D. Mathur

Multiply charged molecules

Isolated metastable multiply charged molecular ions constitute a new class of molecular entity which dissociate much less rapidly than would be expected from conventional perturbative single-particle theories of atomic and molecular collisions. Studies of such entities have witnessed a spectacular growth in the course of the last five years. This report attempts to present an overview of developments in this subject area by presenting recipes of contemporary experimental and theoretical techniques applied to the study of such species, along with illustrative examples of the type of information obtainable. There is strong interplay between theoretical and experimental studies. Experimental effort is generally directed towards obtaining information on the energetics involved in the formation and fragmentation of multiply charged molecules and their interaction with other, neutral atoms and molecules. Theoretical studies primarily address the question of the metastability of multiply charged molecules and focus attention of calculations of ionic potential-energy surfaces.

Laser photodetachment of C60− and C70− ions cooled in a storage ring

Abstract Results are reported of a new type of experiment in which a heavy ion storage ring is utilized to cool fullerene ions, C 60 − and C 70 − , and to carry out laser photodetachment experiments in order to measure the electron affinities of ions possessing the minimum amount of internal energy. Fullerene negative ions are stored in the ring for periods of up to 5 s, and the time evolution of the electron affinity is measured, yielding limiting values of 2.666 ± 0.001 and 2.676 ± 0.001 eV for cold C 60 − and C 70 − , respectively.

Femtosecond laser written channel waveguides in tellurite glass

We have made and characterized a new, erbium-doped tellurite glass that has high glass transition temperature. Addition of phosphate is found to increase the phonon energy. The peak emission cross section is 6 x 10(-21) cm(2) at 1537 nm and the fluorescence lifetime of the (4)I(13/2)-(4)I(15/2) transition is 4.1 ms. We have written 2-D channel waveguides in this glass using focused, 45-fs pulses from an amplified Ti:sapphire laser at different laser energies and writing speeds. Migration of atoms towards the periphery of the waveguides occurs, leading to refractive index changes. Channels show waveguiding at 1310 nm which is promising for the fabrication of integrated lasers and broadband amplifiers.

Raman Tweezers Spectroscopy of Live, Single Red and White Blood Cells

An optical trap has been combined with a Raman spectrometer to make high-resolution measurements of Raman spectra of optically-immobilized, single, live red (RBC) and white blood cells (WBC) under physiological conditions. Tightly-focused, near infrared wavelength light (1064 nm) is utilized for trapping of single cells and 785 nm light is used for Raman excitation at low levels of incident power (few mW). Raman spectra of RBC recorded using this high-sensitivity, dual-wavelength apparatus has enabled identification of several additional lines; the hitherto-unreported lines originate purely from hemoglobin molecules. Raman spectra of single granulocytes and lymphocytes are interpreted on the basis of standard protein and nucleic acid vibrational spectroscopy data. The richness of the measured spectrum illustrates that Raman studies of live cells in suspension are more informative than conventional micro-Raman studies where the cells are chemically bound to a glass cover slip.

Long-lived, doubly charged diatomic and triatomic molecular ions

Lifetimes of doubly charged diatomic and triatomic molecules have been measured by monitoring the decay curves of such ions in a heavy-ion storage ring. CO2+, N22+, CO22+, CS22+ and SH2+ are all found to possess long-lived components which survive for time periods greater than a few seconds. All these dications are found to be essentially stable and their ultimate destruction is due to interactions with residual gases in the ring. CO2+ possesses many more lifetime components in the millisecond range than the isoelectronic N22+ ion. Translational energy spectrometry experiments on the latter species also fail to reveal any short-lived (microsecond) components. Ab initio configuration interaction calculations have been carried out and the potential energy curve for the lowest-energy metastable state of N22+ (1 Sigma g) has been determined, along with Franck-Condon factors for vertical transitions to different vibrational levels from the ground state of neutral N2; tunnelling times of each vibrational level have been computed.

Torque-generating malaria-infected red blood cells in an optical trap

We have used optical tweezers to trap normal and Plasmodiuminfected red blood cells (iRBCs). Two different facets of the behavior of RBCs in infrared light fields emerge from our experiments. Firstly, while the optical field modifies both types of RBCs in the same fashion, by folding the original biconcave disk into a rod-like shape, iRBCs rotate with linearly polarized light whereas normal RBCs do not. Secondly, and in the context of known molecular motors, our measurements indicate that the torque of rotating iRBCs is up to three orders of magnitude larger.

Probing oxidative stress in single erythrocytes with Raman Tweezers

Raman Tweezers have been successfully applied to characterize chemically-induced oxidative stress on optically-trapped live, single erythrocytes. There is significant enhancement in Raman peak intensities corresponding to SS and C-S stretching modes that are induced by oxidative stress. This is consistent with the formation of mixed disulphides between protein SH groups and low-molecular-mass thiols such as glutathione during oxidative damage to cells. Enhancement in glutathione level as a protective response against oxidative stress has been observed. Principal component analysis of the data yields good discrimination between spectra of normal and stress-induced red blood cells.

All-optical switching with bacteriorhodopsin

All-optical, mirrorless switching is demonstrated with bacteriorhodopsin (bR) film using a very simple geometry. A low-power, 532 nm laser beam modulates the transmission of a cw laser beam at 635 nm, a wavelength that corresponds to peak absorption of the O-excited state in the bR photocycle. The switching contrast depends on the pulse width and average power of the modulating laser. Varying the pulse width and frequency of the modulating laser controls the phase of the switching characteristics. Simulations based on a rate equation approach consider a six-state model of the bR photocycle that successfully reproduce the experimental results.

Measuring erythrocyte deformability with fluorescence, fluid forces, and optical trapping

A laser-based method has been developed for experimentally probing single red blood cell (RBC) buckling and determining RBC membrane rigidity. Our method combines a liquid flow cell, fluorescence microscopy, and an optical-trap to facilitate simple measurements of the shear modulus and buckling properties of single RBCs, under physiological conditions. The efficacy of the method is illustrated by studying buckling behavior of normal and Plasmodium-infected RBCs, and the effect of Plasmodium falciparum-conditioned medium on normal, uninfected cells. Our simple method, which quantifies single-RBC deformability, may ease detection of RBC hematological disorders.

Highly efficient white light generation from barium fluoride

We demonstrate highly efficient white light generation by focusing 45 fs long pulses of 800 nm laser radiation with 1 mJ energy onto a 10 cm long barium fluoride crystal. The entire wavelength band from 400-1000 nm was generated with efficiency greater than 40%. We also observe multiphoton absorption induced fluorescence in the crystal. By employing line focusing geometry at low intensity, we show that white light fringes are formed with a single laser beam, indicative of the coherent property of the white light that is produced.