研究成果概要


1. 超高分解能レーザー・分子分光システムの開発・製作


　市販の波数可変レーザーの出力光の一部を温度・気圧を制御した安定化エタロンに導入し、波数のふらつきに起因するエラーシグナルを 得て、波数可変レーザーの波数のふらつきを減少させた。これにより線幅が0.00003 cm-1 以下で 0.8 cm-1 の波数域を連続掃引できる 可視域(450-900 nm)発振の分子分光用波数自動掃引単一モードレーザーを製作した。 光共振器内に倊波発生用結晶を設置した外部光共振器に可視域発振の波数自動掃引単一モードレーザー光を導入し、紫外域(225-450 nm) 発振の分子分光用波数自動掃引単一モードレーザーを製作した。 これらレーザーの波数掃引の線形性を補正するために、先ずNd:YAGレーザー(線幅0.000001 cm-1)の波数をヨウ素分子の特定の超微細 構造線の一つに固定し、その波長変動を0.00003 mm/s 以下にした・・・基準レーザー。基準レーザーの光をエタロンに導入し、その共 振器長を共鳴条件を満足する様に固定した・・・基準エタロン。基準エタロンに分子分光用波数自動掃引単一モードレーザーの光の一部 を導入し、透過光強度を波数掃引時に記録する様にした・・・これにより0.001 cm＊1 毎にフリンジマーク(目盛)が得られる。 　各フリンジマークの絶対波数を知るために、分子分光用波数自動掃引単一モードレーザーの光の一部を用いてヨウ素分子のドップラー フリー高分解能吸収スペクトルを、波数掃引時に測定記録する様にした・・・次に記述するDoppler-Free High Resolution Spectral Atlas of Iodine Molecule 15000 to 19000 cm-1 を参照することにより、0.0001 cm-1 以上の精度で絶対波数を決定できる。


2. Doppler-Free High Resolution Spectral Atlas of Iodine Molecule 15000 to 19000 cm-1 の製作・出版


　可視・紫外域の絶対波数校正のためには、各種原子の発光線を利用した“M.I.T. Wavelength Tables ”(M.I.T. Press, 1939 and 1969 editions, 精度 0.02 cm-1) とフランスのＣＮＲＳ（国立科学研究院）より出版された “Atlas du Spectre D’absorption de la Molecule D’iode 14800 – 20000 cm-1” が世界中で広く活用されている。後者のスペクトル集は高分解能フーリェ変換分光器を 用いて測定されたもので、光の絶対波数を精度 0.01 cm-1 で波数校正することを可能にするものである。 本プロジェクトで開発・製作した超高分解能分光測定装置では、分解能が0.00003 cm-1 あるのだから、より高精度の絶対波数校正用 スペクトル集が必要である。従来もその必要性を痛感していたので、沃素分子（I2）の波数 15000～19000 cm-1 領域のドップラーフリー 高分解能スペクトルを測定し、この波数領域における光の絶対波数が、精度 0.0001 cm-1 で波数校正することを可能にするスペクトル 集を製作した。これをまとめたものを日本学術振興会から“Doppler-Free High Resolution Spectral Atlas of Iodine Molecule 15000 to 19000 cm-1 ” として出版していただいた。このスペクトル集は4000 ページを越えるもので、４分冊の印刷本（４枚のＣＤ付） として発売されている。


