The focus of this work is controlling orientation and bond-selective dissociation of hydrogen-bonded
anions of the type XHY- using ultrashort, "few-cycle" laser pulses.
For these quantum dynamical simulations, two triatomic hydrogen-bonded anions and their neutral counterparts,
FHF-/FHF and OHF-/OHF, are chosen as model systems.
Geometry optimizations of FHF- and OHF- are performed
at QCISD and UMP4 levels of theory, respectively, to obtain equilibrium bond lengths and rotational constants.
For the triatomic molecules, two-dimensional (2D)
potential energy surfaces (PES) and dipole surfaces are constructed in bond coordinates,
R1 and R2. The molecules
are assumed to retain a collinear geometry with C∞v symmetry
throughout all simulations. From the PES, 2D vibrational wave functions are calculated,
as well as anharmonic symmetric and asymmetric vibrational stretching frequencies.
Isotope effects are studied for the deuterated species, FDF- and ODF-.
Absorption spectra are simulated and the frequencies and intensities of the
resulting peaks are analyzed and compared with experimental data, as well as with
vibrational frequencies obtained from the 2D wave functions.
The challenge of controlling molecular orientation prior to bond-selective dissociation
is treated explicitly for OHF- with rotational wave packet simulations.
A single half-cycle IR pulse of nearly one field polarity is applied to orient the molecule
in the direction of the laser field. Resulting rotational wave packet dynamics
are analyzed to measure the degree of molecular orientation. Even after the field is removed,
moderate molecular orientation is periodically obtained for several picoseconds.
These rotational revivals appear on the time scale of the rotational period, and they recur as long as
coherence in the wave packet is maintained.
To control bond-selective dissociation of the pre-oriented FHF- and OHF- anions,
few-cycle infrared (IR) pulses are designed to induce bond compressions and extensions in the anion species.
In this context, isotope effects are also examined for the heavier isotopomer FDF-.
These pulses are constructed out of several half-cycle IR pulses, such that each half-cycle drives the
oscillatory motion of the center hydrogen atom between the heavy end atoms. For FHF- and
FDF-, this IR pulse gives rise to dynamical symmetry breaking.
When the resulting wave packet has reached the turning point of its oscillation, a well-timed, ultrashort
UV pulse is applied to the anion. The UV pulse detaches an electron from the anion and vertically excites the
wave packet to a bond-selective region of the neutral PES. Branching ratios of the resulting dissociation can
For FHF-, this few-cycle IR +UV pulse scheme enhances the natural branching ratio of
(FH + F):(F + HF) from 0.50:0.50 to 0.75:0.25. For the heavier isotopomer FDF-, the adjusted pulse
parameters give rise to an even larger (FD+F):(F+DF) branching ratio of 0.80:0.20. In OHF-, a natural
branching ratio of (O+HF):(OH+F)=0.84:0.16 is enhanced to 0.95:0.05. Breaking the strong HF bond is challenging,
and with re-optimized IR+UV pulses, the branching ratio can only be enhanced marginally in favor of the products
OH+F to 0.80:0.20.
The spatial separation of dissociation products for FHF- is also investigated for a range of
molecular orientations in the space-fixed laboratory frame. For molecular orientations in the +Z or -Z
laboratory directions, one obtains an optimal yield of spatially separated molecular (FH) and atomic (F)
To summarize, this thesis investigates the orientation and bond-selective dissociation of
the model systems FHF- and OHF- with a control scheme based on ultrashort,
few-cycle IR and UV laser pulses.
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