5. Content of this thesis - General Introduction

Chapter I General Introduction

I- 5. Content of this thesis

The rest of this thesis is organized as follows. In Chapter II, I describe the multichannel low-frequency Raman spectrometer constructed in the present study. The working principle of this technique is mentioned in detail. In addition, the major components of the apparatus such as the iodine vapor filter, single-mode Ar-ion laser, and multichannel detector are described.

The performance and advantages of the apparatus are also discussed in comparison with single-channel detection. In Chapter III, the application of the constructed low-frequency Raman system to the crystal polymorphs and melting process of 1,1′-binaphthyl is presented.

The Raman spectra of the two crystal forms are compared especially for the spectral range below 200 cm⁻¹, which provide useful information about intermolecular interactions between 1,1′-binaphthyl molecules in the crystal structure. We also discuss the relationship between the Raman spectra of the two crystal forms and their crystal structures. Finally, I conclude my master work in Chapter IV and discuss the future perspective of low-frequency Raman spectroscopy in order to improve the spectral quality for better elucidation of low-frequency Raman spectra.

Chapter II

Construction of a Fast Multichannel

Low-Frequency Raman Spectrometer

II-1. Introduction

The low-frequency region (<200 cm⁻¹) of Raman spectra contains a wealth of information on intermolecular vibrations and lattice vibrations of molecular and ionic compounds in the condensed phase. In order to investigate the change of the crystal structure during the melting process, it is important to record a low-frequency Raman spectrum in a short measurement time (<1 sec). By using a multichannel detector such as a charge coupled device (CCD) camera, fast Raman measurements become possible. In the present study, we have constructed a fast multichannel Raman spectrometer, which can cover a wide spectral region (−200–1100 cm⁻¹). An absorption band of iodine vapor, having as narrow as 0.03 cm⁻¹ bandwidth, is selected to eliminate gigantic Rayleigh scattering.

As mentioned in Chapter I, the iodine vapor had been used with a single-mode Ar-ion

Two of the I2 vapor

Figure II-1. Laser gain curve and cavity modes of an Ar-ion laser.

One of the absorption bands of iodine vapor is extremely close to the laser gain maximum (1.5 GHz higher).

laser in the 1970–80s for eliminating strong Rayleigh scattering [25-30]. Two of the iodine vapor absorption bands lie very close (1.5 GHz higher) to the gain maximum of an Ar-ion laser (figure II-1). These two absorption bands have a broader bandwidth (~500 MHz) than one laser cavity mode (40 MHz). Hence, The Rayleigh scattered light can be removed by this I₂ absorption band with high efficiency. Unfortunately, this excellent ability of I₂ vapor to get rid of Rayleigh scattering has rarely been used for Raman measurements because the superfluous artifacts caused by I₂-vapor absorption structure appear in the observed Raman spectra.

Not only the Argon ion laser but other single-mode laser can be combined with the I₂ vapor filter, because the iodine vapor has many vibronic transitions in the visible region [41, 42]. For the measurements of filtered Rayleigh scattering in flow field imaging [32, 33] or airborne lidar [34], a single longitudinal mode frequency-doubled Nd:YAG laser is combined with the iodine vapor filter (figure II-2). Also, a large number of the iodine vapor absorption bands are used as the spectral standard. Some of these absorption bands are used for stabilizing the frequency of various single-mode lasers; both gas lasers such as Ar-ion (514.5 nm) [43], Kr-ion (568.2, 530.9, 520.8 nm) [44], He-Ne (632.8 nm) laser [45-47], and solid-state lasers such as a frequency-doubled Nd:YAG (532.0 nm) [48, 49]. The versatility of the iodine vapor filter has been revealed above.

In what follows, the constructed low-frequency Raman spectrometer, which is composed of a single-mode Ar-ion laser, an iodine vapor filter, a polychromator and a CCD camera, is fully discussed. In addition, the advantages of the multichannel detection, especially high wavenmber reproducibility (<1 cm⁻¹) and short measurement time (<1 sec), are also shown.

★

★ (a)

(b)

Figure II-2. Calculated absorption lines of I2 in the visible frequency region accessible by (a) Ar-ion (514.5 nm), (b) Nd:YAG (532.0 nm) lasers.