3. 多原子分子の構造とダイナミックスに関する研究と新しい研究領域の創生


サブドップラー分子線・レーザー交差型励起スペクトル (MLCE; 衝突の影響がない、振動・回転温度を極低温にできるのでスペクトルが 単純になる)、ドップラーフリー２光子吸収励起スペクトル (DFTAE; 究極の高分解能が可能)、ドップラーフリーレーザー偏光スペクトル (DFLP; 高感度であり吸収スペクトルを直接測定するので発光が無くても可)、ドップラーフリー光・光二重共鳴レーザー偏光ラベル スペクトル (DFOOPL; 同定が容易)等各種ドップラーフリー高分解能分光法による測定装置を開発・製作した。 グリオキザール、ベンゼン、ナフタレンのDFTAEスペクトルを測定し、多原子分子の電子スペクトルで世界一の分解能、波数精度を達成 した。高い精度で遷移波数を測定できたので、分光定数も高い精度で決定できた。またスペクトル線の微小な分裂や強度異常が観測され、 数ガウスの弱い磁場でも大きく変化することより三重項状態との摂動であることが明確になった。 紫外域発振の分子分光用波数自動掃引単一モードレーザーを活用することにより二硫化炭素、ベンゼン、ナフタレンの回転構造まで 分離したMLCEスペクトルとの外部磁場による影響を調べた。　 多原子分子のスペクトル測定例の無かったDFLP法およびDFOOPL法で、ナフタレン、フェノールのスペクトル測定・解析に成功した。 この手法は、多原子分子の高分解能分光法として極めて有用であり、今後主要な分光法として活用・発展していくと思われる。　 従来の分光法では２原子分子や常磁性分子に限られていたゼーマン分裂の観測が、分解能の向上により1 T以下の磁場で、ベンゼンや ナフタレンの一重項間遷移でも観測可能になった。ゼーマンスペクトルは分子の磁気的性質、一重項・三重項相互作用、磁場効果等に 関する情報を与えてくれるだけでなく、複雑な多原子分子のスペクトル同定にも有用である。 　ナフタレン程度の大きさの分子であれば、室温でも回転構造まで分離したスペクトルが観測でき、スペクトルの同定が可能であること を立証した。従って、研究対象となる分子は、数百とあり、より多くの原子からなる分子についての研究もこれからである。紫外・真空 紫外で波数自動掃引可能な単一モードレーザーの開発もこれから進展するであろう。高分解能になれば、スペクトル線数も増大し、解析 に時間も掛かるが、それに見合う詳細で正確な情報が得られる。着実で重要な基礎研究の新しい分野を創生した。


Outline of 10 Main Contributions

1. Magnetic circular dichroism of spin-forbidden transitions in the Fe3+ high-spin system

Scientific Paper 14).

The magnetic circular dichroism (MCD) of the aqueous solution of potassium trismalonatoferrate(III) was measured at room temperature over the region 400-700 nm. A huge MCD was found, and the MCD of the spin-forbidden d-d transitions of high-spin iron (III) complexes was analyzed on the basis of ligand field theory. The electric dipole transition moments were calculated for two cases where d-d transitions are allowed by odd crystal field and by odd vibrations. It was demonstrated that the vibrations of T1u mode are effective in promoting transition 6A1g → 4T2g(1). By comparing the theoretically derived Faraday parameters with the observed MCD, it was shown that the band at 640 nm is the transition 6A1g → 4T2g(1) and that the band at 445 nm is composed of two transitions: transition 6A1g → 4A1g in the higher energy region and transition 6A1g → 4Eg(1) in the lower energy region. The huge MCD of these transitions was demonstrated to arise mainly from the C term, which has its origin in the spin degeneracy of the ground state.


2. Rotational assignments in excited iodine and reorientation by elastic and inelastic collisions from circularly polarized emission

Scientific Paper 19).

The circular polarization of the resonance fluorescence series from excited molecular iodine, prepared by excitation with the 5145 Å and 5017 Å lines of an argon laser, were measured. The sign of the circular polarization of the rotationally resolved features was useful to assign the spectrum. The circular polarization measures the populations of the MJ states in the excited molecule. Experiments to investigate the dependence of this polarization on the foreign gas pressure and laser power were reported. The results demonstrated that there is a strong persistence of the initially selected distribution of MJ states throught elastic and rotationally inelastic collisions.


3. Molecular structure and dynamics of Cs2 studied by various techniques of laser spectroscopy

Scientific Paper 25), 28), 36), 40), 49), 50), 55), 59), 60), 72), 75), 77), 84), 95).

