IACS Institutional Repository

Jana, Debnarayan

Chen, Li-Chyong

Chen, Chun Wei

Chen, Kuei-Hsien

Indian J. Phys. 81 (1) 41 -45 (2007) 
$ 
Ab-initio study of optical conductivity of B^ Cy 
nano-composite system 
Debnarayan Jana1", Li-Chyong Chen2, Chun Wei Chen3 and Kuei-Hsien Chen4 
department of Physics, University of Calcutta, 92 A P C Road, Kolkata-700 009, India 
2Centre for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan 
department of Material Science and Engineering, National Taiwan Unh/ersity, Taipei 106, Taiwan 
institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan 
E-mail djphy(a'caluniv ac in 
Abstract : We compute the imaginary part of the dielectric constant t2{Q -> O.a) of Single Wall Carbon Nanotubes 
(SWCNTs) alloyed with Boron (B) through fixed C-C bond length ab-initio Density Functional Theory (DFT) It is observed that the 
dielectric constants (both real as well as imaginary) essentially depend on B-doping concentration as well as the direction of 
polarization In particular, we demonstrate that the maximum value of the optical conductivity of this composite system depends 
strongly on the B-concentration in a non-linear way with a unique minimum at a particular critical concentration This study may be 
of importance in discussing the nature of collective excitations in B-alloyed SWCNT system 
Keywords : Optical conductivity, Nano Composite System, ab-initio calculation 
PACS Nos. : 31 15 Ar, 73 22 -f, 78 67 Ch 
1. Introduction 
Recent years have seen considerable advancement in research in C^ and carbon 
nanotubes [1,2]. It is well known that the intrinsic carbon nanotubes are unable to 
detect the highly toxic gases, water molecules and bio-molecules [3]. To improve the 
nanosensor reliability as well as quality, the importance of substitutional doping of 
impurity atoms such as boron, nitrogen atoms have been demonstrated in the literature 
[4]. Apart from this, an enhancement of electron field emission [5] has been observed 
in B doped system. In general, through the reaction with B203 with CNT under Ar 
atmosphere [6] boron atorn/s can be substituted in the carbon atoms/s of SWNT. 
Recently, electronic structure and optical properties of boron doped single wall carbon 
nanotubes (SWCNT) have been studied in detail and it is found that boron is in spr2 
configuration [7]. It has been also shown recently that even a small amount dopant can 
drastically change the mesoscopic conductivity of chemically boron doped carbon 
nanotubes [8]. All the above examples show the importance of the study of composite 
nanotubes and invite further investigation about the optical properties of the doped 
"Corresponding Author © 2007 IACS 
42 Debnarayan Jana, Li-Chyong Chen, Chun Wei Chen and Kuei-Hsien Chen 
system. Since this doping can alter the band structure (such as band gap, the density 
of states near Fermi energy) considerably [9,10], we would naturally expect some 
dramatic changes in the response of the B-doped carbon nanotubes under an 
electromagnetic field. The optical properties of 4 A diameter pure single wall nanotube 
have been investigated [11-13] recently by first principles calculation to explain the 
experimental results. 
In this paper, we study the response of the boron doped pure semiconductor 
carbon nanotube under the action of a uniform electric field with various polarization 
directions through ab-initio calculations with chemical doping of boron in a semiconductor 
(8,0) nanotube. We finally discuss the variation of optical conductivity as a function B 
doping concentration in different polarizations as well as un-polarized case. 
2. Numerical methods 
The optical properties of any system are generally studied by the complex dielectric 
function defined by 
D((D) = e(cu)E((o) = [e,(co) + e2{(o)]E(o)). (1) 
However, a\(co) and e2(o>) are not independent of each other. If one knows the values 
of any part for a wide enough range of frequency, then it is possible to get the other 
one via Krammers-Kronig relation. The conductivity a(co) is related to the dielectric 
constant via relation 
o(o)) a a, + io2 = ~{t^ -1) . (2) 
We calculate the imaginary part of the dielectric function using first order time 
dependent perturbation theory as implemented in CASTEP code [14]. For the exchange 
and correlation term, the Generalized Gradient Approximation (GGA) as proposed by 
Perdew-Burke-Ernzerhof [15] was adopted. The norm-conserving pseudo-potential in 
reciprocal space was invoked for the optical calculation. Compared to standard Local 
Density Approximation (LDA) (with appropriate modifications) used mostly in electronic 
band structure calculation, the optical properties are standardized by spin un-polarized 
GGA. A cutoff energy of 470 eV for the grid integration was adopted for computing the 
charge density. We have used 6 Monkhorst [16] /c-points for Brillouin zone integration 
along the tube axis. The detailed numerical procedure will be published elsewhere [17]. 
Geometry optimization of the doped SWNT has not been considered in this short 
paper. 
3. Results and discussion 
(A) Study of variation of dielectric function with boron doping : 
Before we discuss the optical properties, we show in Figure 1 the typical stick and 
ball model of (8,0) B3C system. We compute the imaginary part of the dielectric 
Ab-initio study of optical conductivity ofBfiy nano-composite system 43 
constant within the specified frequency range for semiconductor SWCNT. We note that 
the static value (strictly speaking co - * 0, but in our numerical computation to = 0.0150 
Hz) of the dielectric constants for both pure and doped system is always positive. We 
note that the change of value of static dielectric constant depends on both polarizations 
as well as on the nature of CNT. The real parts of the ptatic dielectric constant at 
to s 0.0150 Hz. for various B concentrations are shown ifl Figure 2. 
1200 
Parallel Polarization 
1000 jtfnpolarfcad Light 
perpendicular Polarization 
800 
600 \ \ 
400 
200 V"l! i o-2 °-J ° 4 <>•• 
0.0 0.2 0.4 0.6 0.8 1.0 
Boron Dopling Concentration 
Figure 2. Variation of static (real) dielectric constant of 
