3Tp00S

人工原子 ・分子が拓く量子の世界（東京大学・理・物理）樽茶清悟

電子をド・ブロイ波長程度の小さい空間に閉じ込めると、電子の運動は完全に量子化され、エネルギーはとびとびの値しかとれなくなる（零次元準位）.

このような微小構造は量子ドットと呼ばれ、量子物理の新しい研究対象として注目されている.

最近では、形状や電子数を厳密に制御できる半導体量子ドットが開発され、本物の原子に類似の殻構造やフント則の存在も確認されている（「人工原子」）. また、人工原子を連結することによって分子に類似の構造（「人工分子」）を作ることもできる.

本講演では、このような人工原子・分子の電子状態の特徴を説明した後、少数電子系の相互作用とスピン効果、量子計算への応用などについて紹介する.

また、スピン効果に関して、ナノチューブや分子接合系への適応性にも言及する.

人工原子
ここで紹介する人工原子は薄い円盤状の半導体でできていて、高さ方向には強い閉じ込めによる単一の量子準位、面内方向には放物形の緩い閉じ込めによる複数の量子準位が作られている. 電子の軌道は面内に広がっていて、その寸法は数 10nm と原子の千倍程度大きい.

さらに、人工原子は電界効果トランジスタ形の素子に埋め込まれていて、ゲート電極を使って電子数を零個から一個ずつ正確に変えられる.

このような素子を用いて、ソース、ドレイン間のコンダクタンスを測定することにより電子系の電気化学ポテンシャルが求められる.

これにより、特定の電子数のとき閉殻形成による電子系の安定化とフント則に従った半閉殻形成による安定化が確認される. ここで、殻構造は 2 次元調和形ポテンシャルで閉じ込められた固有状態の縮退（1s、2p、3s3d、…の軌道縮退）に関係し、これらの固有状態で作られる少数多体系の量子数は、スピンを含めて、磁場依存性の測定から正確に決めることができる.

加えて、人工原子では、磁場が電子状態を制御するための有効なツールで、例えば数テスラ以下の磁場印可により、軌道縮退の成否を自在に操作することができる.

このとき、軌道縮退時の電子占有には、フント則に従った高スピン状態が出現することから、フント則の一般性を確認することができる.

人工分子
人工分子は、通常、2 個の量子ドットを同一面内で横に並べたり、縦に重ねたりして作られ、その電子状態はドット間のトンネル結合や静電的結合などの大小に依存する.

トンネル結合が大きく、また 2 個のドットが対称なときには、結合、反結合状態が良く形成されていて結合状態

に優先的な電子占有が、そうでない時にはそれぞれのドットに局在した電子占有が起こる.

こ

れらの様子は、トンネル結合を様々に変えて前述と同様な電気的測定を行うことによって確認される.

スピン効果
人工原子、分子を用いることにより、スピンの関与する様々な実験を行うことができる.

こ

こでは、典型的なスピン効果である近藤効果とパウリ則について紹介する.
(1)近藤効果：

近藤効果は、一般には、局在スピンと伝導スピンのスピン１重項結合を介した高次のコヒーレント散乱問題として考えられる.

人工原子では、ドットに捕らえられた電子スピンが局在スピン、ドットとトンネル結合した電極（ソース、ドレイン）のフェルミ電子が伝導スピンに置き換えられる.

人工原子の大きな特徴は、電極とドットの間のトンネル結合や、ドットの電子状態、特にスピン状態を様々に変えられることにある.

とくに、トンネル結合が強い人工原子を使うことにより、ドット中の電子数が奇数の場合、即ち、不対の局在スピンが存在する場合に、明瞭な近藤効果が観測される. これは、通常の近藤効果であるが、それ以外にも、磁場を印可して、軌道縮退を起こさせると極めて強い近藤効果が出現するなど、人工原子特有の新しい近藤問題も提起されている.
(2)パウリ則：

パウリ則によれば、2 個以上の電子は同一の量子状態を占めることができない．

電気伝導は、いわば電子による各量子状態間の移動現象であり、常にパウリ則を満たす必要がある．しかし、通常は、物質には無数の量子状態が縮退し、各量子状態の占有確率は 1 より十分小さいので、パウリ則が重要になることはない．ここで、人工２原子分子を考え、各ドットには唯一の量子状態が存在するとすれば、電子の伝導はパウリ則の影響を受ける．とくに、電極間に直列に挟まれた人工分子で、一方のドットにのみ常に 1 個の電子が捕らえられているような場合には、1 個の電子スピンに反応する整流作用が出現する.
(3)量子計算：

人工原子、分子の電子スピンの量子ビットへの応用は、有力な候補の一つとして注目されている.

