Jeffrey A. Cina

Fluorescence‐detected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phase‐locked pulses

We introduce a novel spectroscopic technique which utilizes a two‐pulse sequence of femtosecond duration phase‐locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in‐phase, in‐quadrature, and out‐of‐phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct de...

Fluorescence‐detected wave packet interferometry. II. Role of rotations and determination of the susceptibility

The recently developed technique of time‐resolved spectroscopy with phase‐locked optical pulse pairs is further explored with additional experimental data and more detailed comparison to theory. This spectroscopic method is sensitive to the overall phase evolution of an optically prepared nuclear wave packet. The phase locking scheme, demonstrated for the B←X transition of gas phase molecular iodine, is extended through the use of in‐quadrature locked pulses and by examination of the dispersed fluorescence signal. The excited state population following the interaction with both pulses is detected as the resultant two‐field‐dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from rovibrational wave packet dynamics of the molecule on the excited state electronic potential energy curve. Quantum interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two‐pulse phase‐locked interferograms are in...

On the preparation and measurement of superpositions of chiral amplitudes

We examine the preparation and detection of superpositions of chiral amplitudes of a handed molecule, showing that specific sequences of phase‐controlled ultrashort light pulses enable the preparation and measurement of chiral coherences. It is found that certain choices of relative optical phase between the pulses of the preparation sequence set up left–right superpositions that would be inaccessible by tunneling dynamics alone.

What can short-pulse pump-probe spectroscopy tell us about Franck-Condon dynamics?

We examine the signal from pump-probe spectroscopy of a model system—nonrotating I2—at short time delays and compare signals calculated without approximation (a full quantum calculation), with a semiclassical Franck-Condon approximation, and with a classical simulation of the nuclear wave packet. In order to assess the complications of simulation and interpretation when the probe window lies in the spectroscopically and dynamically important Franck-Condon region, we concentrate on a case where pump and probe resonances are at the same internuclear distance. We find that the common practice of ignoring the pump-truncation effects of pulse overlap leads to an overestimate of the signal at short times. Moreover, both classical simulations and semiclassical Franck-Condon treatments can deviate significantly in form from the actual signal even with proper treatment of pulse overlap. The sources of these deviations can be seen in the evolution of the excited-state nuclear distributions calculated classically an...

Nonlinear wavepacket interferometry for polyatomic molecules

We investigate the application of a previously considered nonlinear wavepacket interferometry scheme to molecules with a single stable conformation in the electronic ground state. It is shown that interference experiments with pairs of phase-locked ultrashort pulse-pairs can be used to determine the complex overlaps of a nonstationary nuclear wavefunction evolving in an excited electronic state with a collection of compact displaced wavepackets moving in specified ways in the ground-state potential.

Can chirp enhance cumulative pre‐resonant impulsive stimulated Raman excitation?

Simple arguments are presented and numerical calculations are performed which show that frequency chirp increases the amplitude of vibrational motion induced in the ground state of I2 by optimized sequences of ultrashort, pre‐resonant optical pulses. Sequences with a variety of constraints on pulse chirp rates and pulse center frequencies were generated by time local optimization and are compared. A sequence with pulse‐by‐pulse variable chirp and variable center frequency is shown to be considerably more effective than the most effective sequence with a fixed negative chirp rate and constant pulse center frequency, which is in turn much more effective than a sequence of unchirped fixed‐center‐frequency pulses considered previously. The best sequence of pulses induces a 0.4 A circuit in the expectation value of the I2 stretch from cumulative pre‐resonant impulsive stimulated Raman scattering.

Thomas–Fermi theory in a weak, slowly varying vector potential

The gauge invariant kinetic energy of an inhomogeneous electron gas in the presence of a weak, slowly varying vector potential is determined by a generalization of Schwinger’s method. The kinetic energy is a function of the charge density and current density. The resulting energy should prove useful in determining the linear responses of closed shell systems to magnetic fields.

Wave packet interferometry for short-time electronic energy transfer: Multidimensional optical spectroscopy in the time domain

We develop a wave packet interferometry description of multidimensional ultrafast electronic spectroscopy for energy-transfer systems. After deriving a general perturbation-theory-based expression for the interference signal quadrilinear in the electric field amplitude of four phase-locked pulses, we analyze its form in terms of the underlying energy-transfer wave packet dynamics in a simplified oriented model complex. We show that a combination of optical-phase cycling and polarization techniques will enable the experimental isolation of complex-valued overlaps between a “target” vibrational wave packet of first order in the energy-transfer coupling J, characterizing the one-pass probability amplitude for electronic energy transfer, and a collection of variable “reference” wave packets prepared independently of the energy-transfer process. With the help of quasiclassical phase-space arguments and analytic expressions for local signal variations, the location and form of peaks in the two-dimensional interferogram are interpreted in terms of the wave packet surface-crossing dynamics accompanying and giving rise to electronic energy transfer.

TOWARD PRERESONANT IMPULSIVE RAMAN PREPARATION OF LARGE AMPLITUDE VIBRATIONAL MOTION

This article investigates a new approach to the optical generation of large‐amplitude coherent molecular vibrations in condensed media. On the basis of analytical results using pulse propagators in the classical Franck approximation, we are led to investigate the efficacy of driving vibrational motion in the electronic ground state by impulsive stimulated Raman scattering with a timed sequence of electronically preresonant femtosecond laser pulses. Numerically exact computations are performed on a model system of dilute molecular Iodine in a low‐temperature host crystal. Vibrational relaxation is incorporated via Redfield theory. The results indicate that under a variety of conditions, chemically significant (greater than 0.1 A) displacements can be produced in a Raman active mode with a fair measure of control over wave packet spreading, and without substantial population loss due to electronic absorption.

Resonant short-pulse effects on nuclear motion in the electronic ground state

Abstract We investigate several approximate treatments of the pulse propagators describing the effects of electronically resonant ultrashort light pulses on the states of molecules in condensed phases, including the classical Franck approximation and a quadratic Hamiltonian treatment. Using a Morse oscillator model of iodine, we examine the effects of the initial form and motion of the pump-induced nuclear density matrix increment on pump-probe signals. We also clarify the conditions of high temperature and short-pulse duration under which the contribution of nuclear dynamics in the electronic ground state to a pump-probe signal will bear a definite sign.