Raman

scattering, coherent anti-Stokes Raman scattering, coherent Stokes Raman

scattering and stimulated Raman gain or loss spectroscopy.

1.2 Normal Raman Scattering

Rayleigh and Raman scattering:

Light scattering phenomena such as Rayleigh and Raman scattering play a key

role in an understanding of static and dynamic properties in condensed phase.

Although vibrational spectra are observed experimentally as infrared and

Raman spectra, physical origins of these two types of spectra are different.IR

2

spectra originate in transition between two vibrational levels of the molecule in

the electronic ground state and are usually observed as absorption spectra in the

IR region. On the other hand, Raman spectra originate in the electronic

polarization caused by UV or visible light.

When a photon of light that is too low in energy to excite the molecule into an

electronic excited state, interacts with a molecule it can be scattered in either of

three ways. Suppose a molecule is irradiated by monochromatic radiation of

frequency ω1 then most of it is transmitted without change but, in addition, some

scattering of the radiation occurs. If the frequency content of the scattered

radiation is analysed, there will be observed to be present not only the frequency

ω1 associated with the incident radiation but also, in general, pairs of new

frequencies of the type ω1±ωM . . Here ωM is the vibrational transition frequency.

The scattering without change of frequency is called Rayleigh scattering, and

that with change of frequency is called Raman scattering. Thus vibrational

frequencies are observed as Raman shift from incident frequency ω1. Raman

bands at frequencies less than the incident frequency (i.e. of the type ω1-ωM) are

referred to as Stokes bands, and those at frequencies greater than the incident

frequency (i.e. of the type ω1+ωM) as anti-Stokes bands. The schematic

representation of Rayleigh and Raman scattering is shown in figure 1. Since in

the case of anti-stokes Raman scattering, the transition occurs from an excited

vibrational state, it requires sufficient population in that state .But normally, at

3

room temperature, the population in excited vibrational level is sufficiently

low ,as a result of which anti stokes Raman Scattering is less intense than the

corresponding Stokes Raman scattering. Thus it is customary to measure stokes

line in Raman spectroscopy. The disadvantage of the Raman spectroscopic

technique is that the Raman signal is generally very weak requiring large

concentration of the sample and often the Raman signal is found to be obscured

by competing physical processes such as fluorescence.

Use of laser operating in the IR overcomes the problem of fluorescence, which

normally occurs following the absorption of only visible or ultraviolet radiation.

The advantage of using an infrared laser is more than counteracted by the fact

that the intensity of Raman scattering decreases as the fourth power of the

wavelength, as Equation (1) indicates, making detection extremely difficult.

For Raman transition between two states ׀i› and ׀f › of a

scattering system, the intensity of light scattered at 90ᵒ to the direction of

irradiation is given by the following equation (1).

Ifi(π /2) =¿2/ε02)(ν0±νs)4 I0 ∑

ρ , σ

¿¿ρσ]fi[αρσ]fi

* ........................................................................ (1)

Where, I0 is the irradiance of the incident radiation, ϵ 0 is the permittivity of

vacuum and [αρσ]fi is the ρσth component of the transition polarizability tensor,

which is expressed as

(2)

4

Where,[μ ρ]fr is the ρ-th component of the transition dipole moment associated

with the transition ׀r› to ׀f ›, iҐr is the damping factor, which is related to the

lifetime of the state ׀r› and c is the velocity of light.

Figure 1: Schematic representation of Rayleigh, Stokes Raman, Anti-Stokes

Raman.

1.3 Resonance Raman Scattering

All light-scattering processes are characterized by the fact that, unlike direct

absorption processes, the energy of an incident photon is not required to be

equal to the energy corresponding to the difference between two discrete energy

levels of the material system. It is a matter of experimental observation

however, that as the energy of an incident photon gets closer to an electronic

transition energy associated with a transition from the ground electronic state to

an excited electronic state of the material system, the intensity of the scattering

5

is enhanced. This enhancement increases rapidly as the energy of the incident

photon approaches electronic transition energy. Such enhanced scattering is

called resonance scattering. The characteristic properties of resonance scattering

differ in some important respects from those of normal scattering.

Therefore Resonance Raman spectra are obtained when the wave number of the

exciting radiation is close to, or coincident with, that of an electronic transition

of the scattering species. Such spectra are usually characterized by a very large

enhancement of the intensities of particular Raman bands. The technique

provides detailed information about excited electronic states because it is only

the vibrational modes associated with the chromophores that are resonance-

Raman active. The sensitivity is also enhanced and can be applied successfully

even in case where the concentration is very low (10-6mol/L). The intensity

enhancement in Resonance Raman scattering is well understood from the

equation (2). As ν0 approaches to νfi there occurs an increase of [αρσ]fi and

consequently an enhancement of intensity.

