Sankar De

Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields

We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules.

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C(8)H(5)F) and dissociating, laser-aligned 1,4-dibromobenzene (C(6)H(4)Br(2)) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.

Selective breaking of bonds in water with intense, 2-cycle, infrared laser pulses

One of the holy grails of contemporary science has been to establish the possibility of preferentially breaking one of several bonds in a molecule. For instance, the two O-H bonds in water are equivalent: given sufficient energy, either one of them is equally likely to break. We report bond-selective molecular fragmentation upon application of intense, 2-cycle pulses of 800 nm laser light: we demonstrate up to three-fold enhancement for preferential bond breaking in isotopically substituted water (HOD). Our experimental observations are rationalized by means of ab initio computations of the potential energy surfaces of HOD, HOD(+), and HOD(2+) and explorations of the dissociation limits resulting from either O-H or O-D bond rupture. The observations we report present a formidable theoretical challenge that need to be taken up in order to gain insights into molecular dynamics, strong field physics, chemical physics, non-adiabatic processes, mass spectrometry, and time-dependent quantum chemistry.

Beam-single and beam-two-foil experimental facility to study physics of highly charged ions

A facility for lifetime measurement of metastable states in highly charged ions using the beam-foil technique with a single-foil and a two-foil target has been developed. In the two-foil technique, one foil moves with respect to the other and the option of varying the thickness of the fixed foil online has been implemented. A holder with multiple foils is used as a fixed target, and moved along x, y, and θ, the angle of rotation with respect to beam direction along the z axis. Using this facility, the He-like 1s2pP2o3 and Li-like 1s2s2pP5∕2o4 titanium lifetimes have been measured and compared with earlier values. In addition to this, the processes which occur when excited states collide with carbon foils of different thicknesses have also been investigated. Preliminary results suggest the scope of studying intrashell transitions during ion-solid collision using this setup. In this article, the setup is described in detail and representative results are briefly discussed.

Transition between Mechanisms of Laser-Induced Field-Free Molecular Orientation

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 × 10(13) W/cm(2), the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrating absolute rates of nonperturbative strong-field molecular ionization.

Observation of electromagnetically induced transparency in six-level Rb atoms and theoretical simulation of the observed spectra

We report the observation of electromagnetically induced transparency (EIT) in a six-level Λ-type system in atomic Rb vapor containing both 87Rb and 85Rb. The experimental observation includes five velocity selective optically pumped (VSOP) absorption dips for both 87Rb and 85Rb. The EIT signal appears on the background of one such VSOP absorption dips. The measured EIT linewidth () shows sub-natural ( ) values for both lower and higher values of pump Rabi-frequencies. The density matrix based theoretical model for the six-level system is developed and solved numerically by taking into account the Doppler broadening. A complete analytical solution (non perturbative) for a three level Λ-type system has been obtained and compared with the experimentally observed sub-natural EIT linewidth. The simulated spectra are in good agreement with the experimental findings.

A Current Controller for Tunable Near Infrared Diode Laser

A semiconductor diode laser current controller is described for room temperature operation. A current limiting circuit has been introduced in the controller to avoid the breakdown of the diode laser at high current. The diode current can be modulated for frequency modulation of the source and can be tuned over small wavelength ranges. Using the controller the absorption of the weak overtone band transitions of C2 H2 around 788 nm have been observed using the lock-in detection technique with a path length of only 1.5m.

Splitting of electromagnetically induced absorption signal in a five-level V-type atomic system

Electromagnetically induced absorption (EIA) and velocity-selective transparency peaks are observed in the probe absorption signal of V-type five-level atomic systems interacting with copropagating control and probe fields. At high control beam intensity, a splitting of EIA occurs. The experiment shows similar results for both Rb87 and Rb85 isotopes at room temperature. A density matrix-based theoretical model of this five-level system has been developed and solved numerically under steady-state condition. The simulated spectra agree quite well with the experimental findings. An analytic expression for the probe response is derived and used to find out the causes for the splitting of EIA.

Dynamic field-free orientation of polar molecules by intense two-color femtosecond laser pulses

We present the first experimental observation of dynamic field-free orientation of a heteronuclear molecule (CO) induced by intense two color (800 and 400 nm) femtosecond laser pulses. We have used the two color pulse as pump to orient the molecules and a more intense 800 nm pulse as probe to measure the angular distributions. In addition to dynamic alignment seen in time dependence of , we observe clear orientation in traces, which revives with the rotational period and can be reversed by changing the relative phase of the two colors. We studied the dependence of degree of orientation on the pump pulse intensity, and compared the results with theoretical calculations.

IR-assisted ionization of He by attosecond XUV radiation

In this work we characterize the underlying processes behind the IR-assisted ionization of helium by extreme ultra violet (XUV) attosecond radiation. We show that, if used in the form of an attosecond pulse train, the XUV radiation can have a good spectral resolution as well as good temporal resolution. Our experimental data and associated theoretical calculations show how to use the properties of attosecond XUV radiation to induce a desired excited state of helium, and how to tune the IR laser electric field to control the ionization pathways.