PREFACE
The Thesis entitled “Spectroscopic studies on certain cometary
diatomic molecules” deals with results of the investigations carried out by the
author
on
the
electronic
band
spectra
CO,CCf,CH,eir,CN, CS, N2\ N2,OH,S2,
no
of
the
diatomic
molecules
and NO+. Detailed account of the
work carried out by the author is presented in the form of five chapters. These
chapters are devoted exclusively for the constructions of potential energy curves
and the estimation of dissociation energies of comet diatomic molecules from
band spectroscopic data. In order to study the variation of intensity of different
bands, r-Centroids and Franck-Condon factors are estimated for different band
systems of the comet diatomic molecules. The results related to these chapters are
published in different journals.
Comets may represent the oldest and best-preserved material in the
solar system and are probably the only specimens offering information about the
very early phase of the solar nebula and the processes that led to the formation of
the planets. Comets (Arpigny et.al., 1994) offer of course, a great intrinsic interest
to learn more about the nature and origin of these fascinating objects and to have a
better interpretation of their sometimes puzzling and unique behaviour. However,
they are even more important owing to the numerous connections that exist
between their study and a number of fundamental areas of physics, chemistry and
astrophysics, such as low temperature physics and chemistry, spectroscopy,
photochemistry, the physical chemistry of surfaces, plasma physics, interplanetary
space physics, the study of the past and present of the solar system of the
interstellar medium.
A bright (W.F.Huebner et.al., 1990) active comet also provides a
spectacular display on the night sky. It is estimated that there are several times
|0U comet nuclei in the Oort cloud. For these reasons comets are interesting
objects in there own right that deserve to be investigated scientifically. However,
when reasons are given why comets should be investigated and why they are
interesting to science, the most commonly quoted objectives are the
interdisciplinary information that comets reveal about:
(1) The early history of the solar system, i.e., the chemical composition and
thermodynamic conditions of the solar or presolar nebula region in which comets
formed.
(2) The formation of planetesimals from cometary’s sub bodies.
(3) The evolution of planets through comet impacts, i.e., the formation of oceans
and atmospheres and the general enrichment of volatiles to the inner solar system.
(4) The origins of life from the influx of comets on Earth. Two additional, but
separate and equally important objectives can be raised.
(5) To understand the processes that give rise to plasma structures in comets that
might be identifiable from earth so that comets can be used as probes of the inter
planetary medium, particularly in regions where it is difficult and energetically
expensive to make in situ measurements. Still, the prime reason for studying
comets stems from the cosmogonic significance attached to them.
The most prominent bands (Herzberg, 1979) in the spectra occurring
in the tail of the comet are the 2n - 2Z bands of CO+ which are named as the
comet tail bands. The electronic spectrum of CCT consists of four electronic
band systems, namely, the comet - tail (AzIIj - XT'), the first negative
(B 2r - X 2Z+). The Baldet-Johnson (B 2I+-A 2rii), and the Marchand - D Inco Janin (C Ar - A 11;) band systems of CO are observed in the region between 1800
and 8500A0.
CH radical oresence has been observed in G. K. R and N-tvpe stars
(Stranghan.B.P, 1976) Herzberg first reported A*A - XTl system of CH molecule
observed in the optical spectral region between 3889A0 and 4315 A0. The CH+ ion
radical emission spectra of six bands of the A'TI-X'Z system in the region 395-399
nm have been also obtained by Bembenek. Large amounts of organic dust and
variety of species like CN ,NH,CO+, CS,NH+, S2, SO, CN+,OH+ are observed in
the comet Halley (Arpigny.C, 1994., Altwegg. K et al 1999). Prior to Halley,
neutral CO had been observed directly in only two comets; the CO fourth positive
group near X ~ 1500A0 was detected during sounding rocket observations of comet
west
and during IUE observations of comet Bradfield
(Heam.M.F., 1982).
Some of the bands of the CN red system (A2I1 - X 2Z+) occurring around
A,~8000A° were first seen in the comet mrkos based on photographic spectra. The
(W.F.Huebner, 1990) radical CS has been clearly identified in cometary ices by its
bands around 260nm.