(8,0) B doping for parallel and perpendicular CNT with 
polarization un-polarized one. Inset shows along with the 
extended version of the same graph is depicted up to 0.5 
doping. 
It is evident from the figure that unlike the single B-doped system, for BC3 tubes 
(corresponding to 0.25 concentration) in all three cases, the static values (both real as 
well as imaginary) are increased as compared to pure (8,0) tube. The most remarkable 
fact is that such a small boron concentration is enough to change the optical behavior 
drastically in compared to pure case. With increase of boron concentration, the static 
values decrease and reach very close to pure case. Since boron atom has one 2p 
electron and with increase of boron atom, the system resembles more or less with the 
pure one. For parallel and un-polarized case, the enhancements of the static values are 
appreciable at a particular concentration in compared to the perpendicular one. 
However, at 0.75 and above boron doping, we find a significant giant enhancement of 
the static dielectric constant for all direction of polarizations including the un-polarized 
case. This could be due to the availability of large free charge carriers in the system. 
(B) Study of the optical conductivity of the doped system : 
The optical conductivity is studied from the dielectric constant with zero DC conductivity 
and 0.5 eV Drude damping. It is noticed that real part of the conductivity ay is always 
positive throughout the range of the frequency while the imaginary part t% oscillates 
Between the positive and negative values. This feature is common in any direction of 
3lectromagnetic field including the un-polarized one. The multiple peaks in lower 
frequency region in pure (8,0) have been smoothen in B3C. The maximum value of the 
Figure 1. Ball and stick model of (8,0) B3C 
tube in 3d triciinic structure. 
44 Debnarayan Jana, U-Chyong Chen, Chun Wei Chen and Kuei-Hsien Chen 
peak in B3C has been substantially enhanced In compared to pure case. This is due 
to increase of free electrons in B3C. Besides, in contrast to pure situation, the peak 
position in B3C has been shifted to lower frequency region. In Figure 3, we show the 
variation of the maximum optical conductivity (both real as well as imaginary) with 
boron doping concentration for parallel polarization and un-polarized light with incidence 
(1.0, 0.0, 0.0). A typical polynomial fit suggests the existence of a unique minimum 
value of a^Q)) at a particular boron doping concentration in all three cases. The 
values of the concentration at which the minima occur differ in each case signaling the 
dependence of the nature of incident electromagnetic field. We show in the following 
table (Table 1) the typical values of the minima of a^co) and the corresponding doping 
concentration for three situations. 
Table 1 . Result of minima of optical conductivity. 
Polarization 
Paralle 1 
Perpendicular 
Un-polarized 
Minimum value 
of a™*(*V) 
0.709 
1.454 
1.225 
Respective boron 
doping concentration 
0.421 
0 182 
0.389 
Minimum value 
of c r ^ e V ) 
0.364 
0.734 
0.561 
Respective boron 
doping concentration 
0.413 
0.200 
0.382 
(a) Parallel Polarization 
18 
14 
^ 12 
& 10 
T a 
I e 
O* 4 
2 
• Real 
• Imaginary ~J\ 
(b) Unpolarized Light 
10 
& , 
X 6 
3 
V 
1.0 0.0 0.2 0.4 0.6 0.8 
Boron Doping Concentration 
ol~ 0.0 0.2 * 0.4 O.Tj 0.8 ' 1.0 
Boron Doping Concentration 
Figure 3. The maximum optical conductivity v^w) as a function of boron doping is shown for (a) parallel 
polarization, (b) un-polarized light with incidence (1,0,0). 
4. Conclusions and perspectives 
From the first principles optical study of the b-doped SWCNT (8,0) semiconductor 
system, we observe that above a certain critical concentration of boron doping, the 
static values of the dielectric constant are considerably enhanced in compared to pure 
case. The real as well as the imaginary part of the maximum value of the optical 
conductivity show a non-linear behavior with boron doping having unique minima in 
polarized as well as un-polarized cases. 
Acknowledgments 
We would like to thank the National Science Council (NSC) of the Republic of China 
for financially supporting this research under Contract No. NSC93-2811-M-002-034. 
Discussions with Dr. C L Sun and Dr. Yue-Lin Huang are gratefully acknowledged. 
Ab-initio study of optical conductivity ofBfiy nano-composite system 45 
References 
[1] R Saito, G Dresselhaus and M S Dresselhaus Physical Properties of Carbon Nanotubes (London : 
Imperial College Press) (1998) 
[2] S Reich, C Thomson and J Maulzsch Carbon Nanotubes : Basic Concepts and Physical Properties 
(Weinheim: Wiley-VCH) (2004) 
[3] J Kong, N Franklin, C Zhou, M Chapline, S Peng, K Cho and M Dai Sconce 2B7 622 (2000) 
[4] $ Peng and K Cho Afeno Lett. 3 513 (2003) f 
[5] J-C Charlier, M Terrones; M Baxendale, V Meunier, T Zacharia, N | Rupesinghe, W K Hsu, N Gobert, 
H Terrones and G A L Amartunga Nano Lett. 2 1191 (2002) -
[6] W Han, J Cumings, X Huang, K Bradley and A Zettl Chem. Phys. Left. 346 368 (2001) 
[7] T Picher, E Borowiak-Palen, G G Fuentes, M Knupfer, A Graff, J fink, i Wirtz and A Rubio AlP Conference 
Proceedings 685(1) p361 (2003) 
[8] S Latil, S Roche, D Mayou and Jean-Christophe Charlier Phys. Rev. Lett. 92 256805 (2004) 
[9] L D Carroll, P Redlich, X Blas6, J-C Charlier, S Curran, P M Ajayan, S Roth and M Ruhle Phys. Rev. Lett. 81 
2332 (1998) 
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P M Ajayan, W Blau, M Ruhle and L D Carroll Nano Lett. 1 457 (2001) 
[11] M Machon, S Reich, C Thomsen, D SanchezrPortal and P Ordejon Phys. Rev. 866 155410 (2002) 
[12] X P Yang, H M Weng and J Dong Euro. Phys. J. B 32 345 (2003) 
[13] H J Liu and C T Chan Phys. Rev. B 66115416 (2002) 
[14] MD Segall, P L D Lindan, M J Probert, C J Pickard, P J Hasnip, S J Clark and M C Payne J. Phys.: Cond. 
Matter U 2717 (2002) 
[15] J P Perdew, K Burke and M Ernzerhof Phys. Rev. Lett. 77 3865 (1996) 
[16] H J Monkhorst and J D Pack Phys. Rev. B13 5188 (1992) 
[17] D Jana, L-C Chen, C W Chen, S Chattopadhyay and K H Chen A First Principles Study of the Optical 
Properties of BxCy Single Wall Nanotubes (Submitted to Carbon) (2006)

