R. Zadoyan

The breaking and remaking of a bond: Caging of I2 in solid Kr

The caging of I2 in solid Kr is followed in real‐time following its dissociative excitation on the A(3Π1u) surface. The experiments involve pump–probe measurements with a time resolution of ≥150 fs. The experimental signals are reproduced using classical molecular dynamics simulations, and the classical Franck approximation. The comparison between experiment and simulation allows an unambiguous interpretation of features in the observed signal as being due to the initial impulsive stretch of the I–I bond, collision of the atoms with the cage wall, recoil and recombination, and the subsequent coherent oscillations of the nascent I2 molecule. These detailed observations are possible due to retention of coherence along the I–I coordinate throughout the caging process. The extent of coherence is dictated mainly by the initial impact parameters of the molecule‐cage collision, which in turn is controlled by the thermal and zero‐point amplitudes of lattice vibrations. The caging is well‐described as a sudden pro...

Quantum control of I2 in the gas phase and in condensed phase solid Kr matrix

We present experimental results and theoretical simulations for an example of quantum control in both gas and condensed phase environments. Specifically, we show that the natural spreading of vibrational wavepackets in anharmonic potentials can be counteracted when the wavepackets are prepared with properly tailored ultrafast light pulses, both for gas phase I2 and for I2 embedded in a cold Kr matrix. We use laser induced fluorescence to probe the evolution of the shaped wavepacket. In the gas phase, at 313 K, we show that molecular rotations play an important role in determining the localization of the prepared superposition. In the simulations, the role of rotations is taken into account using both exact quantum dynamics and nearly classical theory. For the condensed phase, since the dimensionality of the system precludes exact quantum simulations, nearly classical theory is used to model the process and to interpret the data. Both numerical simulations and experimental results indicate that a properly ...

Femtosecond dynamics of coherent photodissociation—recombination of I2 isolated in matrix Ar

Abstract Pump—probe studies of I 2 in matrix Ar, with a time resolution of 180 fs, are reported. The experiments are simulated using classical molecular dynamics. Dissociative excitation of I 2 (A) results in coherent cage-induced recombination on the A/A′ surfaces. The recombinant molecule vibrates coherently even after extensive energy loss.

Interrogation and control of condensed phase chemical dynamics with linearly chirped pulses: I2 in solid Kr

The effect of linearly chirped pulses in condensed phase ultrafast pump–probe experiments is investigated by classical simulations for the model system of I2 isolated in a Kr matrix. The central frequency of the probe laser is selected to monitor exclusively the event of first collision and recoil of atoms from the host cage. It is shown that a chirped probe pulse enables characterization of the magnitude and sign of the momentum of the evolving trajectory flux. This can be understood by transforming the frequency–time profile of the probe pulse to coordinate–time space, and noting that the observable signal is a function of the relative group velocities of the traveling wave packet and the traveling window function. The effect of the pump pulse chirp, is a measure of the controllability of the evolving dynamics. In the particular case studied, breaking and remaking of the I2 bond near the dissociation limit of the bare molecule, it is shown that the memory of the system outlasts the collision with the ca...

Photodynamics in superfluid helium: Femtosecond laser-induced ionization, charge recombination, and preparation of molecular Rydberg states

Femtosecond pulses (790 nm) are used for nonresonant laser excitation of superfluid liquid helium to prepare ionic and neutral excited states at energies above 18 eV. Measurements of laser-induced fluorescence and photocurrent enable a detailed description of the primary photoprocesses. A controllable excitation regime unique to femtosecond pulses is realized at laser intensities below the dielectric breakdown threshold, I<5×1013 W/cm2. A steady state of the long-lived triplet excimers He2*(3a) (lowest Rydberg state) is established; the concentration decays between laser pulses through diffusion-controlled bimolecular annihilation to ∼1012 cm−3 at a laser repetition rate of 500 Hz. The triplet population is amplified with each pulse in a sequence that involves: (1) ionization of the Rydberg electron of He2* via complete Coulomb barrier suppression; (2) cascade electron impact ionization of the ground-state He atoms by the ponderomotively accelerated quasifree electrons in liquid He; (3) localization and t...