S2 molecule (A’Heam.M.F. et al 1983) was discovered
serendipitously in comet IRAS-Araki-Alcock when the comet was at its closest
approach to earth, 0.032AU. The spectrograms of comet 1941 I Cunningham’s
nucleus region, taken by Pol Swings. Revealed for the first time the ultraviolet
bands due to the hydroxyl radical OH at 3078-3100A0. The study of the 18 cm
line of OH, in the radio region should also give information about the production
rates and the velocity fields in the coma. Spectra of natural molecular nitrogen N2
and its ion N2+ play a significant role in atmospheric and astrophysical
phenomena. The identification of the spectra of comets is one of the active areas
of study. This is due to the combination of many factors such as better
instruments, availability of space vehicles, computation techniques, etc. The
identification of the lines in the spectra of a comet is quite a complex and difficult
task. The usual procedure is to look for the coincidences between the laboratory
wavelength of lines and the observed wavelengths in the spectra of a comet, which
have been corrected for the velocity effect. With this method many of well-known
in
atoms and molecules that are seen in other astronomical objects have been
identified in the spectra of comets. Many more are still not identified.
Spectroscopy happens to be the most versatile remote sensing tool of
astronomers. It only gives the physical conditions in the source like temperature,
pressure etc., but also yields information about its chemical composition. Recently
studies of coma of comet Halley by the Giotto space craft of ESA and the Vega
Space Craft of the Soviet Union Provides Wealth of data on cometary materials.
As the comet approaches the sun, some diatomic species like CO, N2, CS, NO, 02,
OH, NH, CH, S2, CN, SO, SiO and C2 are observed and also in the tail of comet,
we see ionic species like CO+, OH+, CH+, CbT, NHT and N2\
Much physico-chemical information and data is not available on
comet comae and comets.
Reliable atomic and molecular data on cometary
materials are required for models of photon-driven physical and chemical process
and for models of the steady state properties of astronomical objects that absorb or
emit photons.
The preparation of laboratory spectra under simulated space
conditions is crucial for the identification and abundance determination of
molecular species in space.
The comparison of laboratory spectra with the
astronomical data from ground and space-based observations allows us to define
parameters of the environment. A detailed spectroscopic information of the above
cometary diatomic molecules is not available.
As such physico-chemical
parameters such as potential energy curves, dissociation energies, and ionization
energies, Franck-Condon factors, r-Centroids, for cometary molecules and the
spectroscopic information related to the above materials have to be studied in
detail. The study of spectra of comets in the infrared, visible, and ultraviolet
regions have now become important. It may be mentioned that such studies are
IV
useful for obtaining better understanding of matter and of the laws of the atomic
and molecular physics.
In chapter I a general survey is made of existing knowledge on
the nature of the ground and excited electronic states of the different comet
diatomic molecules. The importance of the calculation of the true potential energy
curves is mentioned. It is pointed out that using the potential energy curves, the
dissociation energies of the ground states for the above molecules have been
calculated. In addition, the r-Centroids evaluation and their importance have been
mentioned along with Franck-Condon factors.
Chapter II gives an account of the calculations of the true
potential energy curves from band spectroscopic data.
Using the well known
RKRV (Rydberg-Klein-Rees as modified by Vanderslice et al) method the
potential energy curves have been calculated for thirty three electronic states of
twelve diatomic molecules namely CO,CO+,CH+,CH,CN,CS,N2\ N2,OH,S2, NO
and NO+. The main results of this chapter brought out as four papers.
Chapter III deals with the determination of ground state
dissociation energies belonging to the twelve different comet diatomic molecules
for which potential energy curves are constructed.
The true potential energy
curves (RKRV) have been used to estimate the dissociation energies of diatomic
molecules in a number of cases by fitting an empirical potential energy curve. The
Hulbert - Hirschfelder function gives best average results and in general gives best
fit of the potential for all the cases studied. As has been observed by Steele,
Lippincott and Vanderslice the average error is not more than 1 to 2 percent in
|V-VrkrvI / De value. The dissociation energy thus evaluated compare very well
with the values obtained from other methods.
An accurate knowledge of the CN dissociation energy is of
paramount importance in deducing the nitrogen abundance of numerous
astrophysical objectives in which this radical appears, especially of red giants
(Costes et al 1990). The consideration of all D0 (CN) values reported so far in the
literature shows a scattered one. Our estimated dissociation energy of CN is 7.63
± 0.18eV is in good agreement with values 7.65, 7.60 and 7.70 eV recommended
by Sneden and Lambert (1982), Lambert et al.(1986), Banschlicher et al (1988)
and Gaydon (1968) respectively. The estimated dissociation energies CO, CN,
and CS are used in the cycle of ionization potential and dissociation energy,
CO,CN and CS ionization potentials are estimated. The obtained ionization
potentials of CO, CN and CS are 13.92,14.00,12.15 eV respectively.