English

Ab-Initio Study of Optical Conductivity of BxCy Nano-Composite System

Behera, Banarji

Nayak, P

Choudhary, R N P

Dielectric and Electrical Properties of NaBa2V5O15 Ceramic

Prasad, K

Lily

Kumari, K

Suman, C K

AC Impedance Analysis of Modified PbTiO3 Ceramic

Yadav, B S

Yadav, Amit Kumar

Seema

Chand, Subhash

Indian J. Phys. 81(3) 395-401 (2007) 
Electronic spectral studies of 2-chloro-6-methylpyridine 
in various solvents 
2 
B S Yadav*, Amit Kumar Yadav, Seeroa1 and Subhash Chand 
Molecular Spetroscopy and Biophysics Laboratory, D.N (PG) College, Meerut-250 002, 
Uttar Pradesh, India 
department of Chemistry, J V (PG) College Baraut, Bagpat-250 602, Uttar Pradesh, India 
department of Chemistry, Meerut College, Meerut-250 001, Uttar Pradesh, India 
Received 8 June 2006, accepted 8 February 2007 
Abstract: The electronic transition bands of 2-chloro-6-methyl pyridine molecule in liquid state and the effect of 
^ -halogen atom on the absorption intensity of the 8-band (characteristic band of aromatic ring) have been 
studied The n-iC transition (E2-band) has shown the usual behavior towards the substitution and the polarity of 
the solvents But the rviC transition (R-band) has evidenced a usual behavior towards the solvent polarity (/ e. 
the Thif transition has shown blue shift with respect to the n-if system of pyridine) The halogen atom and/or 
orhto para directing group (having high potential for a large inductive effect) have shown a marked influence on 
the sff electrons of the N-atom of the ring As a result, the 8-band has evidenced an unusual behavior / e it has 
not shown a hyperchromic effect with increase in the polarity of the solvent 
Keywords : Ultra violet absorption spectra, solvents effect on electronic transitions 
PACS Nos : 78.40. Dw, 31. 70 Dk 
1. Introduction 
The N-Heterocyclic compound such as Pyridine and their derivatives are of considerable 
biological importance. They have been known to act as herbicides, insecticides, rodenti-
cides and plant growth regulators [1-4] etc. However very little spectroscopic studies are 
reported for them in literature. Pyridine shows in addition to the usual n-n* system, a 
shoulder band towards longer wave lengths. This system which is symmetry allowed and 
polarized perpendicular to the molecular plane, has been assigned as due to t>*? transition 
by Kasha [5]. Further, a systematic and complete Vibrational analysis of electronic spectra 
of Pyridine and some of its deuterated analogues has been made by some previous 
workers [6-8]. On substitution, the n-rt and t>if system of Pyridine normally produce 
* Corresponding Author © 2007 IACS 
396 B S Yadav, Amit Kumar Yadav, Seema and Subhash Chand 
bathochromic and hypsochromic shifts respectively, the vibrational frequencies also get 
modified. As regards the earlier studies, the spectral data of some of the mono and 
disubstituted pyridine are provided among other [5, 9-15]. In the present work, we have 
measured the n-n* and n-n* transitions of 2-chloro-6-methyl pyridine (here after referred as 
2,6-CMP) in the region of 400-200 nm in liquid state using polar solvents with a view to 
investigate the effect of solvent and effect of halogen and/or ortho para directing groups on 
the position and intensity of the electronic absorption bands. 
2. Experimental procedure 
The spec-pure grade sample of 2,6-CMP was obtained from M/s Sigma Aldrich Chemicals 
Co. West Germany and used as such. The Purity of the compound was further confirmed 
by boiling point (64°) determination and elemental analysis. We have measured the n-K*l 
n-n*II and n-if transitions of 2-chioro-6-methyl pyridine in a uv/vis double beam spectro-
photometer Model Backman M-35 in the region of 400-200 nm liquid state, using various 
polar solvents (wz-ethonol-»methanol~>water). The solvent used for preparing the solutions 
of the said compound were of spectroscopic grade. 
3. Results and discussion 
The molecular structure of 2, 6-CMP is shown in Figure 1. The near ultra violet absorption 
spectra (region 400-200nm) of 2, 6-CMP in various solvents (a) Ethanol (b) Methanol and 
(c) Water are shown in Figure 2. The electronic transitions of the compound observed in 
various solvents are given in Table 2. 
s*** 
Figure 1. The molecular structure of 2-chloro-6-methy! pyridine. 
3.1 Band due to n-n* transition: 
The spectra of heteroaromatic compounds are on the whole similar to their corresponding 
hydrocarbons [16,17] The spectrum of pyridine is comparable with that of benzene. 
Benzene itself displays three electronic absorption bands [16,17] at 184nm (e, the molar 
