Conversion Of Electronic Energy Research Articles

Role of contact complexes in singlet-triplet energy transfer

(T-T) energy transfer from the acceptor to donor. If the properties of the solvent cage lead to the same probability for these two energy transfer processes between donor and acceptor molecules, there should be no accumulation of triplet acceptor molecules in solution. In this case, acceptor molecules act as a catalyst leading to an increased population of triplet donor states. In addition to the above scheme of energy conversion in a contact complex, one can also expect that during the lifetime of the solvent cage only the S-T stage of electronic energy transfer will be completed. In this case the excited complex decays before the reverse T-T energy transfer from the acceptor to donor takes place and therefore there is a high probability that the unreacted triplet acceptor molecules will migrate beyond the boundary of the solvent cage. Therefore, depending on whether or not the triplet acceptor molecules are found in solution, one can make a conclusion about the number of elementary steps responsible for the electronic excitation energy conversion in a contact complex during its lifetime in the solvent cage. Answering the above-mentioned questions is important for understanding the properties of the solvent cage and mechanisms of energy conversion in excited contact complexes. Therefore, we conducted studies on decay channels for contact complexes, in which one of the partners from a pair was in the singlet excited state. In order to simplify the problem as much as possible and reduce it to considering only two processes: the S-T energy transfer from the donor to acceptor and the reverse T-T energy transfer from the acceptor to donor, the excited complex must fulfill a number of conditions. One of them is the lack of overlap between the fluorescence spectrum of donor and the absorption spectrum of acceptor. The fulfillment of this condition allows us to neglect the sing!et-singlet (from SoD to SoA) energy transfer relative to the efficiency of S-T energy transfer between donor and acceptor. Moreover, some experimental data [i] indicate that the donor triplet level TiD of the studied pair should be considerably lower than the acceptor triplet level TiA (AE T > i000 cm-i). The larger the energy difference between TiD and TiA , the higher the probability of appearance of acceptor molecules in the triplet state in solution at the expense of S-T energy transfer relative to the probability of T-T energy transfer from the donor to acceptor.

Diode laser probing of vibrational product state distributions in metal–molecule collisions: Hg(6 3P1)–CO2(m n l p)

Diode laser probing has been used to follow the time dependent changes in the populations of low lying vibrational states of CO2 produced via quenching of Hg(6 3P1) initially excited by a pulse from a doubled dye laser. The conversion of mercury electronic energy into CO2 vibrational energy is quite efficient and mode specific. The ratio of the number of bending mode quanta to the number of asymmetric stretch quanta produced by the quenching process is 40±12. The rate of filling of both the bending and asymmetric stretch levels is identical and corresponds to 80 CO2/CO2 gas kinetic collisions. This data, taken in conjunction with the known quenching rate of Hg(6 3P1) by CO2 (four gas kinetic collisions) suggests that the quenching process produces a metastable state of CO2 which is either a bent electronic triplet or a highly excited vibrational level of the ground electronic state.

Ir absorption spectrum of CrO 2Cl 2 molecules for high-lying states of the vibrational quasi-continuum

A new approach to the spectroscopy of highly excited vibrational states of polyatomic molecules has been elaborated. The molecules of CrO 2Cl 2 were prepared in states with a vibrational energy of the ground electronic term A 1 of ≈ 19000 cm −1 by means of internal conversion of electronic energy from the electronic state B 1 excited by laser radiation. The spectroscopy of the vibrationally excited molecules has been carried out in the region of the ν 6 and ν 1 bands with diode and CO 2 lasers. The fwhm of the obtained spectrum was ≈ 15 cm −1. The intermode interaction in CrO 2Cl 2 has been theoretically analyzed, and the calculated spectrum compared with that measured experimentally. The time evolution of the spectrum of vibrationally excited CrO 2Cl 2 molecules has been studied. The average energy transferred per one collision with unexcited CrO 2Cl 2 molecules was equal to 〈δ E〉 ≈ 1200 cm −1.