Chapter IV deals with the determination of r-Centroids for ten
band systems belonging to seven diatomic molecules CO , CN, CO, N2\ N2, NO
and NO+. It has been shown as has been observed by Nicholls and Jarmain that
the r-Centroid increases or decreases as wavelength increases. Accordingly, a
constant difference is found in the r-Centroid values in a given sequence in
accordance with the conclusions drawn by Nicholls and Jarmain.
The Ar values for the band systems a — X and A— X of CO and
A— X of CN are >0.01 A0, which indicates that the potentials are wide. The
sequence difference Ar = rv>+]i v»+j- rvV' has been found to be constant in
accordance with the observations of Nicholls and Jarmain (1956).
Since re) > re2
in the case of a— X ,A— X systems of CO and A-X system of CN, r-Centroids
estimates are expected to increase with wavelength, which is the trend observed in
a red-degraded band system.
The condition framed by Nicholls and Jarmain
(1956) are not fulfilled in the case of B— X system of CN and a — X system of
CS, as such r-Centroids are not evaluated. The sequence difference in the above
system in very less, which indicates the nature of the potential is very narrow.
vi
The At values for the band system of A — X of N 2+ and A — X and B— X of
N
2
are >0.01A°, which indicates that the potentials are wide. The sequence
difference Ar = rv +i, V”+i- Av’ has been found to be constant accordance with the
observations of Nicholls and Jarmain. Since intemuclear distance of the upper
state is greater than the ground state (reI > re2) in all cases, r-Centroids estimates
are expected to increase with wavelength, which is the trend observed in a reddegraded band system.
The last chapter V describes the determination of the FranckCondon factors for fourteen band systems belonging to COT,CN,CO, CS,N2+,N2i
OH, S2,NO+ and NO. The method of Fraser and Jarmain (with and without
re-
shift) have been employed in the evaluation of the Franek-Condon factors. The
results obtained by these methods compared wherever found possible and seen to
be in excellent agreement. From the magnitudes of the Franek-Condon factors
some of the band systems belonging to these molecules, it is explained how some
bands are missing or not observed experimentally.
In some cases since the
vibrational sum rule is not satisfied for v1 and v11 progresssions as far as the bands
are observed. It has been necessary to excite more new bands in the spectrum of
some of these band systems belonging to these molecules. Due to high cosmic
abundance of Carbon and Oxygen, CO is one of the more important molecules in
astrophysics; consequently, many of the spectroscopic constants of CO are well
known, particularly for the ground state. It is seen from the magnitude of FC
factors that the (0,0),(0,1),(0,2),(1,0) and (2,0) bands are strongest and that the FC
system of the successive bands in the Av=0 sequence fall off rapidly in the A-X
system of CO as v1 and v11 increase.
To understand the physical conditions of
the emitter from the relative intensities of the bands of a molecule, one must have
a theoretical knowledge of the corresponding vibrational transition probabilities
for the respective band heads in a band system.
The vibrational sum rule is
satisfied for v'=0 with v11 > 2 and vn=0 with v1 > 2 . (0,3),(3,0) FC factors are
small and they may not be probably be noticed experimentally. (0,0) band is the
strongest one among all the bands observed in the a~X system of CS molecule.
The results obtained by these methods compared wherever found possible and
seen to be in excellent agreement with other methods.
Major portion of the work presented in the thesis has been published by the
author in the following scientific journals.
1.
Spectroscopic investigations on cometary molecules CO", CH and CH"
J. Quant. Spectro. Radia. Transfer, 85 (2004) 105-113.
2. Estimation of Potential Energy Curves, Dissociation Energies, FranckCondon Factors and r-Centroids of Comet interesting Molecules.
Astrophysics and Space Science, 286 (2003) 419-436.
3. Spectroscopic Studies of molecules observed in Comets.
IndianJournal of Pure and Applied Physics, 43(2005) 237-245.
4. Spectroscopic studies of atmospheric interest on NO and NO".
J. Quant. Spectro. Radia. Transfer, (Accepted for publication) 2005.
viii