Chapter 3
Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy Siva Umapathy, Khokan Roy, Surajit Kayal, Nilesh Rai, and Ravi Kumar Venkatraman
3.1
Introduction
Spectroscopy is the study of interaction between electromagnetic radiations with matter. To understand the architecture of atoms and molecules we use many spectroscopic techniques. For example, X-rays are used to determine the crystal structure of molecules, while vibrational spectroscopy is used to study molecular structures. When a photon is incident on a molecule, the electromagnetic field will distort the charge (electron density) cloud around the molecule. In that process it can be absorbed or scattered by the molecule giving rise to different spectroscopic processes. When the frequency of the incident field νi, matches with the frequency of the energetics of the molecule νm, resonance is established and electromagnetic radiation is absorbed. In UV-Visible spectroscopy the incident radiation promotes the molecule from the ground electronic state to higher electronic states, providing information about the electronic energy levels. The nature of chemical bonding in molecules can be probed by infrared or Raman spectroscopy. The shape of the electronic absorption band is associated with vibrational transitions coupled to the electronic excitation are functions of equilibrium bond length. To understand the vibrational structure in the electronic spectra of molecules we apply the Franck-Condon principle. It states that as the nuclei are so much heavier than electrons, electronic transition takes place much faster than the nuclei can respond [1]. During the electronic excitation the initially stationary nuclei instantly experience a new force field. To adjust within the new force field, the molecule starts to vibrate. The equilibrium separation in the ground state becomes the turning point in the excited electronic state, this is known as the vertical transition. The relative displacement of the electronic potential energy determines the vibrational structure in the electronic spectrum, and the excited state potential is normally more displaced than the ground state potential. The separation S. Umapathy (*) • K. Roy • S. Kayal • N. Rai • R.K. Venkatraman Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560012, India e-mail:
[email protected] J.A.K. Howard et al. (eds.), The Future of Dynamic Structural Science, NATO Science for Peace and Security Series A: Chemistry and Biology, DOI 10.1007/978-94-017-8550-1_3, © Springer Science+Business Media Dordrecht 2014
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Fig. 3.1 Representation of electronic absorption and Franck-Condon principle
of the vibrational lines depends on the vibrational energies of the excited electronic state. By applying quantum mechanics we can have more insight into the FranckCondon Principle. In quantum mechanics we say that during electronic transition nuclei retain their initial dynamic state. This dynamic state is represented by wavefunction ψ. So the nuclear wavefunction does not change during electronic transition. At normal temperature most of the molecules are in the lowest vibrational state of the ground state potential energy function. The information about the distribution of molecules can be obtained by Maxwell-Boltzmann distribution formula [2]: N ¼ eΔE=RT No (where No, N are the population in the ground state and excited state, ΔE is the energy difference between the two energy states). The V ¼ 0 vibrational level in the ground electronic state has a maximum in the centre, indicating the region of maximum probability. So after photo-excitation the most probable transition occurs from the V ¼ 0 vibrational level (Fig. 3.1 [3]). The time taken for the electronic transition is of the order of 1015 s, while the time period for a vibration is around 1012 s, which is 1000 times slower than the electronic transition. As a consequence the internuclear distance does not change during the absorption of light, hence the transition process can be represented by the vertical
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transition which is parallel to the potential energy axis, originating from the lower potential curve to the upper curve. This essentially says that it is difficult to convert rapidly electronic energy into vibrational kinetic energy and the most probable transitions between different electronic and vibrational levels are those for which the momentum and position of the nuclei do not change very much. The quantitative form of the Franck-Condon principle can be obtained by applying quantum mechanics through the transition dipole moment. The transition dipole moment operator is a sum over all nuclei and electrons in the molecule: !