Sputtering of ices: a review

The sputtering of low temperature rare-gas and molecular-gas solids by ion bombardment occurs through two different routes. Collision cascades initiated by momentum transferring collisions of ions with atoms of the solid are effective as they are in metals. The apparent surface binding energy is, however, much lower than the sublimation energy and there is a substantial low energy component to the ejected energy distribution that is suggestive of a thermal or collisional spike. In the ices sputtering is also produced by electronic excitation by ion or electron bombardment. This arises from conversion of electronic energy to translational kinetic energy and at low excitation densities may involve individual excitons or repulsive recombination events. At higher densities, cooperative effects lead to nonlinear sputtering yields. The current status of both the collisional and the electronic regimes of ice sputtering is briefly reviewed.

Electronic relaxation in rare-gas solids: Ejection of atoms by fast charged particles

Recent experiments show that the sputtering yields for condensed-gas solids bombarded by fast ions are large, even after allowance for the small surface binding energies. These yields are related to the electronic excitation produced by the incident ions in constrast to yields for metals and semiconductors. To understand the measured yields for condensed rare-gas solids we examine the conversion of electronic energy to lattice, nuclear motion. Thermal-spike models indicate that about 10–20% of the electronic energy deposited must be converted to nuclear motion in the vicinity of the ion track within 10 −11 s to account for the observed yields. It is the theme of the present article to point out that results and data from recent spectroscopic and luminescence studies show that this is indeed the case. On electronic excitation of these closed-shell, van der Waals solids, new covalent bonds form. These bonds cause significant lattice displacements and thus provide a mechanism for the electronic recombination. Relaxation to low-lying electronic states then occurs via repulsive potentials, by which means sufficient electronic energy is converted to nuclear motion to account for the observed ejection (sputtering) from these solids by fast light ions.

The role of excited electronic states in ion fragmentation: C 6H 6+

The present article presents a critical review of our understanding of the fragmentation behavior of the electronically excited C 6H 6 + system. This system produces a number of primary ionic fragments at low excitation energy. The determination of the structure of these fragments is discussed. The evidence for structural isomerization is reviewed, both for fragmenting and non- fragmenting C 6H 6 + ions. The energetics of various ionization and fragmentation processes is established and presented, leading to conclusions about the relative stability of various C 6H 6 + isomer states. The competition between fragmentation, internal conversion, and reradiation of electronic excitation energy is examined. Lastly, the methodology for estimating heats of formation of molecules and ions of unusual structure is briefly discussed.

Mechanism of electron-excitation-induced defect creation in alkali halides

Abstract Mechanisms of the defect creation in alkali halides induced by electronic excitation, suggested within these three decades, are surveyed, emphasizing how they are related to the overall yield of defect creation in solids. The experimental and theoretical studies which are critical check points for proposed mechanisms are discussed in detail. It is pointed out that the processes of localization of the wave functions of the electrons and holes and the conversion of localized electronic energy to the motion of the lattice atoms should be understood. The mechanisms of the dynamic motion of atoms to which the electronic energy is imparted and of the stabilization of the primary products, the neutral interstitials and vacancies in the halogen sublattice, are also discussed.

Monte carlo analysis of a free electron laser in a storage ring

This paper contains studies of the operation of a one-dimensional storage ring free-electron laser (FEL) using a Monte Carlo technique to generate the electron energy fluctuations produced by the FEL. The energy and phase equations of motion have been numerically integrated to calculate equilibrium values of: a) electron energy spread, b) electron phase spread (e.g. electron bunch length), and c) efficiency of conversion of electron energy into laser radiation. The operation of the storage ring free-electron laser was studied for five different FEL magnet designs. It is found that a “one-dimensional” storage ring free-electron laser can operate on a steady-state basis only with reduced overall efficiency due to the inability of the system to damp effectively the electron energy fluctuations produced by the FEL. Results of operation of a SRFEL in a pulsed mode are also presented.

