Studies of highly excited and super-excited states of atoms and molecules (which typically lie in the VUV and Soft X-ray region) are crucial for understanding photochemical processes in the upper atmosphere, astrophysical objects, plasmas, environmental sciences, etc. The Atomic Molecular and Optical Sciences (AMOS) beamline is designed for investigating the electronic structure as well as dynamics of molecules consequent to photon excitation like photoionization, photodissociation, and fluorescence. This beamline is also useful for X-ray absorption studies in the soft X-ray region, important for probing absorption edges of materials of low Z elements. The AMOS beamline utilizes an undulator port, which provides high brilliance and tunable radiation in the VUV and soft X-ray regions, and thus enables a unique combination of tunability, high flux and high resolution at the sample position.
Photograph of the beamline and gas phase end-station
Beamline parameters & Optical layout
Parameters
Typical values
Source
Undulator (U1)
Photon Energy Range
6-800 eV
Wavelength
~ 2000-15 Å
Monochromator
Varied Line Spacing Plane Grating Monochromator (VLSPGM) with four gratings (6–18 eV, 15–45 eV, 42–126 eV, 90–800 eV)
Optical layout of the AMOS Beamline
The AMOS beamline is installed on U1 undulator of Indus-2 storage ring
The pre-optics section of the beamline comprises a combination of plane mirror (PM) and toroidal mirror (TM1) which focuses the SR beam onto the entrance slit of the monochromator.
The monochromator used is a varied line spacing plane grating monochromator (VLSPGM). Four gratings (6-18 eV, 15-45 eV, 42-126 eV and 90-800 eV), interchangeable in-vacuum, facilitate coverage of a wide photon energy range. A second toroidal mirror (TM2) is used to focus the beam onto the exit slit.
The post optics part uses a third toroidal mirror (TM3) to direct and focus the dispersed radiation onto the sample position.
The beamline is equipped with interchangeable experimental stations, enabling both gas phase and solid phase studies depending on specific experimental requirements.
Experimental facilities
A gas cell for molecular gas phase photoabsorption studies in the 6 -11.8 eV energy range (limited by the LiF window) and a chamber with sample manipulator for soft X-ray studies on solid samples are currently available for users. In addition, a differential pumping system at the AMOS beamline enables gas-phase experiments in the higher-energy, windowless region. An advanced experimental station, designed for investigations of photoionization and photodissociation processes in molecules using the velocity map imaging technique, is under development. Additionally, a low-temperature setup for matrix isolation spectroscopy intended for experiments on molecular ices and molecules trapped in inert gas matrices at cryogenic temperatures is under development.
(Left) Solid state chamber with sample manipulator. (Right) pre-optics section of the beamline
Application Areas
Studies of highly excited states of molecules provide deep insights into molecular geometries, electronic state ordering, and conformational effects.
Photoionization and photo-fragmentation dynamics of molecules are crucial in deciphering formation and decay pathways, thereby aiding the understanding of processes relevant to astrophysical environments.
Photoabsorption studies of molecules and radicals, important in atmospheric sciences and astrochemistry, provide essential data for modelling their roles in these environments.
Photochemistry of molecular ices and molecules trapped in inert gas matrices at cryogenic temperatures is useful to probe chemical properties and reactivity under low temperature conditions.
X-ray absorption spectroscopy (XAS) of K edges of low Z elements, L-edges for intermediate Z elements and M-edges for high Z elements has wide-ranging applications in material characterization, including in areas such as batteries and energetic materials.
X-ray absorption spectroscopy (XAS) of K edges of low Z elements, L-edges for intermediate Z elements and M-edges for high Z elements has wide-ranging applications in material characterization, including in areas such as batteries and energetic materials.
1.
Commissioning and first result of undulator based atomic, molecular & optical sciences (AMOS) beamline at Indus-2 synchrotron radiation source Param Jeet Singh, Tamaghna Maitra, A.K. Das, A.K. Yadav, Ravi Kumar, Ankur Agarwal, Ashutosh Dwivedi, K.K. Gorai, A. Shastri, D. Bhattacharyya and D.V. Udupa,
Rev. Sci. Instrum. 95, 123307 (2024)