μ ¼ e
X
r i i
þe
X I
Z i RI
(where the vectors are the distances from the centre of the charge of the molecule). The intensity of the transition is proportional to the square modulus, |μ|2, of the transition dipole moment. The overall state of the molecule consists of an electronic part, ψe, and a vibrational part, ψv. So according to Born-Oppenheimer approximation, the transition dipole moment factorizes as follows [4]: Z n X o X μfi ¼ ψ sf ψ vf e i r i þ e I Zi RI ψ si ψ vi dτ Z Z X Z XZ ¼ e i ψ sf r i ψ si dτs ψ vf ψ vi dτv þ e I Zi ψ sf ψ si dτs ψ vf RI ψ vi dτv The second term on the right hand side is zero, due to the orthogonality of two different electronic state. So the expression for the transition dipole moment becomes Z XZ μfi ¼ e i ψ sf r i ψ si dτs ψ vf ψ vi dτv ¼ μsf εv S vf ; vj (where the matrix element μsf εv is the electronic-transition dipole moment operator. S (vf,vi) is the overlap integral between the vibrational states ψ vi in the lower electronic state and the ψ vf in the final electronic state). From the above equation we can see that intensity of a transition depends on the overlap integral also known as Franck-Condon Integral. From the classical electromagnetic theory, the polarization induced in the matter due to the external field is obtained by the vector sum of the individual induced dipoles divided by the volume, this is called the polarization density. The relation between the incident field strength and the polarization density (linear approximation) of the matter is given by the following equation, PðvÞ ¼ fεr ðvÞ 1gε0 EðvÞ;
0
00
εr ¼ εr þ iεr
(where P – Polarization density of matter, εr – Relative permittivity/Dielectric constant of the matter, ε0 – Permittivity of the vacuum, E – Electric field).
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Fig. 3.2 Real and imaginary dielectric constant as a function of frequency
The dielectric constant is a function of frequency of the electromagnetic 0 radiation and it is a complex function. The real function εr gives dispersion while 00 the imaginary function εr gives the absorption of the matter as a function of frequency as shown in Fig. 3.2. At different region of electromagnetic spectrum like Microwave, IR and UV-VIS, different molecular motions like orientation, distortion and electronic polarization respectively are in resonance [5]. In general, electronic transitions of most of the organic molecules fall in the UV-VIS region of electromagnetic spectrum. For a transition to be allowed there should be an oscillating dipole moment corresponding to that of the molecular motion in resonance with the frequency of the incident field of the electromagnetic radiation. This is known as transition dipole moment.
3.1.1
Photophysical Processes [6]
During an electronic transition, the molecules nuclei positions are fixed, this is because, the masses of the nuclei are heavier than those of the electrons; this is known as the Franck-Condon principle. As a result of the Franck-Condon principle, the higher vibrational levels of various modes of the singlet electronic excited state are populated during an electronic transition. The higher vibrational level of the singlet electronic excited states then relaxes back to the ground vibrational level of the electronic excited state with the same multiplicity by Internal Conversion (IC) or to a different multiplicity (triplet) by Inter System Crossing (ISC). Generally emission starts from the lowest electronic excited state to the ground state and is called Kasha’s rule [7], albeit there are some exceptions [8]. The lowest singlet
3 Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy
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Fig. 3.3 Jablonski diagram
electronic excited state relaxes to ground state, either radiatively by fluorescence or non-radiatively by IC and similarly the triplet electronic excited state relaxes to ground electronic state, either radiatively by phosphorescence or non-radiatively by ISC as shown in the Fig. 3.3.
3.1.2
Photochemical Processes
Despite the photophysical processes, the excited state can take part in chemical reactions, with reactions starting from either singlet or triplet excited state. In general bimolecular photochemical reactions start from triplet states rather than from singlet states, since triplet states are long-lived they undergo more effective collisions with reactants to give products. Photochemical reactions are faster compared to thermal reactions because the activation energy (Ea) needed for the photochemical reactions is much less than that required for thermal reactions and the enthalpy change of the reactions becomes favourable as illustrated in Fig. 3.4, for hydrogen atom abstraction reaction [9]. There are a range of photochemical reactions including, isomerisation along >C═C< bond, Norrish type I and II reactions, etc. [10, 11].
3.1.3
Photoinduced Electron Transfer Reactions (PIET)
Electronic excitation of a molecule increases the Electron Affinity (EA) and decreases the Ionisation Potential (IP) of the molecule as depicted in the Fig. 3.5. Thus molecules in the excited state show enhanced reactivity towards electron transfer reactions (redox reactions) which are known as PIET reactions [12, 13].
