Optical pumping by excitation of molecular vibrations

A new optical pumping mechanism is observed for dilute Yb³⁺ impurities in a single crystal of yttrium ethyl sulphate at low temperatures. The polarization of Yb³⁺ spins is reduced by illumination with near infrared radiation. This reduction in spin polarization is caused by excitation of molecular vibrations of the ligand molecules surrounding the paramagnetic impurity.     

The dynamics of the SiF4 - sensitized processes initiated by a pulsed CO2 laser

A model enabling the examination of the dynamics of the SiF₄ - sensitized processes initiated by a pulsed CO₂ laser is outlined, taking into account three effects : energy absorption, energeticity of chemical reactions and expansion of hot gas created upon interaction of molecules with the infrared photon field. This approach was used to elucidate some aspects regarding the decomposition of PH₃ and GeH₄.     

Fast folding of RNA pseudoknots initiated by laser temperature-jump

RNA pseudoknots are examples of minimal structural motifs in RNA with tertiary interactions that stabilize the structures of many ribozymes. They also play an essential role in a variety of biological functions that are modulated by their structure, stability, and dynamics. Therefore, understanding the global principles that determine the thermodynamics and folding pathways of RNA pseudoknots is an important problem in biology, both for elucidating the folding mechanisms of larger ribozymes as well as addressing issues of possible kinetic control of the biological functions of pseudoknots. We report on the folding/unfolding kinetics of a hairpin-type pseudoknot obtained with microsecond time-resolution in response to a laser temperature-jump perturbation. The kinetics are monitored using UV absorbance as well as fluorescence of extrinsically attached labels as spectroscopic probes of the transiently populated RNA conformations. We measure folding times of 1-6 ms at 37 °C, which are at least 100-fold faster than previous observations of very slow folding pseudoknots that were trapped in misfolded conformations. The measured relaxation times are remarkably similar to predictions of a computational study by Thirumalai and co-workers (Cho, S. S.; Pincus, D.L.; Thirumalai, D. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 17349-17354). Thus, these studies provide the first observation of a fast-folding pseudoknot and present a benchmark against which computational models can be refined.     

XeCl准分子激光引发香芹酮的化学反应

本文报道了用XeCl准分子激光引发香芹酮(1)化学反应的研究结果.根据反应产物香芹酮樟脑(2)和1-挂,5-二甲基-顺2-[(乙氧羰基)甲基]双环[2.1.1]己烷(3)与激光照射能量的关系,得出香芹酮的1→2→3的反应过程.实验表明,在低激光强度照射时,每照射1焦耳能量生成2和3的量随着激光强度的增加而增加,并显示出饱和趋势.同时在照射过程中观察到浓度效应和发光.     

Chemical reaction of carvone initiated by XeCl excimer laser

The results of chemical reaction of carvone (1) initiated by the irradiation of XeCl excimer laser were reported in this paper. According to the relations of the products of carvonecamphor (2) and 1-exo, 5-dimethyl-syn-2 [(ethoxycarbonyl)-methyl] bicyclo [2, 1, 1] hexane (3) to the irradiation energy of the laser, the reaction process of 1→2→3 can be defined. The experiments showed that the production rates of 2 or 3 per joule of the irradiation energy was increasing with the increase of the incident laser beam intensities at low light intensity and had a tendency to saturate at high intensity. Besides, the concentration effect of the sample and luminescence phenomenon could also be observed during the laser irradiation process.     

CO2 laser excitation of triethylsilane: Time resolved luminescence of diethylsilyl radical

The infrared multiphoton excitation of triethylsilane in the gas phase, with a pulsed CO₂ laser at high intensities (I > 700 MW/cm²), produced an intense luminescence. The spectrum and time profile of this luminescence was studied as a function of pressure, and laser frequency. The radiative lifetime of this emission was 357 ± 10 ns, and the quenching rates by Cl₂ and NO were determined from lifetime measurements. A reasonable mechanism for the interpretation of this luminescence involves the initial infrared multiphoton decomposition of triethylsilane, followed by the secondary infrared multiphoton excitation of the primary photofragment diethylsilyl radical, which subsequently undergoes relaxation to an excited electronic state. The addition of O₂ resulted in a new chemiluminescence at shorter wavelengths, which corresponds to the SiO* chromophore group. © 1994 John Wiley & Sons, Inc.     

