Raman and stimulated Raman spectroscopy of chlorinated hydrocarbons

Transcription

1 Department of Chemistry Physical Chemistry Göteborg University KEN140 Spektroskopi Raman and stimulated Raman spectroscopy of chlorinated hydrocarbons WARNING! The laser gives a pulsed very energetic and intense light which can give severe eye damage! - Always use safety glasses (dark glasses) when the laser is running! - Be careful with where the laser hits: do not look at the reflections. Remove rings and watches since reflections from them can be very dangerous for your eyes! Jan Davidsson; Rene Andersson 1997, Leif Holmlid

2 ,QWURGXFWLRQ When light passes through a transparent medium, part of it is scattered by the molecules in the medium. The electromagnetic field which is part of the light interacts with the molecules in the medium, and the molecules emit light in other directions than the incoming light. This can be thought as due to oscillations of the dipole moments which are induced in the molecules by the electric field in the light. The scattered light contains mainly light of the same frequency as the incoming light (Rayleigh scattering). A small part of the incoming light is scattered inelastically, i.e. the scattered light has another frequency than the incoming light (Raman scattering). If the shift is towards smaller frequency, the scattering is called Stokes Raman scattering, while a shift towards higher frequencies is called anti-stokes Raman scattering. The effect was predicted by A. Smekal in 1923 and was observed by C.V. Raman in C.V. Raman received the 1930 Nobel Prize in Physics for his discovery. When the light passing the medium is strong coherent light as for example from a laser, the scattered light is also coherent since the Raman scattering process is extremely fast. This means that the Raman scattered light also gives a beam of light proceeding through the medium. Under certain conditions the Raman shifted light reaches high intensity and replaces the laser beam, so that the initial laser beam is shifted in frequency, even in many consecutive steps. In principle, both Stokes and anti-stokes scattering are possible, and scattered waves may move both forward in the direction of the initial laser beam and backwards. Stimulated Raman scattering can be used to shift laser light to other wavelengths, for example in the UV or IR ranges. Stimulated Raman scattering gives very high intensity transitions in a medium relative to the ordinary Raman scattering. With an intense laser, the stimulated process is often much easier to observe than ordinary Raman. 0HFKDQLVP The process of Raman scattering does not depend directly on the frequency of the incoming light. The molecules on absorbing a photon from the light are brought for a very short time to a state which does not have to be a stationary state (corresponding to any quantum number). Instead, it usually lies between two such normal states in the molecule. Due to its very short lifetime it is called a virtual state. The absorbed quantum of light is re-emitted from the molecule very quickly with a small change in frequency, i.e. energy. The difference between the incoming and the outgoing photon frequencies (the Raman shift) corresponds to a change in the state of the molecule, for example a change in the rotational-vibrational energy from one rotational-vibrational state to another. If a vibrational transition is active both in IR and in Raman, the Raman shift will have the same size as the quantum absorbed in the IR absorption. In the case of vibrational transitions, the higher vibrational states are less occupied, as seen from the Boltzmann distribution. This means that it is more likely for the atom to absorb energy from the incoming photon in the Raman scattering, which gives a so called Stokes line at a lower frequency of the light. The process that a higher vibrational state adds energy to the scattered photon and gives a higher frequency light is a so called anti-stokes transition. For vibration, the anti-stokes lines are weaker than the Stokes lines due to the small thermal population of the high vibrational states. 2