(Taken from Gregory S. Elliott and Thomas J. Beutner, Process in Aerospace Science 35 799 (1999).)

The 19429.82 cm⁻¹ and 1889.90 cm⁻¹ absorption lines (marked by stars) are used for laser line elimination of the single-mode Ar-ion laser and Nd:YAG laser, respectively.

II-2. Multichannel low-frequency Raman spectrometer using a single-mode Ar-ion laser (514.5 nm) and I

vapor filter

The fast multichannel low-frequency Raman system used in the present study was originally developed by Okajima and Hamaguchi [31] and subsequently constructed as a new setup at NCTU by us. In this section, the combination of a multichannel Raman spectrometer and an I2

vapor filter is described. A single-mode Argon ion laser is used as the excitation source, which is the same as the previous low-frequency Raman studies [25-30] with an I2 vapor filter. The advantages of this technique are also discussed.

II-2-1. Experimental setup

Iodine vapor filter

Figure II-3 shows the cylindrical cell (purchased from Sacher Lasertechnik) we used.

It is made of glass with a diameter of 2.5 cm and a length of 10 cm, and contains iodine solids.

Few milligrams of iodine are placed inside the cell and sealed at a high vacuum level (<10⁻³ torr). The purity level of the natural elements used in manufacturing I₂ vapor cell is higher than 98%.

10 cm

2.5 cm

Figure II-3. Iodine vapor filter used in the present study.

(Purchased from Sacher Lasertechnik )

The iodine inside the cell is in the gas-solid equilibrium, and its vapor pressure depends on the temperature. Figure II-4 shows the pressure of iodine vapor under the gas–solid equilibrium estimated from the following equation [50].

log ( ) = 3.36429 − ( ^.

. ) (311.8≦T≦456) (II-1) where P is the vapor pressure of the I₂ vapor filter (bar), T is the temperature inside the cell

(K). The vapor pressure increases almost exponentially with the temperature. A small fluctuation in the filter temperature causes a large change in the vapor pressure, which is proportional to the transmittance of the vapor filter. Therefore, the cell temperature should be highly stabilized during the experiment. In our I₂ vapor filter, we used a rubber heater, which wraps the whole glass cylinder in order to elevate the filter temperature (figure II-5). A chromel-alumel thermocouple placed between the rubber heater and filter is used to monitor the cell temperature. In each experiment, the cell temperature is kept at 95 °C and regulated within ±1 °C during the measurement.

Figure II-4. Temperature dependent of iodine vapor pressure in the gas–solid equilibrium.

Single-mode Ar-ion laser

A water-cooled Ar-ion laser is used as the excitation source (Beam Lok 2060 with Z-Lok option, Spectra Physics). Figure II-6 shows a schematic diagram of this laser. An etalon consists solely of two partial reflecting surfaces with parallel alignment to each other.

When it is inserted within a laser cavity, the partial reflectors create multiple, overlapping beams, which are directed out of the cavity by an intentional small tilt of the etalon with respect to the laser beam. Because of the “destructive interference” of the reflected beams at particular frequencies, almost no light actually comes out from the cavity. However, once the off-peak mode beams are reflected out of the cavity, the single-mode laser line is generated.

Because the cavity length of the laser is 1.1 meters, the mode spacing of each adjacent cavity modes is about 136 MHz. The full width at half maximum (FWHM) of the single-mode laser line is less than 40 MHz, which is much narrower than that of the iodine vapor absorption (~1 GHz). In order to fine tune the single-mode laser line to the I2 vapor absorption band, the single cavity longitudinal mode located at the etalon loss minimum is selectively operated (figure II-1). By slightly changing the etalon temperature, the etalon loss minimum can be

Figure II-5. Photographs of the I2 vapor filter were taken at different filter temperatures (a) 24 °C (room temperature)

(b) 95 °C

tuned, and one longitudinal mode hops to next mode (mode-hopping). Also, the cavity length can be changed by slightly adjusting the piezoelectric (PZT) stage of the output coupler, a minor shift of the laser frequency (<100 MHz). In the experiments, both the etalon temperature and the PZT stage of the output coupler are controlled so that the observed Rayleigh scattering of the sample is maximally reduced.

