Cross Sections For H2O Research Articles

The rotational excitation of the water isotopologues by molecular hydrogen

ABSTRACT We present cross-sections and rate coefficients for rotational transitions in the water isotopologues D2O, H$_2^{18}$O, and HDO induced by collisions with para-H2(j2 = 0) and ortho-H2(j2 = 1). Quantum scattering calculations are performed at the full close-coupling level with the isotopic variants of an accurate full-dimensional H2O−H2 interaction potential. The D2O, H$_2^{18}$O, and HDO cross-sections are compared to the corresponding cross-sections for H2O. Large isotopic effects are observed in the case of D2O and HDO, in particular for collisions with p-H2(j2 = 0), while the 18O isotopic substitution is found to be negligible. Rate coefficients are provided for rotational transitions among all para-D2O, ortho-D2O, and HDO levels with internal energy below 300 cm−1 and for kinetic temperatures in the range 5–300 K. Non-LTE radiative transfer calculations show that the HDO 225.9 GHz and H$_2^{18}$O 203.4 GHz transitions recently detected with ALMA in the young proto-planetary disc V883 Ori should be inverted (weak masers) in a large fraction of the disc.

H2O and O(3PJ) photodesorption from amorphous solid water deposited on a lunar mare basalt

H2O (ν=0) and O(3PJ=2,1,0) desorbates were measured with resonance-enhanced multiphoton ionization following 157-nm irradiation of amorphous solid water (ASW) deposited on a lunar mare basalt. Both H2O photodesorption and O(3PJ) photodissociation products of ASW were studied in the attempt to better understand the competition between photodesorption and photodissociation of water in the condensed phase on a lunar surface. The oxygen atom time-of-flight (TOF) spectrum was measured as a function of spin–orbit state, H2O exposure, and 157-nm irradiation time. Maxwell–Boltzmann distributions with translational temperatures of 10,000K, 1800K, 400K, and 89K fit the four TOF components. For high H2O exposures, diffusion out of pores in the lunar substrate made a large portion of the O(3PJ) signal appear to be sub-thermal. Water depletion cross sections were measured at exposures between 0.1 and 10Langmuir (1L=10−6Torrs). These cross sections decreased with increasing coverage and matched previously measured cross sections from a lunar impact melt breccia. Additionally, non-resonant ionization was employed to detect vibrationally excited water indirectly through its fragments. The OH+ fragment of H2O (ν*) and the O(3PJ) photodissociation product increased in intensity during prolonged irradiation as hydroxyl groups accumulated on the surface and then recombined. For an initial exposure of 5L H2O, after reaching maximum signal, the cross sections for H2O (ν*) and O(3P2) depletion were measured to be 1.2×10−19cm2 and 6.7×10−20cm2, respectively.

Theoretical study of influence of H2O on parameters of low-temperature plasmas in humid mixtures

Monte Carlo (MC) electron dynamic simulations in dry and humid N2, O2, CO2 and air were performed in order to obtain electron energy distribution functions (EEDFs), mean free path (λ), drift velocity (v d) and collision frequency of electrons (ν), their mean energy \((\bar \varepsilon )\) and rate coefficients of electron impact reactions, as functions of reduced electric field strength (E/N) and concentration of H2O. The aim was to test the latest compilation of cross-sections for H2O, the applicability of the MC method used, and the effect of the H2O concentration.

Bond-selective photofragmentation of jet-cooled HOD at 193 nm: Vibrationally mediated photochemistry with zero-point excitation

Photofragment yields are reported for supersonically cooled H2O, D2O, and HOD via one photon, 193 nm photolysis in a slit jet expansion, with OH and OD fragments monitored by laser induced fluorescence methods. Detailed analysis of the dependence of OH vs OD photofragment signals on isotopic composition is used to extract relative photolysis cross sections and branching ratios for bond-selective cleavage in HOD, H2O, and D2O samples. Specified relative to the 193 nm cross sections for H2O→H+OH, the ratios are 0.392(20), 0.032(20), and 0.0157(19) for (i) HOD→H+OD, (ii) HOD→D+OH and D2O→D+OD channels, respectively. Specifically, these results indicate a propensity for H–OD vs D–OH bond cleavage in HOD of 12(8):1. This strong H/D isotopic selectivity reflects extreme non-Franck–Condon photolysis out of classically of forbidden regions of the ground-state wave function, i.e., bond-selective photochemistry mediated solely by zero-point vibrational excitation. However, when compared with theoretical predictions from full three-dimensional quantum scattering calculations on the ground (X̃ 1A1) and excited (Ã 1B1) potential-energy surfaces (PES) of water, the observed HOD branching ratio is found to be too low by an order of magnitude. These results provide additional evidence that photodissociation of water in the extreme non-Franck–Condon region is not adequately explained by current theoretical models and suggest that contributions from other electronic surfaces may be important.

