Pressure Shift Research Articles

P.760 Antidepressant-like activity of cyclo-l-prolylglycine and its possible mechanism

Two quantum cascade lasers were used to measure line intensities and line shape parameters of nitric oxide rovibrational transitions in the R-branch of the fundamental band near 1900 cm−1 in a heated static cell with temperatures from 294 to 802 K and in a shock tube from 1000 to 2500 K. Pressure in the static cell and shock tube experiments ranged from 0.025 to 1 atm and 1.5 to 3.2 atm, respectively. Collision broadening and pressure shift coefficients along with their temperature dependence were determined for nitrogen, argon, and air collision partners. The line shapes of high J” rovibrational transitions (i.e. R(39.5)–R(43.5)) were measured for the first time at elevated temperatures, where these transitions become sufficiently strong. Additionally, several hot band transitions were measured. Measured line intensities show good agreement with those measured by other researchers and tabulated in the HITRAN databases. Comparisons between the temperature exponents measured in the static cell and shock tube provide evidence that the power law often used to define the temperature dependence of collisional effects does not hold outside the temperature range studied.

Exploration of the thermal shift parameters for the six luminescence peaks in Nd3+-doped YAlO3

In this paper, we analysis the thermal shifts of six luminescence peaks (R1 → Y2,3,5, R1 → Z3, R2 → Y1, R2 → Z3) in Nd3+-doped YAlO3 crystal with a full expression including both the static effect (characterized by the static parameter A) from lattice thermal expansion and the dynamic effect (characterized by the electron-phonon coupling parameter α′) originating from the electron-phonon interaction. The static effect is evaluated from the pressure shift of the spectral peak. The two thermal shift parameters A and α′ for these peaks are attained by means of an approximate treatment on the full expression. The results indicate that differing from the previous opinion, even in the ‘hard’ crystal YAlO3:Nd3+, the static effects to thermal shifts of most peaks are important and cannot be ignored. The estimated thermal shift parameters A and α′ for the spectral peaks of YAlO3:Nd3+ in order of magnitude are the same as those of various 4fn ions in crystals found previously and can be seen as reasonable. The cause of negative static parameters A (which were not reported previously) for some peaks is discussed.

A Cr3+ luminescence study of natural topaz Al2SiO4(F,OH)2 up to 60 GPa

Abstract Topaz [Al2SiO4(F,OH)2] is a subduction-related mineral that is found in metasediments and has a large pressure and temperature stability field. Here, we use luminescence spectroscopy of Cr3+ to probe the Al site in topaz at pressures up to ~60 GPa, which corresponds to a depth of ~1400 km in the Earth. This technique allows us to probe all three unique Al environments (i.e., [AlO4(OH)2]7–, [AlO4(F)2]7–, and [AlO4OH,F]7–) simultaneously under high pressure. We find that the R-line luminescence from all three Al environments shift linearly to longer wavelength to ~40 GPa. Above ~40 GPa, they shift nonlinearly and begin to flatten out at ~48 GPa, with a pressure shift of ~0 cm–1/GPa from ~48–55 GPa. Our results, combined with previous high-pressure single-crystal X-ray diffraction studies to ~45 GPa, strongly indicate that there is a change in the compression mechanism in topaz above ~40 GPa. Our high-pressure room-temperature results show that the metastable persistence of topaz on compression represents one of the most extreme cases among tetrahedrally coordinated silicates.

Efficient two-color two-step laser ionization schemes of λ1∼ 250 nm and λ2 = 307.9 nm for heavy refractory elements-Measurements of ionization cross-sections and hyperfine spectra of tantalum and tungsten.

We demonstrated efficient two-color two-step laser ionization schemes in the combined use of λ1 ∼ 250 nm and λ2 = 307.9 nm, which are applicable to heavy refractory elements with an atomic number in the wide range of Z = 69-78. We investigated newly observed ionization schemes of tantalum and tungsten atoms in an argon-gas-cell-based laser ion source for the efficient ionization of atoms of unstable nuclei through the two-color two-step laser resonance ionization technique. We experimentally determined the ionization cross sections from the measured saturation curves by solving the rate equations for the ground, intermediate, and ionization continuum populations. Hyperfine structures of these elements were also studied to deduce the isotope-shift, pressure-shift, and pressure-broadening in the resonance spectra of the excitation transitions in the argon gas cell. The electronic factor F255 of the excitation transition λ1 = 255.2115 nm between the ground and intermediate states was deduced from the measured isotope shifts of stable 182,183,184,186W isotopes. The ionization schemes investigated here are applicable to extract any isotopes of these elements by considering the measured pressure shift and nuclear isotope shift in optimizing the wavelength λ1.

