Elke Plonjes

Time-resolved Fourier transform infrared spectroscopy of optically pumped carbon monoxide

Abstract The paper discusses measurements of vibration-to-vibration (V–V) energy transfer rates for CO–CO using time-resolved step-scan Fourier transform infrared spectroscopy of optically pumped carbon monoxide. In the experiments, time evolution of all vibrational states of carbon monoxide excited by a CO laser and populated by V–V processes (up to v∼40) is monitored simultaneously. The V–V rates are inferred from these data using a kinetic model that incorporates spatial power distribution of the focused laser beam, transport processes, and multi-quantum V–V processes. Although the model predictions agree well with the time-dependent step-scan relaxation data, there is variance between the model predictions and the up-pumping data, however. Comparison of calculations using two different sets of V–V rates with experimental spectra showed that the use of the semi-empirical V–V rates of DeLeon and Rich provides better agreement with experiment. It is also shown that the multi-quantum V–V rates among high vibrational quantum numbers, calculated by Cacciatore and Billing, are substantially overpredicted. The results provide some new insight into nonequilibrium vibrational kinetics, and also demonstrate the capabilities of the step-scan Fourier transform spectroscopy for time-resolved studies of molecular energy transfer processes and validation of theoretical rate models.

Vibrational energy storage in high pressure mixtures of diatomic molecules

Abstract CO/N 2 , CO/Ar/O 2 , and CO/N 2 /O 2 gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T V =1900–2300 K for N 2 , T V =2600–3800 K for CO, and T V =2200–2800 K for O 2 . The translational–rotational temperature of the gases does not exceed T =700 K. Line-of-sight averaged CO vibrational level populations up to v =40 are inferred from infrared emission spectra. Vibrational level populations of CO ( v =0–8), N 2 ( v =0–4), and O 2 ( v =0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.

Synthesis of single-walled carbon nanotubes in vibrationally non-equilibrium carbon monoxide

Single-walled carbon nanotubes (SWNTs) are synthesized in a gas-phase non-equilibrium plasma process.The carbon producing CO disproportionation reaction is driven very efficiently in a flow reactor, in which extreme disequilibrium between the vibrational and translational mode of the carbon monoxide gas is maintained even at low translational temperatures by using a powerful and efficient carbon monoxide gas laser.In the presence of metal catalysts, the vibrationally excited CO reacts to form CO2 and structured carbon molecules, notably SWNTs.The individual tubes form ropes or flat ribbons and these are aligned parallel to each other into larger structures of SWNT material without any post-synthesis treatment. 2002 Published by Elsevier Science B.V.

Electron density and recombination rate measurements in CO-seeded optically pumped plasmas

Electron production rate and electron density in cold optically pumped CO–Ar and CO–N2 plasmas in the presence of small amounts of O2 and NO have been measured using a Thomson discharge probe and microwave attenuation. Nonequilibrium ionization in the plasmas is produced by an associative ionization mechanism in collisions of highly vibrationally excited CO molecules. It is shown that adding small amounts of O2 or NO (50–100 mTorr) to the baseline gas mixtures at P=100 torr results in an increase of the electron density by up to a factor of 20–40 (from ne<1010 cm−3 to ne=(1.5–3.0)×1011 cm−3). This occurs while the electron production rate either decreases (as in the presence of O2) or remains nearly constant within a factor of 2 (as in the presence of NO). It is also shown that the electron–ion recombination rates inferred from these measurements decrease by two to three orders of magnitude compared to their baseline values (with no additives in the cell), down to β≅1.5×10−8 cm3/s with 50–100 mTorr of oxy...

