Resume' of Research Activities
In my research career of past one and half decade I have worked on both experimental and theoretical
aspects of spectroscopic investigations of atoms and molecules. The experimental
aspect is concerned with the application of accelerators and Fourier transform emission spectroscopy to the study of atoms
and molecules while the theoretical part is concerned with elucidation of absorption features in the characteristic IR spectra
of solid H2 on the one hand and to a comprehensive study of the radiative channels and lifetimes of certain metastable
levels in Hg-like and rare gas atoms, on the other. Following is a brief summary
of my work done so far.
1. Fourier Transform Emission Spectroscopy
of Atoms and Molecules : Recently, we initiated emission spectroscopic experiments
on a commercial FTS (BOMEM Model DA3.002) after switching it to the emission mode and augmenting the optics and detection
systems. The motivations behind such studies on the FTS are : (i) to use it as
an alternative to photographic spectroscopy in the uv and visible regions, (ii) to have a quantitative idea about the rotational intensity distribution in complex electronic
transitions of molecules and model it theoretically, and (iii) to study molecular electronic transitions in the near IR and
mid IR regions inaccessible by photographic photometry. To start with, the FT
spectra of rare gases Ne, Ar, Kr and Xe in the near infrared were recorded. The
motivation has been to augment, if necessary, the existing line listing, and to look into the possibility of dimeric transitions
as well as certain forbidden atomic transitions in the rare gases. The inherent
sensitivity of FTS has enabled the identification and classification of the 62 new, extremely weak atomic transitions in the
first spectra of the rare gases. Based on these observations, it has been possible
to establish 10 energy levels of Kr I more accurately and to determine the energies of 3 new levels belonging to the 3p5
(2P3/2) 7g configuration in Ar I. Besides the rare gas
atomic
spectra, the source also produced dimeric Rydberg ® Rydberg transitions between excited states of the dimer. Vibrational
analysis has been proposed for these transition features as well. Simultaneously,
the rotational intensity distribution in the first positive B3Pg
- A3Su+ system
of N2, taken up as a test case for further intensity, was also studied
to an accuracy that enabled detection of small perturbation due to a far off electronic state. Presently, the study of near infrared emission spectra of CO and
NO molecules recorded on FTS is in progress.
2. Accelerator Based Spectroscopy of Atoms
and Molecules : Among the developments that spectroscopy witnessed in the last three
decades, the application of particle accelerators for spectroscopic investigations is one of the most important. While the synchrotron radiation sources in the form of electron/positron storage rings, offer very intense
and tunable electromagnetic radiation with unique properties suitable for carrying out a variety of spectroscopic experiments,
especially in the VUV and X-ray regions, the heavy ion accelerators in the low-medium energy regimes have heralded the beam-foil
spectroscopy research suited for the study of highly stripped atoms. I commenced
my research career with accelerator based spectroscopy group at BARC. By participating
in the beam-foil spectroscopy experiments on Ar the significant results are : (i) the correction of many discrepancies in
earlier line assignments in Ar II, Ar III and Ar IV ions, based on the investigations of the excitation functions, and (ii)
determinations of lifetimes of more than 10 levels of these ions for the first time.
Concurrently, I participated in developing a high resolution VUV beam-line at the 450 MeV INDUS-I synchrotron radiation
storage ring. For design considerations of the beam-line the photon flux was
calculated. With the objective of initiating preliminary VUV spectroscopy experiments
an old 3m VUV concave grating spectrograph was refurbished and put in good working order.
Presently, experiments are being planned using the INDUS-I synchrotron radiation source.
3. Elucidation of Absorption Features in
the Characteristic IR Spectra of Solid H2 : Molecular hydrogen for which
pure rotational and rovibrational electric dipole transitions in the free states are symmetry-forbidden, exhibits an infrared
spectrum in the condensed phase caused by multipolar induction. We have developed theory
pertaining to the intensities of zero-phonon trnasitions in solid hydrogens and derived intensity formulas for the following types of transitions: (i) double transitions involving para-H2
- ortho-H2 pair; (ii) single and double transitions in solid HD, HT
and
DT;
(iii) double transitions involving a homonuclear-heteronuclear pair of hydrogen molecules in condensed phase; and (iv)
double transitions involving orthohydrogen pair in solid parahydrogen crystal. The
methodology adopted is novel; the prediction of a new kind of T + T transititions (DJ = 3 in two molecules) in the asymmetric isotopomers is a new and
unexpected result. In order to correlate the theoretical
and experimental intensities, we have performed elaborate computations of the rovibrational matrix elements of multipole moments
and of polarizabilities for different isotopomers of hydrogen molecules from first principles. Experiments are being planned on the infrared spectroscopy of solid hydrogen using
FTS.
