Biomedical subjects

Alexander Portnov

Publications and source records attributed to Alexander Portnov.

5 recordsLinked to original sources

PubMed2006

Vibrationally mediated photodissociation of ethene isotopic variants preexcited to the fourth C-H stretch overtone.

H and D photofragments produced via vibrationally mediated photodissociation of jet-cooled normal ethene (C2H4), 1,2-trans-d2-ethene (HDCCDH), and 1,1-d2-ethene (CH2CD2), initially excited to the fourth C-H stretch overtone region, were studied for the first time. H and D vibrational action spectra and Doppler profiles were measured. The action spectra include partially resolved features due to rotational cooling, while the monitored room temperature photoacoustic spectra exhibit only a very broad feature in each species. Simulation of the spectral contours allowed determination of the band types and origins, limited precision rotational constants, and linewidths, providing time scales for energy redistribution. The H and D Doppler profiles correspond to low average translational energies and show slight preferential C-H over C-D bond cleavage in the deuterated variants. The propensities toward H photofragments emerge even though the energy flow out of the initially prepared C-H stretch is on a picosecond time scale and the photodissociation occurs following internal conversion, indicating a more effective release of the light H atoms.

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PubMed2006

Vibrational structure and methyl C-H dynamics in propyne.

Our previous study [J. Chem. Phys. 122, 224316 (2005)] presented the photoacoustic and action spectra of the V=2, 3, 4, and 5 manifolds of the C-H methyl stretching vibrations of propyne and their analysis in terms of a simplified joint local mode/normal mode model. In the current paper the C-H transition intensities were calculated using B3LYP6-311++G(d,p) level of theory to obtain the dipole moment functions. The diagonalization of the vibrational Hamiltonian revealed new model parameters obtained by least square fitting of the eigenvalues to the action spectra band origins, while examining the correspondence between the calculated intensities and simulated band areas. The newly derived parameters predict well the band positions and the observed intensities, allowing new assignment of the features. The derived Hamiltonian was also used to obtain the overall temporal behavior of the C-H stretches as a result of the Fermi couplings and interactions with the bath states. These results indicate that any specificity attained by suitable excitation of the methyl C-H stretches is lost on picosecond time scale, primarily due to strong interactions with doorway states in the lower overtone and coupling with bath states in the region of the higher ones.

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PubMed2005

Overtone spectroscopy of C-H ethyl stretches of 1-butyne.

Room-temperature photoacoustic (PA) spectra and jet-cooled action spectra of the first to third overtone regions of the ethyl C-H stretches in vapor phase 1-butyne, CH3CH2C[Triple Bond]C-H, were measured. Both the PA and action spectra exhibit a complex multiple peak structure being better resolved and more pronounced in the latter, due to inhomogeneous structure reduction. The observed manifolds were analyzed in terms of a simplified joint local-/normal-mode (LM/NM) model accounting for two types of C-H stretches (methyl and methylene) and for Fermi resonances between stretches and deformations. The retrieved parameters, used for calculation of the eigenstates, come from the best-fit parameters based on the diagonalization of the vibrational Hamiltonian in the LM/NM basis. The parameters were obtained by comparing the eigenvalues and the sum of the squares of the expansion coefficients of the eigenvectors of the C-H stretches of methyl and methylene to the action spectra peak positions and intensities, respectively. This approximate model vibrational Hamiltonian is proposed to explain most observed spectral features, corresponding to C-H stretch bands and to combinations of C-H stretches and deformations, indicating the importance of the Fermi resonance. The model was also applied to calculate the dynamics of the C-H stretching modes resulting from coupling with the deformations, implying rapid initial state decay on subpicosecond time scale. Decays of several picoseconds were found for complete transfer of probability from the initially prepared state of methylene and methyl to the counterpart LM states.

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PubMed2004

Vibrational spectroscopy and intramolecular dynamics of 1-butyne.

Photodissociation of jet-cooled vibrationally excited 1-butyne, C(2)H(5)C[Triple Bond]C[Single Bond]H, coupled with mass spectrometric detection of H photofragments, facilitated measurements of action spectra and Doppler profiles, expressing the yield of the ensuing fragments versus the vibrational excitation and UV probe lasers, respectively. Both the action spectra and the simultaneously measured room temperature photoacoustic spectra in the 2nu(1), 3nu(1), and 4nu(1) C[Single Bond]H acetylenic stretch regions exhibit unresolved rotational envelopes with significant narrowing of the former due to temperature-related change in the rotational structure. The narrowing of the action spectrum in the 3nu(1) region exposed a resonance splitting, implying intramolecular vibrational energy redistribution (IVR) time of approximately 1 ps. Asymmetric rotor simulation of the band contours provided the rotational constants and estimates for the homogeneous broadening arising from IVR to the bath vibrational states. The homogenous linewidth of 4nu(1) is anomalously narrower than that of 2nu(1) and 3nu(1), indicating a longer lived 4nu(1) state despite the increasing background state density, suggestive of a lack of low-order resonances or of mode-specific coupling with the bath states. The Doppler profiles indicate that the H photofragments are released with low average translational energies, pointing to an indirect dissociation process occurring after internal conversion (IC) to the ground electronic state or after IC and isomerization to butadiene.

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PubMed2003

Emission following laser-induced breakdown spectroscopy of organic compounds in ambient air.

Laser-induced breakdown spectroscopy (LIBS) was applied to nitroaromatic (NC) and polycyclic aromatic hydrocarbon (PAH) samples in ambient air to characterize their resultant emission. Compounds covering various surface, were ablated by use of the second (532-nm) or the fourth (266-nm) harmonic of a nanosecond pulsed Nd:YAG laser. The emission consisted of spectral features related mostly to CNand C2 molecular fragments and to C, H, N, and O atomic fragments. The transitions of the molecular fragments correspond to the CN (B 2sigma+ - X 2sigma+) violet system and the C2 (d 3IIg - a 3IIu) Swan system; the intensity of the former is higher in NCs than in PAHs. The intensity ratios between C2 and CN and between O and N correlate to the molecular structure, suggesting the possibility of distinguishing one chemical class from another and in optimum cases even identifing specific compounds by use of LIBS.

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