absorptivity or the extinction = 60,000), at 203.5nm (e=7900) and at 256nm (e=200) due 
to K-i? transitions. The itense band at 180nm results from an allowed transition [17], 
whereas the weaker bands at 200 and 260nm result from forbidden transitions in the highly 
symmetrical benzene molecule. Different notations have been used to designate the 
absorption bands of benzene, these are summarized in Table 1. We shall discuss these 
Electronic spectral studies of 2-chloro-6-methylpyridme etc 397 
5.000-
oooo H 
-5.000 A 
'H i 
I 
I 
j j j , , , , , ,
 1 j ( — ^ j j j j j ( ( j ( ^ 
200 400 |00 800 
Wavelength (nm) 
(a) 
0100 
0.000 
-0100H 
t 
/» 
1 « ) 
vy 
— I — i — i — i — i — r — T — i — i — i — « — i — i — | — i — i — r -
260 280 
-i—i—|—i—i—i—r-
300 
- T — I — I — I — I — I -
Wavelength (nm) 
(b) 
Abs 
0.000 
—,— 
250 
300 
Wavelength (nm) 
(c) 
Figure 2. Near ultra violet absorption spectra (region 400-200 nm) of 2-chloro-6-methyl pyridine in 
(a) ethanol (b) methanol and (c) water. 
using braude's Eand Snotations[17]. The ©-band has evidenced an unusual behavior i.e. 
it has not shown a hyperchromia effect with increase in the polarity of the solvent. Such 
type of bands i.e. unusual behaviors of B-band arises due to n-n* transition in aromatic or 
hetro-aromatta molecules (the n-n* transition has shown red shift with respect to the n-x* 
system of pyridine). Benzenes shows absorption peaks between 230-270 nm. When a 
chromophoric group is attached to the benzene ring, the 8-bands are observed at longer 
398 6 S Yadav, Amit Kumar Yadav, Seema and Subhash Chand 
wave-lengths than the more intense K-bands. The position as well as the intensity of the 
S-band is not shifted by increasing the polarity of the solvent. But in heterocyclic aromatic 
compounds a marked hyperchomic shift (increases in e ^ is observed by increasing the 
polarity of the solvent. This type of transition occurs in the unsaturated centers of the 
molecule, i.e., in compounds containing double or triple bonds and also in aromatics. The 
excitation of n electron requires smaller energy and hence, transition of this type occurs 
at longer wavelength. A n electron of a double bond is excited to n* orbital. S-band is 
generally found in between 230-270 nm. 
Table 1. Electronic absorption bands of benzene melecule (16). 
184nm 
Et-and 
204nm 
E2-and 
K-band 
256nm 
S-band 
tf-band 
References 
(18) 
(19) 
Between benzene and pyridine, the S-band of pyridine is more intense with some what 
diminished fine structure where as in benzene, the S-band at 256nm displays fine structure 
i.e. the band contains multiple peaks. This transition is allowed for pyridine but forbidden 
for more symmetrical benzene [16,17]. The band observed at 282nm, 283nm and 277nm 
in the study in all the three solvents (ethanol, methanol and water) are originated from 
TT-TT*, transition and designated as S-band. 
3.2 Bands due to n-n * transition : 
The f?-band expexted for an n-n* transition for pyridine molecule has been observed in 
vapour phase spectra. The spectrum is determined in solution [17] because of intermolecu-
lar hydrogen bonding found between the N-atom of pyridine ring and the solvent molecule. 
The ff-bands found in this study, have originated from n-n*l and n-7t*U transitions due 
to the attachment of groups containing lone pair electrons i.e. -CI or -CH3 groups and 
occured at 298nm (Methanol), 291 nm (Ethanol/Methanol) and 281 nm (Water) respectively. 
4. Effect of substitution on electronic transitions 
An Auxochrome (Le. a saturated group with non bonded electrons eg: - OR,-ON,-NH2, and-
CI etc.) attached to a chromphore (a covalentaly unsaturated group responsible for elec-
tronic absorption eg. aromatic ring, -C-C-.-CsC-, etc.) alters both the wave length and the 
intensity of the absorption [16]. Auxochromic substitution on the aromtic ring shifts S-band 
to longer wave length, frequently with intensification of the 6-band and loss of its fine 
structure because of *>*;* conjugation [17]. Generally, the S-band occured in between (230-
270nm) and displays a fine structure i.e. the band contain multiple peaks. In the present 
investigation S-bands (n-n*) observed at 282nm, 283nm and 277nm in 
ethonol-»methanol->water have large intensity without a fine structure. In auxochromic 
substitution. The S-bands shifts to longer wavelength frequently increase in intensity. 
Electronic spectral studies of 2<htoro-6-methylpyridine etc. 399 
CM, & ' ^ V ^ ^ ^ CM, 
(O) (t» 
Figure 3. The combined inductive and mesomeric (resonance) effects of chloro and methyle group 
