Introduction

The low frequency region (< 200 cm-1) of Raman spectra provides valuable information on inter-moleculardynamics in condensed-phase materials. Commercially available multi-channel Raman spectrometers with aholographic notch filter can hardly record this spectral region, because such kind of filters have considerableelimination bandwidths (typically >200 cm-1) and they eliminate not only Rayleigh scattering but also lowfrequency Raman scattering as well. Multiple stage monochromators have thus been commonly used for lowfrequency Raman measurements. These spectrometers are used with single-channel detectors likephotomultipliers and require a long measurement time, typically more than 10 minutes. Furthermore, eachportion of a spectrum cannot be obtained simultaneously and therefore observed spectra are sensitive tofluctuations in measuring conditions. It is almost impossible to trace real-time Raman spectral changes duringphase transitions by using the conventional techniques.

In the present study, I newly develop a multi-channel low frequency Raman spectrometer by using aniodine vapor filter for fast (< 1 sec) measurements of low frequency Raman spectra. The iodine vapor was firstdemonstrated in 1970s[1]. Vibronic absorption lines of iodine vapor, which exist in 500-600 nm region, havevery narrow bandwidth of 0.03 cm-1. It is possible to use these bands for laser line elimination, if thewavelength of a longitudinal single-mode laser line coincides with that of the iodine absorption line. In spite ofits high Rayleigh line elimination efficiency, it was seldom used for low frequency Raman measurements,because the iodine absorption lines cause spike-like artifacts in the observed Raman spectra.

I revisit this technique and combine it with a multi-channel Raman spectrometer. The developedspectrometer records the low frequency (> 5 cm-1) region with retaining all the advantages of the multi-channeldetection, such as short exposure time (~0.1 sec), simultaneous recording of the whole spectra, and highwavenumber reproducibility. I also applied this new technique to real-time low frequency Ramanmeasurements during phase transitions.

Multi-channel low frequency Raman spectrometer (Chapter 2)

The developed apparatus is shown in Figure 1. A 90。 Raman scattering geometry is used for collectingRaman scattering. A heated 10-cm glass cell, which contains a few grams of iodine, is placed in front of a60-cm F/6.2 polychromator. The Raman scattered light is dispersed by the polychromator and detected with a liquid nitrogen cooled CCD camera. The read-out time of the detector is about 0.1 sec. The light source is an Arion laser (514.5 nm) or a frequency-doubled Nd:YVO4 Laser (532.2 nm) operating in a single longitudinalmode. The spectral widths of the laser lines are less than 40 MHz, which is one fiftieth of the iodine absorptionbandwidth. The operating wavelength of the laser is monitored by the wavelength-meter to set the laser linestrictly to the iodine absorption lines (514.5295 nm or 532.0500 nm). The laser line transmittance of the iodinevapor filter is less than 10-7, while the continuum light transmittance in the same spectral region is 10~50 %.The laser line elimination efficiency is thus over 106.

Because the observed Raman spectra contain spike-like artifacts due to the iodine absorptions, theintensity correction of the spectra is extremely important. It is necessary to measure the transmittance spectrumof the iodine vapor filter with good wavelength reproducibility. A flipper mirror is used to introduce continuumlight from a tungsten lamp into the filter before and after each measurement. The intensity corrected spectrumis obtained by dividing observed Raman spectrum by the continuum light spectrum. The artifact from iodinevapor no longer exists in the corrected spectrum.