1.3 COHERENT ANTI-STOKES RAMAN SCATTERING (CARS):

In all the light scattering processes discussed so far, the incident radiation has

consisted of one monochromatic wave of frequency ω1 .Here we now consider

the experimental situation illustrated in Fig. 2 where the incident radiation

consists of two overlapping coherent monochromatic beams of frequencies ω1

6

and ω2, with ω1¿ω2 . As the overlapping beams of radiation propagate through

the material system, new radiation is produced with frequencies corresponding

to the various combinations of ω1 and ω2.

Coherent Anti-stokes Raman scattering depends on the general phenomena of

wave mixing. In CARS as a result of four-wave mixing, radiation of wave

number ω3 is produced where ω3 =2ω1-ω2 . If we vary ω2 while keeping ω1

constant we find that the intensity of the scattering increases dramatically when

ω1 -ω2 = ωM, where ωM is a molecular frequency that can be observed in Raman

scattering. When this frequency-matching condition is satisfied then

ωs=ω1+ωM, because ωs = 2ω1 - ω2 = ω1 +ω1 -ω2=ω1+ωM. This is evident

from fig 3(a). The scattered frequency ω1+ωM have the form of an anti-Stokes

Raman frequency relative to ω1. As this scattered radiation is coherent, it is

called Coherent anti- Stokes Raman Scattering, or CARS. By varying ω2 over a

range of values that covers the desired values of ωM a CARS spectrum can be

obtained. Again, four wave mixing by involving one photon of ω1and two

photon of ω2 to produce ω3=2ω2-ω1=ω2-ωM is equally probable. This process is

referred to as Coherent Stokes Raman scattering (CSRS). There is no net energy

change in the material system. The material system acts as a facilitating agent as

it were for the exchange of energy between radiation of different frequencies

and this is very effective when ω1 -ω2 = ωM.

CSRS is also of course a passive or parametric process.

7

Fig: 2 Diagrammatic representations of CARS, CSRS, SRGS, SRLS

Fig: 3 Energy level diagram of CARS and CSRS

8

CHAPTER: 2

Stimulated Raman

Spectroscopy

2.1 Introduction:

9

Time-resolved Raman Spectroscopy is a valuable tool for revealing structural

dynamics in ultrafast chemical and biological reaction. Spontaneous Raman

scattering can be used to obtain spectra over a ≈1500 cm-1 spectral window, but

requires picoseconds or longer duration pulses to obtain adequate spectral

resolution. A significant benefit of Raman scattering is resonance enhancement,

which allows observation of the vibrational spectrum of a specific

chromospheres in a complex system but electronic resonance is often

accompanied by fluorescence backgrounds that can easily overwhelm the

spontaneous Raman signal.

To overcome these problems of time resolution and spectral quality, the many

nonlinear Raman techniques, such as coherent anti-Stokes Raman scattering

(CARS), stimulated Raman spectroscopy have been developed. The main

disadvantage of CARS is the inevitable contributions to the signal from the non-

resonant background. This project report presents a study of stimulated Raman

gain and loss spectra of different solvents by using Femtosecond Stimulated

Raman Spectroscopy (FSRS). Like CARS it is also a nonlinear four wave

mixing process.

10

2.2 Theory

2.2.1 Stimulated Raman gain spectroscopy:

We can consider another interaction that can arise instead of CARS when there

is present in the material system radiation of frequencies ω1 (Raman pump) and

ω2 (Raman probe) with ω1¿ω2 and ω1-ω2 = ωM. The interaction of the radiation

with the material system can involve the annihilation of a photon of energy ђω1

and creation of photon of energy ђω2 which exactly matches with one of the

frequency of Raman probe. Thus the intensity of Raman pump decreases and

that of Raman probe increases. So we can say that there is gain in Raman probe

and loss in Raman pump. And the Raman gain in the direction of probe pulse is

measured. The scattered radiation now has frequency ω2=ω1-ωM. Thus the

overall process is a Stokes Raman process (fig 2.1) relative to ω1 because the

radiation at ω1-ωM is produced but, unlike the normal Stokes Raman process this

process occurs in presence of frequency ω2(Raman probe), the frequency of the

Stokes Raman radiation itself. This process is termed a stimulated one, the

presence of radiation of frequency ω2 = ω1– ωM being said to stimulate the

Stokes Raman process and produce a gain in intensity of the radiation of

frequency ω2. This gain can be exponential and lead to very substantial transfer

of energy from radiation of frequency ω1 to that of frequency ω2 and

consequently substantial population of the final state f. If probe is not there then

stimulated Raman process tends towards the normal Raman process. The main

11

advantages of this process are that it is insensitive to the non resonant

background signal present in CARS. The main disadvantages of SRS is that it

requires an extremely stable cw probe laser in order to obtain high-resolution

spectra, and the observed signal gain may be complicated due to fluorescence or

“hot luminescence.”

