Nitro- and inorganic-based energetic material is vaporized at atmospheric pressure using nonresonant, 70
femtosecond laser pulses prior to electrospray post-ionization and transfer into a time-of-flight mass spectrometer for
mass analysis. Measurements of a nitro-based energetic molecule, cyclotrimethylenetrinitramine (RDX), adsorbed on
metal and dielectric surfaces indicate nonresonant vaporization of intact molecules, demonstrating the universality of
laser electrospray mass spectrometry (LEMS) technique for explosives. In addition, RDX is analyzed at a distance of 2
meters to demonstrate the remote detection capability of LEMS. Finally, the analysis and multivariate statistical
classification of inorganic-based explosives containing ammonium nitrate, chlorate, perchlorate, black powder, and an
organic-based explosive is presented, further expanding the capabilities of the LEMS technique for detection of energetic
materials.
Multiple foci are created along the propagation direction of a laser beam using simultaneous spatial and
temporal focusing of an ultrafast laser pulse in conjunction with parametric pulse shaping. The pulses
are characterized with scanning SEA TADPOLE. The longitudinal and transverse positions of the foci
are controlled using phase and amplitude shaping in a 4-f laser pulse shaping system. Measurements of
the pulse duration as a function of spatial position of the foci are in agreement with the predictions of a
Fourier optics model.
Atmospheric pressure mass analysis of solid phase biomolecules is performed using laser electrospray mass
spectrometry (LEMS). A non-resonant femtosecond duration laser pulse vaporizes native samples at atmospheric
pressure for subsequent electrospray ionization and transfer into a mass spectrometer. LEMS was used to detect
a complex molecule (irinotecan HCl), a complex mixture (cold medicine formulation with active ingredients:
acetaminophen, dextromethorphan HBr and doxylamine succinate), and a biological building block
(deoxyguanosine) deposited on steel surfaces without a matrix molecule.
Broadband, coherent radiation in the optical frequency range is generated using micro-plasma channels in
atmospheric gases in a pump-probe experiment. A micro-plasma medium is created in a gas by a focused intense
femtosecond pump pulse. A picosecond probe pulse then interacts with this micro-plasma channel, producing broad,
coherent sidebands that are associated with luminescence lines and are red- and blue-shifted with respect to the laser
carrier frequency. These sidebands originate from the induced Rabi oscillations between pairs of excited states that
are coupled by the probe pulse. These excited states become populated in the process of plasma cooling. Thus, the
sideband radiation intensity tracks the micro-plasma evolution. The sidebands incorporate Rabi shifts corresponding
to varying value of the electric field magnitude in the probe pulse: this makes them broad and malleable to tuning.
The intensity of the probe beam ~ 1010 W cm-2, creates a maximum sideband shift of > 90 meV from the carrier
frequency, resulting in an effective bandwidth of 200 meV. The sidebands may be effectively controlled by the
intensity and temporal profile of the probe pulse. The giant Rabi shift is both tunable and coherent over a wide range
of frequencies and over a wide range of atomic transitions. The fact that the coherence is observed in a micro plasma
demonstrates that Rabi cycling is possible at high temperature with moderately high laser intensities (1010 W cm-2)
as long as transitions close to the driving frequency (▵ ~ 2% ωc) are available.
A new multiplexed stimulated Raman spectroscopic technique encompassing a single-shot spectral
measurement range of over 3900 cm-1 is presented. Impulsive excitation of all Raman active vibrational
modes present in a medium is achieved by self-compression of a laser pulse undergoing filamentation in
air, creating coherent vibrational wave-packets. These wave-packets create a macroscopic polarization of
the medium that imparts sidebands on a delayed narrowband probe pulse. The background-free
measurement of impulsively excited Raman modes in gas-phase N2, O2, H2, CO2, toluene, ammonia, and
chloroform with a spectral resolution of 25 cm-1 is presented.