CO inner-shell excitation studied by electron impact spectroscopy

The inner-shell excitation and decay of the CO molecule have been studied in electron impact experiments. The dipole-forbidden transition (1sσ_c)⁻¹(2pπ) ³Π has been characterized by angular resolved electron energy loss spectroscopy and its decay via the measurement of resonant Auger spectra. The contribution of the (1sσ_c)⁻¹(2pπ) ³Π state to the CO resonant Auger spectrum in the region of the “spectator transitions” has been isolated and the population of CO⁺ quartet final states has been observed.     

Ab initio design of picosecond infrared laser pulses for controlling vibrational-rotational excitation of CO molecules

Optimal control of rovibrational excitations of the CO molecule using picosecond infrared laser pulses is described in the framework of the electric-nuclear Born-Oppenheimer approximation [G. G. Balint-Kurti et al., J. Chem. Phys. 122, 084110 (2005)]. The potential energy surface of the CO molecule in the presence of an electric field is calculated using coupled cluster theory with a large orbital basis set. The quantum dynamics of the process is treated using a full three dimensional treatment of the molecule in the laser field. The detailed mechanisms leading to efficient control of the selected excitation processes are discussed.     

Selective excitation of coupled CO vibrations on a dissipative Cu(100) surface by shaped infrared laser pulses

In a previous paper [Beyvers et al., J. Chem. Phys. 124, 234706 (2006)], the possibility to mode and state selectively excite various vibrational modes of a CO molecule adsorbed on a dissipative Cu(100) surface by shaped IR pulses was examined. Reduced-dimensionality models with stretching-only coordinates were employed to do so. This model is now extended with the goal to include rotational modes. First, we present an analysis of the bound states of the adsorbed CO molecule in full dimension; i.e., six-dimensional eigenstates are obtained by diagonalizing the six-dimensional Hamiltonian containing the semiempirical potential of Tully et al. [J. Vac. Sci. Technol. A 11, 1914 (1993)]. This is achieved by using a contracted iterative eigensolver based on the coupled two-term Lanczos algorithm with full reorthogonalization. Reduced-dimension subsystem eigenvectors are also computed and then used to study the selective excitation of the molecule in the presence of dissipation within the density matrix formalism for open systems. In the density matrix propagations, up to four degrees of freedom were included, namely, r (the C-O distance), Z (the molecule-surface distance), and phi and theta (the azimuthal and polar angles of the molecular axis with respect to the surface). Short, intense laser pulses are rationally engineered and further refined with optimal control theory, again with the goal for mode and state selective excitation. Also, IR-laser induced desorption is studied. For the calculations, the previous two-mode (r,Z) dipole surface is extended to include the angular dependence and the model for the coupling of the molecule to the surface electronic degrees of freedom is refined.     

Dynamics of a CO chemical laser initiated by flash photolysis of CS2NO2

The time-resolved spectrum and small signal gain of CO laser emission obtained from flash photolysed CS₂NO₂ are reported and analysed. It is suggested that NO₂ efficiently quenches high vibrational levels of CO. A method is described for using the gain characteristics to estimate the quenching rates.     

Molecular-mass distribution during radical polymerization of acrylates initiated by pulse laser radiation

The analytical expressions for the calculation of the molecular-mass distributions for the polymerization of acrylates initiated by the pulse periodic radiation are derived. The intramolecular chain transfer to the polymer is shown to be responsible for the known abnormalities in the molecular-mass distributions during the polymerization of acrylates. A relationship is derived for the characterization of the periodic structure of the molecular-mass distributions and for confirmation of the applicability of the first peak for estimating the avarage rate constant of chain growth.     