Results and Discussions

(1) Low frequency Raman measurements of L-cystine (Chapter 2)

In order to evaluate Rayleigh scattering elimination efficiency, Raman spectra of microcrystalline powderL-cystine, which is often used for testing Raman spectrometers, are measured (Figure 2 (a)). Thanks to a highRayleigh scattering elimination efficiency of the iodine vapor filter, the remaining Rayleigh scattering at 0 cm-1has nearly the same area intensity as that of the intense S-S stretching Raman band at 498 cm-1. The Rayleighscattering makes almost no interference in the Raman spectra, even though both the Stokes and anti-Stokessides are simultaneously recorded. The low frequency Raman spectra can be recorded with a small Rayleighgap (-5 cm-1 to 5 cm-1), and many Raman bands including as low as 9.8 cm-1 band of L-cystine are measurable.Note that these low frequency Raman bands are very difficult to detect by using conventional methods.Because of multi-channel detection, the measurement time is determined only by the exposure and read-outtime of the detector. As shown in Figure 2 (b), 0.1 sec exposure, which is as short as the read-out time, isenough to measure a high S/N low frequency Raman spectra of L-cystine. It means sub-second real-time lowfrequency Raman measurements are possible by using this apparatus. If the CCD camera having shorter read-out time is used, the measurement time can be shortened.

(2) Real-time low frequency Raman spectral changes during the melting process of anthracene

(Chapter 3)

Next I applied the developed technique to monitoring the quick melting of an anthracene crystal. The lowfrequency Raman bands of anthracene are thoroughly studied by using polarized Raman measurements of asingle crystal [2]. A small piece of crystalline anthracene in a 1-mm glass capillary is quickly heated by using aheating gun. The Raman spectral change is continuously measured in 0.2 seconds (Figure 3(a)). Three Ramanbands in both Stokes and anti-Stokes sides are assigned as a librational lattice vibration of anthracene crystal.After heating, these bands gradually shift to lower frequency side and disappear in 15 seconds, whichcorresponds to the change of the crystal structure and its disappearance (melting).

In the multi-channel measurements, Stokes and anti-Stokes sides of Raman spectra are simultaneouslymeasured, and the intensity ratios of both sides, which correspond to the temperatures, are obtained accurately.The temperature estimated from the anti-Stokes/Stokes intensity ratio of the lattice vibrational Raman bands areplotted in Figure 3(b). There is a plateau in the temperature profile, and it has almost the same as the meltingpoint of anthracene (491 K). This profile is consistent with known melting process. It is interesting that evenafter the temperature rise over the melting point, the lattice vibrational bands remain for a few seconds (10~15sec). Because the temperature is estimated from the lattice, this result indicates that the remaining crystal hastemperatures higher than the melting point (super heating).

(3) Melting processes of an ionic liquid compound bmimCl (Chapter 4)

Finally, I applied this developed technique to the melting of bmimCl (1-butyl-3-methyl-imidazoliumchloride), which is a prototype of imidazolium-based ionic liquids. It is known that bmimCl has crystalpolymorphism dues to the trans-gauche rotation isomerism of butyl chain in the cation [3]. In solid phase thebutyl conformation is either in the trance or gauche form, while in the liquid phase the two conformers coexist.In order to understand how bmimCl forms liquid, it is important to study the conformation changes of the butylchain during the melting. The multi-channel low frequency Raman measurement can simultaneously record both lattice vibrational bands in the lowfrequency region and the marker bands of thetrans and gauche conformations (625 cm-1 and603 cm-1 respectively).

A small piece of crystalline bmimCl isheated quickly and the Raman spectral changesare measured in 0.5 sec (Figure 4). The lowfrequency Raman bands below 200 cm-1gradually changes after heating and sharpspectral features disappear after 54 sec, whilethe marker band of trans isomer does not appear.The trans isomer band appears only a fewseconds after the disappearance of the latticevibrational bands. This result indicates that thelost of the crystal structure and the butyl chainconformational change do not simultaneouslyoccur.

Conclusion

A newly constructed multi-channel lowfrequency Raman spectrometer using an iodinevapor filter is described. It records down to 5cm-1 in both the Stokes and anti-Stokes sidessimultaneously with a short (~ 0.2 sec)measurement time. It is shown that this newtechnique can detect real-time low requencyspectral changes, and can be applied toinvestigate structural changes during the phasetransitions.