Laser induced fluorescences of Cs2 excited by the Ar+ laser lines were measured. The diffuse fluctuation band with a maximum of intensity at 16400 cm-1 was found, and the band was identified as a transition to a dissociative continuum. Shortening of the lifetime of several levels of the D 1Σ+u state by an application of magnetic field was found, and it was identified as originating from the magnetic predissociation, which is caused by a combination of the spin-orbit interaction between the D 1Σ+u and (2) 3Πu states and the electronic Zeeman interaction between the (2) 3Π0u and (2) 3Σ+u states. The orientation of dissociated atom was found, and we demonstrated that it is induced by the interference between the indirect predissociation, which occurs by a combination of the spin-orbit coupling of the D 1Σ+u state with the (2) 3Π0u state and the L-uncoupling and Zeeman interactions between the (2) 3Π0u and dissociative (2) 3Σ+u states, and the dissociation following a direct excitation to the (2) 3Σ+u state, which is allowed by spin-orbit coupling of the (2) 3Σ+u state with the B 1Πu state. In the presence of the external magnetic field, even when all degenerated molecular M sublevels are excited by the light linearly polarized parallel to the field, the atomic orientation of dissociated atoms is induced through the interference between the predissociation and the dissociation following the direct excitation to the dissociative state.


4. Molecular structure and dynamics of NaK studied by various techniques of laser spectroscopy

Scientific Paper 27), 30), 80), 41), 93), 94), 54), 56), 63), 64), 67), 73).

Intensity distributions of both bound-bound and bound-continuum transitions of NaK d 3Π1 → a 3Σ+ fluorescence were analyzed, and the potential curves of the d 3Π1 and a 3Σ+ states were estimated. The J' dependencies of energy and line width of the 23Na39K B 1Π(v'=30-43,J') ←X 1Σ+(v''=2,J'') transitions were measured up to the breaking-off points, where NaK dissociates to the Na(3s 2S1/2) + K(4p 2S3/2) atoms. Line broadenings were observed for transitions to the B 1Π(v'=30,J'≥42), (v'=31,J'≥35), (v'=32,J'≥27), (v'=33,J'≥14), and (v'≥34, all J') levels, and were attributed to the predissociation via the c 3Σ+ state to the Na(3s 2S1/2) + K(4p 2P1/2) atoms. The (v',J') dependence of the predissociation threshold was attributed to the potential barrier due to rotation. Below and near the threshold, a series of the perturbation centers which converge to the predissociation threshold was observed for each v', and the perturbing state was identified as the c 3Σ+ state. Rotational perturbations were observed also above the predissociation threshold, and the perturbing state was identified as the b 3Π1 state. The line widths were observed to change drastically around the maximum perturbation, and this was identified as originating from the interference effect which arises because both the B 1Π and b 3Π1 states interact with the dissociative continuum of the c 3Σ+ state. In the transitions to levels near the breaking-off points of the B 1Π(v'≥37), the line splittings into two lines were observed for each J'. This splitting was identified as originating from the S-uncoupling interaction between the B 1Π and b 3Π states at a long internuclear distance. Similar line splittings are observed for the B 1Π(v'=30, all J') levels, but are not observed for v'=31-36. An accidental coincidence of the level energies of the B 1Π(v'=30) and b 3Π(v) levels is presumed, and the origin of the line splitting is identified as the S-uncoupling interaction. This is confirmed by the analysis of the hyperfine structures observed for the split lines.
We found a new interference effect in predissociation, and it is a quantum effect in photodissociation. We demonstrated the importance of phases of wave functions whose levels concerns a chemical reaction.


5. Energy levels and line intensities of diatomic molecules. Application to Alkali metal molecules

Scientific Paper 62).

The procedures for making quantum mechanical calculation of the rotational energy levels and rotational line intensities in the electronic transitions of diatomic molecules were described. The method was applied to lower electronic states of alkali metal diatomic molecules. The spin-orbit interaction, rotational interaction, spin-rotation interaction, spin-spin interaction, hyperfine interaction, and the Zeeman interaction were considered. The procedures are not new, but many matrix elements were listed, and the method calculations are described.