Magnetic‐Resonant Modulation of Photoconductivity of Crystalline Charge Transfer Complexes. Anthracene‐Tettacyanbenzene

AbstractA magnetic‐resonant modulation of the photoconductivity of polycrystalline samples of CT complex anthracene–TCNB is observed. The modulation is due to microwave induced transitions between Zeeman sublevels of the excited triplet CT state. The magnetic resonance spectrum of the CT triplet state is registered, the lifetime of the latter is estimated to be about 10−9 s. Switching‐on the resonant microwave field decreases the photoconductivity. The nonresonant effect of the permanent magnetic field on photocurrent and fluorescence of anthracene–TCNB crystals is investigated. A scheme is given representing the processes of the electronic excitation energy conversion in the anthracene–TCNB CT complex.

Application of Stimulated Electromagnetic Shock Radiation to the Generation of Intense Submillimeter Waves

A new synergistic effect, resulting in tunable high-frequency high-power narrow-band radiation, is described. The effect, called stimulated electromagnetic shock radiation (SESR), combines the Doppler shift in frequency of Compton backscattering of electromagnetic radiation from moving electrons with the formation of an electromagnetic shock front in a medium in which the Mach condition is satisfied. The underlying physical mechanism of SESR is contrasted with those of Cerenkov radiation and laser action. The characteristics of SESR and Cerenkov radiation are compared. It is found that SESR is tunable in frequency and can be generated with much greater intensity and in a much narrower frequency band than Cerenkov radiation. The efficiency of conversion of electron energy into SESR is expected to be high (greater than 50 percent) under suitable conditions wherein the process is made self-limiting. The potential of SESR for generating submillimeter and far-infrared waves is discussed qualitatively.

Studies in the mechanism of radiationless conversion of electronic energy. I. p-xylene in solution

The fluorescence quantum yields of p-xylene in isooctane excited at 184.9 nm have been determined for the transitions S3→S0 (Φ3), S2→S0 (Φ2), and S1→S0 (Φ1) as functions of the concentration of p-xylene (cx) and of the concentration of added quenchers (CCl4, CHCl3, c−C7F14), and compared with the S1→S0 fluorescence quantum yield (φ1) for excitation at 253.7 nm in the same solutions. Also vapor fluorescence quantum yields have been redetermined for the S3→S0 and S2→S0 transitions. It has been found that the total nonradiative decay rate constants of S3 (2.5×1014 sec−1) and of S2 (1.0×1013 sec−1) are essentially the same for dilute solution and vapor phases. Also it is found that in dilute solution, Φ3 is independent of concentration of quencher, whereas Φ2 is strongly quenched and to about the same extent as is the ratio β=Φ1/φ1. In more concentrated solutions (cx≳3M), Φ3 rapidly declines to unobservable levels, whereas Φ2 and β appear to increase—but only slightly. On the basis of these results the following is concluded for a dilute solution of p-xylene in isooctane excited into S3 (at 184.9 nm): (i) The S3 state either radiates (kr=3.0×109 sec−1) or makes internal conversion to S2 (k=2.5×1014 sec−1). No autoionization occurs nor do any other decay channels such as S3→S1 or S3→S0 have appreciable probability. (ii) The S2 state either radiates (kr=1.3×108 sec−1), makes internal conversion to S1 (k=1.0×1013 sec−1), or autoionizes (k≳1014 sec−1). All other decay channels are of negligible importance. (iii) In the absence of a quencher, the ejected electron returns to the parent positive ion to regenerate S2 and not a lower state. In the presence of quencher the electron is scavenged and no longer can contribute to any emission. Possible mechanisms for the disappearance of Φ3 at high cx are considered.

Spatial distribution of energy deposited in nitrogen by electrons

The spatial distribution of the energy deposited by kilovolt electrons moving through gaseous molecular nitrogen was measured. The range of electrons of initial energy 300 eV to 5 keV was obtained and can be expressed by the formula R=K1+K2EA1−K3EA2, where R is the range, E is the initial energy of the electrons, and Ki,Ai are constants. The range, in this energy interval, is greater than that determined by previous measurements. A source of error, not previously discussed, is considered. The energy region (above 1 keV) where the simpler expression R=KEA holds is discussed. It is shown that this is the energy region where the energy and range dependence of the energy deposition curve can be removed and a normalized, characteristic energy deposition curve λ can be obtained. The efficiency of conversion of electron energy at 1 keV and 280 μ pressure to energy of photons at 3914 Å was measured to be (0.28±0.03) %.