Phase-Dependency of Medial-Lateral Balance Responses to Sensory Perturbations During Walking.

The human body is mechanically unstable during walking. Maintaining upright stability requires constant regulation of muscle force by the central nervous system to push against the ground and move the body mass in the desired way. Activation of muscles in the lower body in response to sensory or mechanical perturbations during walking is usually highly phase-dependent, because the effect any specific muscle force has on the body movement depends upon the body configuration. Yet the resulting movement patterns of the upper body after the same perturbations are largely phase-independent. This is puzzling, because any change of upper-body movement must be generated by parts of the lower body pushing against the ground. How do phase-dependent muscle activation patterns along the lower body generate phase-independent movement patterns of the upper body? We hypothesize that when a sensory system detects a deviation of the body in space from a desired state that indicates the onset of a fall, the nervous system generates a functional response by pushing against the ground in any way possible with the current body configuration. This predicts that the changes in the ground reaction force patterns following a balance perturbation should be phase-independent. Here we test this hypothesis by disturbing upright balance in the frontal plane using Galvanic vestibular stimulation at three different points in the gait cycle. We measure the resulting changes in whole-body center of mass movement and the location of the center of pressure of the ground reaction force. We find that the magnitude of the initial center of pressure shift in the direction of the perceived fall is larger for perturbations late in the gait cycle, while there is no statistically significant difference in onset time. These results contradict our hypothesis by showing that even the initial CoP shift in response to a balance perturbation depends upon the phase of the gait cycle. Contrary to expectation, we also find that the whole-body balance response is not phase-independent. Both the onset time and the magnitude of the whole-body center of mass shift depend on the phase of the perturbation. We conclude that the central nervous system recruits any available mechanism to generate a functional balance response by pushing against the ground as fast as possible in response to a perturbation, but that the different mechanisms available at different phases in the gait cycle are not equally strong, leading to phase-dependent differences in the overall response.

Oxygen broadening and shift coefficients in the ν6 band of methyl iodide (12CH3I) at room temperature

In this study we report the high-resolution measurements of oxygen pressure- broadening and pressure-induced shift coefficients for rovibrational transitions in the ν6 band of methyl iodide (12CH3I), centered at 892.918 cm–1. The results were obtained by analyzing fourteen high-resolution room temperature laboratory absorption spectra with a mono-spectrum non-linear least squares fitting of Voigt profiles. The data were recorded with the Bruker IF125HR Fourier transform spectrometer located at the LISA facility in Créteil, using a White type cell with a path length of 564.9 cm and total pressures up to 295 hPa. The measured oxygen-broadening coefficients range from 0.0648 to 0.1207 cm–1atm–1 at 295 K. The measured shift coefficients were all negative and varied between −0.00044 and −0.04984 cm–1atm–1. The average accuracy on the measured O2-broadening coefficients and pressure shift coefficients was estimated to about 4% and 11%, respectively. The O2-broadening coefficients obtained in the present work are compared with values reported in the literature for the ν5 band of CH3I, showing a satisfactory agreement with an average difference of about 8%. The shift coefficients are compared with values reported in the literature for the ν6 band of CH3F-Ar system, exhibiting the same order of magnitude and trend. The J and K rotational dependences of the O2-broadening coefficients have been observed and the latter modeled using empirical polynomial expansions. On average, the empirical expression reproduces the measured O2-broadening coefficients to within 3%. Using the measured broadening coefficients of the CH3I-O2 and CH3I-N2 [Attafi et al., J Quant Spectrosc Radiat Transf 231 (2019) 1–8] systems, we produced CH3I-air broadening coefficients, ranging from 0.0783 to 0.1385 cm–1atm–1 at 295 K. The present results and the data already available should be valuable not only for predicting the CH3I infrared spectrum in the atmosphere, but also for verifying theoretical calculations of pressure-broadening and pressure-shift coefficients in the ν6 region of methyl iodide spectra.