3 In the case of rotational transitions, the fact that there is a maximum in the distribution at a level higher than the lowest state means that the intensities of the transitions will not be as simple as for vibration. Since the rotational constants usually are small, of the order of 1 cm -1, the pure rotational transitions will group very close to the frequency of the strong exciting light (at the same frequency as the intense Rayleigh scattering). This means that the pure rotational Raman scattering may be very difficult to study, and requires special techniques. Thus, it is easier to study the pure vibrational spectrum by the Raman technique, since the vibrational transitions are of the order of 1000 cm -1 instead of 1-10 cm -1 for the rotation. With enough resolution, also the rotation-vibrational structure in the gas phase can be studied, which means that both Stokes and anti-stokes rotational lines group around each Stokes and anti-stokes vibrational line. 6HOHFWLRQUXOHV The selection rules for Raman scattering are not the same as for IR absorption. If a rotational or vibrational transition will be active in Raman, the requirement is that the polarizability α should vary during the corresponding motion, rotation or vibration. One advantage of the Raman method is that many transitions which are not active in absorption are active in Raman scattering. An important example of this is homonuclear molecules, which are inactive in IR absorption since they do not have a dipole moment. The transition probability is proportional to 5 Y 2, as in absorption, where 5 Y = & y Y a LM y Y Gt. To make this integral non-zero, thus to make the transition allowed in Raman scattering, means that the product of the representations G(y Y & ) % G(a LM ) % G(y Y ) should belong to the totally symmetric representation A Here, α ij is any one of the elements in the polarizability tensor α a = a [[ a [\ a [] a \[ a \\ a \] a ][ a ]\ a ]] For most molecules, the ground state (y Y ) is totally symmetric. It then follows that G(y Y )=G(a LM ) should be valid to make the Raman transition allowed. ([SHULPHQWDO 3

4 To observe the Raman effect, a strong monochromatic light source is needed. In the old days (till 1965) a line in the Hg lamp spectrum was used, but today a laser is normally used. Spectral analysis of the scattered light is done by a monochromator (spectrograph). In the present laboratory study we will study the vibrational Raman and stimulated Raman spectrum of a few chlorinated hydrocarbons in the liquid phase. As the light source, a Nd:YAG pumped dye laser will be used. The wavelength is around 565 nm, using Rhodamine 6G in the dye laser. 9LEUDWLRQDO5DPDQVSHFWUXPRI&&O In CCl 4 there should exist 9 fundamental vibrational modes (31-6). Only five lines in IR and Raman are considerably stronger than the other lines, which means that they could be fundamental lines (see table). 6\PPHWU\ If the molecule possessed a center of symmetry, for example was planar with C in the center of the square formed by the four Cl atoms, no vibration would be visible both in IR and Raman (rule of mutual exclusion). However, the same wave number is observed in IR and Raman (see table) and thus a center of symmetry is excluded. The most likely symmetry is then a tetrahedron with the C atom in the center (point group 7 G ). In this point group there is strong degeneracy (E and F symmetry species) which may be the reason why only five strong lines are observed. )HUPLUHVRQDQFH The two lines at Dm 762 and 791 cm -l have the same intensity, and they are quite close. It is possible that a so called Fermi resonance exists between two different transitions at almost the same wave number. The resonance equalizes the intensities and pushes the two lines apart. Thus, one fundamental and one overtone or combination line should exist close to Dm = 776 cm -l. 3RODUL]DWLRQ The line at 460 cm -1 consists of three lines ( cm -1 ) which are caused by the two different isotopes of Cl. This line is strongly polarized. Further, since this vibration moves Cl (the isotope effect) it is totally symmetric, belonging to the representation $ 1.,5 The lines Dm = cm -l (a strong fundamental) and 314 cm -l are observed in IR. Since the selection rule for absorption in IR gives that they will have the same symmetry as a translation, they must belong to the representation ) which is triply degenerated and has the same symmetry as the three translations. The only vibration that has enough intensity otherwise to be a fundamental is Dm =218 cm -1. The symmetry typesone$ and two ) have been allocated, and thus only = 2 vibrational degrees remain. Thus, Dm = 218 cm -l belongs to the representation (. 6HOHFWLRQUXOHV 4