Absolute partial cross sections for electron-impact ionization of H2O and D2O from threshold to 1000 eV

Absolute partial cross sections for electron-impact ionization of H2O and D2O are reported for electron energies from threshold to 1000 eV. Data are presented for the production of H2O++OH++O+, O2+, H2+, and H+ from H2O and for the production of D2O+, OD+, O+, O2+, D2+, and D+ from D2O. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±4.5% for singly charged parent ions and is slightly greater for fragment ions. The cross sections for H2O and D2O are found to be the same to within experimental uncertainties except for the H2+ cross section which is approximately a factor of 2 greater than the D2+ cross section. Previous results are compared to the present measurements.

High temperature quenching cross sections for nitric oxide laser-induced fluorescence measurements

Collisional quenching cross sections for laser-induced fluorescence from NO A(v′=0) are measured for H2O, CO2, O2, NO, N2, H2, Ar, C2H6, and CO as a function of temperature from 300 to 1750 K. Room temperature cross sections vary widely (≤0.03 Å2 for N2, H2, C2H6, and Ar to ≥100 Å2 for H2O). As temperature increases, cross sections for H2O and CO2 decrease, cross sections for O2 and NO remain constant, and cross sections for N2, H2, CO, and C2H6 increase. These data provide quantitative quenching corrections for laser-induced fluorescence measurements of NO concentrations in nonreacting and reacting flows. Physical quenching models (based upon NO–Q and NO+–Q− collision complexes) are reviewed and used for experimental data correlation and extrapolation. A new quenching mechanism is proposed for weak quenchers.

Molecular beam scattering experiments with polar molecules: measurement of differential collision cross sections for H2O+H2, He, Ne, Ar, H2O and NH3+H2, He, NH3

Differential scattering cross section experiments are reported in which one or both of the scattering partners is a polar molecule. The well resolved diffractive oscillations for H2O+H2, He, Ne and NH3+H2, He and the rainbow maximum observed for H2O+Ar permit the determination of the potential parameters, epsilon , the well depth, and sigma , the zero position for an assumed Lennard-Jones (12,6) potential. The cross sections for H2O+H2O and NH3+NH3 exhibit no diffraction oscillations but the general slope of the cross section permits an estimate of the well depth, epsilon .

799. Investigation of the effects of heating in CO 2 on the oxide-coated cathode surface microtopography and emission characteristics of a triode electron tube : E J Nowak et al,J Vac Sci Technol, 10 (3), May–June 1973, 453–463.

H2O (ν = 0) and O(3PJ=2,1,0) desorbates were measured with resonance-enhanced multiphoton ionization following 157-nm irradiation of amorphous solid water (ASW) deposited on a lunar mare basalt. Both H2O photodesorption and O(3PJ) photodissociation products of ASW were studied in the attempt to better understand the competition between photodesorption and photodissociation of water in the condensed phase on a lunar surface. The oxygen atom time-of-flight (TOF) spectrum was measured as a function of spin–orbit state, H2O exposure, and 157-nm irradiation time. Maxwell–Boltzmann distributions with translational temperatures of 10,000 K, 1800 K, 400 K, and 89 K fit the four TOF components. For high H2O exposures, diffusion out of pores in the lunar substrate made a large portion of the O(3PJ) signal appear to be sub-thermal. Water depletion cross sections were measured at exposures between 0.1 and 10 Langmuir (1 L = 10−6 Torr s). These cross sections decreased with increasing coverage and matched previously measured cross sections from a lunar impact melt breccia. Additionally, non-resonant ionization was employed to detect vibrationally excited water indirectly through its fragments. The OH+ fragment of H2O (ν*) and the O(3PJ) photodissociation product increased in intensity during prolonged irradiation as hydroxyl groups accumulated on the surface and then recombined. For an initial exposure of 5 L H2O, after reaching maximum signal, the cross sections for H2O (ν*) and O(3P2) depletion were measured to be 1.2 × 10−19 cm2 and 6.7 × 10−20 cm2, respectively.