Ortho-para Conversion Rate Research Articles

Interstitial H2 in Si29

The ortho-para conversion of interstitial ${\mathrm{H}}_{2}$ in single-crystalline natural Si ($^{\text{nat}}\mathrm{Si}$) and $^{29}\mathrm{Si}$ (enrichment 96.2%) is studied by Raman scattering. The conversion process was found to be practically independent of the isotope composition of Si. The characteristic ortho-to-para conversion time at 77 K was found to be $220\ifmmode\pm\else\textpm\fi{}35$ and $200\ifmmode\pm\else\textpm\fi{}35$ h for $^{29}\mathrm{Si}$ and $^{\text{nat}}\mathrm{Si}$, respectively, whereas at room temperature the back conversion occurs with a characteristic time of $8.8\ifmmode\pm\else\textpm\fi{}1.5$ and $10.5\ifmmode\pm\else\textpm\fi{}1.5$ h for $^{29}\mathrm{Si}$ and $^{\text{nat}}\mathrm{Si}$, respectively. These values agree very well with the transition rates found in previous IR absorption studies by Peng et al. for $^{\text{nat}}\mathrm{Si}$ [Phys. Rev. B 80, 125207 (2009)]. Our findings imply that an interaction of ${\mathrm{H}}_{2}$ with the nuclear spin of nearby $^{29}\mathrm{Si}$ has marginal, if any, effects on the ortho-para conversion rate of molecular hydrogen in silicon.

On the H2 abundance and ortho-to-para ratio in Titan's troposphere

We have analyzed spectra recorded between 50 and 650 cm−1 by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles to determine simultaneously the H2 mole fraction and ortho-to-para ratio in Titan's troposphere. We used constraints from limb spectra between 50 and 900 cm−1 and from in situ measurements by the Huygens probe to characterize the temperature, haze and gaseous absorber profiles. We confirm that the N2-CH4 collision-induced absorption (CIA) coefficients used up to now need to be increased by about 52% at temperatures of 70–85 K. We find that the N2-N2 CIA coefficients are also too low in the N2 band far wing, beyond 110 cm−1, in agreement with recent quantum mechanical calculations. We derived a H2 mole fraction equal to (0.88 ± 0.13) × 10−3, which pertains to the ~1–34 km altitude range probed by the S0(0) and S0(1) lines. This result agrees with a previous determination based only on the H2-N2 dimer transition in the S0(0) line, and with the in situ measurement by the Gas Chromatograph Mass Spectrometer (GCMS) aboard Huygens. It is 3–4 times smaller than the value measured in situ by the Ion Neutral Mass Spectrometer (INMS) of Cassini at 1000–1100 km. The H2 para fraction is close to equilibrium in the 20-km region. CIRS spectra can be fitted assuming ortho-to-para (o-p) H2 thermodynamical equilibrium at all levels or a constant para fraction in the range 0.49–0.53. We have investigated different mechanisms that may operate in Titan's atmosphere to equilibrate the H2 o-p ratio and we have developed a one-dimensional model that solves the continuity equation in presence of such conversion mechanisms. We conclude that exchange with H atoms in the gas phase or magnetic interaction of H2 in a physisorbed state on the surface of aerosols are too slow compared with atmospheric mixing to play a significant role. On the other hand, magnetic interaction of H2 with CH4, and to a lesser extent N2, can operate on a timescale similar to the vertical mixing time in the troposphere. This process is thus likely responsible for the o-p equilibration of H2 in the mid-troposphere implied by CIRS measurements. The model can reproduce the inferred o-p ratio in the 20-km region, assuming low atmospheric mixing in the troposphere down to 15–20 km and conversion rates with CH4 or N2 slightly larger than obtained from an extrapolation of natural ortho-para conversion rate measured in gaseous hydrogen.

Influence of Molecular Oxygen on Ortho-Para Conversion of Water Molecules

The mechanism of influence of molecular oxygen on the probability of ortho-para conversion of water molecules and its relation to water magnetization are considered within the framework of the concept of paramagnetic spin catalysis. Matrix elements of the hyperfine ortho-para interaction via the Fermi contact mechanism are calculated, as well as the Maliken spin densities on water protons in H2O and O2 collisional complexes. The mechanism of penetration of the electron spin density into the water molecule due to partial spin transfer from paramagnetic oxygen is considered. The probability of ortho-para conversion of the water molecules is estimated by the quantum chemistry methods. The results obtained show that effective ortho-para conversion of the water molecules is possible during the existence of water-oxygen dimers. An external magnetic field affects the ortho-para conversion rate given that the wave functions of nuclear spin sublevels of the water protons are mixed in the complex with oxygen.