Apparatus

The constructed apparatus is schematically shown in figure II-7. The above-mentioned argon ion laser (514.5 nm) is used as the excitation source and the scattered light is collected at 90-degree by a camera lens (f = 50 mm, f/1.2, Nikon). The collimated light is passed through the I2 vapor filter, which filters out most of the elastically scattered light. The transmittance is then focused onto the entrance slit of a polychromator (f = 500 mm, f/6.5, SP-2558, Princeton Instruments), and detected by a back-illuminated, deep-depletion, liquild-N2 cooled CCD detector (Spec-10:100, Princeton Instruments) with 100×1340 pixels operating at −120 °C.

The entrance slit width was typically set to 50 μm. A 1200 grooves/mm grating was used to cover a wide spectral range (>1300 cm⁻¹) with a high spectral resolution of 2.7 cm⁻¹.

Figure II-6. Schematic diagram of a single longitudinal mode Ar-ion laser.

The white light from a tungsten lamp is monitored before and after each measurement to correct the intensity profile of observed Raman spectra. Furthermore, the transmittance of the white light spectrum can be used to remove the superfluous I₂-vapor absorption structure.

In order to introduce the white light into the filter with good position reproducibility, a flipper mirror and an aperture need to be placed in front of the I₂ vapor filter.

II-2-2. Results and Discussion

The intensity correction of the observed Raman spectra

To demonstrate how the intensity correction works, the Raman spectrum of carbon tetrachloride has been measured by using the apparatus in the present study. The observed and the intensity corrected Raman spectra are shown in figure II-8. The observed spectrum shows some superfluous artifacts due to I2-vapor absorption structure. The peak positions and

Figure II-7. Schematic diagram of the multichannel low-frequency Raman spectrometer.

intensities of these artifacts precisely reproduce, indicating that they are not random noises but artifacts inherent to the I₂ vapor filter. These unwanted artifacts can be removed simply through dividing the observed spectrum by the white light spectrum.

It is noteworthy that the intensity and artifacts correction with scanning spectrometers is much more difficult than that with multichannel spectrometers because the wavenumber reproducibility of a scanning spectrometer is only about 1 cm⁻¹. The sharp spikes caused by I2

vapor absorption appearing in the Raman spectrum and the white light spectrum require high spectral reproducibility for a rigorous correction. Thus, simultaneously detecting a wide spectral range (>1300 cm⁻¹) of spectrum without scanning the grating of the spectrometer is desirable [25]. In the case of using multichannel spectrometers, which have high wavenumber reproducibility, the intensity and artifacts correction is simply done as mentioned above. The real power of the combination of I2 vapor filter and multichannel spectrometer is now demonstrated.

T=302 K

Figure II-8. Intensity correction of the Raman spectrum of CCl4. (a) White light transmitted through the I₂ vapor filter.

(b) Observed Raman spectrum.

Laser power at the sample point was 4.5 mW, the spectral resolution was 2.7 cm⁻¹, and the exposure time was (a) 50 sec and (b) 60 sec

It is possible to determine the sample temperature by measuring both the anti-Stokes and Stokes Raman spectra. The temperature at the sample point is estimated from the intensity ratio of the anti-Stokes and Stokes Raman lines using the equation [51] :

(anti-Stokes)

(Stokes)

=

⁽ ⁾

( )

e

^⁄ (II-2) where is the wavenumber of the laser line, is the vibrational frequency of a band of the solvent or sample, h is Planck’s constant, c is the speed of light, is Boltzmann constant, and T is absolute temperature of the sample. This equation is based on the canonical distribution; hence it is not applicable when anti-Stokes lines are very weak. It should be noted that this equation is valid only for the spectra obtained under off-resonance conditions.

The temperature of CCl₄ is estimated to be 302 K (the inset of figure II-8(c)), which is close to room temperature (298 K). Such temperature estimation is quite reliable if and only if multichannel detection is employed because both anti-Stokes and Stokes regions are measured simultaneously and the resulting anti-Stokes/Stokes intensity ratio is not affected by the intensity fluctuation of the laser.

