Efficient Vibrational Excitation Research Articles

Electric field distribution in a non-self-sustained RF discharge with ionization generated by Ns discharge pulses

Electric field in a capacitively coupled, non-self-sustained RF discharge nitrogen plasma with external ionization generated by high-voltage ns pulses has been measured by ps Electric field induced second harmonic generation (EFISH). The measurements are made both in the bulk of the plasma and in the sheaths, using the absolute calibration by the Laplacian field between two plane electrodes. The results are compared with the kinetic modeling calculations. The RF electric field in the sheaths is significantly higher compared to that in the plasma, due to the displacement of the electrons by the drift oscillations and the resultant plasma self-shielding. However, the kinetic modeling predictions indicate that the electron impact ionization in the sheaths is largely ineffective, due to the low electron density. The reduction of the electric field in the plasma by the self-shielding in the sheaths is moderate, such that the energy is coupled to the plasma by the below-breakdown RF field. The peak RF field in the plasma is in the range of 15–25 Td, indicating the efficient vibrational excitation of N2 by electron impact. The modeling predictions suggest that the targeted vibrational excitation of molecular species in a non-self-sustained RF discharge with external ionization is scalable to high pressures, electron densities, and discharge powers. The present approach can be extended to the vibrational excitation of other molecular species where the vibrational relaxation is relatively slow, such as CO, CO2, and H2.

Efficient vibrational excitation of molecular nitrogen in low-pressure plasma with ultralow electron temperature

We investigated the vibrational temperature of molecular nitrogen in the downstream of helicon-wave excited helium and argon-based plasmas. It was confirmed by optical emission spectroscopy that the major part of the helium plasma was at a recombining state and it had an ultralow electron temperature of approximately 0.1 eV. In spite of the ultralow electron temperature, the vibrational temperature of molecular nitrogen, which was added into the helium plasma, was higher than that in the argon-based plasma at an ionizing state with an electron temperature of 1.7 eV. According to the relationship between the rate coefficient of electron impact vibrational excitation and the electron temperature, the higher vibrational temperature in the helium plasma is not attributable to the more efficient vibrational excitation. Therefore, the higher vibrational temperature is owing to the less efficient destruction of vibrational excited states in the helium plasma with the ultralow electron temperature.

Vibrationally Mediated Stabilization of Electrons in Nonpolar Matter.

We explore solvation of electrons in nonpolar matter, here represented by butadiene clusters. Isolated butadiene supports only the existence of transient anions (resonances). Two-dimensional electron energy loss spectroscopy shows that the resonances lead to an efficient vibrational excitation of butadiene, which can result into the almost complete loss of energy of the interacting electron. Cluster-beam experiments show that molecular clusters of butadiene form stable anions, however only at sizes of more than 9 molecular units. We have calculated the distribution of electron affinities of clusters using classical and path integral molecular dynamics simulations. There is almost a continuous transition from the resonant to the bound anions with an increase in cluster size. The comparison of the classical and quantum dynamics reveals that the electron binding is strongly supported by molecular vibrations, brought about by nuclear zero-point motion and thermal agitation. We also inspected the structure of the solvated electron, finding it well localized.

Time-dependent modeling on initiation of pre-ionized laser filament in atmospheric air under an external field far below conventional threshold

In this paper, atmospheric laser filament initiation is simulated under an external field 5.7 kV/cm, which is far below the conventional breakdown threshold (25–30 kV/cm). The time-dependent model is based on a self-consistent solution to a coupled electron energy distribution function, vibrational kinetics, gas thermal balance, as well as detailed air chemistry. The model is validated with the experiment results on plasma density decay both with and without an external field. The slow decay of plasma density is ascribed to the decreased electron recombination and increased detachment rate under an external field. The initiation process is facilitated by the increase in the electron temperature, gas heating, and efficient vibrational excitation under the external field. We have varied the detachment rate between oxygen negative ions and nitrogen vibrational states to study its role on the initiation process, and found that larger detachment rates lead to shorter breakdown delay times. If the detachment rate is taken to be 3 × 10−11 cm3 s−1 as an upper limit, the breakdown threshold and delay time are consistent with the previous experiment study at least semiquantitatively. This work is beneficial for studying the initiation of a preformed laser filament under an external field as well as potential applications in lightning initiation and guiding below the conventional gas breakdown threshold.