4. Laser Spectroscopy of Acetylene Molecule : During 2001
– 2003 I worked as a Post-Doctoral fellow at MIT, USA, in the Harrison Spectroscopy laboratory with Professor Robert
Field. Here, along with Prof Field Group’s members, I developed a
suite of complementary spectroscopies (surface electron ejection by laser excited metastables, UV-laser induced fluorescence,
and photofragment IR-laser-induced fluorescence) and statistical pattern recognition schemes by which the detailed mechanism
of intersystem crossing in small polyatomic molecules can be characterized. Using
this apparatus, pulsed
supersonic jet spectra of acetylene have been recorded in the vicinity of the A-X V^3_0 band in two mutually exclusive detection channels: ultraviolet laser induced fluorescence (UV-LIF) and Surface Electron
Ejection by Laser Excited Metastables (SEELEM). No eigenstate of the upper state(s)
can appear via transitions in both UV-LIF and SEELEM channels. The UV-LIF channel is blind to transitions into eigenstates that have lifetimes longer than a few microseconds,
while the SEELEM channel is sensitive to transitions into electronically excited eigenstates that have lifetimes longer than
~100 microseconds. It is observed that intersystem crossing from the (S1)
bright state to the dark triplet states is mediated through a single vibrational level of the T3 electronic state. We have characterized the bright state, the doorway state, and the dark states (singlet
as well as triplet) illuminated by the bright state. Rotational transitions that
belong to many new vibrational-symmetry-allowed but nominally Franck-Condon forbidden S1ßS0
electronic-vibrational bands are identified in the UV-LIF spectrum. The UV-LIF
spectrum reveals that the electronic symmetry of the T3 state is 3Bu and the K¢ value of the observed T3ßS0
band segment is 1. From the SEELEM spectrum, nine vibrational levels of dark
triplet states are observed, spread over ~2.42 cm-1. Unique N, K, and e/f quantum numbers
are assigned for all of the observed triplet spin-rovibronic levels. Electronic
symmetries for seven of the nine triplet states are determined. Most of the triplet
state vibrational levels observed in the SEELEM spectrum belong to the T1 surface (3Bu and
3B2) at energy above the barrier to linearity. Despite
the high vibrational state density the upper levels of all of the assigned spin-rovibronic transitions fall onto surprisingly
regular rotational term value plots [E¢ vs N¢(N¢+1)] and the
intensities fall onto surprisingly smooth Boltzmann plots. The most remarkable
thing about the SEELEM spectrum is the regularity of the spectra combined with a vibrational density of states so high that
anharmonic and a–type Coriolis
coupling among the T1 and T2 vibrational levels is essentially complete.
The evidence for the completeness of the anharmonic and a-type Coriolis
mixing is that the spectroscopically observed density of K-assigned vibrational
states is approximately twice the calculated symmetry-sorted state density!
5. Radiative Lifetimes Calculations
: A comprehensive calculation of the radiative
lifetimes of the first excited 3P2,0o metastable levels of Hg-like atoms and rare gas atoms
was performed prior to recording the spectra of the rare gases on FTS. All the possible decay modes are explored including the hyperfine interaction-induced electric dipole transition
channel that is specific to the ‘odd’ isotopic atoms. The present
work fills important gaps in the available data on the lifetimes of the first excited 3P2,0o
metastable
levels of these atoms. As a spin off, the potential
importance of some of the transitions (3P0o ® 3P1o in rare gas atoms or 3P2o
® 3P1o in Hg-group atoms) for developing IR
lasers is inferred and the peak gain coefficient b for the proposed laser transitions calculated.
