Electron temperature in nitrogen afterglow: Dependence of theoretical results on the adopted set of cross sections and on the type of molecular distribution over vibrational levels

The electron energy distribution function in nitrogen afterglow is simulated using different available sets of cross sections for electron scattering by vibrationally excited molecules. The calculations are performed for two types of molecular distribution over vibrational levels, namely, for the Boltzmann and Treanor-Gordiets distributions. It is shown that the calculated values of the electron temperature in nitrogen afterglow depend strongly on the set of cross sections used and on the type of molecular distribution over vibrational levels. The validity of comparison between theoretical and experimental results is discussed.     

Pink splash of active nitrogen in the discharge afterglow

Results are presented from experimental studies of the glow dynamics of active nitrogen in the stage of its excitation by a current pulse and during the discharge afterglow. The mechanism is proposed for the generation of a light splash in a highly activated nitrogen after the end of its pulsed excitation. The key role in the generation of this splash is played by the D-V processes, by which the dissociation energy is transferred to the vibrational degrees of freedom in the course of recombination of nitrogen atoms, and the V-E processes, by which the vibrational energy of highly excited molecules N₂(X, v ≥ 25–27) is transferred to the emitting electronic states N₂(B, v) after the V-V delay. Results of simulations based on the mechanism proposed are also presented.     

Electron temperature in a decaying molecular nitrogen plasma in the presence of weak electric fields

The electron energy distribution function in an afterglow molecular nitrogen plasma is studied both experimentally and theoretically under the conditions of weak electric fields such that the electron gas is heated by superelastic collisions of electrons with vibrationally excited molecules. Based on the mean electron energy balance, it is established that, depending on the degree of plasma ionization and the vibrational temperature of nitrogen molecules, an afterglow plasma may evolve into two states, differing in electron temperature. This kind of bistability is found to stem from the difference in the main mechanisms for electron energy losses in the two stable states. The prediction that the shape of the electron energy distribution function should change in a jumplike manner when a weak electric field is imposed has been confirmed experimentally.     

Investigation of the electron energy distribution function in hollow-cathode glow discharges in nitrogen and oxygen

The mechanism for the formation of the inverse electron distribution function is proposed and realized experimentally in a nitrogen plasma of a hollow-cathode glow discharge. It is shown theoretically and experimentally that, for a broad range of the parameters of an N₂ discharge, it is possible to form a significant dip in the profile of the electron distribution function in the energy range ε=2–4 eV and, accordingly, to produce the inverse distribution with df(ε)/d?>0. The formation of a dip is associated with both the vibrational excitation of N₂ molecules and the characteristic features of a hollow-cathode glow discharge. In such a discharge, the applied voltage drops preferentially across a narrow cathode sheath. In the main discharge region, the electric field E is weak (E<0.1 V/cm at a pressure of about p～0.1 torr) and does not heat the discharge plasma. The gas is ionized and the ionization-produced electrons are heated by a beam of fast electrons (with an energy of about 400 eV) emitted from the cathode. A high-energy electron beam plays an important role in the formation of a dip in the profile of the electron distribution function in the energy range in which the cross section for the vibrational excitation of nitrogen molecules is maximum. A plasma with an inverted electron distribution function can be used to create a population inversion in which more impurity molecules and atoms will exist in electronically excited states.     

Vibrational distribution of nitrogen molecules in the C3Πu state in a near-surface microwave plasma in nitrogen at pressures of 1–5 Torr

The radiation of the second positive nitrogen system has been used to study the spatial dependence of the vibrational distribution of nitrogen molecules in the C³Π_u state in the near-surface plasma layer of an electrode microwave discharge in nitrogen at pressures of 1–5 Torr. It has been shown that the vibrational distribution changes at a scale of 100 μm. It has been concluded that this state is populated owing to the electron impact from the ground state. The possibility of using the local approximation for the electron energy distribution function to explain the experimental results has been analyzed.     