A Resonant Pressure Microsensor With a Stress Isolation Layer

This paper presents a resonant pressure microsensor composed of a glass cap for hermetic package, an SOI layer with sensing elements and a glass base for stress isolation. Tested pressure under measurement bends the pressure sensitive diaphragm, producing frequency shifts of underlining resonators while the stress isolation layer can work as a cushion layer to minimize environmental temperature and mechanical disturbances. Numerical simulations were conducted, finding that the stress isolation layer can decrease the axial stresses of the resonators from +16 to +2 MPa (high-temperature challenging), from −12 to −5 MPa (low-temperature challenging), and from 2.5 to 0.5 MPa (mechanical challenging). The proposed microsensor was fabricated based on conventional microfabrication and characterized with comparison to the counterpart without the stress isolation layer, producing an accuracy of 0.01% FS vs. 1% FS, a temperature shift of 37 ppm versus 10088 ppm, a pressure shift of 45 ppm versus 777 ppm, a temperature hysteresis error of 21 ppm versus 187 ppm, a pressure hysteresis error of 22 ppm versus 265 ppm, a temperature repeatability error of 15 ppm versus 703 ppm, a pressure repeatability error of 21 ppm versus 36 ppm, and a long-term stability of >7 months versus 69 days. These results validated the functionality of the proposed resonant pressure microsensor with the stress isolation layer, which may be potentially used as an effective tool in the fields of aerospace aviation, industrial control, and meteorological monitoring.

H2-induced pressure broadening and pressure shift in the P-branch of the ν3 band of CH4 from 300 to 655 K

For accurate modelling of observations of exoplanet atmospheres, quantification of the pressure broadening of infrared absorption lines for and by a variety of gases at elevated temperatures is needed. High-resolution high-temperature H2-pressure-broadened spectra are recorded for the CH4ν3-band P-branch. Measured linewidths for 143 transitions between 2840 and 3000 cm−1 with temperature and pressures ranging between 300 and 655 K, and 10 and 933 Torr, respectively, were used to find rotation- and tetrahedral-symmetry-dependent coefficients for pressure and temperature broadening and pressure-induced lineshifts. The new pressure-broadening data will be useful in radiative-transfer models for retrieving the properties of observed expolanet atmospheres.

Resonant Pressure Micro Sensors Based on Dual Double Ended Tuning Fork Resonators.

This paper presents resonant pressure micro sensors based on dual double ended tuning fork (DETF) resonators, which are electrostatically excited and piezoresistively detected. In operation, the barometric pressure under measurement bends the pressure sensitive diaphragm functioning as the anchor of DETF resonators and therefore produces eigenfrequency shifts of the resonators. Theoretical analyses and finite element analyses (FEA) were conducted to optimize the key geometries of the DETF resonators with enhanced signal to noise ratios (SNRs). In fabrications, key steps including deep reactive ion etching (DRIE) and anodic bonding were used, where sleeve holes were adopted to form electrical connections, leading to high-efficiency structure layout. Experimental results indicate that the presented micro sensors produced SNRs of 63.70 ± 3.46 dB in the open-loop characterizations and differential sensitivities of 101.3 ± 1.2 Hz/kPa, in the closed-loop characterizations. In addition, pressure cycling tests with a pressure range of 5 to 155 kPa were conducted, revealing that the developed micro sensors demonstrated pressure shifts of 83 ± 2 ppm, pressure hysteresis of 67 ± 3 ppm, and repeatability errors of 39 ± 2 ppm. Thus, the developed resonant pressure micro sensors may potentially function as an enabling tool for barometric pressure measurements.

R-branch line intensities and temperature-dependent line broadening and shift coefficients of the nitric oxide fundamental rovibrational band

Two quantum cascade lasers were used to measure line intensities and line shape parameters of nitric oxide rovibrational transitions in the R-branch of the fundamental band near 1900 cm−1 in a heated static cell with temperatures from 294 to 802 K and in a shock tube from 1000 to 2500 K. Pressure in the static cell and shock tube experiments ranged from 0.025 to 1 atm and 1.5 to 3.2 atm, respectively. Collision broadening and pressure shift coefficients along with their temperature dependence were determined for nitrogen, argon, and air collision partners. The line shapes of high J” rovibrational transitions (i.e. R(39.5)–R(43.5)) were measured for the first time at elevated temperatures, where these transitions become sufficiently strong. Additionally, several hot band transitions were measured. Measured line intensities show good agreement with those measured by other researchers and tabulated in the HITRAN databases. Comparisons between the temperature exponents measured in the static cell and shock tube provide evidence that the power law often used to define the temperature dependence of collisional effects does not hold outside the temperature range studied.