Ionization measurements in optically pumped discharges

The kinetics of ionization and electron removal in optically pumped non-equilibrium plasmas sustained by a CO laser are studied using non-self-sustained dc and RF electric discharges. Experiments in optically pumped CO/Ar/N2 mixtures doped with O2 and NO demonstrated that associative ionization of CO produces free electrons at a rate up to S = 1015 cm-3 s-1. The ionization rate coefficient, inferred from the CO vibrational population measurements, is kion = (1.1-1.8)×10-13 cm3 s-1. It is shown that excited NO and possibly O2 molecules also contribute to the vibrationally stimulated ionization process. In a CO/Ar plasma, applying a dc bias to the cell electrodes resulted in the rapid accumulation of a deposit on the negative electrode due to a large cluster ion current. The average mass of an ion in this plasma, estimated by measuring the mass of the deposit, is m250 amu, which is consistent with the mass spectrometer analysis of the deposit. The deposit did not accumulate when small amounts of O2 and NO were added to the CO/Ar plasma, which presumably indicates the destruction of the cluster ions. It is demonstrated that adding small amounts of O2 to the optically pumped CO/Ar plasmas significantly increases the electron density, from ne = (4-7)×109 cm-3 to ne = (1-2)×1011 cm-3. This effect occurs at a nearly constant (within 50%) electron production rate S, indicating a substantial reduction in the overall electron removal rate. This reduction can be qualitatively interpreted as the destruction of rapidly recombining cluster ions in the presence of the O2 additive, and their replacement by monomer ions with a slower recombination rate. Further studies of the ion composition in optically pumped plasmas are suggested.

Electron-mediated vibration–electronic (V–E) energy transfer in optically pumped plasmas

The paper discusses experiments on vibration-to-electronic energy transfer in CO laser pumped CO–Ar and CO–N2 plasmas. Ionization in these strongly nonequilibrium plasmas occurs by an associative mechanism, in collisions of two highly vibrationally excited CO molecules. The experiments show that removal of the electrons from the optically pumped plasmas using a saturated Thomson discharge results in considerable reduction of the UV/visible radiation from the plasma (CO 4th positive bands, NO c bands, CN violet bands, and C2 Swan bands). At some conditions, the removal of electrons results in a nearly complete extinguishing of the UV/visible glow of the plasma. This effect occurs even though electron removal results in an increase of the high vibrational level populations of the ground electronic state CO(X 1 R, v � 15–35). On the other hand, deliberate electron density increase by adding small amounts of O2 or NO to the optically pumped CO–Ar plasmas produced a substantial increase of the UV/visible radiation intensity, which strongly correlates with the electron density. The results of the present experiments indicate that the vibration-toelectronic (V–E) energy transfer process CO(X 1 R ! A 1 P), and, possibly, analogous processes populating radiating excited electronic states of NO, CN, and C2, in optically pumped plasmas, may be mediated by the presence of electrons which are created in the absence of an electric field, with low initial energies. Most importantly, this effect occurs at ionization fractions as low as ne=N � 10 � 9 –10 � 7 . 2002 Elsevier Science B.V. All rights reserved.

Radio frequency energy coupling to high-pressure optically pumped nonequilibrium plasmas

This article presents an experimental demonstration of a high-pressure unconditionally stable nonequilibrium molecular plasma sustained by a combination of a continuous wave CO laser and a sub-breakdown radio frequency (rf) electric field. The plasma is sustained in a CO/N2 mixture containing trace amounts of NO or O2 at pressures of P=0.4–1.2 atm. The initial ionization of the gases is produced by an associative ionization mechanism in collisions of two CO molecules excited to high vibrational levels by resonance absorption of the CO laser radiation with subsequent vibration-vibration (V-V) pumping. Further vibrational excitation of both CO and N2 is produced by free electrons heated by the applied rf field, which in turn produces additional ionization of these species by the associative ionization mechanism. In the present experiments, the reduced electric field, E/N, is sufficiently low to preclude field-induced electron impact ionization. Unconditional stability of the resultant cold molecular plasma ...

Effect of electron density on shock wave propagation in optically pumped plasmas

This article discusses experimental studies of spark-generated shock wave propagation in CO-laser sustained optically pumped CO–Ar–O2 plasmas. The rotational-translational temperature of the plasma is measured by Fourier transform infrared emission spectroscopy. The electron density in the plasma is determined by microwave attenuation. The line-of-sight averaged density distribution across the propagating shock is detected by photoacoustic deflection (PAD). The measurements show that adding small amounts of oxygen (up to 0.1%) to the baseline optically pumped CO–Ar plasma increases the electron density and the ionization fraction by more than an order of magnitude (up to ne=0.9×1010 cm−3 and ne/N=0.8×10−8, respectively), while the gas temperature remains nearly constant, within 3%–5%. Therefore this approach allows varying the electron density in the plasma nearly independently of the gas temperature. The PAD measurements show considerable apparent weakening and dispersion of a shock wave propagating in t...