Rotational Temperature Dependence Research Articles

Development of a wavy Stark velocity filter for studying interstellar chemistry.

Cold polar molecules are key to both the understanding of fundamental physics and the characterization of the chemical evolution of interstellar clouds. To facilitate such studies over a wide range of temperatures, we developed a new type of Stark velocity filter for changing the translational and rotational temperatures of velocity-selected polar molecules without changing the output beam position. The translational temperature of guided polar molecules can be significantly varied by exchanging the wavy deflection section with one having a different radius of the curvature and a different deflection angle. Combining in addition a temperature variable gas cell with the wavy Stark velocity filter enables to observe the translational and rotational temperature dependence of the reaction-rate constants of cold ion-polar molecule reactions over the interesting temperature range of 10-100 K.

Study of argon–oxygen flowing afterglow

The reaction kinetics in argon–oxygen flowing afterglow (post-discharge) was studied using NO titration and optical emission spectroscopy. The flowing DC post-discharge in argon–oxygen mixture was created in a quartz tube at the total gas pressure of 1000 Pa and discharge power of 90 W. The O(3P) atom concentration was determined by NO titration at different places along the flow tube. The optical emission spectra were also measured along the flow tube. Argon spectral lines, oxygen lines at 777 nm and 844.6 nm and atmospheric A-band of were identified in the spectra. Rotational temperature of was determined from the oxygen atmospheric A-band and also the outer wall temperature of the flow tube was measured by a thermocouple and by an IR thermometer. A zero-dimensional kinetic model for the reactions in the afterglow was developed. This model allows the time dependencies of particle concentrations and of gas temperature to be calculated. The wall recombination probability for O(3P) atoms and wall deactivation probability for (b ) molecules were determined from the fit of model results to experimental data. Sensitivity analysis was applied for the analysis of kinetic model in order to reveal the most important reactions in the model. The calculated gas temperature increases in the afterglow and then decreases at later afterglow times after reaching the maximum. This behavior is in good agreement with the spatial rotational temperature dependence. A similar trend was also observed at outer wall temperature measurement.

Rotational temperatures of N2(C,0) and OH(A,0) as gas temperature estimates in the middle pressure Ar/O2 discharge

Radiative transitions N2(C–B,0–0) and OH(A–X,0–0) were used in order to estimate rotational temperatures of N2(C,0) and OH(A,0) in the middle pressure Ar/0.5% O2 RF discharge. N2(C,0) and OH(A,0) rotational temperature dependence on power density was determined and quenching/rotational relaxation frequencies of N2(C,0) and OH(A,0) were calculated. It was found that under our conditions rotational temperature of OH(A,0) can be used for estimation of gas temperature while N2(C,0) rotational temperature overestimates the gas temperature.

On the rotational temperature and structure dependence of electric field deflection experiments: A case study of germanium clusters

Molecular beam electric field deflection experiments offer a probe to the structural and dielectric properties of isolated particles in the gas phase. However, their quantitative interpretation is still a formidable task. Despite the benefits of this method, the analysis of the deflection behavior is often complicated by various experimental and theoretical problems, including the amount of energy stored in internal and rotational modes of the deflected particle and the amount of structural asymmetry. In this contribution, we address these issues by discussing the experimentally observed field-induced deflection of Ge(9), Ge(10), and Ge(15) clusters in comparison to quantum mechanical and classical deflection models. Additionally, we derive simple formulas to describe how the molecular beam deflection depends on the rotational temperature and the symmetry of the particle. Based on these results, we discuss to what extend molecular beam electric field deflection experiments can be used as a tool for structure determination of isolated clusters in the gas phase.