2.2.2 Stimulated Raman loss spectroscopy:

The interaction of the radiation in the material system can also result in the

creation of a photon of energy ђω1 and the annihilation of photon of energy ђω3

where ω3-ω1=ωM. In this case Raman pump has the frequency ω1 and Raman

probe has the frequency ω3. Thus there occurs gain in intensity of Raman pump

and loss in intensity of Raman probe and the scattered radiation now has a

frequency ω1 which exactly matches with the Raman pump. Now Raman loss in

the direction of probe pulse is measured. The loss radiation from probe pulse

has the frequency ω3=ω1+ωM. That is why we get loss spectra in the anti stokes

side of stimulated Raman spectrum.

2.3 Femtosecond Stimulated Raman Spectroscopy

12

In the previous section we have seen that the stimulated Raman effect occurs

when two coherent optical fields, the Raman Pump at ωp and the Raman probe

at ωs , propagate through a system with a vibrational resonance at ωvib=ωp-ωs. In

femtosecond broadband stimulated Raman spectroscopy, the Raman pump field

is provided by a narrow bandwidth picoseconds pulse and the Raman probe by a

femtosecond NIR continuum pulse. The broadband probe pulse allows

simultaneous observation of a large range of stokes and anti stokes frequencies.

Measurement of probe spectrum with and without the Raman pump and then

calculation of the pump-on: pump off ratio generates a Raman gain spectrum

and Raman loss spectrum depending on the Stokes and anti Stokes region of

Raman probe with respect to Raman pump.

Fig. Stimulated Raman scattering (SRS). Raman pump (λp) and Raman probe (λs) on simultaneous interaction form a vibrational coherence (|n + 1⟩⟨n|), which decays with vibrational dephasing time (T2vib). During dephasing, another coupling of the Raman pump ( λp) with the system occurs followed by subsequent emission of aphoton ( λs). SRS is a self-matched process

13

CHAPTER 3

EXPERIMENTAL SETUP OF

FEMTOSECOND STIMULATED

RAMAN SPECTROSCOPY

Our Femtosecond Stimulated Raman Spectroscopy setup involves two pulses,

viz. a narrow bandwidth (7- 25 cm-1) ps pulse and a white light (WL) continuum

14

covering the molecular vibrational frequencies. Both the pulses are generated

from a 100 fs laser system as discussed below. The experimental setup for the

stimulated Raman spectroscopy is given in figure bellow.

Fig.4 Experimental setup for femtosecond stimulated Raman spectroscopy

3.1 Experimental design:

3.1.1 Laser system:

Laser system [Fig.4] includes a Ti: Sapphire Regenerative Amplifier (Spitfire,

Spectra Physics) seeded by a Mode-Locked Ti: Sapphire laser (110 fs, 8.75 nJ,

80 MHz, Tsunami, Spectra Physics). The amplifier generates a 105 fs pulse at a

repetition rate of 1 KHz and having a pulse energy of 2.2 mJ centered at 788

15

nm. About ~1 mJ of the amplifier output is used to pump an Optical Parametric

Amplifier (OPA).

MODELOCKED Ti: SAPPHIRE LASER:

The Ti3+ ion is responsible for the laser action of Ti: sapphire. Ti: sapphire is a

crystalline material produced by introducing Ti2O3 into a melt of Al2O3.

3.2 Raman pump (Pico-second pulse):

The OPA generates 86 fs pulse centered at 550 nm, which is used to produce the

ps pulse using a spectral filter. The spectral filter consists of two gratings

(1200g/mm, 750nm blaze), two lenses (150mm focal length) and an adjustable

slit. The design of the spectral filter and the generation of picoseconds pulse is

shown in Fig. 5.

Fig. 5 Spectral filter

16

The OPA output is attenuated by an aperture to a beam of size 4 mm and energy

3.7 J and incident on the first grating. Then a ps pulse centered at 550.3 nm

with BW and pulse energy of 25 cm-1 and 209 nJ respectively obtained using the

spectral filter is shown in Fig. 5. All the components of the spectral filter are

positioned at focal distance.

3.3 Raman probe pulse (White light continuum):

Rest of the amplifier output is used to produce the WL using a nonlinear crystal,

Sapphire (Sa). This is used as probe beam during pump-probe experiment. After

attenuating the amplifier output to a beam of size 3 mm and energy 1.5 J, it is

focused onto a 2mm Sa crystal for generating WL. A stable and smooth WL is

obtained by adjusting the focal point in the crystal and the input amplifier beam

energy with the help of the combination of a neutral density filer and an iris.