Influence of plasma kinetic processes on electrically excited CO2 laser performance

Reported values of electron-molecule energy-transfer rate coefficients are combined with heavy-particle and radiative energy-transfer rate coefficients in an analysis of the factors influencing the conversion of electron energy to optical energy at 10.6 μm in an electric discharge containing CO2, N2, and He. The analytical procedure and the important energy-transfer processes involved are described in detail, and the parametric behavior of small-signal gain, saturation intensity, and optical power density is presented as a function of electron number density, gas temperature, pressure, and mixture composition. The temperature and pressure dependence of the cross section for stimulated emission of 10.6-μm radiation is also illustrated for a typical CO2 electric discharge laser gas mixture. Maintenance of gas temperature below approximately 700°K is shown to be of fundamental importance to CO2 laser operation.

Quenching rates for CO + 2 emission excited by 58.4nm irradiation of CO2

Rate constants have been determined for quenching of emission from the (0,0,0) level of CO+2(B2Σ+u) and the (3,0,0) level of CO+2(A2Πu) by CO2, H2, N2, O2, CO, N2O, C2H6, Ar, Ne and He. For most of the reactants the quenching rates exceed 2 × 10–9 cm3 molecule–1 s–1, with smaller values observed for He and Ne. The measured rates represent reaction cross-sections 1–40 times greater than gas-kinetic values. Such efficient quenching is explained in terms of a charge-transfer mechanism which results in rapid conversion of electronic energy into translation.

QUANTITATIVE ANALYSIS OF THE DEPENDENCE OF CO2 LASER PERFORMANCE ON ELECTRIC DISCHARGE PROPERTIES

Reported values of electron-molecule energy transfer rate constants are combined with atom/molecule-molecule and radiative energy transfer rate constants in an analysis of the conversion of electron energy to optical energy at 10.6μ in an electric discharge containing CO2, N2, and He. The small-signal gain and the saturation intensity are calculated as functions of the electron density and gas temperature in the discharge. The calculated values of these quantities are consistent with experimentally determined values. The detrimental effect of high gas temperature on small-signal gain is found to be the single most important factor influencing the efficiency of the discharge plasma as an energy conversion medium.

Nonlinear Thoery of Ultrasonic Wave Amplification and Current Saturation in Piezoelectric Semiconductors

A nonlinear theory based on numerical calculation is developed for the amplification of ultrasonic waves in piezoelectric semiconductors. The theory is one-dimensional and applies to semiconductors at room temperature in which the mean free path of the carriers is small compared with the acoustic wavelength. To provide a physical understanding of the problem, the following quantities are investigated in detail: (1) the carrier and the piezoelectric potential distributions and their relative phases, (2) the harmonic content of the elastic wave caused by the electronic interaction, (3) the acoustoelectric current and the nonuniform distribution of the dc electric field in the crystal, and (4) the effect of nonelectronic loss. As the intensity of the elastic wave increases, the drift velocity of the carriers starts from the Ohmic velocity and gradually decreases to the speed of sound. Similarly, the acoustic gain expressed in nepers per unit distance starts from the small-signal value and decreases slowly toward zero. The results obtained for the current and gain saturations described above in various conditions of amplification are presented in universal curves. We find that the current saturation occurs because the carriers are trapped in the troughs of the large piezoelectric potential excited by the elastic wave and are forced to move along with the wave at the speed of sound. We also show that this concept of the current saturation is entirely consistent with the calculation of the acoustoelectric current. Later, in a study of the acoustoelectric current and the Weinreich relation by means of simple power relations, we calculate the efficiency for the conversion of electronic energy to elastic energy. Among other findings we show that in a nonlinear theory, the Weinreich relation can be written for each harmonic independently. Finally, the theory is applied to various phenomena of acoustoelectric origin. Several simulation calculations are made for the propagation of high-field domains, and they verify the phenomena of "pinned" domains and the associated damped current oscillations observed experimentally in photoconductive CdS. Good agreement is also obtained between the present theory and the existing measurements of gain saturation.