Performance check: Low-frequency Raman measurement of L-cystine

In this section, we demonstrate the ability of our apparatus to measure low-frequency anti-Stokes Raman spectra in the low-frequency region can be recorded with a small Rayleigh

gap (−5 to +5 cm⁻¹). The most intense Raman band of L-cystine at 498 cm⁻¹ has nearly the same area intensity as that of the remaining Rayleigh scattering band at 0 cm⁻¹.

The constructed spectrometer enables us to record a wide spectral range (1300 cm⁻¹), including not only lattice vibrational modes but also intramolecular vibrational modes simultaneously. The low-frequency region of the Raman spectra of the L-cystine is shown in figure II-10. The ±9.8 cm⁻¹ band, which is usually used as a test of the performance of a low-frequency spectrometer, can be clearly observed. Because the present spectrometer employs a multichannel detection, it is not necessary to scan the spectrograph during the Raman measurement. Hence, the measurement time is, in principle, determined only by the exposure and read-out times of the CCD camera. The CCD read-out time in our case is 0.14 sec. As shown in figure II-10, a 0.2-sec exposure time is enough to measure a high S/N low-frequency Raman spectrum of L-cystine. Thus, it is possible to record a Raman spectrum with a sub-second time resolution.

(a)

Observed spectrum

(b)

Filter transmittance

(c)

Corrected spectrum

Figure II-9. Raman spectra of L-cystine.

(a) Observed Raman spectrum of L-cystine.

(b) Transmittance spectrum of I2 vapor filter.

The measurement was done with 2.7 cm⁻¹ spectral resolution, 10 sec exposure, and 70 mW laser power.

(a) 0.2 sec

(b) 1.0 sec

(c) 10 sec

Figure II-10. L-cystine Raman spectra in the low-frequency region.

The measurement was done with different exposure times:

(a) 0.2 sec (b) 1.0 sec (c) 10 sec

Laser power was 70 mW, and the spectral resolution was 2.7 cm⁻¹

Comparison of I2 vapor filter with commercial notch filters

The Raman spectra of L-cystine measured with three different filters are compared in figure II-11. As mentioned in Chapter I, typical notch filters having a broad Rayleigh rejection band (>200 cm⁻¹) can be seen in the blue line spectrum. Though the green line spectrum detects Raman bands below 100 cm^-1, the Raman bands at 67 and 78 cm⁻¹ give incorrect relative intensity. A possible reason is that these two Raman bands were detected at the edge of the Rayleigh rejection bandwidth. The red line spectrum measured with the iodine vapor filter showing ±9.8 cm⁻¹ bands of L-cystine has already been discussed before. The excellent Rayleigh scattering elimination efficiency of the iodine vapor filter has been demonstrated here.

Figure II-11. Comparison of the Raman spectra of L-cystine measured with three different filters. Laser power at the sample point was 20 mW, the spectral resolution was 2.7 cm⁻¹, and the exposure time was 1 sec.

Artifacts due to the resonant fluorescence of iodine vapor

Although no fluorescence quencher is added into the I2 vapor filter, resonant fluorescence of I2 vapor makes no appreciable interference in the Raman spectrum of

L-cystine as shown in figure II-9. This result contrasts with metal vapor filters such as potassium vapor-containing cell, in which resonant fluorescence is so strong that the addition of a quenching gas is necessary [36].

Fluorescence of iodine vapor is observed in the spectrum only when the strong laser light directly comes into the I2 vapor filter. It occasionally happens when the laser light is reflected by the wall of a glass capillary and enters the collecting optics. Figure II-12(a) shows the Raman spectrum of L-cystine measured under such conditions. Extra bands other than the Raman bands of L-cystine are observed in this spectrum. The band at −13 cm⁻¹, which is close to the Rayleigh light, is due to spontaneous emission of the Ar-ion laser. Other bands (212, 424, 637, and 844 cm⁻¹) result from the resonant fluorescence of the iodine vapor [52]. To reduce these annoying signals, the strong reflected laser light should be blocked. In a 90-degree scattering system, the removal of the reflected light is not so difficult. By placing an aperture in front of the I2 vapor filter (figure II-7), the removal of the reflected light can be achieved easily. If a back or forward scattering geometry is employed for the measurement, quenching the fluorescence of the iodine vapor becomes much important. As a result, the fluorescence quencher may be required to be added to the filter similarly to the case of the metal vapor filters.