Two-dimensional simulation of the development of an inhomogeneous volume discharge in a Ne/Xe/HCl gas mixture

The kinetic processes accompanying plasma column formation in an inhomogeneous discharge in a Ne/Xe/HCl gas mixture at a pressure of 4 atm were investigated by using a two-dimensional model. Two cathode spots spaced by 0.7 cm were initiated by distorting the cathode surface at local points, which resulted in an increase in the field strength in the cathode region. Three regimes differing in the charging voltage, electric circuit inductance, and electric field strength at the local cathode points were considered. The spatiotemporal distributions of the discharge current; the electron density; and the densities of excited xenon atoms, HCl(v = 0) molecules in the ground state, and HCl(v > 0) molecules in vibrational levels were calculated. The development of the discharge with increasing the electron density from 10⁴ to 10¹⁶ cm^?3 was analyzed, and three characteristic stages in the evolution of the current distribution were demonstrated. The width of the plasma column was found to depend on the energy deposited in the discharge. The width of the plasma column was found to decrease in inverse proportion to the deposited energy due to spatiotemporal variations in the rates of electron production and loss. The calculated dependences of the cross-sectional area of the plasma column on the energy deposited in the discharge agree with the experimental results.     

Effect of the vibrational excitation of CO molecules on the parameters of an RF discharge

A one-dimensional model of an RF discharge in CO-containing gas mixtures is developed. The model takes into account the effect of the degree of vibrational excitation of CO molecules on the structure of the discharge and on its parameters. Experimental data are presented from measurements of the voltage-power characteristics of RF discharges in gas mixtures with different CO contents in the pressure range of 10–100 torr. The model developed is used to calculate the dependence of the root-mean-square discharge voltage on the specific power deposition in an RF discharge under our experimental conditions. The experimental data are compared to the results of numerical simulations. For working gas pressures of about 100 torr, which are typical of the operation of slab CO lasers, the calculated voltage-power characteristics of an RF discharge agree satisfactorily with those obtained experimentally. The theoretical model predicts that the vibrational excitation of CO molecules leads to a redistribution of the RF field in the discharge gap and to an increase in the laser efficiency.     

Multidimensional spectroscopy of β-carotene: Vibrational cooling in the excited state

Pump-degenerate four wave mixing (Pump-DFWM) is used for investigating the vibrational dynamics in the excited state of β-carotene in solution. In this 2D technique, an initial pump pulse promotes the system to the excited state, which is then probed by the succeeding DFWM sequence. We focus particularly on the internal conversion between the S₂ and S₁ state with high temporal and spectral resolution. The frequency shift of the excited state vibrations is measured and is explained as mode-specific vibrational cooling. Our results suggest an internal conversion in a time range between 260 and 500 fs without any intermediate states.     

A study of vibrational relaxation of B-state carbon monoxide in the heme pocket of photolyzed carboxymyoglobin.

The vibrational energy relaxation of dissociated carbon monoxide in the heme pocket of sperm whale myoglobin has been studied using equilibrium molecular dynamics simulation and normal mode analysis methods. Molecular dynamics trajectories of solvated myoglobin were run at 300 K for both the delta- and epsilon-tautomers of the distal histidine, His64. Vibrational population relaxation times were estimated using the Landau-Teller model. For carbon monoxide (CO) in the myoglobin epsilon-tautomer, for a frequency of omega0 = 2131 cm-1 corresponding to the B1 state, T1epsilon(B1) = 640 +/- 185 ps, and for a frequency of omega0 = 2119 cm-1 corresponding to the B2 state, T1epsilon(B2) = 590 +/- 175 ps. Although the CO relaxation rates in both the epsilon- and delta-tautomers are similar in magnitude, the simulations predict that the vibrational relaxation of the CO is faster in the delta-tautomer. For CO in the myoglobin delta-tautomer, it was found that the relaxation times were identical within error for the two CO substate frequencies, T1delta(B1) = 335 +/- 115 ps and T1delta(B2) = 330 +/- 145 ps. These simulation results are in reasonable agreement with experimental results of Anfinrud and coworkers (unpublished results). Normal mode calculations were used to identify the dominant coupling between the protein and CO molecules. The calculations suggest that the residues of the myoglobin pocket, acting as a first solvation shell to the CO molecule, contribute the primary "doorway" modes in the vibrational relaxation of the oscillator.     

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density N _e and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N₂. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C ³Π _u state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of N _e and E. A kinetic model of the processes governing the steady-state density of the C ³Π _u nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.     