Millimetre-wave spectroscopy of deuterated vinyl radicals, observation of the ortho–para mixing interaction and prediction of fast ortho–para conversion rates

Ortho-para mixing interaction due to the coupling of nuclear and electron spins was detected for the first time by millimetre-wave spectroscopy of deuterated vinyl radicals, H2CCD and D2CCD, of which the ground states are split by the tunnelling motion of the α deuteron into two components 0+ and 0−, whose separations have been determined to be ΔE 0 = 1186.644(16) and 771.978(18) MHz, respectively. The observed tunnelling-rotation spectra are significantly perturbed by the ortho–para mixing interaction expressed by , where I β1 and I β2 are spins of the two hydrogen nuclei in the β position and S is the electron spin, which connects rotational levels in the 0+ and 0− states, one being an ortho level and the other a para level. The constants for H2CCD and D2CCD have been determined to be 68.06(53) and 10.63(94) MHz, respectively. The ortho and para states are mixed by about 0.097% and 0.0123% due to this interaction. The rate constant of para to ortho (I β = 0 → 1) conversion is predicted as 1.2 × 105 s−1 torr−1 for H2CCD, suggesting extremely rapid mutual conversion between ortho and para nuclear spin isomers of H2CCD, which is more than 106 times faster compared with that in closed shell molecules such as H2CO and H2CCH2.

The effect of n-H2 impurity on the heat capacity of solid Ne

The alloy Ne-5.1% n-H2 is investigated calorimetrically under equilibrium vapor pressure in an interval of 0.9–25K. The magnitude and temperature dependence of the heat capacity Csol(T) of the alloy at T=11–20K are found to be strongly dependent on the conditions of sample preparation. The temperature dependence of Csol(T) for the sample prepared in a calorimeter by direct condensation of a gas mixture at T≈15K exhibits a smeared maximum near the triple-point temperature of hydrogen and a phase transition at T=17.1K. The detected features of Csol(T) indicate that preparation of solid Ne–n-H2 from the gas phase leads to the formation of a long-lived nonequilibrium phase of Ne with high H2 concentrations and a small portion of H2 inclusions with low Ne concentrations. The phase transition is caused by decomposition of this phase. The phase does not recover on cooling of the sample after the phase transition. It is found that the rate of ortho-para conversion of the H2 molecules in the Ne–n-H2 solid solutions is higher than in solid H2.

Low temperature Raman spectra of hydrogen in simple and binary clathrate hydrates

The Raman spectrum of hydrogen clathrate hydrates has been measured, as a function of temperature, down to 20 K. Rotational bands of H(2) and HD, trapped into the small cages of simple (H(2)O-H(2)) and binary (H(2)O-THF-H(2)) hydrates, have been analyzed and the fivefold degeneracy of the molecular J=2 rotational level has been discussed in the light of the available theoretical calculations. The vibrational frequencies of H(2) molecules encapsulated in the large cages of simple hydrates turn out to be well separated from those pertaining to the small cages. Comparison with the equivalent D(2) spectra allowed us to assign the large cavity vibrational frequencies to three couples of Q(1)(1)-Q(1)(0) H(2) vibrational modes. Populations of ortho and para species have been measured as a function of time from rotational spectra and the rate of ortho-para conversion has been estimated for both simple and binary hydrates. We suggest, using the H(2) vibrational spectra, a model to analyze the cage population in simple hydrates.

Nuclear spin conversion of formaldehyde in protostar environments induced by non reactive collisions

Context. Formaldehyde is an important diagnostic of the physical conditions in star forming regions. The temperature of formation is determined by measuring the relative abundance of para-H2CO and ortho-H2CO. The key hypothesis for this determination is that the ortho-para interconversion is strictly forbidden once the molecule is formed. However H2CO nuclear spin conversion mechanisms do exist in a gas phase either involving reactive or non-reactive collisions. This last process is governed by internal properties of the molecule, such as the mixing of energetically closed ortho-para level pairs, which are coupled through magnetic intramolecular interactions. This mixing is interrupted by a collision that makes the molecule leave the mixed state and puts it in a pure state, ortho or para, with a non zero probability for both isomeric forms. Aims. This model allows us to estimate the spin conversion induced by non reactive collisions in different conditions encountered in the interstellar medium. Methods. We calculated the ortho-para conversion rate in H2CO for different temperatures and H2 abundances. Results. It is shown that the characteristic conversion time is always much longer than the H2CO lifetime. Conclusions. Consequently, the conversion probability is zero in gas-phase protostar environments for these non reactive collisions.