Time resolved coherent anti-Stokes Raman scattering of I2 isolated in matrix argon: Vibrational dynamics on the ground electronic state

Time-resolved, electronically resonant, coherent anti-Stokes Raman scattering is used to prepare and interrogate vibronic coherences of molecular iodine in matrix Ar. Coherences that involve evolution on the excited B(3Π0u) state, first- and third-order coherences, decay in less than one vibrational period (τ<300 fs). In contrast, as many as 200 vibrational periods of motion can be observed for Raman-prepared wave packets consisting of zero-phonon vibrational superpositions on the ground electronic state (second-order coherence). Packets consisting of v=4, 5 and v=3, 4, 5 on the X(1Σg) state decay with a half-life of 10±1 ps at 31 K, allowing a more accurate measure of vibrational level spacings and decoherence time than has been possible in frequency domain. The harmonic frequency of the molecule is reduced by 1.5 cm−1 (0.7%) in the matrix. The lack of recurrence in the excited electronic state ensures that the resonant anti-Stokes scattering arises only from the negative momentum component of the Raman ...

A direct interrogation of superfluidity on molecular scales

Time-resolved, pump–probe measurements are used to directly interrogate dissipative fluid dynamics in bulk He-II, on molecular scales, as a function of temperature and pressure. The Rydberg transitions of the triplet He2* excimers, which solvate in bubble states in liquid helium, are used as nanoscale transducers to initiate and to directly monitor the motion of the fluid in the form of damped oscillations of a 13 A spherical bubble. The oscillations are damped out after one period, with a temperature-dependent period that directly tracks the normal fraction. As such, the bubble oscillator acts as a nanoviscosimeter. Through simulations of the observed signals, it is established that the coherent response of the bath obeys hydrodynamic equations of motion of a continuum subject to two-fluid flow. Dissipation occurs through two distinct channels: (a) Radiation of sound in the farfield, driven by the acceleration of volume in the compressible fluid; (b) temperature-dependent drag in the near-field. The drag...

Predissociation dynamics of I2(B) in liquid CCl4 observed through femtosecond pump-probe measurements: Electronic caging through solvent symmetry

Direct observations of the solvent induced electronic predissociation of I2(B) in liquid CCl4 are made using femtosecond pump–probe measurements in which fluorescence from spin–orbit excited I*I* pairs, bound by the solvent cage, is used as detection. Data is reported for initial preparations ranging from the B state potential minimum, at 640 nm, to above the dissociation limit, at 490 nm. Analysis is provided through classical simulations, to highlight the role of solvent structure on: recombination, vibrational relaxation, and decay of coherence. The data is consistent with an anisotropic I2(X)–CCl4 potential which, in the first solvent shell, leads to an angular distribution peaked along the molecular axis. The roles of solvent structure and dynamics on electronic predissociation are analyzed. The data in liquid CCl4 can be understood in terms of a curve crossing near v=0, at 3.05 A<Rc<3.8 A, and the final surface can be narrowed down to 2g or a(1g). This nonadiabatic u→g transition is driven by static...

Dynamics and the breaking of a driven cage: I2 in solid Ar

Pump-probe measurements of I2 in solid Ar are reported and analyzed to extract a description of cage response to impulsive excitation, from the gentle kick, up to the breaking point. The most informative data are obtained through wavepacket motion on cage-bound, but otherwise dissociative, potentials where the chromophore acts as a transducer to drive the cage and to report on the local dynamics. This general class of dynamics is identified and analyzed as a function of energy in Ar, Kr, and Xe. The overdriven cage rebounds with a characteristic period of 1.2 ps that shows little dependence on excitation amplitude in all hosts. After rebound, the cage rings as a local resonant mode in Ar, with a period of 1 ps and dephasing time of 3 ps. This mode remains at the Debye edge in Kr and Xe, with periods of 630 and 800 fs, and dephasing times of 8 and 6 ps, respectively. In the bound B-state, the cage fluctuates toward its dilated equilibrium structure on a time scale of 3 ps, which is extracted from the down-chirp in the molecular vibrational frequency. When kicked with excess energy of 4 eV, the Ar cage breaks with 50% probability, and the molecule dissociates. The kinetics of polarization selective, multiphoton dissociation with Gaussian laser intensity profiles is delineated and the ballistics of cage breakout is described: The photodissociation proceeds by destruction of the local lattice, by creating interstitials and vacancies. During large amplitude motion on cage-bound potentials, sudden, nonadiabatic spin-flip transitions can be observed and quantified in space and time. The spin-flip occurs with unit probability in Ar when the I*-I bond is stretched beyond 6 A.

Femtochemistry of the liquid phase laser-induced harpoon reaction between Cl2 and Xe. Observation of bond formation in real time

The liquid phase, two-photon induced intermolecular charge transfer transition, Cl2 t Xe t 2hu+Xe+ClF, in which the photon is resonant with the dissociative Cl1 (‘ll,J intermediate state, is shown to proceed on a time scale shorter than 100 fs. The process is therefore classified as a coherent, cooperative absorption. The subsequent evolution of the harpoon reaction, Xe+Clr + Xe+Cl- +CI, which in liquid Xe further proceeds to form Xe