Comparison of the Active Species in the RF and Microwave Flowing Discharges of N2 and Ar–20 %N2

We report a detailed comparison between RF and microwave (HF) plasmas of N2 and Ar–20 %N2 as well as in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5–6 mm), gas pressure (6–8 Torr), flow rate (0.6–1.0 slm) and applied power (50–150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (~25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10−2 s) of the active species in the N2 RF plasma [compared to that (10−3 s) of HF], providing a condition for an efficient vibrational excitation of N2(X, v) by (V–V) climbing-up processes, making the RF plasma more vibrationally excited than the HF one. As a result of high V–V plasma excitation in RF, the densities of the vibrationally excited N2(X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N2(X, v) due to the tube wall is estimated to be very similar between the two system as can be inferred from the γv destruction probability of N2(X, v > 3–13) on the tube wall (2–3 × 10−3 for both cases) obtained from a comparison between the density of N2(X, v > 3–9) in the plasmas to that of the N2(X, v > 13) in the long afterglows. Interestingly enough, densities of N-atoms and N2(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N2 + ions than expected in the HF afterglow is rationalized from a high oxygen impurity in our HF setup since N2 + ions are very sensitive to oxygen impurity .

Alpha to gamma mode transition in hydrogen capacitive radio-frequency discharge

Electron energy distribution functions (EEDFs) were measured with increasing discharge voltages in hydrogen capacitively coupled plasmas by means of radio-frequency compensated Langmuir probe. The results are compared with EEDF in argon plasmas. It was found that, in the hydrogen capacitive discharge, abnormally low-energy electrons became highly populated and the EEDF evolved to a non-Maxwellian distribution as the discharge voltage was increased. This voltage dependence of the EEDF in the hydrogen is contrary to argon capacitively coupled plasma, where at high discharge voltage, low-energy electrons are significantly thermalized due to γ heating and the EEDF evolves to the Maxwellian distribution. The highly populated low-energy electrons at high gas pressure, which was not observed in capacitively coupled argon plasma, show that the γ heating mechanism is somehow inefficient in terms of the molecular gas in capacitive discharges. It appears that this inefficient γ heating seems to be attributed to an efficient vibrational excitation in hydrogen capacitive plasma.

Combined effects of gas pressure and exciting frequency on electron energy distribution functions in hydrogen capacitively coupled plasmas

The combined effects of the variation of hydrogen pressure (40–400 mTorr) and exciting frequency (13.56–50 MHz) on the electron energy probability function (EEPF) and other plasma parameters in capacitively coupled hydrogen H2 discharge at fixed discharge voltage were investigated using rf-compensated Langmuir probe. At a fixed exciting frequency of 13.56 MHz, the EEPF evolved from Maxwellian-like distribution to a bi-Maxwellian distribution when the H2 pressure increased, possibly due to efficient vibrational excitation. The electron density largely increased to a peak value and then decreased with the increase of H2 pressure. Meanwhile, the electron temperature and plasma potential significantly decrease and reaching a minimum at 120 mTorr beyond, which saturated or slightly increases. On the other hand, the dissipated power and electron density markedly increased with increasing the exciting frequency at fixed H2 pressure and voltage. The electron temperatures negligibly dependent on the driving frequency. The EEPFs at low pressure 60 mTorr resemble Maxwellian-like distribution and evolve into a bi-Maxwellian type as frequency increased, due to a collisonless (stochastic) sheath-heating in the very high frequency regime, while the EEPF at hydrogen pressure ≥120 mTorr retained a bi-Maxwellian-type distribution irrespective of the driving frequency. Such evolution of the EEPFs shape with the driving frequency and hydrogen pressure has been discussed on the basis of electron diffusion processes and low threshold-energy inelastic collision processes taking place in the discharge. The ratio of stochastic power to bulk power heating ratio is dependent on the hydrogen pressure while it is independent on the driving frequency.

