66 research outputs found
Monitoring Nonadiabatic Avoided Crossing Dynamics in Molecules by Ultrafast X-Ray Diffraction
We examine time-resolved X-ray diffraction from molecules in the gas phase
which undergo nonadiabatic avoided-crossing dynamics involving strongly coupled
electrons and nuclei. Several contributions to the signal are identified,
representing (in decreasing strength) elastic scattering, contributions of the
electronic coherences created by nonadiabatic couplings in the avoided crossing
regime, and inelastic scattering. The former probes the charge density and
delivers direct information on the evolving molecular geometry. The latter two
contributions are weaker and carry spatial information of the transition charge
densities (off-diagonal elements of the charge-density operator). Simulations
are presented for the nonadiabatic harpooning process in the excited states of
sodium fluoride
Utilizing Microcavities to Suppress Third-order Cascades in Fifth-order Raman Spectra
Nonlinear optical signals in the condensed phase are often accompanied by
sequences of lower-order processes, known as cascades, which share the same
phase matching and power dependence on the incoming fields and are thus hard to
distinguish. The suppression of cascading in order to reveal the desired
nonlinear signal has been a major challenge in multidimensional Raman
spectroscopy, i.e., the signal being masked by cascading signals
given by a product of two processes. Since cascading originates
from the exchange of a virtual photon between molecules, it can be manipulated
by performing the experiment in an optical microcavity. Using a quantum
electrodynamical (QED) treatment we demonstrate that the cascading
contributions can be greatly suppressed. By optimizing the cavity size and the
incoming pulse directions, we show that up to 99.5\% suppression of the
cascading signal is possible.Comment: 15 pages, 2 figures; Accepted by J. Phys. Chem. Let
Detecting Electronic Coherence by Multidimensional Broadband Stimulated X-Ray Raman Signals
Nonstationary molecular states which contain electronic coherences can be
impulsively created and manipulated by using recently-developed ultrashort
optical and X-ray pulses via photoexcitation, photoionization and Auger
processes. We propose several stimulated-Raman detection schemes that can
monitor the phase-sensitive electronic and nuclear dynamics. Three detection
protocols of an X-ray broadband probe are compared - frequency dispersed
transmission, integrated photon number change, and total pulse energy change.
In addition each can be either linear or quadratic in the X-ray probe
intensity. These various signals offer different gating windows into the
molecular response which is described by correlation functions of electronic
polarizabilities. Off-resonant and resonant signals are compared
Catching Conical Intersections in the Act; Monitoring Transient Electronic Coherences by Attosecond Stimulated X-Ray Raman Signals
Conical intersections (CoIn) dominate the pathways and outcomes of virtually
all photophysical and photochemical molecular processes. Despite extensive
experimental and theoretical effort, CoIns have not been directly observed yet
and the experimental evidence is being inferred from fast reaction rates and
some vibrational signatures. We show that short X-ray (rather than optical)
pulses can directly detect the passage through a CoIn with the adequate
temporal and spectral sensitivity. The technique is based on a coherent Raman
process that employs a composite femtosecond/attosecond X-ray pulse to detect
the electronic coherences (rather than populations) that are generated as the
system passes through the CoIn
X-Ray sum frequency generation; direct imaging of ultrafast electron dynamics
X-ray diffraction from molecules in the ground state produces an image of
their charge density, and time-resolved X-ray diffraction can thus monitor the
motion of the nuclei. However, the density change of excited valence electrons
upon optical excitation can barely be monitored with regular diffraction
techniques due to the overwhelming background contribution of the core
electrons. We present a nonlinear X-ray technique made possible by novel free
electron laser sources, which provides a spatial electron density image of
valence electron excitations. The technique, sum frequency generation carried
out with a visible pump and a broadband X-ray diffraction pulse, yields
snapshots of the transition charge densities, which represent the electron
density variations upon optical excitation. The technique is illustrated by ab
initio simulations of transition charge density imaging for the optically
induced electronic dynamics in a donor/acceptor substituted stilbene
Monitoring Nonadiabatic Electron-Nuclear Dynamics in Molecules by Attosecond Streaking of Photoelectrons
Streaking of photoelectrons has long been used for the temporal
characterization of attosecond extreme ultraviolet pulses. When the
time-resolved photoelectrons originate from a coherent superposition of
electronic states, they carry an additional phase information, which can be
retrieved by the streaking technique. In this contribution we extend the
streaking formalism to include coupled electron and nuclear dynamics in
molecules as well as initial coherences and demonstrate how it offers a novel
tool to monitor non-adiabatic dynamics as it occurs in the vicinity of conical
intersections and avoided crossings. Streaking can enhance the time resolution
and provide direct signatures of electronic coherences, which affect many
primary photochemical and biological events
Non-adiabatic dynamics of molecules in optical cavities
Strong coupling of molecules to the vacuum field of micro cavities can modify
the potential energy surfaces opening new photophysical and photochemical
reaction pathways. While the influence of laser fields is usually described in
terms of classical field, coupling to the vacuum state of a cavity has to be
described in terms of dressed photon-matter states (polaritons) which require
quantized fields. We present a derivation of the non-adiabatic couplings for
single molecules in the strong coupling regime suitable for the calculation of
the dressed state dynamics. The formalism allows to use quantities readily
accessible from quantum chemistry codes like the adiabatic potential energy
surfaces and dipole moments to carry out wave packet simulations in the dressed
basis. The implications for photochemistry are demonstrated for a set of model
systems representing typical situations found in molecules
Cascading and Local-Field Effects in Non-Linear Optics Revisited; A Quantum-Field Picture Based on Exchange of Photons
The semi-classical theory of radiation-matter coupling misses local-field
effects that may alter the pulse time-ordering and cascading that leads to the
generation of new signals. These are then introduced macroscopically by solving
Maxwell's equations. This procedure is convenient and intuitive but ad hoc. We
show that both effects emerge naturally by including coupling to quantum modes
of the radiation field in the vacuum state to second order. This approach is
systematic and suggests a more general class of corrections that only arise in
a QED framework. In the semi-classical theory, which only includes classical
field modes, the susceptibility of a collection of non-interacting
molecules is additive and scales as . Second-order coupling to a vacuum mode
generates an effective retarded interaction that leads to cascading and local
field effects both of which scale as
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