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V K Tripathi

Publications and source records attributed to V K Tripathi.

5 recordsLinked to original sources
PubMed2004

Electron acceleration by a self-diverging intense laser pulse.

Electron acceleration by a laser pulse having a Gaussian radial and temporal profile of intensity has been studied. The interaction region is vacuum followed by a gas. The starting point of the gas region has been chosen around the point at which the peak of the pulse interacts with the electron. The tunnel ionization of the gas causes a defocusing of the laser pulse and the electron experiences the action of a ponderomotive deceleration at the trailing part of the pulse with a lower intensity rather than an acceleration at the rising part of the laser pulse with a high intensity, and thus gains net energy. The initial density of the neutral gas atoms should be high enough to properly defocus the pulse; otherwise the electron experiences some deceleration during the trailing part of the pulse and the net energy gain is reduced. The rate of tunnel ionization increases with the increase in the laser intensity and the initial density of neutral gas atoms, and with the decreases in the laser spot size, which causes more defocusing of the laser pulse. The required initial density of neutral gas atoms decreases with the increase in the laser intensity and also with the decrease in the laser spot size.

Journal Article↗
PubMed2002

LC-ESI-MS analysis of taxoids from the bark of Taxus wallichiana.

LC-ESI-MS analysis was carried out for taxoid profiling of partially purified methanol extracts of the stem bark of Taxus wallichiana growing in different regions of the Himalayas (Kashmir, Himachal Pradesh, UP hills, Sikkim and Arunachal Pradesh). Cone voltage fragmentation of the protonated, ammonium or sodium cationized molecular species resulted in diagnostic fragment ions. Thus, information about the number and nature of substituents and the taxane skeleton (whether it is normal or rearranged) was readily available from the LC-ESI-MS spectra. The rearranged 11(15-->1)-abeo-taxanes showed a characteristic elimination of the hydroxyisopropyl along with an acetoxy group. The identification of the taxoids was achieved by comparison of the ESI mass spectra with those of the authentic taxoids available to us or by interpreting the ESI mass spectra. The results were also corroborated by MS/MS analysis of the partially purified extract injected directly into the ESI source. Paclitaxel, its analogues and their xylosides are present in samples from all the regions. An interesting observation is the detection of a large number of basic taxoids having nitrogen-containing side chains.

Alkaloids↗
PubMed2001

Self-focusing and frequency broadening of an intense short-pulse laser in plasmas.

An intense ultrafast laser pulse propagating through a plasma undergoes self-focusing and self-phase-modulation as a result of relativistic mass nonlinearity. The inclusion of a quartic (r4) term in the expansion of the eikonal in the radial coordinate r allows the modification of the shape of the radial intensity profile. The front of the pulse, under the combined effects of time-dependent self-focusing and frequency downshifting, acquires a severely distorted temporal shape. The radial profile for I(lambda)2(mu) < 2.8 x 1018 W/cm2, where I is the axial laser intensity and lambda(mu), is the laser wavelength in micrometers, is transformed from a Gaussian to a super-Gaussian because of the faster convergence of the marginal rays than the paraxial rays. In the opposite case of I(lambda)(2)(mu) > 2.8 x 10(18) W/cm2 when nonlinear plasma permittivity approaches saturation, the radial profile in the axial region becomes broader than the Gaussian.

Journal Article↗