responsible for shift in n-n* and n-n* transitions. 
The auxochromic groups present on aromatic riilg are responsible for n-n* transition 
(fl-band) due to the presence of non bonding electrons (i.e. lone pair electrons), available 
on the auxochromic group. In 2,6-CMP molecule (/.e.disubstituted molecule), the inductive 
effect of auxochromic groups is responsible for n-n conjugation. The-CI group has three 
lone pair of electrons to that of -CH3 group. Further more, the -CI group is highly electrone-
gative and have much more negative inductive effect in comparison to that of -CH3 group 
and the size of -CI group is much lower than that of -CH3 group. As a result of which 
valence shell electrons of -CI group contribute large nuclear charge in comparison to that 
of -CH3 group. In conjugated systems, the energy separation between the groups at the 
excited states is reduced and the system absorbs at larger wave lengths with an increased 
intensity [16]. (i.e. S-band is intense at longer wave length). The shift in absorption results 
from a combination of inductive and resonance effects are shown in Figure 3. The /?-band 
(due to the additive effect of auxochromic substitutions) shifts to longer wave lengths at 
298nm (Methanol), 291 nm (Ethanol/Methanol) and 281 nm (Water) respectively. 
5. Effect of solvents on electronic transitions 
The B-band of Benzene, an increase in the solvent polarity does not effect either the 
position or the intensity. But Produces a hyperchromic effect (an increase in absorption 
intensity). For pyridine and its homologs [16,17], this effect is assigned to the hydrogen 
bonding through the lone pair of electrons of the nitrogen atom [18,19] In 2,6-CMP molecule, 
an increase in solvent polarity, hyperchromic effect is not observed because of the presence 
of oc-halogen atom (-CI), which has high negative inductive effect (-1) and influence that sff 
electrons of the A/-atom of the ring, As a result, it makes the lone pair electrons of Af-atom 
of the ring less available for H-bonding. The negative inductive efffect of chloro group 
becomes much greater, as the solvent polarity increases by the dipole-dipole interaction 
between the solute and solvent particles. As the solvent polarity decreases, the negative 
inductive effect of-CI group also decreases. As a result, the lone pair electrons of /V-atom 
of the ring are readily available for intermolecular H-bonding to result in the increase in 
absorption intensity of the S-band in less polar solvent than in more polar solvent. 
The n-if transition (f?-band) shows a hypsochromic shift (blue shift) on increasing the 
polarity of the solvent [16]. The n-if transition will be of higher energy i.e. of lower wave 
length (blue shift) which is shown in Figure 4. In the present work the blue-shift has been 
observed by n-n* transition (ft-band) as the polarity of the solvent increased [20,21] i.e. 
400 B S Yadav, Amii Kumar Yadav, Seema and Subhash Chand 
ethanol-»methanol->water (Table 2). The amount of blue-shift has been used as a measure 
of the strength of the H-bonding. 
n-7i* transition is raised 
in energy 
the n-n - energys 
lowered 
solvent a more polar solvent 
Figure 4. Solvent effect on n-rc* and it-it transition. 
The n-it transition has shifted to the longer wave length i.e. red shifted (Lower energy) 
in a more polar solvent [16]. The polar excited state of the n-tf transition is stabilized by 
W-bonding in more polar solvent. Becker et al [22], has suggested that the n-n* transition 
under go red shifts (Lower energy) as the polarity of the solvent increases. This Lowers 
the distance between n and if levels with a subsequent lowering in the energy or raising 
the wave length of the transition shown in Figure 4. We have observed the red shift by the 
n-n* transition (E2-band) as the polarity of the solvent has increased from 
ethanol-*methanol->water (Table 2) in 2, 6-CMP. 
Table 2. Effect of solvents on electronic absorption bands/transition of 2-chloro-6*methyl pyridine 
(all values are in nanometer). 
Solvent 
Ethanol 
Methanol 
Water 
ihif\ 
(f?-band)/nm 
-
298 
-
n-ru 
(fl-band)/nm 
291 
291 
281 
K-lf 
(B-band)/nm 
282 
283 
277 
Acknowledgment 
Authors are thankful to Punjab University, Chandigarh for providing the necessary facility 
for recording the ultraviolet absorption spectra. 
Electronic spectral studies of 2<hloro-6'methylpyridine etc. 401 
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Electronic Spectral Studies of 2-Chloro-6-Methylpyridine in Various Solvents