Ortho-para conversion of hydrogen at high pressures

Ortho-para conversion rates in solid ${\mathrm{H}}_{2}$ measured as a function of pressure up to 58 GPa are examined theoretically. Analyses of the data provide information on the relative role of diffusion versus intrinsic dependences of the conversion rate on ortho concentration. A theory of the conversion has been developed using a closed-form representation of the conversion promoting nuclear magnetic interaction ${H}_{\mathrm{ss}}$ expanded in spherical harmonics. The mechanisms considered include double conversion, excitations in the $J=1$ and $J=2$ manifolds as conversion energy sinks, and a possibility of intermediate states from which the conversion energy is dissipated via the strong electrical quadrupole-quadrupole (EQQ) interaction. Conversion rates were evaluated for a total of 12 new channels; the two other channels considered previously for moderate pressures have been reconsidered to account for factors that influence phonon-assisted energy dissipation, the most important being the compression-related decrease of the conversion energy (gap closing). Contributions from the standard one-phonon channels with single and double conversion yield fairly good agreement with low-pressure data. The proposed new channel identified as responsible for the observed conversion acceleration is the one in which the conversion Hamiltonian ${H}_{\mathrm{ss}}$ only initiates conversion driving the system to a temporarily nonequilibrium state from which the conversion energy is dissipated via EQQ coupling into excitations within the $J=1$ manifold. Our mechanism predicts a strong and abrupt conversion slowdown at still higher compressions. The abrupt decrease in rate observed at a given pressure at longer times (decreasing ortho fractions) can be explained as due to the inability of slow diffusion to restore the random distribution of ortho species and due to the intrinsic inefficiency of the new channel at low c.

Stress dependence of exciton relaxation processes inCu2O

A comprehensive study of the exciton relaxation processes in ${\mathrm{Cu}}_{2}\mathrm{O}$ has led to some surprises. We find that the ortho-para conversion rate becomes slower at high stress and that the Auger nonradiative recombination rate increases with stress, with apparently no Auger recombination at zero stress. These results have important consequences for the pursuit of Bose-Einstein condensation of excitons in a harmonic potential.

Progress in Cryocrystals at Megabar Pressures

Recent high-pressure studies of cryocrystals have provided a wealth of new information about the behavior of simple condensed-matter systems up to megabar (>100 GPa) pressures. Improved spectroscopic measurements with high-throughput spectrographs and chemically ultrapure synthetic diamonds, the extension of electrical conductivity measurements in the megabar pressure range, and several different X-ray diffraction methods made possible numerous new findings. Examples from rare-gas solids, solid hydrogen, and nitrogen are presented, and include pressure effects on many-body interactions, orthopara conversion rate, molecular orientational order-disorder, reconstructive transitions leading to the formation of new polymeric forms, and direct studies of pressure effects on shear strength.

New ortho-para conversion mechanism in dense solid hydrogen.

Analysis of recent measurements of striking changes in the rate of ortho-para conversion of solid H(2) up to 58 GPa shows that the conversion mechanism must differ from that at ambient pressure. A new conversion mechanism is identified in which the emerging excitations are coupled to the converting molecules via electric quadrupole-quadrupole rather than nuclear spin-spin interactions. The latter only initiates conversion while the coupling enhancement associated with the new mechanism is ensured by high compression and a gap closing, with the conversion energy diminishing strongly with increasing pressure.

Pressure-enhanced ortho-para conversion in solid hydrogen up to 58 GPa.

We measured the ortho-para conversion rate in solid hydrogen by using Raman scattering in a diamond-anvil cell, extending previous measurements by a factor of 60 in pressure. We confirm previous experiments that suggested a decrease in the conversion rate above about 0.5 GPa. We observe a distinct minimum at 3 GPa followed by a drastic increase in the conversion rate to our maximum pressure of 58 GPa. This pressure enhancement of conversion is not predicted by previous theoretical treatments and must be due to a new conversion pathway.

Nuclear spin state relaxation in formaldehyde: dependence of the rate constant on pressure

Measurements of ortho–para conversion rates in pure formaldehyde are described. Separation of the nuclear spin isomers was obtained by selective UV laser photolysis of ortho-formaldehyde. Observed total rate constants are of the order of (7±2)×10 −4 s −1 at 1.3 mbar and 300 K. The dependence of the rate constant on the pressure of pure formaldehyde in the range from 0.1 to 13 mbar was determined. At low pressures the interconversion because of wall collisions was observed. After correcting the measurements for the wall collisions, the pressure dependence of the rate constant for the ortho to para conversion in formaldehyde–formaldehyde collisions k 1 could be determined to be k 1=(2.1±0.5)×10 −4 p/ p 0 [s −1], with p 0=1 mbar.

Magnetic Field Acceleration of the ortho-paraH2Conversion on Transition Oxides

A new electron-nucleus resonant mechanism is shown to increase the ortho-para conversion rate of hydrogen molecules adsorbed on transition metal oxides. This mechanism is suggested to interpret the observed conversion rate accelerations by an external magnetic field and similar effects in a variety of chemical reactions. Since the resonances occur for stringent relations between catalyst and molecular parameters, the magnetic field acceleration is very sensitive to catalyst pretreatments and constitutes a valuable tool to improve the catalytic rates.