Femtosecond and picosecond fluorescence of native bacteriorhodopsin and a nonisomerizing analog

The spectrally and temporally resolved fluorescence properties of native bacteriorhodopsin (bR) and bR reconstituted with a nonisomerizing analog of the retinal Schiff base (bR5.12) are examined. The first attempt to experimentally monitor the excited state relaxation processes in both type of pigments using ultrafast fluorescence spectroscopy is reported. The fluorescence is emitted from retinal molecules in an all-trans configuration. Substantial energy relaxation involves very fast intramolecular and intermolecular vibrational modes and these are shown to occur on a time scale faster than isomerization. The possible contribution of dielectric interaction between the retinal Schiff base and the protein environment for the excited state energy relaxation is discussed.     

Excitation of surface waves at a plasma boundary by a short laser pulse

The excitation of surface waves by a laser pulse as it crosses a vacuum-plasma interface is considered. Surface waves are excited by a vortex electric current that is generated at the plasma boundary by the ponderomotive force of the pulse. The question is considered of how the duration and transverse dimensions of the pulse affect the spatiotemporal distribution and the spectral and energy parameters of the excited surface waves.     

Quenching of fluorescence by triplet excited states in chloroplasts

The fluorescence quantum yield in spinach chloroplasts at room temperature has been studied utilizing a 0.5-4.0 mus duration dye laser flash of varying intensities as an excitation source. The yield (phi) and carotenoid triplet concentration were monitored both during and following the laser flash. The triplet concentration was monitored by transient absorption spectoscopy at 515 nm, while the yield phi following the laser was probed with a low intensity xenon flash. The fluorescence is quenched by factors of up to 10-12, depending on the intensity of the flash and the time interval following the onset of the flash. This quenching is attributed to a quencher Q whose concentration is denoted by Q. The relative instantaneous concentration of Q was calculated from phi utilizing the Stern-Volmer equation, and its buildup and decay kinetics were compared to those of carotenoid triplets. At high flash intensities (greater than 10(16) photon . cm-2) the decay kinetics of Q are slower than those of the carotenoid triplets, while at lower flash intensities they are similar. Q is sensitive to oxygen and it is proposed that Q, at the higher intensities, is a trapped chlorophyll triplet. This hypothesis accounts well for the continuing rise of the carotenoid triplet concentration for 1-2 mus after the cessation of the laser pulse by a slow detrapping mechanism, and the subsequent capture of the triplet energy by carotenoid molecules. At the maximum laser intensities, the carotenoid triplet concentration is about one per 100 chlorophyll molecules. The maximum chlorophyll ion concentration generated by the laser pulses was estimated to be below 0.8 ions/100 chlorophyll molecules. None of the observations described here were altered when a picosecond pulse laser train was substituted for the microsecond pulse. A simple kinetic model describing the generation of singlets and triplets (by intersystem crossing), and their subsequent interaction leading to fluorescence quenching, accounts well for the observations. The two coupled differential equations describing the time dependent evolution of singlet and triplet excited states are solved numerically. Using a single-triplet bimolecular rate constant of gammast = 10(-8) cm3 . s-1, the following observations can be accounted for: (1) the rapid initial drop in phi and its subsequent levelling off with increasing time during the laser pulse, (2) the buildup of the triplets during the pulse, and (3) the integrated yield of triplets per pulse as a function of the energy of the flash.     

Observation of terahertz vibrations in Pyrococcus furiosus rubredoxin via impulsive coherent vibrational spectroscopy and nuclear resonance vibrational spectroscopy--interpretation by molecular mechanics

We have used impulsive coherent vibrational spectroscopy (ICVS) to study the Fe(S-Cys)(4) site in oxidized rubredoxin (Rd) from Pyrococcus furiosus (Pf). In this experiment, a 15 fs visible laser pulse is used to coherently pump the sample to an excited electronic state, and a second <10 fs pulse is used to probe the change in transmission as a function of the time delay. PfRd was observed to relax to the ground state by a single exponential decay with time constants of approximately 255-275 fs. Superimposed on this relaxation are oscillations caused by coherent excitation of vibrational modes in both excited and ground electronic states. Fourier transformation reveals the frequencies of these modes. The strongest ICV mode with 570 nm excitation is the symmetric Fe-S stretching mode near 310 cm(-1), compared to 313 cm(-1) in the low temperature resonance Raman. If the rubredoxin is pumped at 520 nm, a set of strong bands occurs between 20 and 110 cm(-1). Finally, there is a mode at approximately 500 cm(-1) which is similar to features near 508 cm(-1) in blue Cu proteins that have been attributed to excited state vibrations. Normal mode analysis using 488 protein atoms and 558 waters gave calculated spectra that are in good agreement with previous nuclear resonance vibrational spectra (NRVS) results. The lowest frequency normal modes are identified as collective motions of the entire protein or large segments of polypeptide. Motion in these modes may affect the polar environment of the redox site and thus tune the electron transfer functions in rubredoxins.     