The WL obtained ranges from 450 nm to 1000 nm. A short wave pass filter

(FES0600) is used to transmit only the region from 450 nm to 600 nm while

importantly removing the fundamental amplifier output at 788 nm [Fig. 6]. This

region covers the Raman shifts ranging from -3305 cm-1 to 1652 cm-1

Fig. 6 Generation of white light

17

Chapter 4

Experimental details

4.1Datacollection:

18

A non-collinear geometry is used for focusing the two beams, viz. picosecond

and White Light continuum, at the sample point. This ensures that no ps pulse is

imaged on to the detector. At first, the white light is aligned with respect to the

optic axis of the spectrometer. Then both the beams are spatially and temporally

overlapped at the sample point. For the stimulated Raman studies, a sample cell

of either 1mm or10 mm path length was used. The probe beam containing the

sample signal is collected using a lens and focused on to the slit of the

spectrometer connected to a Liquid N2 cooled CCD.

The spectrum with the gain or loss is retrieved by subtracting the WL

containing the signal from the WL without the signal, i.e.

URLS spectrum = [WL with ps pulse ON] - [WL with ps pulse OFF]

The spectra obtained are baseline corrected using ORIGIN. We recorded the

spectra for various solvents systems to demonstrate the performance, understand

the principle of stimulated Raman spectroscopy and to compare the stimulated

Raman gain and loss spectra.

19

Fig.7 Stimulated Raman loss spectroscopy

Fig.8 Stimulated Raman gain spectroscopy

20

RATIORaman Pump OFF

Raman Pump ON

Loss Spectrum

1I

Raman Probe

Wavelength (nm)

Raman Pump

1Wavelength (nm)

1Wavelength (nm)

Raman Probe

Wavelength (nm)

Raman Pump

1Wavelength (nm)

1Wavelength (nm)

+

Raman Probe

1

Wavelength (nm)1

Pump ON

2

2

Pump OFF

Raman Probe

Ratio Ra

man

Ga

in

Raman Shift (cm-1)

Raman Gain =Raman Pump ON

Raman Pump OFF

Chapter 5

Result and discussion

5.1 Comparison of Stimulated and Spontaneous Raman spectra

21

The stimulated Raman spectroscopy exhibits higher signal to noise ratio. The

intensity of the stimulated Raman is considerably higher as compared to

spontaneous Raman. The maximum signal is observed at zero delay that is

when there is maximum temporal overlap between the pump and the probe

beams. The intensity of the peaks is increased by an amount of more than 20

times. The spectra are shown in the Fig.9

5.2 SRS of Benzene

The stimulated Raman spectrum of benzene is recorded using the stimulated

Raman spectrometer. The benzene is from sigma Aldrich. The spectrum was

recorded using a 10mm cuvette. The Raman pump energy was at 0.340 μJ the

bandwidth was 0.7309nm. The probe energy was at 3.8 nJ at the wavelength

515nm.The 992cm-1 peak of the C=C ring stretching is observed. The 606cm-1

22

peak corresponds to the ring in plane deformation. We observed that the

intensity of the peaks in the loss side is about two times more as compared to

the intensity of the peaks in gain side.

5.3 SRS of Chloroform

The stimulated Raman of the chloroform was recorded. The chloroform

sample used was from sigma Aldrich. The sample was taken in a 10mm cuvette.

The Raman pump used was centered at 515nm and the bandwidth was

0.7309nm. The probe energy was at 3.8 nJ at the wavelength 515nm.

Figure 10

Figure 11

23

The peak 668cm-1 observed corresponds to the C-Cl symmetric stretching and

366cm-1 and 261cm-1 represents the C-Cl deformations. And the The intensity

of the peaks in the loss side had a higher intensity than the gain side. The

spectra are given in figure 10 and 11.

5.4 SRS of Carbon tetrachloride

The stimulated Raman spectrum of carbon tetrachloride was also recorded. The

sample used was from sigma Aldrich. The sample to be studied was taken in a

10mm cuvette. The Raman pump was centered at 515nm and bandwidth was

0.7309nm the probe energy was 3.8nJ at wavelength 515nm.

Figure 12

24

Figure 13

The 460cm-1 represents the Raman active symmetric stretch and 314cm -1 stands

for the Raman active bending vibration. The intensity ratio of the loss side to

that of the gain side is about two.

5.5 SRS of Acetonitrile

The stimulated Raman spectrum of Acetonitrile was recorded using the

stimulated Raman spectrometer.

Figure 14

25

Figure 15

5.6 SRS of Hexane

The stimulated Raman spectrum of hexane was also collected. The sample was

taken in a 10mm cuvette. The Raman pump was centered at 515nm and

bandwidth was 0.7309nm. The probe energy was 3.8nJ at wavelength 515nm.

The intensity of the peaks in the loss side was observed to be about two times

larger than those in the gain side. The stimulated Raman spectra of hexane is

shown in figure

Figure 16

26

Figure 17

27