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Fig. 3.4 Hydrogen abstraction
Fig. 3.5 IP and EA before and after photoexcitation
3.2
Time Resolved Experiments
In many of the organic chemical reactions, the intermediates and the transient species involved are short-lived and could not be detected by steady state spectroscopy. However time resolved spectroscopy (TRS) can give us insight into the electronic and structural aspects of the intermediates/transient species involved in organic chemical reactions. Time resolved spectroscopy is basically a pump-probe experiment [14], with the pump pulse initiating the reaction while the probe beam, probes the intermediates/transient species formed during the course of the photoinitiated chemical reaction. The pump pulse could initiate chemical reactions by photolysis, radiolysis, temperature-jump, stopped-flow methods, etc. and then the probe comes at various delays from the pump to give the time resolved information of the probed species. In photolysis, both the pump and probe beam are light pulses, typically on the ns-fs time scale. The pulse width of the pump and probe beam are
3 Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy
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Fig. 3.6 Pump-Probe technique
chosen such that they are narrower than the events that need to be examined. Depending upon the nature of the probe beam and detection method, they can be classified as time resolved UV-VIS and IR absorption, time resolved Raman, time resolved emission spectroscopy, etc. Time resolved Raman and IR absorption spectroscopy give us insight into the structure of the intermediate/transient species. Time resolved Raman spectroscopy is a scattering technique. Since many of the bimolecular photochemical reactions take place from triplet states, ns-TRS can be used to probe the intermediates/ transient species formed during the course of the photochemical processes. ns-LASER flash photolysis (LFP) is an absorption technique used to elucidate the electronic structure of the triplet states or the intermediate species which have a lifetime in the range of few hundreds of ns to μs. ns-time resolved resonance Raman spectroscopy (TR3s) is also a pump-probe technique, in which the pump beam wavelength must be in resonance with steady-state UV-VIS absorption spectrum to create a singlet electronic excited state population which then undergoes various photophysical processes as shown in Fig. 3.3. In TR3s the probe beam must be in resonance with a triplet-triplet/intermediate absorption spectrum obtained from LFP. Resonance Raman enhances the Raman activity of certain modes by a factor of up to 106 relative to that of the normal Raman. Thus combining LFP with TR3s can give us both structural and electronic aspects of the triplet state/intermediate species generated by photolysis, a schematic of TR3s is given in Fig. 3.6.
3.2.1
Experimental Setup
The third harmonic (355 nm) from GCR-250, operating at 10 Hz, spectra-physics is used to pump the Optical Parametric Oscillator which can generate wavelengths
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from 410 to 709 nm with time resolution of a few ns and it is generally used as the Raman probe. Similarly, either second harmonic (266 nm) or third harmonic (355 nm) from INDI LASER, operating at 10 Hz, spectra-physics is generally used as pump. The time delay between pump and probe pulses are controlled by a delay generator from Stanford Research Systems, Inc. The pump and probe are made collinear by using a dichroic mirror (pump wavelength) and a 90 scattering geometry. In an LFP setup, either the third harmonic or second harmonic from a DCR-11 is used as the pump, while a pulsed Xenon arc lamp which is synchronized with a LASER pulse is used as a white light probe. The pump and probe are in perpendicular excitation arrangement and the transmitted white light are dispersed and focused on the exit slit of the Monochromator. The transmitted light intensities are detected by a 5-stage photomultiplier tube (PMT). The signals from the PMT are digitized by an oscilloscope which operates at 500 MSa/s and then stored onto a CPU for further data processing.
3.3
TR3 Spectroscopic Studies of Transients Involved in the Photochemical Reactions
TR3s is a highly selective and sensitive probe for studying the structure of the intermediates formed during photochemical reactions. Fluoranil (perfluorop-benzoquinone) is of interest to photochemists because of its unique structurereactivity correlation. The photochemistry of fluoranil [15] involves various intermediates, e.g. the excited state complex, ketyl radicals and radical anions, the UV-VIS spectra for each of these are very different. The pump wavelength, 355 nm is in resonance with the ground state absorption spectrum and by choosing the probe wavelength in resonance with the absorption spectrum of the intermediates, TR3s can probe various intermediates formed during the photolysis. The TR3 spectra of the fluoranil triplet state in CH3CN and CHCl3 show considerable differences in their spectra which could be due to inversion of the nπ* and ππ* triplet states with solvent polarity [15]. By choosing a probe wavelength at 416 nm, which is in resonance with both ketyl radical and radical anion of fluoranil, it is possible to probe the structure of those intermediates. The TR3 spectrum with a probe wavelength at 416 nm was compared with the RR and FTIR spectra of the fluoranil radical anion (generated by chemical method) and suggested that the observed TR3s bands are due to ketyl radical. Another example of the study of reaction intermediates, occurs for the menaquinone (2-methyl-1, 4-napthaquinone) radical anion [16] which is an active intermediate in various biological redox reactions, and understanding the structure of this intermediate will help in elucidating the mechanism of those processes. The TR3s of the napthoquinone and menaquinone triplet state and radical anion intermediates have been carried out, [16] in order to examine the effect of asymmetric substitution of the methyl group on the radical anion structure. The ab initio computational
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calculation suggests that there is more coupling of the C═C stretching modes with C═O modes. The assignment of the modes of these radical anions helps us to identify the vibrational frequencies observed in Raman studies of photosynthetic systems.
