Matthew C. Asplund

Two-dimensional heterodyned and stimulated infrared photon echoes of N-methylacetamide-D

The stimulated infrared photon echo of N-methylacetamide-D [NMAD; CH3(CO)ND(CH3)] was measured and used to determine the vibrational frequency correlation function. The correlation function was modeled as a single exponential plus a constant, and it was found that most of the NMAD vibrational frequency distribution is motionally narrowed with a pure dephasing time of 1.12 ps. The two-dimensional infrared (2D IR) spectrum of NMAD was also obtained by heterodyning the echo field with a weak local oscillator pulse. The real and imaginary portions of the 2D IR spectrum exhibit multiple peaks due to υ=0–1 and 1–2 coherences that are excited, which are not resolved in the absolute magnitude of the 2D IR spectrum. Using the correlation function determined from the stimulated photon echo, the 2D IR spectrum was accurately simulated. Resolution enhancement of the 2D IR spectrum was performed by manipulating the photon echo field with window functions. The enhanced experimental and simulated 2D IR spectra are drama...

Spectrally resolved three pulse photon echoes in the vibrational infrared

Abstract Spectrally resolved vibrational photon echo measurements of the N 3 − ion in D 2 O and carbonmonoxy hemoglobin in D 2 O have been made using 100 fs infrared pulses in the 1800–2100 cm −1 region. These measurements allow the separation of contributions from different Feynman pathways to the echo signal, allowing comparisons of the time dependence and correlation functions for the 1–0 and 2–1 coherence terms. The spectra show good agreement with a theoretical description of the data fitted to previously determined multiexponential correlation functions. Fluctuations in the anharmonicity are found to be negligible for HbCO when the harmonic vibrational relaxation model is used.

Fabrication of calcium fluoride capillary electrophoresis microdevices for on-chip infrared detection.

In this paper, we demonstrate microfluidic capillary electrophoresis (CE) devices made in CaF2 , for optical detection in a broad spectral range. We have designed methods for micromachining and enclosing capillaries in CaF2. The utility of these microdevices has been shown through CE analysis of fluorescently labeled amino acids. We have also performed infrared spectroscopy for analyte identification in microfluidic CaF2 channels. These CaF2 microdevices open the door to microchip separations with optical detection in the ultraviolet, visible, and infrared spectral regions.

One Ring to Bind Them All: Shape-Selective Complexation of Phenylenediamine Isomers with Cucurbit(6)uril in the Gas Phase

We examined complexes between cucurbit[6]uril and each of ortho-, meta-, and para-phenylenediamine using computational methods, Fourier transform ion cyclotron resonance mass spectrometry, and ion mobility spectrometry. These fundamental gas phase studies show that the lowest energy binding sites for ortho- and meta-phenylenediamine are on the exterior of cucurbit[6]uril, whereas para-phenylenediamine preferentially binds in the interior, in a pseudorotaxane fashion. This conclusion is based on reactivity of each of the complexes with tert-butylamine, where the ortho- and meta-phenylenediamine complexes exchange with tert-butylamine, whereas the para-phenylenediamine complex undergoes two slow additions without displacement. Further, under sustained off-resonance irradiation conditions, the ortho- and meta-phenylenediamine complexes fragment easily via losses of neutral phenylenediamine, whereas the para-phenylenediamine complex fragments at higher energies primarily via cleavage of covalent bonds in the cucurbituril. Finally, ion mobility studies show ion populations for the ortho- and meta-phenylenediamine complexes that primarily have collision cross sections consistent with external complexation, whereas the para-phenylenediamine complex has a collision cross section that is smaller, the same as that of protonated cucurbit[6]uril within experimental error. In agreement with experiment, computational studies indicate that at the HF/6-31G* and B3LYP/6-31G*//HF/6-31G* levels of theory external complexation is favored for ortho- and meta-phenylenediamine, whereas internal complexation is lower in energy for para-phenylenediamine. In contrast, MP2/6-31G*//HF-6-31G* calculations predict internal complexation for all three isomers.

The fifth-order contribution to the oscillations in photon echoes of anharmonic vibrators

Abstract Vibrational photon echo experiments on the CO stretching mode of carbon monoxide bound to hemoglobin revealed pronounced oscillations with a beat frequency which corresponds to the anharmonicity of the vibrator. Intensity-dependent measurements show that the oscillatory signal is due to interference between the third- and fifth-order polarization. Such oscillations cannot occur in the third-order signal unless the inhomogeneous width exceeds the anharmonicity. An expression is derived which simultaneously describes the intensity dependence, the phase, and the damping of such oscillatory signals.