Dc excited glow discharges in atmospheric pressure air in pin-to-water electrode systems

Electrical and optical emission properties of non-equilibrium atmospheric air discharges between a metal pin and a tap water anode/cathode are presented. With a water anode the discharges are of the glow type as is derived from short-exposure time plasma imaging and electrical characteristics. Additionally, the validity of extrapolated scaling laws of low pressure glow discharge supports these findings.In the case of a water cathode the plasma is filamentary in nature at the water surface. In the case of a water anode, the plasma is diffuse down to 10 ns. The timescales on which the filaments are visible in the near water cathode region and estimates of the electrical field in the cathode layer are consistent with the assumption that these filaments occur due to the electrical instability of the water surface.Spatially resolved rotational temperature measurements and dependence of the rotational temperature on current are discussed in detail. The rotational temperatures of OH and N2 in the positive column of the plasma are identical and equal to 3250 ± 250 K. A 2500 K temperature drop in the near anode region clearly shows that the water anode acts as an effective heat sink for the discharge. This indicates that apart from the electrical stabilization of the discharge by the water electrode due to its distributed resistance, a water anode also thermally stabilizes the discharge. The rotational temperature of nitrogen near the metal anode is typically two times smaller.

Rotational temperature dependence of the reactions of N+ and C+ with H2, HD, and D2

The reactions of N+ and C+ with H2, HD, and D2 have been studied as a function of translational energy and hydrogen temperature in a guided ion beam mass spectrometer. For the N+ systems, the cross sections for these slightly endothermic reactions depend significantly upon the temperature of the hydrogen reagent below ≊0.3 eV. The effect increases with increasing endothermicity for these reactions, with the maximum effect being a factor of 5 change in cross section for formation of NH+ in the reaction with HD at a kinetic energy of 0.02 eV. The branching ratio for formation of NH+ and ND+ in the reaction with HD at 0.02 eV is 1:3 at 305 K and 1:13 at 105 K. These effects are consistent with rotational energy driving the reaction. The moderately endothermic reactions of C+ with H2 and D2 have also been studied as a function of translational energy and hydrogen temperature. Cooling the hydrogen results in a sharper threshold for reaction because of reduced Doppler broadening and reduced hydrogen rotational energy. Results for these systems can be modeled by using phase space theory after accounting for Doppler broadening and internal energy effects.

Rovibrational coupling in the S1 excited state of 1-methylindole

We have undertaken an extensive study of the spectroscopy and dynamics of the S1–S0 transition in jet-cooled 1-methylindole and 1-methyl(d3)indole. The energy-resolved fluorescence decays resulting from picosecond excitation of S1 vibrational modes display unusual quantum interference effects which have been attributed to the excitation of a large number of quasidegenerate states even at relatively low levels of vibrational excitation. This has been attributed to rovibrational coupling in which the selection rules are much less restrictive than is normally the case with rigid planar molecules. A significant rotational temperature dependence of the vibrational dynamics has been measured which serves to corroborate this model.

Rotational temperature dependence of the branching ratio for the reaction of Kr+(2P3/2) ions with HD

Branching ratios for the reaction of Kr+(2P3/2) with HD, which produces KrH+ and KrD+, have been measured in a variable temperature-selected ion flow drift tube apparatus as a function of average center-of-mass kinetic energy 〈KEcm〉 at two temperatures, 93 and 300 K. At the lowest energy employed, 〈KEcm〉=0.012 eV, the KrD+ channel is favored, and its contribution decreases with increasing kinetic energy. The data are in agreement with previous measurements. The dependence of the branching ratio on the rotational temperature of HD is derived from the data. The rotational temperature dependence is small and appears to be in the opposite direction to the kinetic energy dependence, i.e., increasing rotational energy appears to decrease the fraction of KrH+ produced. The data are compared with models and with the rotational temperature dependence found for the reaction of O+ with HD. Rate constants for the reactions of Kr+(2P3/2) with H2 and HD were measured at 300 K and no drift field and found to be 2.8±0.7×10−10 cm3 s−1 and 4.0±1.0×10−10 cm3 s−1, respectively. The latter number supports the previous beam measurement and disagrees with a previous selected ion flow tube result.

Rotational temperature dependence of the branching ratio for the reaction of O+ with HD

Rate constants and branching fractions for the reaction of O+ with HD have been measured as a function of average center-of-mass kinetic energy (〈KEcm〉) at three temperatures: 93, 300, and 509 K. Both OH+ and OD+ were produced. The rate constants were found to equal 1.2×10−9 cm3 s−1, independent of temperature or 〈KEcm〉. The branching into OH+ was observed to increase with 〈KEcm〉. Differences in the branching fractions were seen at a particular 〈KEcm〉 at different temperatures. These differences are attributed to a rotational temperature dependence such that increasing rotational temperature decreases the fraction of OH+ produced. The data are in agreement with a theoretical calculation and previous measurements.

