Absorption Of Ultraviolet Photons Research Articles

A computer simulation method for studying the ablation of polymer surfaces by ultraviolet laser radiation

A computer algorithm designed to simulate the ablation of polymer surfaces by ultraviolet laser radiation is presented. The method is based on a synthesis of the dressed state picture of radiation-matter interactions and a semiclassical ‘‘surface hopping’’ technique. A primary feature of the algorithm is that the intrinsically quantum mechanical process of ultraviolet photon absorption can be simulated while classical molecular dynamics techniques are simultaneously employed to integrate the equations of motion for the nuclear positions and momenta of the polymers. Illustrative results obtained from the algorithm are presented for the ablation of a model polymer surface. Quite different energy redistribution behavior is observed for electronic excitations to a repulsive electronic potential energy surface versus excitations to surfaces that have the possibility of bound vibrational motion. These results suggest that the postulated ‘‘direct’’ photochemical (bond-breaking) ablation mechanism for 193 nm lasers may be dynamically distinct from thermal ablation processes that result from intense local heating of the material by longer wavelength lasers.

Effects of Ultraviolet Radiation on the Characteristics of Hydrogenated Amorphous Silicon Thin Film Transistors

ABSTRACTThe experimental results regarding to the effects of ultraviolet (UV) light illumination on the characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) have been presented. The device parameters of a-Si:H TFT, such as threshold voltage, field-effect mobility, and subthreshold slope, have been degraded by electrical stress and visible light illumination, but substantially improved by UV radiation. This may be attributed to an annealing effect on the dangling-bond defects, involving a number of phonons generated by absorption of high energy UV photons in the a-Si:H TFT channel. It has been also observed that the off-current of a-Si:H TFT decreases remarkably while the on-current changes very little. From the experimental results, we report that the improved on/off current ratio of a-Si:H TFT may be achieved by UV radiation.

Grains, or Molecules? Thermal, or non-Thermal?

The effects of size and energy on infrared fluorescence (IRF) and on chemical reaction rates are investigated, using polycyclic aromatic hydrocarbons (PAHs) as examples. The range of validity of the Thermal Approximation (TA) is examined. It is found that for properties that have a near-linear dependence on the internal energy, the TA provides an adequate description of the non-thermal, time-dependent processes associated with ultraviolet photon absorption. Since IRF at high energy is nearly linear, the TA is adequate for IRF at high excitation energies, but care must be taken, because the TA fails at low energies. The TA is never adequate for chemical reactions under these conditions.

Ultraviolet pumping of molecular vibrational states - The CO infrared bands

Observable infrared emission from molecular clouds may arise as fluorescence pumped by absorption of ultraviolet photons from a nearby star. General features of such infrared emission are discussed for various diatomic molecules. It is found that the populations of the vibrational levels are commonly described by a single vibrational temperature which is easily estimated for a number of limiting cases. The recent observations of CO overtone bands in Orion are considered in detail to see if the emission could be produced by UV pumping only. Itis concluded that both the observed relative intensities of the three bands and also their absolute strengths are inconsistent with such a model.

Temperature fluctuations in interstellar dust grains

Temperature fluctuations in interstellar dust grains caused by absorption of ultraviolet photons are discussed. Temperature probability distributions are presented for various assumptions about the grain specific heat and far infrared emissivity. Grains smaller than or about 0.01 μ may suffer large temperature fluctuations and would spend most of their time at very low temperatures. The ‘equilibrium temperature’ is not a good measure for most average temperatures of such grains. It is shown that, although small grains spend only a small fraction of their time at the higher temperatures, the emitted far infrared spectrum is significantly shifted towards shorter wavelengths than predicted for grains assumed to be at the equilibrium temperature.

Compound-Atom States for Two-Electron Systems

The energies of the lowest He-like compound-atom or resonance states lying below the $n=2$ hydrogenic level are evaluated variationally for He and ${\mathrm{H}}^{\ensuremath{-}}$. In order to do this, we employ the definition of such states given by Feshbach as eigenvalues of the Hamiltonian with the open channel projected out, together with the modification required by the Pauli principle. Our trial wave function uses a Legendre expansion in the relative angle and a sum of exponentials in the radial coordinates, with appropriate angular factors to obtain the desired symmetry, parity, and orbital-angular-momentum eigenvalues associated with the $^{1,3}S^{e}$ and $^{1,3}P^{o}$ states. Our results are approximations to the actual physical resonances in that the shift and finite width caused by coupling to the neighboring continuum are not included. It appears, however, that the actual shifts are small, so that the positions of these compound-atom states are believed to give a close indication of where the physical resonances may be expected to occur. The results for He and ${\mathrm{H}}^{\ensuremath{-}}$ are compared in detail with the calculations of resonances in $e$-H and $e$-${\mathrm{He}}^{+}$ elastic scattering and with the observation of these states by ultraviolet photon absorption in He (where they are called autoionizing levels) and by inelastic scattering of electrons from He.

SYNTHESIS OF ADENOSINE TRIPHOSPHATE UNDER POSSIBLE PRIMITIVE EARTH CONDITIONS.

In view of the fact that several fundamental coenzymes of intermediate metabolism and plant photosynthesis are nucleoside phosphates, and as all of these molecules contain purines and pyrimidines that have strong ultraviolet absorption maxima near 2600 A deg , the possibility arises that the absorption of ultraviolet photons by purines and pyrimidines provided the bond energy for the synthesis of nucleoside phosphates in primitive tissues. To test the hypothesis, such syntheses were attempted under laboratory conditions. Labeled adenine, adenosine, and adenylic acid were sealed in aqueous solutions in Vycor tubes with approximately stoichiometric quantities of ribose, phosphoric acid, or polyphosphate ester. The solutions were irradiated by four General Electric ultraviolet germicidal lamps. During a one hour irradiation, the samples absorbed a total of approximates 10/sup 8/ ergs. Reaction products were analyzed by paper chromatography, autoradiography, and ultraviolet absorption. Positions of the carriers were detected by shadowgrams. Coincidence both in position and in shape between the carriers on the shadowgrams and the radioactivity on the autoradiograph were chromatographic bases for identification. Ultraviolet excitation of adenine accounts for the synthesis of adenosine, but participation of phosphorus compounds in the reaction is obscure. It was concluded that ultraviolet irradiation of solutions of desoxyribosemore » purines and pyrimidines may have some relevance to the problem of nucleoside phosphate origins. (H.M.G.)« less