3.3.1
Effects of Solvent on Photoinduced Geminate Ion Recombination Process
The effects of solvent polarity on the fluoranil and durene (tetramethyl-benzene) ion pair generated by photolysis was analysed by TR3s [17]. A change in the vibrational frequencies with respect to solvent polarity was observed for C═C stretching mode. In polar solvents the most intense band at 1,667 cm1 corresponds to that of the Solvent Separated Ion Pairs (SSIP) and a less intense band is due to the contact ion pair (CIP). In moderate and non- polar solvents (binary solvent mixture), three peaks corresponding to CIP, SSIP and the ketyl radical were observed by curve fitting. As the polarity of the solvent decreases the intensity of the band corresponding to SSIP decreases while that of CIP and ketyl increases. This suggest that in polar solvents, the charges are stabilised by the solvent and form solvated ions while in the non-polar solvents, the ion pairs formed are stabilised through electrostatic attraction with the oppositely charged species.
3.3.2
Solvent Induced Structural Changes in the Excited State
The effects of solvent polarity on the fluoranil triplet excited state were studied by using TR3s [18], this resulted in Raman spectra which differed in the intensities of certain vibrational frequencies [18]. Based on the experimental data and computational studies, it is concluded that in non-polar solvents the fluoranil triplet excited state has a more distorted structure (chair form) while in polar solvents it achieves a planar structure. The overtone progressions of the ring bending mode, the 418 cm1 peak and alternate intensities in non-polar solvents suggest that there is a doublewell potential, i.e. two chair forms of triplet fluoranil which could interconvert via a planar structure.
3.4
Introduction to Stimulated Raman Spectroscopy
The vibrational spectral features of a molecule are closely related to its electronic structure, so we have to know how the vibrational and electronic structures are related. The simplest way to explain the vibrational energy is through a harmonic
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oscillator where the energy spacing hν, between the energy levels is fixed. But for real molecules the vibrational potential energy is best explained by Morse potential function given by equation 2 V ¼ De 1 eβq (where De is the dissociation energy; β is measure of curvature of the potential). Both infrared and Raman spectroscopy probe vibrational energy levels in a molecule. IR spectroscopy is an absorption technique while Raman is an inelastic scattering phenomena. If a molecule is irradiated by monochromatic radiation by frequency ω1, most of the radiation is scattered at frequency ω1 (Rayleigh scattering), some radiation is scattered inelastically at a frequency ω1 ωm, where ωm is the vibrational frequency of the molecule. Theoretically the intensity of the Raman band between the energy levels |i > and |f > is given by the following equation X I fi ¼ constant: I 0 ðv1 vfi Þ4 ρσ αρσ fi αρσ fi (where I0 is the intensity of the incident radiation, [αρσ ]fi is the ρσth element of the transition polarizability tensor).
3.4.1
Stimulated Raman Spectroscopy
As Raman scattering is a weak phenomena, the weak Raman signals are overwhelmed by fluorescence background and the time resolution in normal Raman scattering is of the order of picoseconds. So in order to achieve high time and spectral resolution many non-linear spectroscopic technique has been developed. Stimulated Raman spectroscopy is one of the third order nonlinear spectroscopic techniques that can probe the dynamics of a molecule on an ultrafast time scale. Here three ultrashort pulses interact with the material system to give vibrational information.