Raman spectral change caused by mode-hopping

During a long hour measurement, the selected laser cavity mode sometimes hops to another cavity mode (mode-hopping), because of the slight change of the etalon temperature.

Once the mode-hopping occurs, the laser-line elimination of the iodine vapor filter become inefficient, and the observed spectrum drastically change in the low-frequency region. Figure II-13 shows the L-cystine Raman spectra using different cavity modes for measurement.

When the mode-hopping occurs by only 0.3 GHz away from the optimum cavity mode, the observation of low-frequency Raman bands below 30 cm⁻¹ is found to be very difficult.

I₂

I2 I₂ (a)

(b)

Figure II-12. Raman spectra of L-cystine obtained in the following two situations:

(a) The laser light reflected by the glass capillary came into the I2 vapor filter.

(b) The reflected laser light was blocked by the 5 mm aperture.

The mark“Ar” stands for spontaneous emission of the Ar-ion laser,“I2” means the resonant fluorescence of I2 vapor.

Hence, in order to avoid mode-hopping, it is crucial to keep the environmental temperature stable.

In particular, when the laser is operated for over five hour, it is necessary to confirm that the selected cavity mode exactly coincides with the iodine absorption band before starting the measurement. It can be easily confirmed by examining whether the Rayleigh scattering intensity still remains minimum or not.

(a) 1.91 GHz higher than the gain maximum (b) 1.77 GHz higher (c) 1.64 GHz higher (d) 1.50 GHz higher

Figure II-13. Raman spectra of L-cystine measured by tuning the frequency of the single-mode Ar-ion laser.

(a)1.91 GHz, (b) 1.77 GHz, (c) 1.64 GHz, and (d) 1.50 GHz higher than the laser gain maximum.

Chapter III

Real-Time Tracing of the Melting Process of the Two Distinct Polymorphs of

Crystalline 1,1 ′ -Binaphthyl

III-1. Introduction

1,1′-binaphthyl and its derivatives represent a special class of biaryl molecules. It is well-known for their application as chiral recognition receptors and chiral catalysts such as 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) [53-55]. Biphenyl is the simplest biaryl molecule, in which two aromatic rings are linked via a C–C single bond. Its rotation barrier around the phenyl–phenyl bond in the gas phase was found to be ~1.4 kcal/mol [56].

1,1′-binaphthyl, in which two naphthalene moieties are linked via a C–C single bond, has a substantially large increased rotation barrier of 23.5 kcal/mol (∆ ) [57]. The large rotation barrier is attributed mainly to the repulsion of the hydrogen atoms at the 8 and 8′ positions.

Besides, 1,1′-binaphthyl no longer possesses a plane of symmetry that biphenyl has in its perpendicular conformation. This fact allows the isolation of the optically active 1,1′-binaphthyl enantiomers. The dissymmetry of 1,1′-binaphthyl is molecular in nature, and enantiomeric interconversion is made possible simply by rotation about the interannular bond

(S)·(＋)·1, 1′-binaphthyl (R)·(－)·1, 1′-binaphthyl

H H

Sa configuration Ra configuration

Interconversion

(b)

Figure III-1. Axial chirality of 1,1′-binaphthyl.

View along this direction (a)

instead of by any bond-breaking process (figure III-1 (a)). The racemization half-life of the enantiomers was found to be 14.5 min at 50 °C. Chiral 1,1′-binaphthyl was discovered by Pincock et al. in 1971 [58]. He found that racemic 1,1′-binaphthyl underwent spontaneous resolution to generate the optically active R or S enantiomer when this compound crystallized from the melt.

The chiral 1,1′-binaphthyl molecule contains a chiral axis other than a chiral center. The

在文檔中架設快速低振動頻率的拉曼光譜儀並應用於即時監控結晶態1,1'-binaphthyl的熔化過程 (頁 17-0)