Prasad, K Durga

Somayaji, T S N

Naidu, P Appala

Rao, D N Madhusudhana

Arunamani, T

Devi, M Indira

Indian 7. Phys. 81 (5 & 6), 521 -530 (2007) 
A two-dimensional model study of latitudinal, seasonal and 
solar activity variation in transition height in the Z)-region 
K Durga Prasad1, T S N Somayaji2, P Appala Naidu3, 
D N Madhusudhana Rao4, T Arunamani4 and M Indira Devi4* 
lFost Graduate Courses, Maharajah's College, Vizianagaram 535 001, Andhra Pradesh, India 
2School of Physics, University of Hyderabad, Hyderabad-500 046, Andhra Pradesh India 
department of Physics, J N TU College of Engineering, Hyderabad-500 072, Andhra Pradesh, India 
department of Physics, Andhra University, Visakhapatnam-500 003, Andhra Pradesh India 
E-mail indiramalladi(a;yahoo com 
Received 20 October 2006, accepted 16 May 2007 
Abstract . The height of transition above which the water cluster ions give way to molecular ions, is an 
important aspect of the D-region of the ionosphere A recent and comprehensive two-dimensional neutral 
constituent model, SOCRATES, from NCAR has been used in conjunction with an appropriate ion chemical 
scheme to obtain ion composition and the transition height The latitudinal and seasonal variation in the transition 
height for low and high solar activity conditions has been studied It is found that the transition from water 
clusters to molecular ions generally occurs at higher altitudes in solar minimum than in maximum At mid latitudes, 
an asymmetry about the equator is discernible A perceptible seasonal variation exists at middle and high 
latitudes. 
Keywords Ionosphere, 0-region, transition height, two-dimensional model 
PACS Nos. : 94.20 Vv, 94.30 Va, 95 85 Bh 
1. Introduction 
The />region of the ionosphere lying between 60 and 90 km is produced mainly by 
photoionization of NO and 02(1Ag) by solar Lyman-a at 1216A and radiations in the range 
1027A and 1118 A, respectively. Also soft X-rays of wavelength less than 100 A contribute 
to ionization in upper D-region. However, the ion chemistry in this region is quite complex 
when compared with that of E and F regions lying above. The complexity arises because 
Corresponding Author 
©2007IACS 
522 K Durga Prasad etal 
of the high pressures obtained here, and a number of neutral minor constituents produced 
as a consequence of three body reactions. So a detailed study of the observed features 
in the electron density profiles of the D-region requires knowledge of neutral composition 
and ion-chemistry. This is largely accomplished by model studies. In spite of the fact 
that extensive investigations, both experimental and theoretical, carried out over the past 
many years, the D-region continues to interest all scientists [1-4]. 
One of the aspects of the D-region electron density profiles is the steep gradient or 
ledge that forms at heights between 80 and 90 km, where the electron density jumps by 
an order or two in magnitude. After the discovery of water cluster ions in the D-region 
[5], this ledge has been explained on the basis of transition from cluster ion dominant 
lower part to molecular ion dominant upper part of the region. Since the rates of 
recombination of water cluster ions are more than those of molecular ions, a steep rise in 
electron density results at the transition level [6,7]. The height at which this transition 
occurs, has been found to vary with solar zenith angle, season and solar activity [7]. 
Model calculations greatly help in understanding this variability. Further, a study of the 
transition height variation gives an insight into the changes in the minor constituent 
distributions since the transition height variability results from the combined influence of 
solar irradiance and the distribution of minor species like NO, 0 and OH. 
2. Methodology 
In order to study the height of transition between water cluster ion dominant region and 
molecular ion dominant region in the D-region, it is required first to obtain the ion 
composition and its distribution, which in turn, requires neutral species distribution and 
appropriate ion-chemical scheme. Most of the earlier studies of the transition height were 
based on one-dimensional models [8-10]. But one-dimensional models suffer from certain 
inherent shortcomings as they do not include transport in its true sense and give only a 
global average picture. These shortcomings are overcome by using two-dimensional models, 
which consider transport in the form of zonal eddies and mean meridional circulation. In 
the present paper, the seasonal, latitudinal and solar activity variations in the height of 
transition are studied employing a comprehensive two-dimensional model, SOCRATES 
(Simulation Of Chemistry, Radiation and Transport of Environmentally important Species), 
developed by NCAR [11,12]. This model is designed to give atmospheric constituent 
distribution from surface to 120 km at 1 km intervals and between latitudes 85°S and 
85°N in steps of 5°. Multiple scattering of air molecules, aerosols and clouds is considered 
in the calculation of heating rates and photolysis rates for stratospheric studies. For 
mesospheric studies, a non-LTE C02 infrared cooling is incorporated in place of simple 
Newtonian cooling. Heating from chemical recombination and energy loss to airglow 
process, important in the mesosphere, are also considered. Dynamical forcing from 
planetary and gravity waves and quasibiennial oscillation (QBO) in temperature are included. 