3.4.2
Description of Stimulated Raman Spectroscopy (SRS)
In spontaneous Raman spectroscopy a monochromatic pump beam ωP is incident on a sample and Stokes ωs and anti-Stokes ωas photons are created. Stokes Raman is accompanied by loss in photon energy of the incident pump beam, whereas a gain in the photon energy is observed in anti-Stokes scattering. Stimulated Raman is a third order (four wave mixing) process in which three ultrashort pulses interact with the system giving a fourth signal field, this signal corresponds to the third order
3 Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy
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Fig. 3.7 Diagram for stimulated Raman process. Dotted arrow represents bra side while solid line represents ket side interaction
nonlinear susceptibility (χ3) of the system. When three different electromagnetic fields interact, there are 108 pairs of possible combinations, the phase matching condition for SRS is given by following equation: [19] ωsig ¼ ωp þ ωpr þ ωp Here two electromagnetic fields are coming from the pump pulse and the third field is from probe pulse. To observe any vibrational features the following condition has to be satisfied: ωv ¼ ωp ωpr (where ωv corresponds to vibrational frequency in the molecular system). Experimentally SRS is performed with two optical pulses, a Raman pump (ωp) and a white light continuum which is known as the probe (ωpr). Interaction of the two fields drives the molecular vibrations in the molecule, the third order interaction of the pulses with the molecule can be understood from the energy level diagram (Fig. 3.7). Simultaneous interaction of the pump and probe creates coherence between vibrational levels a band c. In other-words a macroscopic polarization is created which oscillates with frequency ωv (frequency of vibration in the molecular system). In femtosecond broadband stimulated Raman spectroscopy [20, 21], the Raman pump field is provided by a narrow bandwidth picosecond pulse and the Raman probe by a femtosecond broadband white light continuum pulse, as broadband white light allows simultaneous detection of a large range of frequencies. SRS is a heterodyned technique which means that signal field is created along the probe direction. Measurement of a probe spectrum with and without the Raman pump followed by calculation of the pump-on:pump off ratio generates a Raman gain spectrum and a Raman loss spectrum depending on the Stokes and anti-Stokes region of the Raman probe with respect to Raman pump (Fig. 3.8). In the case of the loss spectrum the peak intensity is ~1.5 times larger than in the gain spectrum. This is termed as Ultrafast Raman Loss Spectroscopy (URLS) [22, 23], and has been found to have more sensitivity than SRS.
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Fig. 3.8 Diagram representing gain and loss in stimulated Raman process
3.4.3
Description of Experimental System
Our laser system consist of a Ti:Sapphire regenerative amplifier (Spitfire, Spectra Physics) seeded by a Mode-Locked Ti:Sapphire Laser (100 fs, 82 MHz, 9 nJ, centered at 790 nm). It is pumped using a Nd:YVO4 (Millenia) laser. The Ti: Sapphire amplifier generates 100 fs, 2.2 mJ, 1 KHz, centred at 790 nm pulses, 1 mJ is used for pumping an Optical Parametric Amplifier (OPA), the residual is divided into two parts, about 1 % is used for generating the white light continuum probe, while the rest is used to pump TOPAS, which is used as an actinic pulse. The output from the OPA is used to generate Raman Pump, the pico-second Raman Pump Pulse is generated by passing through grating pair in 4f configuration. All three beams are focused onto a 1 mm path length quartz cuvette. The sample was flowing constantly to avoid any degradation of the sample.
3.5
Application: URLS of t-Stilbene in n-BuOH
The ultrafast photophysics of trans-stilbene is well studied, it has an excited state S1 life time of around 50 ps in n-BuOH, and after photoexcitation in the UV region trans-stilbene isomerizes to the cis form on ultrafast time scale. However there were not many studies on the IVR and vibrational cooling mechanism, by using URLS it is possible to probe the structure of the transient molecule and the dynamics of the individual modes that can give us information about the coupling between the modes and the mechanism of the IVR process.
3 Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy
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URLS SPECTRUM OF t-STILBENE IN n-BuOH WITH 286 nm UV EXCITATION, RAMAN PUMP AT 600 nm 1560 cm-1 1415 cm-1 1240 cm-1
1178 cm-1
850 cm-1
0 50 fs -2000
Relative Intensity
-4000 -6000 -8000 -10000 -12000 -14000 1450 fs -16000 -18000 -1800
-1600
-1400
-1200
-1000
-800
-600
Raman Shift (cm-1) Fig. 3.9 Transient URLS spectra of trans-Stilbene in n-BuOH
The URLS spectrum of trans-stilbene [24, 25] is recorded using the following experimental conditions: a 4 mM solution of trans-Stilbene (Sigma Aldrich) was prepared and the spectra was recorded using a 1 mm cuvette. The centre wavelength of the Raman Pump was 600 nm which is in resonance with the excited state absorption of trans-stilbene, and the energy used for the Raman Pump was 300 nJ with a bandwidth of 0.72 nm. The actinic pump pulse energy was 2 μJ with wavelength centered at 286 nm. The probe beam was dispersed in a spectrometer and detected using a LN2 cooled CCD detector. All the spectra shown were obtained by subtracting the Raman pump off from Raman pump on condition.
