Our research centers upon measuring and controlling dynamics in atomic and molecular systems using shaped ultrafast laser pulses.
We are working to develop new time resolved spectroscopies to follow the quantum dynamics of electrons and nuclei in molecular systems.
Our approach is to work on experiments of varying complexity, and to understand our experimental results through both simple intuitive models and detailed calculations.
On Left: Wave packet movie illustrating dynamics in the molecule CH2I2 following impulsive excitation with an ultrafast laser pulse.
Velocity Map Imaging of Strong Field Molecular Ionization
We have developed a coincidence velocity map imaging apparatus for studying strong field molecular ionization using shaped laser pulses. For each laser shot, we measure ions and electrons in coincidence using velocity map imaging
to record the vector momentum of the electrons as they leave the molecule. An illustration of our apparatus with a typical measurement is shown below.
Vacuum Ultraviolet(VUV) Light Source and UV/VUV Pump Probe Experiments
We have developed a new ultrafast light source to conduct UV/VUV pump probe experiments to study excited state dynamics such as internal conversion, isomerization, intersystem crossing, and dissociation.
We use a Ti:Sapphire laser to generate UV and VUV pulses. UV(4.8eV) pulses are generated through third harmonic generation in BBO crystals. The VUV(8eV) pulses are generated via non-collinear-four-wave-mixing
in Argon. In some experiments, the UV pulse acts as a pump and the VUV as a probe, while in others the role of the pulses are reversed, allowing us to study excited states at both 4.8 eV and 8 eV.
Filament Based Spectral Broadening for Sub 10 fs Pulses
We recently developed a filament based spectral broadening source capable of spanning an octave of spectral bandwidth.
We compress the pulses using a grating based compressor and have measured pulses as short as 9 fs. We are currently working on compressing the pulses down to 5 fs. The figure below shows the spectrum from our filament
source along with the shortest pulse supportd by this bandwidth in the inset.
Strong field ionization as a function of pulse duration and shape
Making use of our new broadband light source, we are studying the dynamics of strong field ionization
as a function of pulse duration and shape. The figure below illustrates how the photoelectron spectrum
for ionization of CH2
IBr changes as a function of pulse duration. The measurements are based
on velocity map imaging of the photo-electrons and the peaks have been identified using coincidence
measurements with photo-ions. The figure illustrates how the ionization changes with pulse duration, with a single ionic
state dominating the ionization process for the shortest laser pulse. What is particularly interesting about the measurement
is that it demonstrates preferential removal of an electron from a more deeply bound orbital (HOMO-1, leading to the first excited
ionic state D1
), rather than from the highest occupied orbital (HOMO, leading to the ground ionic state,D0
for the shortest pulse
As a complement to the measurement shown above, we also measured the photoelectron spectrum for ionization as a function of central
frequency. The measurements were carried out using our filament based supercontinuum, using a slit to select a ~25 nm window over
a 200 nm range. The result below indicates that most of the ionization consists of removing an electron from the HOMO, in contrast with
the measurements shown above.
UV/VUV Pump Probe Experiment Results
VUV-pump UV-probe Pyrrole results
With our newly developed UV-VUV light source, we have conducted VUV-pump UV-probe experiments in pyrrole(C4
N). Our measurements, in conjuction with electromic structure calculations, indicate that
pyrrole undergoes rapid internal conversion to the ground state in less than 300fs. The internal conversion to the ground state dominates over dissociation. The figure below shows the VUV-pump UV-
probe ion yields and their fits proformed on ethylene and pyrrole. We applied a mono-exponential fit to ethylene which yields a decay constant of 37fs, consistant with values reported in literatures. Both
mono-exponential(a) and dual-exponential(b) fits are applied to the pyrrole data and residuals are shown in the insets. The residuals of the dual-exponential fit is randomly distributed, indicating that this fit is consistant with the measurements.
UV-pump VUV-probe CH2I2 Results
UV-pump VUV-probe experiments were performed on CH2
. The figure below shows the pump probe yield of parent and fragment ions, in which the fits show a 20fs difference in the decay constants between them. This highlights the question how does the fragmentatoion happen in the ionization process. The calculations done by our theory collaborator(Philipp Marquetand) point out that there are rich dynamics during the ionization process such as I and I2
dissociation. The movie below shows the I dissociation when the ionization happens.
We are working on improving the time resolustion and upgrading the detector to conduct velocity map imaging measurements.
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