Time of Flight Mass Spectrometry of Ions Generated by Molecules in Intense Laser Fields Mingtong Han 25 th August, 2013 Abstract Photoionization of acetylene, dimethylacetylene and methylacetylene in an intense laser field (45 fs, 800 nm, 15 2 10 W/cm ) is studied by high-resolution time of flight mass spectrometry (TOFMS). Possible pathways of Coulomb explosions during photoionization of acetylene are studied by careful analysis of the signal profiles and momentum releases of the fragments. The laser field intensity dependence and polarization dependence of Coulomb explosion fragments of acetylene are investigated by mass-resolved momentum imaging (MRMI) of the ion fragments under a sequence of different laser field intensities and rotated laser polarization directions. New features of ejection of triatomic hydrogen ions in photoionization of dimethylacetylene and methylacetylene molecules are also observed and recorded. 1. Introduction Ultrafast laser highly increases the intensity of laser field that can be achieved in experiments, which opens up a new research field of atomic, molecular and optical physics. In intense laser fields, the possibility for multi-photon ionization to happen significantly increases [1], which enables molecules that are stable in laser fields of usual intensity to get ionized and even fragmented. By focusing the intense laser light, it is possible to approach the intensity of the order of 2 petawatt/cm , which is comparable to the magnitude of the electric field inside the molecule. With the irradiation of such strong laser light, molecules are known to dissociate into multiply charged ions which subsequently experience bond fission processes generating charged ion fragments with high kinetic energy. This phenomenon is called Coulomb explosion, which has become a fascinating research target in recent years. By studying the ion yield, the kinetic energy release and other physical quantities of the ion fragments, it is helpful to reveal the dynamics of Coulomb explosion and to obtain information about the parent ions.
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Time of Flight Mass Spectrometry of Ions Generated
by Molecules in Intense Laser Fields
Mingtong Han
25th
August, 2013
Abstract
Photoionization of acetylene, dimethylacetylene and methylacetylene in an intense laser field (45
fs, 800 nm, 15 210 W/cm ) is studied by high-resolution time of flight mass spectrometry (TOFMS).
Possible pathways of Coulomb explosions during photoionization of acetylene are studied by careful
analysis of the signal profiles and momentum releases of the fragments. The laser field intensity
dependence and polarization dependence of Coulomb explosion fragments of acetylene are investigated
by mass-resolved momentum imaging (MRMI) of the ion fragments under a sequence of different laser
field intensities and rotated laser polarization directions. New features of ejection of triatomic hydrogen
ions in photoionization of dimethylacetylene and methylacetylene molecules are also observed and
recorded.
1. Introduction
Ultrafast laser highly increases the intensity of laser field that can be achieved in
experiments, which opens up a new research field of atomic, molecular and optical
physics. In intense laser fields, the possibility for multi-photon ionization to happen
significantly increases [1], which enables molecules that are stable in laser fields of
usual intensity to get ionized and even fragmented. By focusing the intense laser light,
it is possible to approach the intensity of the order of 2petawatt/cm , which is
comparable to the magnitude of the electric field inside the molecule. With the
irradiation of such strong laser light, molecules are known to dissociate into multiply
charged ions which subsequently experience bond fission processes generating
charged ion fragments with high kinetic energy. This phenomenon is called Coulomb
explosion, which has become a fascinating research target in recent years. By
studying the ion yield, the kinetic energy release and other physical quantities of the
ion fragments, it is helpful to reveal the dynamics of Coulomb explosion and to obtain
information about the parent ions.
2. Experiment
Fig 1. The diagram on the top shows the simple structure and the working principle of the TOF mass
spectrometer. Down left is a photo of the inside of the chamber with the incident direction of laser and
ejection of gas indicated with arrows. Down right is a photo of the three extraction plates.
The experimental setup is indicated in Fig 1. Femtosecond laser pulses (800 nm)
are generated by a laser source at a repetition rate of 1 kHz. After the amplifiers and
pulse compressor, the pulse energy has reached 2.53 mJ/pulse, with the temporal
width of 45 fs measured by Single Shot Autocorrelator. The light beam is focused by a
lens into the chamber, where it meets the molecular effusive beam of sample gas
perpendicularly right between the first two of the three parallel extraction plates of the
time of flight mass spectrometer which accelerates the ion fragments produced in the
various processes of photoionization. The focus area of the laser light as measured to
be -5 22.24×10 cm by CCD camera by assumption of a Gaussian spatial profile, with
the field intensity estimated to be15 22.53×10 W/cm .
Ions with different mass-to-charge ratios acquire the same amount of kinetic
energy but different velocities inversely proportional to the square root of
mass-to-charge ratios in the electric field, with which they fly through a vacuum flight
tube and get separated in space, which will result in a difference of their arrival times
at the micro-channel-plate (MCP) detector placed at the end of the tube. For ion
fragments generated in Coulomb explosion, they get additional initial momentum of
opposite directions compared to other ions, and their peaks are shifted to opposite
ways from the central peak which is formed by ions of the same mass-to-charge ratio
experiencing no Coulomb explosion. Mass spectra are obtained by mass assignment
of the temporal signals recorded and averaged over 3000 sweeps by a digital oscillator
with a sampling rate of 1kHz.
It is crucial to achieve a relatively high mass resolution in order to resolve the
signals of the same ion fragments generated from different multiple charged ions of
one parent molecule. Conventionally, the mass resolution is defined as /m m ,
where m is the smallest deviation between two peak at which they can be
distinguished on the mass spectrum. In our case, the mass resolution can be more
directly signified byshiftm , the deviation of the Coulomb explosion peaks from the
central peak, or from the central mass number if there is no central peak in between. A
larger shiftm for the same kind of ion fragments indicates a higher mass resolution. To
improve the value of shiftm , extension of the original flight tube is conducted by
increasing the length of flight from 502.6 mm to 1477.6 mm as shown in Fig 2, and
tests are run with air as the sample gas. By comparing specifically the peaks of
nitrogen ions on the mass spectra both before and after extension, it is clear that the
mass resolution is highly increased, since shiftm is almost doubled, which is reflected
clearly as the large separation between the side peaks from the central peak in Fig 3.
After reaching this high resolution, experiments on photoionization of acetylene,
dimethylacetylene and methylacetylene are run respectively. A variable neutral
density filter is placed before the pulse compressor to change the intensity of the laser
beam and spectra under different intensities are recorded for each gas sample. For
MRMI, a zero-order half-wave plate is introduced to rotate the polarization of the
laser beam with respect to the direction of the detection axis. The half-wave plate is
rotated manually at an interval of 3 , which corresponds to a 6 interval for the