Femtosecond Stimulated Raman Spectroscopy Study

CHAPTER 1

Introduction General: 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 26

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 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. 26

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 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 26

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) =( π2/ε02)(ν0±νs)4 I0 ρ,σ[αρσ]fi[αρσ]fi*

........................................................................

Where, I0 is the irradiance of the incident radiation,

ϵ

0

(1)

is the permittivity of

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

(2) 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.

26

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 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. 26

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 resonanceRaman 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 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 26

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.

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

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Fig: 3 Energy level diagram of CARS and CSRS

CHAPTER: 2 26

Stimulated Raman Spectroscopy

2.1 Introduction: 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, 26

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 nonresonant 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.

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>ω 2 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 26

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 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 26

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 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.

26

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

CHAPTER 3

EXPERIMENTAL SETUP OF FEMTOSECOND STIMULATED 26

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 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: 26

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 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.

26

Fig. 5 Spectral filter 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

26

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

Fig. 6 Generation of white light

Chapter 4

26

Experimental details

4.1Datacollection: 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. 26

Raman Pump ON Raman Probe Raman Pump

+

Wavelength (nm)

λ1

Wavelength (nm)

λ1

Wavelength (nm)

λ1

I

Raman Pump OFF

RATIO

Raman Probe

Loss Spectrum

Raman Pump

Wavelength (nm)

λ1

Wavelength (nm)

λ1

Wavelength (nm)

Fig.7 Stimulated Raman loss spectroscopy

Raman Probe

λ1 λ2

Wavelength (nm)

Ratio

Raman Gain

Pump ON

Raman Shift (cm-1)

Pump OFF Raman Probe Raman Gain =

Raman Pump ON Raman Pump OFF

λ1 λ2

Fig.8 Stimulated Raman gain spectroscopy

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Chapter 5

Result and discussion

5.1 Comparison of Stimulated and Spontaneous Raman spectra 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. 26

The intensity of the peaks is increased by an amount of more than 20 times. The spectra are shown in the Fig.9 120

100

802cm

-1

5

SRS of cyclohexane

802cm

4

Intensity(arb)

Intensity(arb)

80

Raman scattering

-1

60

40

3

2

20 1 0 0 700

800

900

1000

1100

-1

Wavenumbers(cm )

700

800

900

1000

1100

-1

Wavenumbers(cm )

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 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.

26

5.3 SRS of Chloroform 20000

Intensity(arb)

The stimulated Raman of the chloroform was recorded. The chloroform SRS of benzene

-1

992cm

15000 sample used was from sigma Aldrich. The sample was taken in a 10mm cuvette.

The 10000 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. Intensity (arb)

5000 -1

-1

1180cm

606cm

0

-1

668cm

16000

-1

366cm 12000 400

600

800

SRS of chloroform 1200 1400

1000

1600

-1

delta wavenumbers (cm ) 8000

-1

261cm

4000

Figure 10

0

10000

400

800

Intensity (arb)

-1

delta wavenumbers(cm )

chloroform

0

-1

261cm

-10000 -1

366cm

-20000

-30000

Figure 11

-1

668cm

-1200 -800 -400 The peak 668cm-1 observed corresponds to the C-Cl symmetric stretching and -1

delta wavenumbers(cm )

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 26

10mm cuvette. The Raman pump was centered at 515nm and bandwidth was 0.7309nm the probe energy was 3.8nJ at wavelength 515nm.

Intensity(arb)

12000 -1

459cm

SRS of carbon tetrachloride 8000

-1

314cm

4000

-1

218cm

0

Figure 12

-4000

Intensity(arb)

400

-1

800 delta wavenumbers(cm )

1200

0

-1

218cm -1

314cm

-10000 carbon tetrachloride -20000

-1

-30000

-1200

459cm

-800

Figure 13

-1

delta wavenumbers(cm )

-400

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.

26

Intensity(arb)

8000

SRS of acetonitrile -1

918cm

4000 -1

380cm

Figure 14 0

Intensity (arb)

4000 acetonitrile

-4000 400 0

600

800

1000

1200

1400

-1

1600

delta wavenumbers (cm )

-4000

380cm

-1

-8000

-1600

-1

918cm

-1200

Figure 15

-800

-400 -1

delta wavenumbers(cm )

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

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4000

Intensity(arb)

SRS of hexane -1

1434cm

2000

-1

820cm

-1

1308cm

-1

895cm

Figure 16

0

5000

Intensity(arb)

-2000 400

hexane 800 -1 1200 delta wavenumbers(cm )

0 1308cm

-1

820cm 1434cm

-5000

895cm

-1

-1

-1

-10000 -1600

-1200

-800

-400 -1

delta wavenumbers(cm )

Figure 17

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