Figure 1. Schematic diagram of the developed multi-channel low frequency Raman spectrometer

Figure 2. (a) Raman spectra of L-cystine measured by the developed multi-channel spectrometer using an iodine vapor filter (solid line)and using a typical edge filter (dotted line)Each spectrum is obtained with the same exposure time (10 sec) and the same spectral resolution (1.7 cm-1).(b) Low frequency region of the Raman spectrum of L-cystine obtained with 0.1-sec exposure.Laser power: 50 mW (at the sample point) Excitation: 532 nm

Figure 3. (a) Low frequency Raman spectral change of anthraceneduring the meltingEach spectrum was obtained in 0.2 sec (515 nm excitation).*: Spontaneous emission from Ar+ laser.(b) Temperature estimated from anti-Stokes/Stokes ratio of latticevibrational Raman bandsThe temperatures without heating fluctuate around ±6 K from theroom temperature.

Figure 4. Raman spectral change of bmimCl during the meltingExcitation: 532 nm, exposure: 0.5 secT and G in the graph indicate the positions of marker bands of butylchain conformer of bmim+ (trans and gauche respectively).

本論文は、相転移現象を動的に理解するための高速低振動数ラマン分光装置の開発と、その応用について記述されており、全5章から構成される。

第1章では導入として、本研究の目的が結晶化や融解などの相転移現象を分子レベルで理解することにあり、この目的のために分子間相互作用の動的観測が重要であることが述べられている。また、既存の手法において分子間相互作用の高速測定が困難であることを踏まえ、新規の手法としてマルチチャンネル検出を用いた高速低振動数ラマン分光が提案されている。

第2章では、開発された高速低振動数ラマン分光装置の詳細及びその性能について述べられている。ヨウ素蒸気フィルターをレイリー散乱除去に利用することにより、一般のマルチチャンネルラマン分光計の利点を全て残したまま、低振動数領域を測定することが可能となった。その結果、±5 cm-1までの低振動数ラマンスペクトルを0.1秒単位で高速に取得できることが示された。

第3章では、高速低振動数ラマン分光の応用例として、アントラセンの融解過程を実時間観測した結果とその考察が述べられている。融解における低振動数ラマンスペクトルの変化から格子振動が消失してゆく過程が動的に観測され、さらに得られた各ラマンスペクトルから結晶の温度が精度良く見積もられることが確かめられた。また融解における温度変化から、融点以上の温度を持った結晶状態("過熱状態")が数秒間存在することが確認された。この過渡的な状態として、結晶の長距離秩序が失われ局所的な構造が残ったモデルが考えられ、このようなモデルが格子振動の振動数などの実験事実と良く対応することが述べられている。

第4章では、高速低振動数ラマン分光の別の応用例として、イミダゾリウム系イオン液体のプロトタイプである塩化ブチルメチルイミダゾリウムの融解過程について調べられている。格子振動と内部振動とをマルチチャンネル検出で同時に観測することによって、アルキル鎖の回転異性化が結晶構造の消失より数秒遅れて起こることが示された。このことからアルキル鎖間の強い相互作用が推察され、これとイオン液体の液体構造との関連性について論じられている。第5章は以上の研究成果のまとめである。

本研究により、低振動数ラマンスペクトルを高速に取得する新規手法が開発され、分子間相互作用を動的に観測することが初めて可能となった。マルチチャンネル分光計とヨウ素蒸気フィルターとを組み合わせた本手法は独創性が高く、凝縮相の研究において超高速分光などの発展的な研究への応用が期待される手法である。さらに本研究で行ったように、格子振動の消失や生成の過程を動的に観測することは、融解などの相転移現象を理解する上での新しいアプローチになると考えられる。このような新規の観測手法とその有用性を呈示した本論文の業績は高く評価できる。

本論文第2章の主要部分はApplied Spectroscopy誌に公表済み(〓口宏夫との共著)、第2章と第3章の一部は日本分光学会誌「分光研究」に公表済み(〓口宏夫との共著)であるが、論文提出者が主体となって実験および解析を行なっており、その寄与が十分であるので、学位論文の一部とすることに何ら問題はないと判断する。

以上の理由から、論文提出者岡島元に博士(理学)の学位を授与することが適当であると認める。