3.5.1
Results
Figure 3.9 shows the transient URLS spectrum of trans-stilbene within 1,500 fs in the 1,800–400 cm1. Spectra at long delay was also recorded but is not shown here. The peaks >1,300 cm1 corresponds to C═C stretching frequency, the
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1600 1500 1400 Intensity
1300 1200 1100 1000 900 800 700 0
200
400
600
800
1000
1200
1400
1000
1200
1400
Time Delay(fs)
Fig. 3.10 Plot of intensity vs. time for the 1,560 cm1 mode 1568
Peak Position (cm-1)
1566 1564 1562 1560 1558 1556 0
200
400
600 800 Time (fs)
Fig. 3.11 Plot of peak position vs. time for the 1,560 cm1 mode
1,560 cm1 peak corresponds to the ethyl C═C stretching frequency. Figure 3.10 shows the time dependent change in the intensity of the C═C stretching mode, while the change in peak position of this mode with time is plotted in Fig. 3.11. The variation in the peak intensity of 1,175 cm1 mode, which corresponds to C(ph)-H bending motion, is plotted in Fig. 3.12, while Figs. 3.13 and 3.14 shows the time dependent change in width and position of this mode. These preliminary results suggest that Franck-Condon excited vibrational modes are coupled with
3 Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy
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3500 3000
Relative Intensity
2500 2000 1500 1000 500 0 0
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Time Delay (fs)
Fig. 3.12 Plot of peak intensity vs. time for the 1,175 cm1 mode
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Width (cm-1)
25 24 23 22 21 20 0
500
1000 Time (fs)
1500
2000
Fig. 3.13 Plot of peak width vs. time for the 1,175 cm1 mode
other low frequency vibrations to induce coherence, which is likely to be solvent dependent. The impacts of these observations are currently being explored. However, these URLS results clearly demonstrate the ability of the technique to study bond specific dynamics on photoexcitation.
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Peak Position (cm-1)
1181
1180
1179
1178
1177
1176 0
200
400 600 800 Time Delay (fs)
1000
1200
Fig. 3.14 Plot of peak position vs. time for the 1,175 cm1 mode
3.6
Summary
We have demonstrated that time resolved Raman spectroscopy has the potential to explore bond specific changes induced upon photoexcitation of molecular systems. Interestingly, these changes can be correlated to electron density changes across a bond and this could be correlated to the atomic charge density changes observed by crystallographic studies. Acknowledgement We thank DST, DRDO and the Indian Institute of Science for financial support. SU likes to thank DST for the J C Bose fellowship.
References 1. Turro NJ (1991) Modern molecular photochemistry. University Science Books, Sausalito, California 2. Barrow GM (1988) Introduction to molecular spectroscopy. McGraw-Hill Book Co., London 3. Rohatgi-Mukherjee KK (2006) Fundamentals of photochemistry. New Age International Publishers, Revised 2nd edn, New Delhi 4. Struve WS (1989) Fundamentals of molecular spectroscopy. Wiley, Chichester 5. Atkins P, de Paula J (2007) Atkin’s physical chemistry. Oxford University Press, Oxford 6. Turro NJ, Ramamurthy V, Scaiano JC (2010) Modern molecular photochemistry of organic molecules. University Science, Sausalito, California 7. Michael K (1950) Characterisation of electronic transitions in complex molecules. Discuss Faraday Soc 9:14
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8. Takao I (2012) Fluorescence and phosphorescence from higher excited states of organic molecule. Chem Rev 112:4541 9. Previtali CM, Scaiano JC (1972) Kinetics of photochemical reactions. Part I. J Chem Soc Perkin 2:1667 10. Coyle JD, Carless HAJ (1972) Photochemistry of carbonyl compounds. Chem Soc Rev 1:465 11. Wagner PJ, Kelso PA, Zepp RG (1972) Type II photoprocesses of phenyl ketones. J Am Chem Soc 94:7480 12. Grabowski ZR, Rotkiewicz K (2003) Structural changes accompanying intramolecular electron transfer: Focus on Twisted Intramolecular Charge-Transfer states and Structures. Chem Rev 103:3899 13. Mattay J (1987) Charge transfer and radical ions in photochemistry. Angew Chem Int Ed Engl 26:825 14. Sahoo SK, Umapathy S, Parker AW (2011) Time resolved resonance Raman spectroscopy: exploring reactive intermediates. Appl Spectrosc 65:1087 and references therein 15. Balakrishnan G, Mohandas