ここでは、量子演算のために必要な条件として、スピン操作、初期化、読み出しなどの問題について議論する.

なお、人工原子、分子のスピン効果は、分子接合系やカーボンナノチューブでも同様に考えられる.

近藤効果は、既に観測されているが、これを含めて、講演では、分子接合系で期待されるスピン効果について言及する.

2Ba08S
SPECTRA AND STRUCTURE OF SOME FLOPPY MOLECULES COOLED IN
SEEDED JET
Mihir Chowdhury
Department of Physical Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata 700 032, India
The pioneering works of Raman and Mizushima in the area of vibrational spectroscopy
have been widely exploited by physical chemists for obtaining structural information aboutmolecules. This lecture, dedicated to the two great scientists, reviews our work on low-frequency
vibronic spectra of jet-cooled large organic molecules.
A chemical change involves hopping of the system from one well in the multidimensional potential energy surface (PES) to another. A simplified model for this process whichlends itself to further detailed treatment is large-amplitude vibration along the soft mode of a
floppy molecule. It is therefore pertinent that we collect more information about the groundand/or excited state PES corresponding to motions like aryl rotation, ring-puckering, H-bondstretching and intermolecular dissociation.
In the condensed phase the perturbation by the surrounding environment is large enoughto cause deformation and blurring of conformer identities. It is necessary, therefore, to work witha set of collisionless, cold, uniformly moving molecules as provided in a jet. The vibronic
features of the laser-induced fluorescence (LIF) and fluorescence excitation (FE) spectra of thecold molecular beam are often amenable to analysis in terms of a fundamental frequency and its
higher harmonics, which in turn can be correlated with the nature of excited and ground PESs.
The molecules of interest to us are listed in the table. In particular, the power of twocolour IR-UV double resonance technique in determining structures of clusters is illustrated withfive hydrates of tetrahydroisoquinoline.
Acknowledgement: The works have been carried out by my collaborators with DST support. I
thank Professor O. Kajimoto and Prof. Y. Yoshihara for the invitation to deliver this talk.
References:
1. T. Chakraborty, M.Chowdhury; Chem. Phys. Letts. 171, 1990, 25
2. N. Guchait, R. Dutta,P.Dutta,T.Chakraborty, Ind. J. Phys. 66B, 1992, 241
3. T. Chakraborty, M.Chowdhury; Chem. Phys. Letts. 177, 1991, 223
4. T. Chakraborty, D. N. Nath, M.Chowdhury; J. Chem. Phys. 96, 1992, 6456
5. N. Guchhait, T. Chakraborty, D.Majumdar, M.Chowdhury, J.Phy. Chem. 98,1994, 9227
6. A. Ckaraborty et al. J. Chem.Phys. 114, 2001, 865
7. S. Banerjee et.al. Chem.Phys.Letts. 366, 2002, 2118. N. Guchhait et.al. J. Mol.Struct. 327, 1994, 161
9. A.Chakraborty et.al. J. Chem.Phys. 115, 2001, 1
10. N.Guchhait et. al. J. Chem. Phys. (Communicated)

Molecules

Referen-ce
numberTable I
Vibrational modes (cm-1)
GroundExcited1-28

Butterfly (Puckering)(symmetry-equivalent
double well, low barrier)

2

-

53

Butterfly (Puckering)(symmetry-equivalent
double well, low barrier)

3-

90

Torsion(symmetry-equivalent
double well, low barrier)

4

135

26

Butterfly (Puckering) (softmode with barrier less thanzero-point energy)

5105
160

95
179

Puckering Twist
Puckering Bend

6-66, 87
128

Puckering (in phase andout-of-phase boat and chairform)

7188
81

129
60

Ring Puckering twist[in phase and out-of-phase](Sym and antisym twistconformer)
Puckering coupled to Ninversion

