CARS spectroscopy of the (v=0→1v=0\to1) band in T2\rm{T_2}

Molecular hydrogen is a benchmark system for bound state quantum calculation and tests of quantum electrodynamical effects. While spectroscopic measurements on the stable species have progressively improved over the years, high resolution studies on the radioactive isotopologues $\rm{T_2}$, $\rm{HT}$ and $\rm{DT}$ have been limited. Here we present an accurate determination of $\rm{T_2}$ $Q(J = 0 - 5)$ transition energies in the fundamental vibrational band of the ground electronic state, by means of high resolution Coherent Anti-Stokes Raman Spectroscopy. With the present experimental uncertainty of $0.02\,\rm{cm^{-1}}$, which is a fivefold improvement over previous measurements, agreement with the latest theoretical calculations is demonstrated.Comment: 9 pages, 3 figure

Probing strong field ionization of solids with a Thomson parabola spectrometer

Intense ultrashort laser pulses are known to generate high-density, high-temperature plasma from any substrate. Copious emission of hot electrons, from a solid substrate, results in strong electrostatic field that accelerates the ions with energies ranging from a few eV to MeV. Ion spectrometry from laser–plasma is convolved with multiple atomic systems, several charge states and a broad energy spread. Conventional mass spectrometric techniques have serious limitations to probe this ionization dynamics. We have developed an imaging ion spectrometer that measures charge/mass-resolved ion kinetic energies over the entire range. Microchannel Plate (MCP) is used as the position-sensitive detector to perform online and single shot measurements. The well-resolved spectrum even for the low-energy ions, demonstrates that the spectral width is limited by the space-charge repulsion for the ions generated in the hot dense plasma

Relativistic and QED effects in the fundamental vibration of T2_2

The hydrogen molecule has become a test ground for quantum electrodynamical calculations in molecules. Expanding beyond studies on stable hydrogenic species to the heavier radioactive tritium-bearing molecules, we report on a measurement of the fundamental T$_2$ vibrational splitting $(v= 0 \rightarrow 1)$ for $J=0-5$ rotational levels. Precision frequency metrology is performed with high-resolution coherent anti-Stokes Raman spectroscopy at an experimental uncertainty of $10-12$~MHz, where sub-Doppler saturation features are exploited for the strongest transition. The achieved accuracy corresponds to a fifty-fold improvement over a previous measurement, and allows for the extraction of relativistic and QED contributions to T$_2$ transition energies.Comment: 5 pages, 5 figure

Compact acceleration of energetic neutral atoms using high intensity laser-solid interaction

Recent advances in high-intensity laser-produced plasmas have demonstrated their potential as compact charge particle accelerators. Unlike conventional accelerators, transient quasi-static charge separation acceleration fields in laser produced plasmas are highly localized and orders of magnitude larger. Manipulating these ion accelerators, to convert the fast ions to neutral atoms with little change in momentum, transform these to a bright source of MeV atoms. The emittance of the neutral atom beam would be similar to that expected for an ion beam. Since intense laser-produced plasmas have been demonstrated to produce high-brightness-low-emittance beams, it is possible to envisage generation of high-flux, low-emittance, high energy neutral atom beams in length scales of less than a millimeter. Here, we show a scheme where more than 80% of the fast ions are reduced to energetic neutral atoms and demonstrate the feasibility of a high energy neutral atom accelerator that could significantly impact applications in neutral atom lithography and diagnostics

Anisotropic emission of neutral atoms: evidence of an anisotropic Rydberg sheath in nanoplasma

Intense laser-produced plasma is a complex amalgam of ions, electrons and atoms both in ground and excited states. Little is known about the spatial composition of the excited states that are an integral part of most gaseous or cluster plasma. In cluster-plasma, Rydberg excitations change the charge composition of the ions through charge transfer reactions and shape the angular distributions. Here, we demonstrate a non-invasive technique that reveals the anisotropic Rydberg excited cluster sheath by measuring anisotropy in fast neutral atoms. The sheath is stronger in the direction of light polarization and the enhanced charge transfer by the excited clusters results in larger neutralization