6. Molecular structure and dynamics of carbon disulphide (CS2) studied by various techniques of laser spectroscopy

Scientific Paper 65), 66), 68), 69), 76), 79), 82), 88).

A single mode autoscan laser spectrometer operating in the ultraviolet in combination with a collimated molecular beam was used to measure high-resolution fluorescence excitation spectra of the CS2 V 1B2 ← X 1Σ+g transition under collision-free conditions, and the effects of a magnetic field were measured. Rotational and vibrational levels were fully resolved, and Zeeman splittings were observed in many of the perturbed lines. The Zeeman interaction was observed to induce new perturbation, which induces new transition, level splitting, and energy shift. When the magnetic field strength was changed, the magnitude of interaction, which was observed in the absence of magnetic field, was observed to change dramatically depending on the energy shifts of the Zeeman components. It is shown that the V 1B2(1Δu) state is mixed with the B2(3A2) component by first order spin-orbit interaction, and through the mixed component, the Zeeman interaction between the V 1B2(1Δu) and 3A2(3Δu) states is induced. Large Zeeman splittings were observed for most of the background lines of weak intensity, and this demonstrates that the background levels are levels of the 3A2(3Δu) state. The fluorescence decays of single Zeeman components were observed to be single exponential. The lifetimes of the perturbing 3A2(3Δu) levels were determined by deperturbation analysis. Triplet-triplet 3A2(3Δu)→ 3B2(3Σu)+u) emission was confirmed. We have demonstrated that the quenching of the V 1B2 ← X 1Σ+g fluorescence by a magnetic field was caused by a mixing of the 3A2 state with the V 1B2 state and the resulting increase of triplet-triplet emission. In a time-dependent picture, the intersystem crossing from the 1B2(1Δu) and 3A2(3Δu) states is enhanced by the magnetic field.


7. Molecular structure and dynamics of trans-glyoxal (H2C2O2) studied by Doppler-free two-photon excitation spectroscopy

Scientific Paper 75), 78), 87), 92).

Doppler-free two-photon excitation spectra of the A 1Au ← X 1Ag transition of tran-glyoxal were measure, and the effects of magnetic fields were measured up to H=6T. From the analysis of the perturbed A 1Au and perturbing a 3Au levels, the hyperfine constant of the deperturbed a 3Au level was determined to be 88 MHz. The magnetic moment of the a 3Au level was determined to be 0.86 μB from the magnitude of Zeeman splitting of a hyperfine component. From analysis of the Zeeman splittings of unperturbed levels, which were transitions to the A 1Au state and where no appreciable energy shifts were observed, the 1 3Bu state was shown to be slightly mixed into the 1 1Au state. The perturbation between the A 1Au(v7=1(au)) and a 3Au(v'(au)) levels was shown to occur through the vibronic interaction between the mixed 1 3Bu(v''(au)) and the a 3Au(v'(au)) states. The perturbation is appreciable if levels of the A 1Au and a 3Au states are accidentally close in energy. The energy spacing between levels of the A 1Au and a 3Au states states changes with the magnetic field, and the resulting changes of the perturbation are observed.


8. Molecular structure and dynamics of benzene (C6H6 and C6D6) studied by various techniques of laser spectroscopy

Scientific Paper 86), 98), 99), 100), 102), 103).

We measured rotationally resolved spectra and the Zeeman effects of several low-lying vibrational bands of the S1←S0 transition of benzene. Almost all the observed lines were assigned. Perturbations originating from a perpendicular Coriolis interaction, a parallel Coriolis interaction, and an anharmonic resonance interaction were identified. The density of perturbed lines were found to increase, in general, as the rotational and vibrational excess energy increases. Zeeman splitting of benzene was observed to increase in proportion to K2 for a given J, and that of the K=J line was observed to increase in proportion to J. From this, we concluded that (i) Zeeman splittings of the rotational lines were caused by a magnetic moment of the S1 state, and (ii) the magnetic moment lies perpendicular to the molecular plane and originates from an electronic angular momentum induced by J-L coupling between the S1 and S2 states. It was demonstrated from the Zeeman spectra that rotationally resolved levels of the S1 state are not mixed with a triplet state, and all the perturbing levels are a singlet state. We observed no coupling to a triplet state. In the stationary state picture, intersystem mixing was demonstrated not exist in the S1 state. Therefore, ISC could not be the dominating nonradiative process at low lying vibrational states, and the main nonradiative process is presumed to be IC. The S1→S0 IC becomes efficient when the optically excited vibronic levels mix with the background levels in the S1 state, which couple strongly with the highly excited vibronic levels in the S0 state.