We have studied the application of the diffusion mapping technique to dimensionality reduction and clustering in
multidimensional optical datasets. The combinational (input-output) data were obtained by sampling search spaces
related to optimization of a nonlinear physical process, short-pulse second harmonic generation. The diffusion mapping
technique hierarchically reduces the dimensionality of the data set and unifies the statistics of input (the pulse shape) and
output (the integral output intensity) parameters. The information content of the emerging clustered pattern can be
optimized by modifying the parameters of the mapping procedure. The low-dimensional pattern captures essential
features of the nonlinear process, based on a finite sampling set. In particular, the apparently parabolic two-dimensional
projection of this pattern exhibits regular evolution with the increase of higher-intensity data in the sampling set. The
basic shape of the pattern and the evolution are relatively insensitive to the size of the sampling set, as well as to the
details of the mapping procedure. Moreover, the experimental data sets and the sets produced numerically on the basis of
a theoretical model are mapped into patterns of remarkable similarity (as quantified by the similarity of the related
quadratic-form coefficients). The diffusion mapping method is robust and capable of predicting higher-intensity points
from a set of low-intensity points. With these attractive features, diffusion mapping stands poised to become a helpful
statistical tool for preprocessing analysis of vast and multidimensional combinational optical datasets.
Efficient confinement of laser radiation in the core of a photonic crystal fiber (PCF) enhances the nonlinear processes
resulting in supercontinuum generation. The technique of adaptive pulse shaping using an evolutionary algorithm
provides a method to gain control over nonlinear processes. Adaptive pulse shaping of the driving laser radiation
passing through the photonic crystal fiber was employed to modify the shape and composition of the output
supercontinuum. Amplified pulses of a Ti:Sapphire laser system were coupled into a high air fill factor (cobweb) PCF
sample. Alternatively a Cr:Forsterite master oscillator was used as the pump source. Supercontinua acquired with
unshaped pulses are compared for both pump sources. Amplitude and phase shaping of the amplified Ti:Sapphire pulses
was then used to optimize the emission between 500 and 700 nm, as well as a soliton centered at 935 nm. The originally
separated spectral regions near 700 nm eventually merged into a smoother and broader supercontinuum. The intensities
of the broadband emission and of the soliton driven by a shaped laser pulse increased in comparison to unshaped pulses
by factors of 4 and 3, respectively. In addition, the suppression of self-steepening effects in supercontinuum spectra was
demonstrated using a shaped laser pulse.
We report the collision-induced desorption and dissociation of two molecular forms of ammonia bound to a Pt{111} single crystal by a beam of translationaly energetic, neutral noble gas atoms. In this experiment the probability for desorption at low coverage is measured as a function of the collision energy between an Ar atom and chemisorbed ammonia molecule and is found to be independent of the Ar beam angle of incidence. At higher initial ammonia coverages the probability of collision-induced desorption is greater since the NH3/Pt{111} binding energy is much weaker. In addition we observe formation of surface NHx products generated in a process known as collision-induced dissociation.
Efficient sequencing of the human genome will require the development of new methods that are less expensive and orders of magnitude faster than current technology. Recent advances in laser-based methodology suggest that a mass spectroscopic DNA sequencing technique may surmount present limitations. This contribution will focus on the use of laser vaporization and laser ionization to prepare single stranded DNA for high speed sequencing in the gas phase. As a first step in the implementation of a mass spectroscopic sequencing approach, we have shown that single-stranded DNA molecules having chain lengths of over 1000 nucleotides can be laser vaporized into the gas phase with no discernible strand cleavage. This observation provides the basis for the time-of-flight (TOF) mass spectral-based sequencing experiment that we are developing. To determine the DNA sequence the experiment will be repeated for the four complimentary dideoxy sequencing reactions. The realization of this method would allow a 300 base DNA sequence to be determined in less than one second. At that rate, an instrument based on this technology could potentially generate sequencing data in excess of 25 million bases per day.
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