To solve the chemical transport, the model uses a semi-Lagrangian scheme. For obtaining 
the ion composition from the neutral constituent data, the ion chemical scheme suggested 
by Reid [13] has been used. This scheme includes 59 positive and 23 negative ion 
A two-dimensional model study etc 523 
reactions. The required data of solar radiation fluxes and ionization cross sections etc. 
are taken from literature [14,15] 
Concentration (cnrr*j[ 
1 00E+05 1 OOE+06 1 00E+07 1 00E+08 100E+09 1 00E+10 1 OOE+11 
TUU< 
95 
90 
| 8 5 
-8 ao 
I »• 
70 
65 
60-
- — v: ; -—'— 
I ""•^ qrXRAYS I j% * ^^NJ""" 
/ *f^ */* 
*^J&r fjT 
Its * * Jt 
* • A » J T » 
j,^* 750^ 
i^j\ 
100 
95 
90 
85 J 
80 "§ 
75 1 
70 
65 
60 
1 00E-03 1 00E-02 1 00E-01 1 0OE+00 1 00E+01 1 00E+02 1 00E+03 
Electron-Ion Production Rate (cm"3s~1) 
Concentration (cm-3) 
1 00E+O5 1 00E+06 1 00E+07 1 00E+O8 1 00E+09 
1001 ^+10 1 OOE+11. 
1 00E-03 1 00E-02 
100 
95 
90 
85 f 
80 "§ 
75 < 
70 
65 
60 
1 00E-01 1 OOE+00 1 00E+01 1 00E+02 1 00E+03 
Electron-Ion Production Rate (cnrr3s 1) 
Concentration (cm-3) 
1 00E+05 1 00E+06 1 00E+07 1 00E+08 1 00E+09 
100-
1 00E+10 1 00E+11 
100E-03 100E-02 1OOE-01 100E+00 100E+01 10OE+O2 1 dOE+03 
Electron-Ion Production Rate (cnrV1 ) 
Figure 1. (a) Ion production rates due to Lyman-a, UV and X-rays (Lower 
Soale) and NO and H20 concentrations (Upper Scale) for summer solar minimum 
524 K Durga Prasad etal 
Concentration (cm"3) 
100E+05 100E+06 1 00E+07 1 00E+08 100E+09 100E+10 100E+11 
1 00E-03 1 00E-02 100E-01 100E+00 1 00E+01 1 OOE+02 
Electron-Ion Production Rate (crrrV1) 
1 00E+03 
1 00E+05 1 00E+06 
100 
95 
90 
J 85 
| 80 
' 75 
70 
65-I 
- q-LymanAlpha 
-qXRAYS 
Concentration (cm-3) 
100E+07 100E+08 1OOE+09 1 00E+10 1 00E+11 
100 
45°N 
-H2Ql 
95 
90 
I-85 1 
H80 "a 
75 < 
70 
65 
60 J . . . • • L60 
100E-03 1 OOE-02 1 OOE-01 1 OOE+00 1 00E+01 1 00E+02 1 00E+03 - 0 E-02 1 00E-01 100E+00 1 0 E+01 100E+02 1 
Electron-Ion Production Rate (cnrrV1) 
Concentration (cm**3) 
100E+05 100E+06 100E+07 1 OOE+08 1 OOE+09 
100, 
1 OOE+10 
10OE-02 100E-01 100E+00 100E+O1 1 OOE+02 
Electron-ton Production Rate (crirtr1) 
Figure 1. (b) Same as Figure 1 (a) for solar maximum 
1 00E+11 
100 
95 
90 
86 | 
80 "§ 
7 5 * 
70 
65 
60 
100E+03 
A two-dimensional model study etc 525 
The altitude distributions of neutral constituents relevant to mesosphere and lower 
thermosphere, viz, oxygen (O, 03> 02(1Ag))f odd hydrogen (H, OH, H02), odd nitrogen (N, 
NO, N02), H20 and the temperature and air densifjy have been obtained from the model 
described above. The vertical distributions of N2, 02jand C0 2 have been obtained from the 
air density assuming the standard mixing ratios oftthese gases. These data have been 
obtained at 2 km intervals between 60 and 120 kr# and used in conjunction with the ion 
chemical scheme described above to obtain the iqn composition and electron density. 
3. Results 
The neutral composition data along with the radiation fluxes have been used in the ion-
chemical scheme to calculate the ion production rates. Figures 1 (a, b) illustrate the ion 
production rates due to principal ionizing radiations, Lyman-oc (1216 A) and UV (1027-
1118 A) as well as X-rays (<100 A) for summer conditions at representative high (75°N), 
middle (45°N) and low (15°N) latitudes and for the two solar epochs. In the same figure, 
the neutral concentrations of H20 and NO are also shown. The ion composition is then 
obtained from the model calculations and is shown in Figures 2(a-c) for the three 
representative latitudes mentioned earlier, under solar minimum conditions. In these figures, 
the height distribution of various positive ions, between 60 and 100 km in the months of 
December, June and September, representing winter, summer and equinox respectively, is 
depicted. From these figures, it is evident that the total water cluster ion concentration 
dominates over the molecular ion concentration in the lower Z>region at all latitudes and 
in all seasons, as is generally expected. However, a closer examination reveals that 
substantial contribution to the total hydrated clusters comes from third and fourth hydrated 
clusters up to an altitude, which is found to have latitudinal and seasonal dependence. 
While the concentrations of the third and fourth hydrated ions are more below 68 km at 
high latitude in winter, they are dominant up to 78 km in low latitude equinox. A similar 
trend has been noticed in the ion composition during high solar activity also. 
The height of transition between the cluster-ion dominant region and the molecular ion 
dominant region is identified from the ion composition height profiles as the altitude at 
which the contributions of the two kinds of ions are equal. The variation of this transition 
height with latitude in different seasons and for low and high solar activity conditions, is 
shown in Figure 3. The seasons marked in the figure refer to seasons in northern 
hemisphere. Therefore, the transition heights along the lines marked corresponding to 
winter at northern latitudes, correspond to summer in southern latitudes. The following 
observations are made from this figure. The transition height is generally large in solar 
minimum than in solar maximum. A lowest transition height of 71 km is obtained at 75° 
latitude in winter, while the transition height is as high as 88 km in summer at the same 
latitude in solar minimum. 