9. Molecular structure and dynamics of Naphthalene (C10H8 and C10D8) studied by various techniques of laser spectroscopy

Scientific Paper 87), 90), 91), 96), 101).

For the first time Doppler-free laser polarization (DFLP) spectroscopy was successfully applied to a large polyatomic molecule. Rotationally resolved spectra of the 33(b2g)01 vibronic band of the S1←S1 electronic transition of gas phase naphthalene at room temperature have been measured with the technique of DFLP spectroscopy. In order to assign the dense and complicated parts of the spectra, the technique of Doppler-free optical-optical double resonance polarization labeling (DFOOPL) spectroscopy was used. Both V-type and Λ-type double resonances were observed, and these signals were found to be very useful for the unambiguous assignment of the complicated lines of DFLP spectrum. Rotational constants were determined by a least-squares fitting of 4632 spectral lines in the range J=4-154 and Ka=0-40 with a standard deviation of 0.0002 cm-1.
The rotationally resolved fluorescence excitation spectra for collimated molecular beam and DFTPE spectra for gas phase and the Zeeman effects of several low-lying vibrational bands of the S1←S0 transition were measured. For an excitation by linearly polarized photons, only Q(Ka)Q(J)KaKc lines were observed, where the quantum numbers Ka and Kc are projections of the angular momentum J along the a- and c-axis, respectively. Perturbations originating from a parallel Coriolis interaction and from a perpendicular Coriolis interaction were observed. The Zeeman splittings of the lines of a given J were observed to increase in proportion to Kc2, and the Zeeman splittings of the Kc=J lines were observed to increase in proportion to J, when J is large. With similar analysis performed for benzene, we conclude that (i) Zeeman splittings are induced by a magnetic moment of the S1 state, and (ii) the magnetic moment lies perpendicular to the molecular plane and it originates from an electronic angular momentum induced by the J-L coupling between S1 and S2 states.
It became clear from the Zeeman studies that rotationally resolved levels of the S1 state were not mixed with a triplet state. Accordingly, we concluded that nonradiative decay of an isolated naphthalene molecule excited to the low-lying vibrational leves of S1 state does not occur through the S1→ Ti ISC. Since ISC is not the source of the low fluorescence quantum yield, the S1→ S0 IC was presumed to be the source. The IC will be enhanced by anharmonic and Coriolis interactions with vibronic levels in the S1 state, which couple strongly with the highly excited vibronic levels in the S0 state.


10. Doppler-Free High Resolution Spectral Atlas of Doppler-Free High Resolution Spectral Atlas of Iodine Molecule 15000-19000 cm-1

Book 1).

The absorption spectrum of the iodine molecule in the 15000-19000 cm-1 region has been measured by applying the technique of Doppler-free laser specroscopy. Each individual spectrum (0.52 cm-1 in range) consist of
1) The frequency marks of the etalon which can be used to calculate the absolute wavenumber of a given hyperfine line of I2,
2) The excitation spectrum of I2 which is broadened by the Doppler effect,
3) The Doppler-free absorption spectrum of I2.
All of which were measured simultaneously. The absolute wavenumber of a transition line in this region can be obtaine with a standard deviation of 0.000054 cm-1. In addition to this printed document, this spectral data is available on an accompanying CDROM. Using this CDROM, it is possible to obtain the absolute wavenumbers of all the hyperfine components in this attlas. This can be used to calibrate the absolute wavenumber of transition lines with accuracy of approximately 0.0001 cm-1.