Kinetic energy, temperature, and derived rotational temperature dependences for the reactions of Kr+(2P3/2) and Ar+ with HCl

Rate constants for the reactions of Kr+(2P3/2) with HCl and DCl and of Ar+ with HCl have been measured as a function of reactant ion/reactant neutral average center-of-mass kinetic energy (〈KEc.m.〉 ) at several temperatures. The measurements were made using helium as the carrier gas. From these data we have derived the dependences of the rate constants on the rotational temperature of H(D)Cl. Rate constants for the reaction of Kr+(2P1/2) with HCl have also been measured as a function of temperature. The rate constants for all of the reactions were found to decrease with increasing temperature. The rate constants were also found to decrease with increasing 〈KEc.m.〉 at low 〈KEc.m.〉 but then to increase at higher 〈KEc.m.〉 . A significant rotational temperature dependence of the rate constant was derived for the reaction of Kr+(2P3/2) with H(D)Cl. The analogous derivation for Ar+ reacting with HCl showed the rate constant for this reaction to be independent of the rotational temperature of HCl within experimental uncertainty.

Rotational temperature dependences of gas phase ion–molecule reactions

A technique for measuring the rotational temperature dependences of gas phase ion–molecule rate constants is presented. The technique involves measuring the kinetic energy dependences of the rate constants at several temperatures in a variable temperature selected ion flow drift tube. For a monatomic ion, comparing the rate constants at the same center of mass kinetic energy at different temperatures yields the dependence of the rate constant on the internal temperature of the reactant neutral. For neutrals in which the vibrational modes are inactive at the temperatures of the experiment, the internal energy dependence is the rotational temperature dependence. Two examples are presented here, one in which rotational energy significantly influences the rate constants, approximately T−0.5, and one in which it does not. Implications for past drift tube experiments are discussed.

Fluorescence polarization from 1B1 pyrimidine: Evidence for intramolecular vibration–rotation energy transfer in a highly excited molecule

We have measured the polarization of ensemble-averaged fluorescence from the 000 and 5 vibronic regions of 1B1 pyrimidine, in order to determine the extent of intramolecular vibration–rotation energy transfer at high energies. The polarization of the 000 is 15.54%±0.35%, and decreases smoothly to 6.93%±0.22% at Evib=3700 cm−1. From comparisons with model calculations, we conclude that highly excited pyrimidine undergoes nearly statistical rotational motion during its fluorescence lifetime. Additional experiments in a supersonic expansion show that the rotational temperature dependence of the polarization is weak.

Sensitized fluorescence of diatomic free radicals by Hg(6 3P 0) metastables

Mercury 6 3P 0 metastables were shown to sensitize the emission of a number of diatomic free radicals (HgH, HgCl, CN, OH, and NH) of photochemical interest. Evidence is presented that the energy transfer process is a non-Franck-Condon one at least for HgCl and CN. An inverse rotational temperature dependence on nitrogen pressure was observed for the HgH( 2Π 1 2 ) u′ = 0 state and attributed to near resonant rotational-rotational relaxation processes involving the zeroth and first vibrational levels.

A theoretical study of the interpretation of emission-absorption intensity ratio temperature measurements in the absence of thermal equilibrium

Emission-absorption intensity ration temperature measurements of diatomic molecules under thermal non-equilibrium conditions are investigated theoretically. It is shown that the rotational population distribution must be taken into account when a small part of a vibration-rotation band is viewed in experiments for measuring a vibrational temperature. An emission-absorption intensity ratio of unity does not always mean equality of effective brightness temperature of background source and a vibrational temperature. Such an assumption could lead to errors of several per cent in experimentally deduced vibrational temperatures, particularly in situations where the ratio of rotational and vibrational temperatures is less than unity. The rotational temperature dependence is explained in terms of transitions taking place in vibration-rotation bands and this dependence decreases when a larger part of a band is viewed. Emission-absorption measurements are also investigated for cases where the vibrational population distribution deviates from Boltzmann, for polyatomic and for molecular electronic spectra. Finally it is shown that, in principle, both rotational and vibrational temperatures may be determined from two simultaneous emission-absorption measurements.