Aparna Shastri

Development of Photophysics Beamline at Indus-1 Synchrotron Radiation Source

A beamline to carry out photoabsorption and fluorescence experiments using INDUS-1 synchrotron radiation source has been recently commissioned at the Centre for Advanced Technology, Indore India. The optical system for the beamline consists of a pre-focusing toroidal mirror, an indigenously developed one meter Seya-Namioka monochromator and a post-focusing toroidal mirror to cover the wavelength range of 500Å–2000Å. An experimental chamber for performing VUV excited fluorescence on solid samples is coupled to the beamline. A quarter meter gas cell has also been fitted with the experimental station. In this paper, we have presented the salient features of the beamline and the recorded photoabsorption spectra of benzene and benzene-d6.

Electronic state spectroscopy of diiodomethane (CH2I2): Experimental and computational studies in the 30 000–95 000 cm−1 region

The electronic absorption spectrum of diiodomethane in the 30,000-95,000 cm(-1) region is investigated using synchrotron radiation; the spectrum in the 50,000-66,500 cm(-1) region is reported for the first time. The absorption bands in the 30,000-50,000 cm(-1) region are attributed to valence transitions, while the vacuum ultraviolet (VUV) spectrum (50,000-95,000 cm(-1)) is dominated by several Rydberg series converging to the first four ionization potentials of CH2I2 at 9.46, 9.76, 10.21, and 10.56 eV corresponding to the removal of an electron from the outermost 3b2, 2b1, 1a2, and 4a1 non-bonding orbitals, respectively. Rydberg series of ns, np, and nd type converging to each of the four ionization potentials are assigned based on a quantum defect analysis. Time dependent density functional theory calculations of excited states support the analysis and help in interpretation of the Rydberg and valence nature of observed transitions. Density functional theory calculations of the neutral and ionic ground state geometries and vibrational frequencies are used to assign the observed vibronic structure. Vibronic features accompanying the Rydberg series are mainly due to excitation of the C-I symmetric stretch (ν3) and CH2 wag (ν8) modes, with smaller contributions from the C-H symmetric stretch (ν1). UV absorption bands are assigned to low lying valence states 1(1)B2, 1(1)B1, 2(1)A1, 3(1)A1, 2(1)B1, and 2(1)B2 and the unusually high underlying intensity in parts of the VUV spectrum is attributed to valence states with high oscillator strength. This is the first report of a comprehensive Rydberg series and vibronic analysis of the VUV absorption spectrum of CH2I2 in the 50,000-85,000 cm(-1) region. The VUV absorption spectrum of CD2I2 which serves to verify and consolidate spectral assignments is also reported here for the first time.

Vacuum Ultraviolet Absorption Spectrum of Difluoromethane Reinvestigated

ABSTRACT The vacuum ultraviolet (VUV) absorption spectrum of difluoromethane (CH2F2) was studied using synchrotron radiation from the storage ring Indus-1, Indore, India. Spectra were recorded in the spectral region 1050–1500 Å (∼8.3–11.8 eV) at a resolution of 1.5 Å. Three absorption band systems were observed in this region. Overall features observed are in good agreement with previously published work. Some discrepancies in assignments of the observed vibronic bands carried out by previous workers have been resolved. The observed bands have been classified in terms of Rydberg series.

Comment on ‘The vacuum ultraviolet absorption spectrum of diatomic calcium’

A paper entitled ‘The vacuum ultraviolet absorption spectrum of diatomic calcium’ was published earlier in this journal. It is our opinion that the spectra observed belong to doubly excited transitions of the calcium atom rather than to the calcium dimer. Our observations in this regard are recorded.

MEAN LIFETIME MEASUREMENTS OF F II AND F III LEVELS USING BEAM-FOIL TECHNIQUE

Beam-foil spectrum of fluorine was recorded in the wavelength region of 2000–4500 Å using F+ ion beams of energies ranging from 216 to 296 KeV. Some of the spectral lines of fluorine observed during the present investigation are hitherto unknown. Mean lifetimes of a few of the excited levels of F II and F III are reported for the first time.