High-Temperature Limit for the O2-Catalyzed Ortho-Para Conversion Rate in Solid Hydrogen

Ortho-para conversion in solid hydrogen doped with small concentration of molecular oxygen (100 and 1000 ppm) was studied by pulsed NMR in the temperature range 4.2–10.9 K. Conversion rate was mainly determined by the paramagnetic oxygen impurities. The catalyzed conversion rate constant increases with temperature in the range 4.2–8 K and becomes independent of temperature above 8 K. The high-temperature limit is 1.15(8) s−1. The catalyzed conversion rate was shown to become temperature independent when the ortho-H2diffusion time τdfalls below the conversion time τccin the first coordination sphere of O2impurity. The obtained value of τcc=10.5(8) s was used to identify the position of O2in H2lattice as substitutional. The temperature dependence of the ortho-H2diffusion rate near the high temperature limit was determined.

Violation of nuclear-spin symmetry in collisions of symmetric tops

Evidence of nuclear-spin symmetry violation in molecular collisions is presented. Measurements of the ortho-para conversion rates in 12 CH 3F embedded in an gaseous oxygen or nitrogen shows that oxygen speeds up the nuclear spin conversion by 2×10 −4 s −1/Torr contrary to nitrogen which slows down the conversion by 1×10 −4 s −1/Torr. The difference is attributed to direct collisional transitions between the ortho and para states in CH 3F due to the large oxygen magnetic moment, which is much smaller in the case of nitrogen.

A High Pressure Study of Ortho-para Conversion in Hydrogen by NMR

We have developed a system for studying NMR in a diamond anvil cell (DAC) resulting in an increased sensitivity of a few orders of magnitude. The DAC was loaded with solid hydrogen and studied to a pressure of 12.8 GPa in the temperature range of 4.2 to 77.3 K. We measured free induction decays and spin echoes. From this we could determine the ortho-para conversion rate. Conversion measurements were made at both low and high temperatures and are possible because at high pressure the sample remains solid. The rate constant rises to 58 %/hr at our highest pressure.

Natural ortho-para conversion rate in liquid and gaseous hydrogen

We measured natural ortho-para conversion rates within a wide region of hydrogen fluid states: at temperatures 17–32 K in the liquid and at temperatures 40–120 K in the gas for densities up to 0.09 g/cm3. The experimental data for the gas phase are interpreted within the framework of a theory, the basic distinction of which from Wigner’s approach is that we take into account the dependence of the closest distance in a collision of two orthomolecules on their velocity (i.e., temperature). Our theory yields results in good qualitative agreement with experiment in the gas phase. In order to describe the entire bulk of conversion rate data in the hydrogen fluid phases we suggest a convenient interpolation formula, which has reasonable physcial grounds.

Specific heat of quench-condensed hydrogen films

The specific heatC(X, T) of quench-condensed films of H2 has been measured as a function of ortho concentration X with 0.28<X<0.75 for coverages between 24.3 and 92.3 A−2 at temperatures between 0.4 and 3.0 K. The films were condensed on evaporated gold substrates held at several temperaturesT. cond between 1.0 and 3.5 K. The observed specific heat is attributed to orientational ordering of the ortho-H2 molecules. For the films withX = 0.74 condensed atT cond>2.5 K, there is a peak which indicates a bulk-like ordering transition. At temperatures below the peak, there is a large contribution toC, which is not present in bulk H2, and which we attribute to short-range ordering size effects. AsT cond is decreased below 2.5 K, the shape of the specific heat curve changes, and the peak at 1.5 K is replaced by a gradual rise with a sharp drop above 2.6 K. Despite this strong dependence ofC onT cond, the entropy per molecule at 3 K is only weakly dependent onT cond and comparable to that for bulk H2. Film annealing at 3.4 K produces a change in the specific heat curve, and a study of this effect is presented. The ortho-para conversion rate of the films condensed at the various temperatures is found to be same as in bulk, well-annealed H2. As in bulk H2, the transition temperature inferred from the location of the specific heat peak or anomaly decreases withX. Unlike in bulk H2, there is no temperature hysteresis inC for any of the quench-condensed films. This implies that the ordering transitions are not accompanied by a martensitic transformation.

Ortho-para H2 conversion on a cold Ag(111) metal surface.

Electron-energy-loss experiments of H 2 physisorbed on noble metals at low temperature indicate that the ortho-para conversion rate is 2 orders of magnitude faster on a Ag(111) surface than on a Cu(100) one. We suggest a process in which a metal electron is virtually transferred back and forth from a surface band to the molecule antibonding orbital, the ortho-para energy being dissipated by metal (electron-hole) triplet pairs. The ortho-para rates are about 1 min on Ag(111) and 1 h on Cu(100), in agreement with experimental data