Sivarama Krishnan

Photoionization spectroscopy and mass spectrometry of doped helium (He) nanodroplets rely on the ability to efficiently detect ions and/or electrons. Using a commercial quadrupole mass spectrometer and a photoelectron-photoion coincidence spectrometer, we systematically measure yields of ions and electrons created in pure and doped He nanodroplets in a wide size range and in two ionization regimes—direct ionization and secondary ionization after resonant photoexcitation of the droplets. For two different types of dopants (oxygen molecules, O2, and lithium atoms, Li), we infer the optimal droplet size to maximize the yield of ejected ions. When dopants are ionized by charge-transfer to photoionized He nanodroplets, the highest yield of O2 and Li ions is detected for a mean size of ∼ 5 × 1 0 4 He atoms per nanodroplet. When dopants are Penning ionized via photoexcitation of the He droplets, the highest yield of O2 and Li ions is detected for ∼ 1 0 3 and ∼ 1 0 5 He atoms per droplet, respectively. At optimum droplet sizes, the detection efficiency of dopant ions in proportion to the number of primary photoabsorption events is up to 20% for charge-transfer ionization of O2 and 2% for Li, whereas for Penning ionization it is 1% for O2 and 4% for Li. Our results are instrumental in determining optimal conditions for mass spectrometric studies and photoionization spectroscopy of molecules and complexes isolated in He nanodroplets.

Photoionization process of acetylene doped in helium nanodroplets is studied with EUV synchrotron radiation with photon energies between 20 and 26 eV by Photoelectron-Photoion Coincidence (PEPICO) experiment by detecting photoelectrons in coincidence with the photoions using electron velocity map imaging (VMI) spectrometer and ion time of flight (TOF) spectrometer. Acetylene is ionized in the droplet via Penning ionization at 21.6 eV photon energy. For photon energy of 23.9 eV and above the photoionization threshold of He, charge transfer ionization occurs in acetylene following autoionization and direct ionization in the droplet respectively.

The fragmentation dynamics of the gas-phase, doubly charged camphor molecule, formed by Auger decay following carbon 1s ionisation, using soft X-ray synchrotron radiation, is presented in this work. The technique of velocity map imaging combined with a photoelectron-photoion-photoion coincidence (VMI-PEPIPICO) is used for both electron energy and ion momentum (in-sequence) measurements. The experimental study is complemented by molecular dynamics simulation, performed with an NVT (moles, volume, and temperature) ensemble. Velocity Verlet algorithms were used for time integration at various internal energies. These simulations validate observed dissociation pathways. From these, we successfully deduce that the internal energy of the doubly charged molecular ion has a significant contribution to the fragmentation mechanism. Notably, a prominent signature of the internal energy was observed in the experimentally determined energies of the neutral fragment in these deferred charge separation pathways, entailing a more detailed theoretical study to uncover the exact dissociation dynamics. This journal is

Femtosecond laser-induced micro or nanoscale periodic structure on silica glass is still an area of interest due to its application in manufacturing of optoelectronics devices and medical devices. The study of surface modification in micron level was carried out in using Ti: Sapphire femtosecond laser pulse of pulse duration 100 fs at 800 nm at a repetition rate of 1 kHz. The microchannels are created at different fluence and scanning velocity with the linearly polarized pulse. The variation of the width and the depth are optimized with respect to the laser fluence and scanning velocity.

We report the observation of electron-transfer-mediated decay (ETMD) involving magnesium (Mg) clusters embedded in helium (He) nanodroplets. ETMD is initiated by the ionization of He followed by removal of two electrons from the Mg clusters of which one is transferred to the He ion while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. For Mg clusters larger than five atoms we observe stable doubly ionized clusters. Thus, ETMD provides an efficient pathway to the formation of doubly ionized cold species in doped nanodroplets.

Ionization of matter by energetic radiation generally causes complex secondary reactions that are hard to decipher. Using large helium nanodroplets irradiated by extreme ultraviolet (XUV) photons, we show that the full chain of processes ensuing primary photoionization can be tracked in detail by means of high-resolution electron spectroscopy. We find that elastic and inelastic scattering of photoelectrons efficiently induces interatomic Coulombic decay (ICD) in the droplets. This type of indirect ICD even becomes the dominant process of electron emission in nearly the entire XUV range in large droplets with radius ≳40 nm. Indirect ICD processes induced by electron scattering likely play an important role in other condensed-phase systems exposed to ionizing radiation as well, including biological matter.

Helium nanodroplets irradiated by intense near-infrared laser pulses ignite and form highly ionized nanoplasmas even at laser intensities where helium is not directly ionized by the optical field, provided the droplets contain a few dopant atoms. We present a combined theoretical and experimental study of the He nanoplasma ignition dynamics for various dopant species. We find that the efficiency of dopants to ignite a nanoplasma in helium droplets strongly varies and mostly depends on (i) the number of free electrons each dopant donates upon ionization, (ii) the pick-up process, and (iii) the hitherto unexplored effect of the dopant location in or on the droplet.

In micromachining, the extruded material is re-deposited back on the structured surface. In this work, we report on the advantages and results of fs laser-based surface micromachining of methanol immersed of Si comparing the same with the ambient air.

Sub-wavelength periodic nanostructures have unique properties that can lead to various applications in the field of photonics including cloaking, perfect absorption, perfect reflection and negative refractive index. Dielectric structures, unlike their metal counterparts, have low losses thus providing an alternative in various applications. In this work, we study the light-matter interaction in high refractive index dielectric periodic metasurfaces made of Tellurium cubes in air. In our earlier investigations in this direction with smaller periodicities, we observed a novel non-radiative state (anapole) immediately following a highly transparent state at higher frequencies and a reflection band at lower frequencies. In the current paper, we investigate the effect of periodicity of the metasurface on the response spectrum and most importantly on the observed transparent state. By studying the spatial distribution of the electric and magnetic fields and detailed multipole analysis, we see that the response spectrum is significantly affected by the periodicity of the metasurface. As the periodicity is increased, the band-like structure diminishes and a suppression of the electric dipole resonance is observed while the magnetic dipole resonance remains unaffected except for a shift towards lower frequencies. The highly transparent state which is a hybrid mode of electric dipole and quadrupole, however, is found to be independent of the periodicity of the structure, which has not been reported earlier in detail as per our knowledge.

Continuous micro-channels were formed on the surface of the automotive piston rings using two different lasers, namely nanosecond and femtosecond pulsed laser. Tribology measurements were conducted to compare the friction properties of the samples fabricated with the laser pulse width of nanosecond and femtosecond order. The femtosecond laser ablation comparatively minimized the re-deposition layer, heat affected zone around the micro-channel compared to the nanosecond laser ablation. Owing to the localized material ablation, the compressive residual stress was not altered with femtosecond laser processing. The friction test was conducted using reciprocating type friction measurement setup for the loads ranging from 10 to 130 N. The friction coefficients of all the textured samples were reduced compared to the non-textured samples. Furthermore, it is observed that the average friction coefficient was low with the femtosecond laser textured samples compared to that of the nanosecond laser textured samples of the same dimensions. The results show that the thermally induced defects while using the nanosecond laser ablation might have influenced the tribological properties.