Crosslinking of ethylene-propylene (-diene) terpolymer elastomer initiated by an excimer laser

Excimer laser-induced crosslinking of ethylene-propylene-diene elastomer using a trace amount of photoinitiator, 1-hydroxy cyclohexyl phenyl ketone, was evaluated. Gel content measurements on irradiated samples showed that crosslinking was a natural consequence in all, which had undergone exposure to laser radiation. Degree of crosslinking was found to be linear with the laser exposure energy and was found to reach a saturation value of approximately 80% at about 100 J. However, for thicker samples the dependence of gel content i.e. crosslinking was found to be exponential over the range of laser exposure energy in excess of 200 J. As is expected, the tensile strength increased significantly with the increase of crosslinking. The band at 808 cm⁻¹ in the infrared spectra was attributed to the 5-ethylidene-2-norbornene (ENB) content in the polymer. The ENB has a double bond which is considered to be consumed with a high radiochemical yield and thereby contributes to the increase in rate of the formation of intermolecular bridge density.     

Direct excitation of potassium diffuse bands by single mode laser radiation

Direct excitation of the potassium yellow diffuse band was carried out by means of a tunable single mode ring dye laser. The excitation was performed across the whole region of the diffuse band while avoiding simultaneous excitation of discrete molecular bands. The fluorescence spectrum of the band was measured using a high resolution scanning monochromator.     

Resonance spectrum and production of the fluorosulfate radical by laser excitation

Discrete visible emission from the fluorosulfate radical produced in the gas phase by heating the parent dimer S₂O₆F₂ was observed using argon ion laser excitation: Under certain conditions the radical could be generated by laser excitation alone.     

Atomic fluorescence spectrometry with laser excitation

A laser atomic fluorescence spectrometry for the detection of trace concentrations of the elements is described. The detection limits for Pb, Fe, Na, Pt, Ir, Eu, Cu, Ag, Co and Mn in aqueous solutions obtained at present are the best ones for the rapid spectral analytical methods. The analytical potentials of the laser spectrometer are exemplified by the analysis of real samples of different chemical composition.     

Rate coefficients for resonant vibrational excitation of CO

Rate coefficients for vibrational excitation of the carbon-monoxide molecule, via the ²Π shape resonance in the energy region from 0 eV to 5 eV have been calculated. Calculations are performed for a Maxwellian electron energy distribution by using our recent experimentally measured differential cross sections for excitation of the first 10 vibrational levels. By using extended Monte Carlo simulations the electron energy distribution functions (EEDFs) and rate coefficients are determined in non-equilibrium conditions, in the presence of a homogeneous electric field. Calculations are performed for typical, moderate values of the electric field over gas number density ratios, E/N. A difference between Maxwellian and non-equilibrium rate coefficients was found due to a specific shape of the electron energy distribution function under the considered conditions.     

Radical telomerization of ethylene by triethylsilane initiated by Mn2(CO)10 or butyl peroxide

Conclusions Complexes of triethylsilane with Mn₂(CO)₁₀ are effective hydrogen transfer agents in the step involving chain transfer at the Si-H bond in radical telomerization.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2825–2827, December, 1984.     

An atomic boron laser - pumping by incomplete autoionization, or ion-electron recombination

Pulsed lasing was obtained from discharges in mixtures of He (≈ 10 torr) and a number of boron containing species (≈ 25 mtorr). Discharge conditions for optimizing the output are briefly described. Laser emission appears at 2777 ± 1.6 cm⁻¹, with 20–50 μs delay after passage of the discharge pulse (≈ 1 μs half width). The emitters are boron atoms. Among the term values for B I and B II only one pair of levels for B I have a separation close enough to the observed frequency to be considered. One possible mechanism is based on excitation to a metastable level above the first ionization limit. A distant collision may induce transition to captured-electron states at the limit (incomplete autoionization), which constitute the upper lasing levels; lasing them occurs by induced transitions to the lower level, 7s, ²S_1/2, which is rapidly depleted. Another possibility is the capture of an electron by B⁺ into a very large Rydberg orbit, at the ionization limit, which are the upper lasing states.