DihydroanthraceneO

Description of modes andpotential energy surface

Xanthene

Dihydrophenanthrene
FFF

FFF
F

F

Octafluoronaphthalene

TetralinHexahydropyrene

Octahydroanthracene
845
119

NHIndoline

9N

H

95
144

9298

3717
3377
3352

-

Tetrahydroisoquinoline
N. H 2OH

THIQ (H2O)1 cluster

10Out-of-plane N-puckering(axial and equatorial form)
Free OH
H-bonded OH
H-Bonded NH(Five clusters with five
distinct IR-signatures)2Cp01S

Atmospheric applications ofvibrational local modes

Henrik G. Kjaergaard, Department of Chemistry,
University of Otago, Dunedin, New Zealand

We have calculated fundamental and overtone OH-stretching vibrational band
positions and intensities of molecules and mono-hydrated complexes ofatmospheric interest. The spectral intensities were determined with aharmonically coupled anharmonic oscillator local mode model and ab initio
calculated dipole moment functions. The basic principles of the local mode
model will be outlines and the effect of various ab initio methods on these typesof calculations discussed. Atmospheric implications of our results arehighlighted.
The water dimer is a weak van der Waals complex consisting of a hydrogen
doner and a hydrogen acceptor unit.

Our calculations predict that the frequencies of the hydrogen-bonded OHstretching transitions are red-shifted compared to those of the monomers andthat the shifts increase with increasing overtone. For the fundamental region, wesee a significant increase in intensity for the hydrogen-bonded transitions
compared to the monomer transitions. For the first overtone, the hydrogenbonded transitions are weaker than the monomer transitions, and for the higher
overtones the intensities are comparable. Our calculated water dimer spectrum
compares well with recent matrix isolation spectra in the fundamental and firstovertone region. The OHb bond of the hydrogen donor unit is the bond mostly
affected by the complexation. We show that transitions to higher overtones in
the OHb-stretching vibration are well seperated from water monomer transitions.

Resent atmospheric observation of one of these OHb-stretching transitions has
provided the first atmospheric detection of the water dimer. Our calculated
spectra of water dimer and hydrated complexes of oxygen, nitrogen, and argon
combined with estimates of their atmospheric abundance have been used toaccess the impact these complexes have on absorption of solar radiation.
Sulfuric acid is one of the major sulfur species in our upper atmosphere. The
concentration and distribution of sulfur compounds in the stratosphere have beenof increasing interest due to the important role they play in the formation of
atmospheric aerosols. Atmospheric field measurements and models of thestratospheric sulfate aerosol layer led to the suggestion that sulfuric acid must
photolyze at high altitudes.
High level ab initio calculations found that the lowest lying electronic transitionin sulfuric acid occurs at wavelengths of about 144 nm. However, very few
photons with this much energy are available in our atmosphere and thus
photolysis via electronic transitions is not very likely.
We propose that excitation of vibrational overtones of sulfuric acid and itshydrate leads to photolysis forming sulfur trioxide and water in the near infraredand visible.

The calculated spectra of fundamental and overtone OH-stretching transitions insulfuric acid are in very good agreement with observed OH-stretching transitionin vapor phase sulfuric acid spectra. Based on our calculated spectra, themeasured stratospheric and mesospheric sulfur dioxide concentrations and theobservation of the sulfate layer can be explained.
These findings will allow scientists to improve their models of the Earth’s
atmosphere and will no doubt change our understanding of the atmosphere.
It is likely to affect climate modelling and our planet’s energy budget throughradiative and chemical effects.

3Aa08S

The Spin Structure of Linked Nitronyl Nitroxide Radicals
H. Heise and F. H. Köhler

Anorganisch-chemisches Institut, Technische Universität München, D-85747 Garching, Germany
Abstract: Solid-state 1H, 13C, and 31P MAS NMR spectroscopy
has been used for the investigation of compounds wherein up to
three nitronyl nitroxides are linked via various potential coupling
units. When the signal shifts are transformed into spin densities,
the radicals’ spin structure including most of their nuclei can bedetermined. Effects like changes of the spin sign uponcomplexation and magnetic interaction between the radicals will
be demonstrated.
Nitronyl nitroxides are rather stable organic
radicals, which are known for more than 30years.1 There was a renaissance of thischemistry, when nitronyl nitroxides were used
as ligands bound to paramagnetic transition
metal ions and when this resulted in not only
ferromagnetic
interaction,
butalso2
spontaneous magnetization. A bit later, itturned out that in some cases the transition
metal ion was not compulsory and that thesolid sample of a nitronyl nitroxide alone may
become a magnet.3 This raised the question on
the origin of the magnetic interactions. Since
theoretical results suggested that HeitlerLondon-type interactions (summarized as
McConnell-I mechanism4) are responsible5,
the spin density distribution in the nitronyl
nitroxides became a focus of current research.
Therefore, this contribution first briefly
compares the advantages and weaknesses ofcurrent methods for determining the spin
structure of radicals. It will be alsodemonstrated briefly by showing a simple
example, what data can be obtained from
NMR spectroscopy.6 More recent preparative
efforts aimed at coupling nitronyl nitroxides inorder to change the spin state and toinvestigate the efficiency of the coupling units.
While compounds of this sort are known7 theirspin structure is still essentially unexplored.