Generation of energetic negative ions from clusters using intense laser fields

Intense laser fields are known to induce strong ionization in atoms. In nanoclusters, ionization is only stronger, resulting in very high charge densities that lead to Coulomb explosion and emission of accelerated highly charged ions. In such a strongly ionized system, it is neither conceivable nor intuitive that energetic negative ions can originate. Here we demonstrate that in a dense cluster ensemble, where atomic species of positive electron affinity are used, it is indeed possible to generate negative ions with energy and ion yield approaching that of positive ions. It is shown that the process behind such a strong charge reduction is extraneous to the ionization dynamics of single clusters within the focal volume. Normal and well-known charge transfer reactions are insufficient to explain the observations. Our analysis reveals the formation of a manifold of Rydberg excited clusters around the focal volume that facilitate orders of magnitudes more efficient electron transfer. This phenomenon, which involves an active role of laser-heated electrons, comprehensively explains the formation of copious accelerated negative ions from the nano-cluster plasma

Bacterial cells enhance laser driven ion acceleration

Intense laser produced plasmas generate hot electrons which in turn leads to ion acceleration. Ability to generate faster ions or hotter electrons using the same laser parameters is one of the main outstanding paradigms in the intense laser-plasma physics. Here, we present a simple, albeit, unconventional target that succeeds in generating 700 keV carbon ions where conventional targets for the same laser parameters generate at most 40 keV. A few layers of micron sized bacteria coating on a polished surface increases the laser energy coupling and generates a hotter plasma which is more effective for the ion acceleration compared to the conventional polished targets. Particle-in-cell simulations show that micro-particle coated target are much more effective in ion acceleration as seen in the experiment. We envisage that the accelerated, high-energy carbon ions can be used as a source for multiple applications

Precision measurement of the fundamental vibrational frequencies of tritium-bearing hydrogen molecules: T2_2, DT, HT

High-resolution coherent Raman spectroscopic measurements of all three tritium-containing molecular hydrogen isotopologues T$_2$, DT and HT were performed to determine the ground electronic state fundamental Q-branch ($v=0 \rightarrow 1, \Delta J = 0$) transition frequencies at accuracies of $0.0005$ cm$^{-1}$. An over hundred-fold improvement in accuracy over previous experiments allows the comparison with the latest ab initio calculations in the framework of Non-Adiabatic Perturbation Theory including nonrelativisitic, relativisitic and QED contributions. Excellent agreement is found between experiment and theory, thus providing a verification of the validity of the NAPT-framework for these tritiated species. While the transition frequencies were corrected for ac-Stark shifts, the contributions of non-resonant background as well as quantum interference effects between resonant features in the nonlinear spectroscopy were quantitatively investigated, also leading to corrections to the transition frequencies. Methods of saturated CARS with the observation of Lamb dips, as well as the use of continuous-wave radiation for the Stokes frequency were explored, that might pave the way for future higher-accuracy CARS measurements.Comment: 15 pages, 13 figure

Luminous, relativistic, directional electron bunches from an intense laser driven grating plasma

Bright, energetic, and directional electron bunches are generated through efficient energy transfer of relativistic intense (~ 1019 W/cm2), 30 femtosecond, 800 nm high contrast laser pulses to grating targets (500 lines/mm and 1000 lines/mm), under surface plasmon resonance (SPR) conditions. Bi-directional relativistic electron bunches (at 40° and 150°) are observed exiting from the 500 lines/mm grating target at the SPR conditions. The surface plasmon excited grating target enhances the electron flux and temperature by factor of 6.0 and 3.6, respectively, compared to that of the plane substrate. Particle-in-Cell simulations indicate that fast electrons are emitted in different directions at different stages of the laser interaction, which are related to the resultant surface magnetic field evolution. This study suggests that the SPR mechanism can be used to generate multiple, bright, ultrafast relativistic electron bunches for a variety of applications