5 G(y Y ) is totally symmetric = $ (ground state). G(a [[ )=G([ $ [) = ) x ) = $ O + ( + ) + ) and so on, or see a character table. G(y Y ) may be $, ( or ) in agreement with discussion above. 1) $ O x) x ) x $ = $ + ( + ) +) contains $, thus it is active in Raman. 2) $ O x ) x ) x ( = $ + $ + ( + ) + ) active 3) $ x ) x ) x ) = $ + $ +( + ) + ) active The combination line in the Fermi resonance can be 458 cm cm -1 = 772 cm -1, that is m 1 + m 4. According to the formula for the selection rule on gets $ l x ) x ) x) = $...(as above). Thus, this combination will be Raman active. The fundamental lines are now identified as: Dm (FP 1 ) normal modes ν 1 ν 2 ν 3 ν 4 representation $ ( ) ) For the following discussion, see the figure with normal vibrations. ν 1 should be a symmetrical C-Cl stretch ($ ) ν 3 has considerably higher wave number than ν 2 and ν 4.This vibration could thus be thought to mainly involve motion of the C atom, while the two other vibrations mainly involve motion of Cl. ν 3 is then the vibration (triply degenerated since the C atom can move along the three axis x, y and z) in which the Cl atoms only move relative to C and not relative to each other. If C in ν 2 and ν 4 is assumed not to move, two possibilities for the vibration can be imagined. The first one means Cl-C-Cl angular changes and change of the binding distance in the CCl 2 planes which can be arranged in the molecules. Since three such planes can be brought to pass each Cl atom, this type of vibration is triply degenerate (which means ν 4 ). The second type means changes in the angles between the Cl atoms, and can be shown to be doubly degenerate (thus ν 2 ). ([SHULPHQWDO /DVHU The Nd:YAG laser and the dye laser will be started by the supervisor. The Nd:YAG laser needs cooling water. Use the dark glasses (spectacles) provided for your safety and health! 5

6 Dye laser beam Cuvette holder with cuvette Monochromator Fig 1. Typical experimental setup for Raman spectroscopy. The frequency doubled light with 532 nm wavelength is used to pump the dye laser. The doubling is taking place in a nonlinear crystal built into the laser enclosure. The Nd:YAG laser is Q-switched with a Pockels cell, and the light is coming in strong 5 ns pulses with a repetition rate of 10 Hz. The green 532 nm light is separated from the 1064 nm IR light from the YAG laser by two dichroic mirrors, which transmit the IR light but reflects the green light at 532 nm into the dye laser. The dye laser is studied further in another laboratory experiment in the course. The dye Rhodamine 6G is used in the laser, giving intense pulsed 565 nm light. A typical experimental setup is shown in Fig. 1. The cuvette should be placed so that the laser beam passes directly through the cuvette. A dump for the laser beam is needed on the other side of the cuvette. The light is usually directed into the monochromator by 1-2 concave mirrors, which increases the signal intensity considerably. A so called half-wave plate of birefringent material is placed in the laser beam in front of the sample. It is used to rotate the plane of polarization of the laser light, so that a signal can be obtained with both vertical and horizontal polarization of the laser light. This means that when the Raman scattering is detected in the horizontal plane, horizontal polarization of the laser light gives minimum Raman signal for strongly polarized (type A) vibrational transitions 9LEUDWLRQDO5DPDQVSHFWUXP A similar monochromator and computer program is used also in the dispersive IR laboratory experiment that started this course. Refer there to the handling of the monochromator and the computer readout system. The detection of the Raman scattering is done by locking fast peaks in the scattered light by a sample-and-hold circuit, which locks and stretches the short pulse so it can be measured by the analog-to-digital converter and the computer. Scan the monochromator from 570 nm to longer wavelengths, and from 560 nm to shorter wavelengths. Avoid to measure at the laser wavelength 565 nm, since the intense radiation from the laser destroys the photo multiplier detector. Measure the wave length and wave numbers for the lines observed and calculate the Raman shifts Dm, as: 6

7 Dm = m ODVHU m for Stokes lines Dm = m m ODVHU for anti-stokes lines where m is the observed wave number. 7RGR Measure the positions of Raman peaks around 565 nm and make tables of lines and polarization state. Observe that Raman overtones and combination bands may be formed due to the intense laser light. Determine which representations belong to which lines, using reference spectra and character tables. Attempt to discuss the assignment of the lines for CHCl 3 and CH 2 Cl 2. If not possible, you will hear more about it during the discussion of the laboratory results later in the course. /DERUDWRU\UHSRUW A short description of the theoretical background. Describe the experimental procedure. Answer the questions "to do" above. Also answer the following experimental questions: v During how large fraction of time on average does the laser light exist, i.e. how large is the so called duty factor? v What would the benefits and drawbacks be of instead using a laser in the infrared for Raman studies? v Will the Raman signal increase if the laser beam is focused into the sample? v What factors are of importance for the stimulated Raman scattering? 7