P, Umapathy S (2005) Time resolved resonance Raman, Ab Initio Hartree-Fock, and density functional theoretical studies on the transients states of Perfluoro-pBenzoquinone. J Phys Chem A 105:7778 and references therein 16. Balakrishnan G, Mohandas P, Umapathy S (1996) Time-resolved resonance Raman spectroscopic studies on the radical anions of menaquinone and naphthoquinone. J Phys Chem 100:16472 and references therein 17. Mohapatra H, Umapathy S (2009) Influence of solvent on photoinduced electron-transfer reaction: time-resolved resonance Raman study. J Phys Chem A 113:6904 and references therein 18. Balakrishnan G, Sahoo SK, Chowdhury BK, Umapathy S (2010) Understanding solvent effects on structure and reactivity of organic intermediates: a Raman study. Faraday Discuss 145:443 19. Mukamel S (1995) Principles of nonlinear optical spectroscopy. Oxford University Press, New York 20. Kukura P, McCamant DW, Mathies RA (2007) Femtosecond Stimulated Raman Spectroscopy. Annu Rev Phys Chem 58:461 21. Lee SY, Zhang D, McCamant DW, Kukura P, Mathies RA (2004) Theory of femtosecond stimulated Raman spectroscopy. J Chem Phys 121:3632 22. Mallick B, Lakshmanna A, Radhalakshmi V, Umapathy S (2008) Design and development of stimulated Raman spectroscopy apparatus using a femtosecond laser system. Curr Sci 95:1551 23. Umapathy S, Lakshmanna A, Mallick B (2009) Ultrafast Raman Loss Spectroscopy. J Raman Spectrosc 40:235 24. Weigel A, Ernsting NP (2010) Excited Stilbene: Intramolecular vibrational redistribution and solvation studied by femtosecond stimulated Raman spectroscopy. J Phys Chem B 114:7879 25. Hamaguchi H, Kato C, Tasumi M (1983) Observation of the Transient Raman spectra of the S1 state of trans-stilbene. Chem Phys Lett 100:3
Chapter 3
Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy Siva Umapathy, Khokan Roy, Surajit Kayal, Nilesh Rai, and Ravi Kumar Venkatraman
3.1
Introduction
Spectroscopy is the study of interaction between electromagnetic radiations with matter. To understand the architecture of atoms and molecules we use many spectroscopic techniques. For example, X-rays are used to determine the crystal structure of molecules, while vibrational spectroscopy is used to study molecular structures. When a photon is incident on a molecule, the electromagnetic field will distort the charge (electron density) cloud around the molecule. In that process it can be absorbed or scattered by the molecule giving rise to different spectroscopic processes. When the frequency of the incident field νi, matches with the frequency of the energetics of the molecule νm, resonance is established and electromagnetic radiation is absorbed. In UV-Visible spectroscopy the incident radiation promotes the molecule from the ground electronic state to higher electronic states, providing information about the electronic energy levels. The nature of chemical bonding in molecules can be probed by infrared or Raman spectroscopy. The shape of the electronic absorption band is associated with vibrational transitions coupled to the electronic excitation are functions of equilibrium bond length. To understand the vibrational structure in the electronic spectra of molecules we apply the Franck-Condon principle. It states that as the nuclei are so much heavier than electrons, electronic transition takes place much faster than the nuclei can respond [1]. During the electronic excitation the initially stationary nuclei instantly experience a new force field. To adjust within the new force field, the molecule starts to vibrate. The equilibrium separation in the ground state becomes the turning point in the excited electronic state, this is known as the vertical transition. The relative displacement of the electronic potential energy determines the vibrational structure in the electronic spectrum, and the excited state potential is normally more displaced than the ground state potential. The separation S. Umapathy (*) • K. Roy • S. Kayal • N. Rai • R.K. Venkatraman Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560012, India e-mail:
[email protected] J.A.K. Howard et al. (eds.), The Future of Dynamic Structural Science, NATO Science for Peace and Security Series A: Chemistry and Biology, DOI 10.1007/978-94-017-8550-1_3, © Springer Science+Business Media Dordrecht 2014
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