Role of Carotenoids in Photosynthesis of Green Plants

In contrast to other hypotheses, β-carotene is responsible for the photic reduction of triphosphopyridine nucleotide during the photosynthetic cycle of electron transfer and energy conversion in green plants.

THE TRANSFER AND CONVERSION OF ELECTRONIC ENERGY IN SOME ‘MODEL’ PHOTOCHEMICAL SYSTEMS*

Abstract— Recent studies of the effects of molecular structure and reaction environment on the mechanism of primary photochemical processes involving transfer and conversion of electronic energy in relatively ‘simple’ organic molecules are presented and discussed. A quantitative i.r. spectroscopic method for studying intramolecular and intermolecular photoprocesses of u.v. irradiated substrates dispersed in solid alkali halide matrices at room temperature is described. Current data for the substrates ortho‐nitrobenzaldehyde, anthracene and benzophenonebenzhydrol are presented.A series of ‘model’ ketones containing cyclopropyl groups have been synthesized and while their absorption spectra are similar, the efficiency of vapor‐phase photodissociation into radicals is shown to be strongly dependent on molecular structure.Butyrophenone and a series of ring substituted derivatives have been photolyzed in the liquid phase using the quantum yield of the photoelimination of ethylene (Type II split) as a “probe” to determine the effect of substituents on the internal H atom abstracting power of the excited carbonyl chromophore. Φc2H4 is very sensitive to ring substitution, dropping from 0.24 in butyrophenone to 0.20, 0.058 and 0.00 in the p‐CH3, p‐OCH3 and p‐NH2 derivatives respectively, and to 0.00 in both ortho and para hydroxy derivatives. This effect is correlated with their absorption spectra in terms of the lowest states of these alkyl aryl ketones being 3(π, π*) rather than 3(π, π*) in character.While several ‘classic’ photochemical reactions, unimolecular and bimolecular, proceed efficiently in solid KBr matrices giving the same product as in liquid systems, the ‘model’ cyclopropyl compounds and the alkyl aryl ketones did not undergo their usual intramolecular processes. Implications of this molecular environment effect are pointed out.

Optical Radiation from Nitrogen and Air at High Pressure Excited by Energetic Electrons

The spectral characteristics and fluorescent efficiencies for electron excitation of nitrogen and air at 600 Torr have been determined. Results are presented for the efficiency of conversion of electron energy to optical radiation in approximately one hundred resolved spectral components of air and nitrogen between 3200 and 10 800 Å when bombarded by 50-keV electrons. The total fluorescent efficiency under these conditions is (0.14±0.02)% for nitrogen and (6.7±1.0)×10—3% for air. In nitrogen the first positive (B 3Πg→A 3Σu+), second positive (C 3Πu→B 3Πg), Gaydon green, Herman infrared, and Goldstein—Kaplan (C′ 3Πu→B 3Πg) band systems of N2 and the first negative (B 2Σu+→X 2Σg+) band system of N2+ were observed. The (0–2) and (0–3) transitions of the Herman infrared system and [N I]32(2D—2P) forbidden doublet at 1.04 μ, previously unreported in the laboratory, were observed. In air the first positive and second positive band systems of N2 and the first negative band system of N2+ were observed as well as atomic spectra of nitrogen, oxygen, and argon.

A Simple Synthesis of Bicyclo [1.1.0]butane and its Relation to the Internal Conversion of Electronic Energy in 1,3-Butadiene

<b>A Simple Synthesis of Bicyclo [1.1.0]butane and its Relation to the Internal Conversion of Electronic Energy in 1,3-Butadiene</b>