Effect of molecular nitrogen on the electron mobility in a mixture of argon and optically excited sodium vapor

A parametric study of the electron energy distribution function (EEDF) and the electron mobility in the mixture Na + Ar + N2 is carried out. An analysis is made of the conditions that obtain in a photoplasma when the detachment of the mean electron energy from the neutral gas temperature is due to superelastic collisions (collisions of the second kind) with excited sodium atoms. The case of low ionization of the medium at low vibrational temperatures of the ground state of the nitrogen molecules is considered. To find the EEDF a numerical solution of the Boltzmann transport equation is carried out. It is found that in the indicated mixture the presence of nitrogen leads to a depletion of the EEDF in the region of efficient vibrational excitation of the molecules and promotes the formation of inversion in the EEDF ∂f(ɛ)/∂ɛ>0 in the energy range corresponding to the Ramsauer minimum in the cross section of elastic collisions of electrons with the argon atoms. It is shown that the nonequilibrium character of the EEDF leads to a complicated dependence of the electron mobility on the partial ratios of the components of the mixture, the degree of ionization of the medium, and the population of the resonantly excited sodium atoms.

Comment on ‘‘Reduction of hot electron degradation in metal oxide semiconductor transistors by deuterium processing’’ [Appl. Phys. Lett. 68, 2526 (1996)

Lyding et al. have recently reported significant improvements in the lifetime of metal oxide semiconductor ~MOS! transistors due to incorporation of deuterium ~D!, rather than hydrogen ~H!, at the Si/SiO2 interface. This remarkable achievement indicates that the Si–D bond is more resistant to hot-electron excitation than the Si–H bond. Lyding et al. pointed out that the phenomenon is probably analogous to the observed reduction in desorption of deuterium versus hydrogen from hydrogenated Si~100!:H surfaces using the scanning tunneling microscope ~STM!. In this comment, we propose a specific pathway for the dissociation of Si–H and Si–D bonds, providing a natural explanation for the difference in dissociation rates. Shen et al. have proposed that the low-voltage STMinduced desorption of Si–H bonds from Si~100! proceeds via a multiple-vibrational excitation by tunneling electrons. Electrons excite Si–H vibrational transitions with a rate proportional to the tunneling current. The extent to which vibrational energy can be stored in the bond depends on the lifetime, i.e., on the rate at which energy is lost by coupling to phonons. Because the lifetime of H on Si is long, efficient vibrational excitation is expected. In the quantitative analysis of Ref. 3, it was assumed that the vibrational energy is deposited in the stretch mode of the Si–H bond, which has a frequency around 2100 cm. The same assumption is usually implicitly made in discussions of dissociation of Si–H bonds. Our main purpose here is to point out that both the vibrational lifetime and carrier-enhanced dissociation mechanisms are most likely controlled by the Si–H bending modes. The vibrational frequency of the bending mode for Si–H is around 650 cm, and the estimated frequency for Si–D is around 460 cm. This value is close to the frequency of bulk TO phonon states at the X point ~463 cm21). We therefore expect the coupling of the Si–D bending mode to the Si bulk phonons to result in an efficient channel for deexcitation. While it is quite possible to reach a highly excited vibrational state in the case of Si–H, this will be more difficult for Si–D. Deuterium should therefore be much more resistant to STM-induced desorption and hot-electron induced dissociation, due to the relaxation of energy through the bending mode.