Figure 1. 13C MAS NMR spectrum of twonitonyl nitroxides fixed at ferrocene.
As metallocenes are potential coupling
units, ferrocene- and ruthenocene-bridged
nitronyl nitroxides have been investigated byusing solid-state NMR spectroscopy undermagic angle spinning.8 Figure 1 shows how
well resolved the spectra can be. Spectrum
analysis reveals that considerable spin density
is transferred from the nitronyl nitroxides tothe ferrocene ligands. Nevertheless, the signalsof the radical moieties appear in ranges thatare known from isolated nitronyl nitroxides,9
while magnetic measurements show antiferro-Figure 2. 31P MAS NMR spectrum of themolecule shown.

Figure 3. 1H MAS NMR spectrum of ethylene-bridged nitronyl nitroxides.

magnetic interaction. For a ruthenocenebridged biradical it turns out that
conformational changes may lead to signal
shifts that could also result from antiferromagnetic interactions.
When nitronyl nitroxides are linked via
a phosphorus atom, potential donors fortransition metals result.10 31P MAS NMR
studies (see Fig. 2) uncover a sign change ofthe spin density at phosphorus, when anitronyl nitroxide-substituted phosphane iscoordinated to a metal fragment. The magnetic
interaction across the phosphorus is weak.
Conjugated non-aromatic bridges are
more efficient couplers between two nitronylnitroxides. This is obvious from strongly
reduced NMR signals shifts (e. g. Fig. 3).11
Temperature-dependent 13C MAS NMR
studies are shown to be a valuable tool for thedetermination of magnetic interactions. Theinteraction constant thus obtained agrees withthose determined by independent methods.

(1991). K. Itoh, M. Kinoshita, Eds.,
Molecular Magnetism, Kodansha, Tokyoand Gordon and Breach Science
Publishers, Australia, 2000, chapter 5.
4 H. M. McConnell, J. Chem. Phys.,
39,1910 (1963).
5 J. J. Novoa, M. Dumal, Struct. Bonding(Berlin), 100, 33, (2001).
6 F. H. Köhler in Magnetism: Molecules toMaterials, Models and Experiments; J. S.
Miller, M. Drillon, Eds.; Wiley-VCH,Weinheim, 2001.
7 J. Veciana, H. Iwamura, MRS Bulletin, 36,
41 (2000). P. M. Lahti, Ed. Magnetic
Properties of Organic Materials; Marcel
Dekker, New York, 1999.
8 C. Sporer, H. Heise, K. Wurst, D.
Ruiz-Molina, H. Kopacka, P. Jaitner, F. H.
Köhler, J. J. Novoa, J. Veciana, submitted
to Chem. Eur. J.
9 H. Heise, F. H. Köhler, F. Mota, J. J.
Novoa, J. Veciana, J. Am. Chem. Soc.,
121, 9659 (1999).
10 C. Rancurel, H. Heise, F. H. Köhler, U.
Schatzschneider, E. Rentschler, J. Vidal, J.
Veciana, J.-P. Sutter, manuscript inpreparation.
11 C. Stroh, R. Ziessel, H. Heise, G.
Raudaschl-Sieber, F. H. Köhler, P. Turek,
N. Claiser, C. Lecomte, manuscript inpreparation.

References
1 J. H. Osiecky, E. F. Ullman, J. Am. Chem.
Soc., 90, 1078 (1968).
2 A. Caneschi, D. Gatteschi, R. Sessoli, Acc.
Chem. Res., 22, 392 (1989).
3 P. Turek, K. Nozawa, D. Shiomi, K.
Awaga, T. Inabe, Y. Maruyama, M.
Kinoshita, Chem. Phys. Lett., 180, 327