At equator and low latitudes, there is little seasonal variation in transition height. At 
other latitudes, the summer values are higher than those in winter. An interesting 
observation that can be made is that while the transition heights at low and high latitudes 
are nearly symmetrical about the equator, at middle latitudes, there appears to be some 
526 K Durga Prasad etal 
asymmetry with higher values in southern latitudes than in northern latitudes, especially 
in winter. 
1 OOE-08 1 OOE-07 1 OOE-06 1 00E-05 1 00E-04 1 00E-03 1 OOE-02 1 00E-01 1 OOE+00 1 00E+01 1 00E+02 1 00E+03 1 00E+04 1 00E+05 
Concentration^ - ions per c c 
1 OOE-08 1 OOE-07 1 OOE-06 1 00E-05 1 OOE-04 1 OOE 03 1 OOE-02 1 00E-01 1 OOE+00 1 0OE+01 1 OOE+02 1 00E+03 1 00E+04 1 00E+05 
Concentration - ions per c c 
1 OOE-08 1 OOE-07 1 OOE-06 1 OOE-05 1 OOE-04 1 00E-03 1 OOE-02 1 00E-01 1 OOE+00 1 00E+01 1 OOE+02 1 00E+03 1 OOE+04 1 OOE+05 
Concentrat ion - Ions per c.c. 
Figure 2. (a) D-region ion composition in different seasons at 75° N 
A two-dimensional modei study etc 527 
The transition height and its variability obtained in the present model study, are consistent 
with earlier experimental and theoretical results. Ganguly [16] from incoherent scatter 
1 0OE-O7 1 O0E-06 1 00E-05 1.00E-04 1 00E-03 1.00E-02 1 00E-01 1 OOE+00 1 00E+01 1 00E+02 1.00E+03 1.00E+04 
Concentration - ions per c c. 
92 
871 
I 82 
h 
< 
721 
67 
nrer 
• H+(H20) 
• H+JH20 2 
• H+(H20)3 
- H*(H20)4 
• H«.|H20)5 
• H+(H20)6 
• NO++02+ 
62 
1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1 00E+02 1.00E+03 1.00E+04 
Concentration - Ions per c.c 
1.00E-08 1.00E-07 1.00E-06 1.00E-05 1 00E-04 1.00E-03 1.00E-02 1.00E-01 t.00E+00 1.00E+01 t.OOE+02 1.O0E+03 1.00E+04 
Concentration - ions per c.c. 
Figure 2. (b) Same as Figure 2 (a) at 45° N. 
528 KDurga Prasad etal 
1 OOE-08 1 OOE-07 1 OOE 06 1 OOE-05 1 OOE-04 1 OOE-03 1 OOE-02 1 OOE-01 1 00E+00 1 OOE+01 1 OOE+02 1 OOE+03 1 00E+04 
Concentration - ions per ex 
1 OOE-08 1 OOE-07 1 OOE-06 1 OOE-05 1 OOE-04 1 OOE-03 1 OOE-02 1 OOE-01 1 00E+00 1 00E+01 1 OOE+02 1 OOE+03 1 00E+04 
Concentration • 
1 OOE-08 1 OOE-07 1.00E-06 1 OOE-05 1 OOE-04 1 OOE-03 1 OOE-02 1 OOE-01 1 00E+00 1 00E+01 1 OOE+02 1 OOE+03 1 00E+04 
Concentration - ions per c.c 
Figure 2. (c) Same as Figure 2 (a) at 15° N. 
A two-dimensional model study etc 529 
measurements observed that the transition height changes by as much as 10 km from 80 
to 90 km. Narcisi and Bailey [17] from rocket experiments showed that at equator, the 
transition height occurs at around 82 km similar to, what has been seen in the present 
7 0 H 1 r — i 1 1 1 1 1 1 1 1 1 1 1 r r 
-90 -60 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 
LATITUDE 
Figure 3. Latitudinal variation of transition height with solar activity. 
computations. The variation of transition height is generally explained on the basis of 
variations in temperature and water vapour content in mesosphere [18]. A detailed 
investigation to study the sensitivity of the transition height to these quantities is being 
made and the results will be published elsewhere. 
4. Conclusions 
The latitudinal, seasonal and solar activity variations in the height of transition between 
the regions of water cluster ion dominance and the molecular ion dominance in the D-
region have been studied using a two-dimensional minor neutral constituent model along 
with an appropriate ion-chemical scheme. The salient results of the study are as follows. 
Major contribution to water cluster ions comes from third and fourth order hydrates. 
The altitude extent of their contribution to the total hydrated clusters shows seasonal and 
latitudinal variability; while it is small in winter at high latitudes, it is large in equinox at 
low latitudes. As for the transition height itself, it is observed that this height is generally 
large in solar minimum than in maximum. Summer values are greater than winter values 
except at equator and low latitudes where there is little seasonal variation. Also it is 
found that while at low and high latitudes, the transition height is symmetrical about the 
equator, at mid-latitudes, there is a discernible asymmetry, especially in winter. 
Acknowledgment 
The authors wish to thank the anonymous referee for his valuable suggestions to improve 
the manuscript. 
530 KDurga Prasad etal 
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A Two-Dimensional Model Study of Latitudinal, Seasonal and Solar Activity Variation in Transition Height in the D-Region

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[ej U Fano and A R P Rao Atomic Collisions and Spectra (New York : Academic) Vol 1, Ch 2, Sec 4, p 25 
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[7] T Atsumi, T Isihara, M Koyama and M Matsuzawa Phys. Rev. A42 6391 (1990) 
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[14] SB Hansen, K B MacAdam and L G Gray 12th hit. Conf. on Atomic Physics (Ann. Arbor) Abstracts px-
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[ 19] W Fritsch (private communication) (1988) 
[211 K B MacAdam (to be published) (1991) 
[23] T Roy PhD Thesis (University of Calcutta, India) (1992) 
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836 Notes for Contributors 
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Notes for Contributors

Indian Association for the Cultivation of Science

Not a member yet

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