Ballistic and dissociative collisions of a rare-gas atom with a halogen molecule

Efficient vibrational excitation and dissociation of halogen molecules have been observed in molecular dynamics simulations of shock-heated rare-gas clusters. We discuss the propensity for such transfers in a single atom—molecule collision and present classical trajectory computational results. Rather large cross sections are reported. It is expected that in a real experiment some of the flux will be diverted to electronic excitation.

Efficient vibrational excitation of gaseous CF 4 and C 2F 6 by low-energy electrons

Evidence is presented from a linewidth study of electron cyclotron resonance (ECR) absorption for efficient T → V energy transfer between low-energy electrons (<0.5 eV) and the fluorocarbons CF 4 and C 2F 6. It is suggested that this excitation proceeds by means of a direct mechanism.

CW CARBON MONOXIDE LASER WITH MICROWAVE EXCITATION IN THE SUPERSONIC FLOW

A new technique of efficiently coupling microwave energy into a supersonic gas flow is described. The stable self-sustained microwave discharge is applied to excite a cw CO gasdynamic laser. Electron densities well above the cut-off value have been achieved together with a reduced field strength within the discharge that is appropriate for efficient vibrational excitation. With an energy loading of 0.15 eV/CO a laser power of 340 Watts is obtained at an overall efficiency of 7 %. The effect of some additives on stability and laser performance is studied.

Cw carbon monoxide laser with microwave excitation in the supersonic flow

A new technique of efficiently coupling microwave energy into a supersonic gas flow is described. A stable self-sustained 2.45-GHz microwave discharge is applied to excite a cw CO gasdynamic laser. Electron densities well above the cutoff value have been achieved together with a reduced field strength within the discharge that is appropriate for efficient vibrational excitation. With an energy loading of 0.15 eV/molecule CO a laser power of 285 W was obtained in a small-scale flow channel at an overall efficiency of 6%.

Distribution of reaction products (theory). VIII. Cl+HI, Cl+DI

The dynamics of the thermal (300°K) reactions Cl+HI → ClH+I and Cl+DI → ClD+I have been examined by the classical trajectory method, in 3D. The Cl+HI reaction has also been studied at an enhanced (6 kcal mole−1) collision energy. The potential-energy hypersurface was the same as that used earlier [J. Chem. Phys. 49, 5189 (1968)]. Though it is a highly repulsive energy surface it is able to account for the efficient vibrational excitation of the molecular product for the mass combination characteristic of this reaction. The effect of changing the mass combination from H+LH (heavy+light-heavy; masses mCl+mHI) to L+HH (light+heavy-heavy; masses mH+mCl2) on the Cl+HI surface has been explored using a full 3D set of trajectories at 300°K. The effect is to markedly reduce the ``mixed energy release'' responsible for the efficient vibrational excitation on the repulsive surface. Vibrational and rotational excitation in the reaction products is correspondingly diminished, and translational excitation is enhanced. The efficient vibrational and rotational excitation for H+LH (Cl+HI), and contrasting behavior for L+HH (H+Cl2) have been observed in infrared chemiluminescence experiments. The present findings are therefore in accord with earlier proposals that both these reactions involve predominantly ``repulsive'' energy release. The computed product angular distribution for the Cl+HI reaction at 300°K was almost isotropic, in contrast to the exclusively backward-hemisphere scattering for the L+HH mass combination on the same energy surface. At 6 kcal mole−1 collision energy the computed angular distribution of HCl from the Cl+HI reaction showed exclusively sharply-forward scattering, in accord with the results of recent molecular beam experiments [J. D. MacDonald and D. R. Herschbach (unpublished)]. Enhanced collision energy gave rise to a small decrease in the computed mean product vibrational excitation, a small increase in mean product rotational excitation and a large increase in product translational excitation. These changes in product energy distribution are in qualitative accord with the findings from infrared chemiluminescence and molecular beam studies at enhanced collision energy. The overall conclusion is that the repulsive LEPS (London, Eyring, Polanyi, Sato) potential-energy hypersurface used here and in our earlier work, provides an acceptable (though not unique) first approximation to the actual interaction potential.