Directing polyallylamine adsorption on microlens array patterned silicon for microarray fabrication

The selective adsorption of reagents is often essential for bioarray and lab-on-a-chip type devices. As the starting point for a bioarray, alkyl monolayer terminated silicon shards were photopatterned in a few nanoseconds with thousands of wells (spots) using an optical element, a microlens array. Polyallylamine (PAAm), a primary amine containing polymer, adsorbed with little selectivity to the spots, i.e., silicon oxide, over the hydrophobic background. However, at appropriate concentrations, addition of a cationic surfactant to the PAAm deposition solution, cetyltrimethylammonium chloride, prevented the nonspecific adsorption of PAAm onto the hydrophobic monolayer, while directing it effectively to the active spots on the device. A nonionic surfactant was less effective in preventing the nonspecific adsorption of PAAm onto the hydrophobic monolayer. The localized reactions/interactions of adsorbed PAAm with four species that are useful for bioconjugate chemistry: glutaric anhydride, phenylenediisothiocyanate, biotin NHS ester, and an oligonucleotide (DNA) were shown in the spots of an array. The reactivity of PAAm was further demonstrated with an isocyanate. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) played an important role in confirming selective surface reactivity and adsorption. X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry, and wetting confirmed PAAm reactivity on planar substrates.

Rapid and convenient method for preparing masters for microcontact printing with 1–12 μm features

Mechanical scribing can be employed to create surfaces with recessed features. Through replica molding elastomeric copies of these scribed surfaces are created that function as stamps for microcontact printing. It is shown that this new method for creating masters for microcontact printing can be performed with a computer-controlled milling machine (CNC), making this method particularly straightforward and accessible to a large technical community that does not need to work in a particle free environment. Thus, no clean room, or other specialized equipment is required, as is commonly needed to prepare masters. Time-of-flight secondary ion mass spectrometry confirms surface pattering by this method. Finally, it is shown that feature size in the scribed master can be controlled by varying the force on the tip during scribing.

Carbon/ternary alloy/carbon optical stack on mylar as an optical data storage medium to potentially replace magnetic tape.

A novel write-once-read-many (WORM) optical stack on Mylar tape is proposed as a replacement for magnetic tape for archival data storage. This optical tape contains a cosputtered bismuth-tellurium-selenium (BTS) alloy as the write layer sandwiched between thin, protective films of reactively sputtered carbon. The composition and thickness of the BTS layer were confirmed by Rutherford Backscattering (RBS) and atomic force microscopy (AFM), respectively. The C/BTS/C stack on Mylar was written to/marked by 532 nm laser pulses. Under the same conditions, control Mylar films without the optical stack were unaffected. Marks, which showed craters/movement of the write material, were characterized by optical microscopy and AFM. The threshold laser powers for making marks on C/BTS/C stacks with different thicknesses were explored. Higher quality marks were made with a 60× objective compared to a 40× objective in our marking apparatus. The laser writing process was simulated with COMSOL.

Femtosecond IR Studies of Alkane C-H Bond Activation by Organometallic Compounds

The mechanism of alkane C-H bond activation by transition metal complexes such as CpM(CO)2 (M=Rh, Ir) has been intensely studied because it represents a first step in a catalytic process using unreactive hydrocarbons.[1] The bond activation reaction starts with the formation of monocarbonyl intermediates such as CpRh(CO). These species have been detected in the gas phase[2] and in liquefied rare Kr and Xe[3] by μs time resolved IR spectroscopy. Unfortunately, the subsequent oxidative insertion of CpRh(CO) into the C-H bond is not well understood due to its rapid rate and low quantum yield (~1%) for formation of the C-H activated product. These properties have hindered previous femtosecond and picosecond time-resolved studies of activation reaction in room temperature alkane solution. [4]

Alkyl Monolayers on Silica Surfaces Prepared from Neat, Heated 3-Glycidoxypropyldimethylethoxysilane Analyzed by XPS

Silane monolayers on silica, prepared from mono-, di-, and trichlorosilanes, are widely used in industry for surface functionalization and modification. However, unlike di- and trichlorosilanes, monochlorosilanes are particularly easy to work with because they can dimerize, but not polymerize, upon reaction with water. Typically, an organic solvent is used when depositing a silane monolayer. Here we show XPS spectra of monolayers of 3-glycidoxypropyldimethylethoxysilane (CAS# 17963-04-1) on silicon oxide (silicon wafer) prepared using a rapid, solvent-free approach. Reaction conditions are 100 °C for 10 min using the neat (pure) compound, and no inert atmosphere or special treatment of the compound is required.