Effect of a small C3O2 additive on the vibrational distribution function of CO molecules in a low-temperature plasma

The concentration of carbon suboxide (C₃O₂) in the plasmas of sealed-off discharges in mixtures of CO with noble gases is measured for the first time by mass-spectroscopic technique. It is shown that the production of C₃O₂ (and, possibly, more complex carbon oxides) in a gas-discharge plasma significantly boosts the vibrational relaxation of CO molecules and thus greatly affects their vibrational populations. Adding xenon to a He: CO mixture reduces the concentration of C₃O₂. The effect of pulsed UV radiation on the vibrational populations of CO molecules is studied experimentally. It is shown that UV irradiation of the gas mixture after long-term discharge operation increases vibrational populations in the plateau region up to the values observed at the beginning of the discharge. This effect is attributed to the decay of C₃O₂ molecules under the action of UV radiation.     

Multidimensional spectroscopy of beta-carotene: vibrational cooling in the excited state

Pump-degenerate four wave mixing (Pump-DFWM) is used for investigating the vibrational dynamics in the excited state of beta-carotene in solution. In this 2D technique, an initial pump pulse promotes the system to the excited state, which is then probed by the succeeding DFWM sequence. We focus particularly on the internal conversion between the S(2) and S(1) state with high temporal and spectral resolution. The frequency shift of the excited state vibrations is measured and is explained as mode-specific vibrational cooling. Our results suggest an internal conversion in a time range between 260 and 500 fs without any intermediate states.     

Beam-plasma interaction in a hybrid plasma waveguide in the pulsed mode of microwave generation

Results are presented from experimental studies of the energy spectra of an electron beam in a model beam-plasma oscillator based on a hybrid plasma waveguide in the pulsed mode of microwave generation with a pulse duration of 1 µs or shorter. The beam energy spent on sustaining the beam-plasma discharge in a slow-wave structure is measured. A correlation between the type of excited waves and the generation of a group of accelerated beam electrons with energies exceeding the injection energy is revealed. It is shown that the pulsed mode of microwave generation is related to the time variations in the plasma density profile in the waveguide and the trapping of beam electrons by the excited microwave field.     

Ionisation of fullerenes and fullerene clusters using ultrashort laser pulses.

We give a brief review of the literature concerning the ultra-short pulse ionisation of fullerenes in the gas phase. Emphasis is placed on the excitation time dependence of different ionisation regimes as manifested by photoelectron spectroscopy. The ionisation rates are modelled for the intermediate situation where the excitation energy is equilibrated between electronic degrees of freedom but not yet coupled to vibrational degrees of freedom. The model is shown to describe many aspects of the experiments. New results are presented on the intra-cluster molecular fusion of fullerene molecules when van der Waals bound clusters of fullerenes are exposed to ultra-short laser pulses. Pump-probe measurements give a decay time constant for the intra-cluster fusion reaction of 520 +/- 55 fs. A comparison with monomer ionisation results suggests that the time window for the fusion reaction is influenced by the coupling of the electronic excitation energy to vibrational degrees of freedom of the molecules in the cluster.     

Critical analysis of objections against the biological molecular energy machines.

In the past, two important objections against McClare's idea of biological molecular energy machines were raised. One of the criticisms was concerned with the origin of energy gained in ATP cleavage and with an interpretation of McClare's "excited vibrational state." The former argument reveals a failure of the critics to comprehend McClare's approach. As to the excited vibrational state, it can be identified with nonequilibrium conformational states of the unit rather than with a single vibrational mode. The other criticism based on Brillouin's energy cost of measurement argued that reversible operation of biological molecular energy machines would be virtually impossible. Using propagation velocities of deformations of the unit's structure (instead of velocity of light), the objections against reversibility are invalidated even in the framework of the critic's approach. McClare's idea and relevant definitions are thus physically correct.