Impact of fluid property shift and capillarity on the recovery mechanisms of CO2injection in tight oil reservoirs

AbstractThe phase equilibria with the confinement effect could shift in nano‐pores, which could have a great impact on the recovery mechanisms of CO2injection in tight oil reservoirs; this has not been systematically studied. In this paper, the confinement effect with property shift and capillarity effect is introduced into the flash calculation of confined fluids. The Soave modification of the Redlich–Kwong equation of state is extended by the molecular‐wall collision parameter to describe the shifted pressure–volume–temperature properties of confined fluid, and the Young–Laplace equation is applied to evaluate the capillary pressure. This developed model could effectively be applied for phase equilibrium calculation in tight porous media because of the verification of experimental results. A binary mixture is investigated to study the different effect of capillary pressure and property shift on phase equilibria. Subsequently, a typical hydrocarbon fluid from Middle Bakken tight oil reservoirs is studied with CO2injection. Results illustrate that the confinement effect could play an increasingly important part in the phase equilibrium state. The CO2solubility and mass transfer driving force in tiny pores would be greater than those in large pores under the same conditions. The gas phase saturation would be smaller with the same compositions, which could extend the single‐phase region of fluid flow in porous media. Furthermore, bubble‐point pressure, the minimum miscible pressure of CO2/hydrocarbon, and the viscosity of tight oil dissolved with CO2both decrease with the pore size, which has a good influence on tight oil recovery. In general, the confinement effect could effectively reinforce the recovery mechanisms of CO2injection, which is conducive to the enhancement of tight oil recovery. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Absolute frequency measurement of molecular iodine hyperfine transition at 534 nm

We report absolute frequency measurements of 19 hyperfine components of the rovibrational transition of molecular iodine R(53) 31-0 transitions at 534 nm with an optical frequency comb. The 534 nm laser, which is frequency-doubled from a 1068 nm external-cavity diode laser, is frequency stabilized to a hyperfine component of the I2127 R(53) 31-0 transition using modulation transfer spectroscopy. A frequency stability of 8×10−12 at 1 s averaging time is achieved when its frequency is stabilized to the a21 component. To obtain the absolute frequency, the pressure shift and power shift of the a21 component are investigated. The absolute frequency of the laser locked to the a21 hyperfine component of the R(53) 31-0 transition is measured to be 561 420 951 318±8 (type-A) ±12 (type-B) kHz. The observed hyperfine transitions of the iodine can be good frequency references at 534 nm and are especially useful for research on the aluminum ion optical clock, because the frequency of the iodine hyperfine transitions is close to the second sub-harmonic frequency of the S01-P13 transition of the aluminum ion at 267 nm.

Whole body ultrasound in the operating room and intensive care unit.

Whole body ultrasound can be used to improve the speed and accuracy of evaluation of an increasing number of organ systems in the critically ill. Cardiac and abdominal ultrasound can be used to identify the mechanisms and etiology of hemodynamic instability. In hypoxemia or hypercarbia, lung ultrasound can rapidly identify the etiology of the condition with an accuracy that is equivalent to that of computed tomography. For encephalopathy, ocular ultrasound and transcranial Doppler can identify elevated intracranial pressure and midline shift. Renal and bladder ultrasound can identify the mechanisms and etiology of renal failure. Ultrasound can also improve the accuracy and safety of percutaneous procedures and should be currently used routinely for central vein catheterization and percutaneous tracheostomy.

Zircon and quartz inclusions in garnet used for complementary Raman thermobarometry: application to the Holsnøy eclogite, Bergen Arcs, Western Norway

Mineral inclusions are common and have been widely used to investigate complex geological history. When a rock undergoes cooling and decompression after the entrapment of an inclusion into a host mineral, residual pressure may develop within the inclusion because of the differences in thermal expansivity and compressibility between the inclusion and host. By combining laser Raman spectroscopy and experimental data relating hydrostatic pressure and Raman shift, it is possible to estimate the entrapment pressure–temperature (P–T) conditions using an isotropic elastic model. In this study, we report Raman spectroscopic data on both zircon and quartz inclusions in garnet host from the Holsnoy eclogite, Bergen Arcs, Norway. Averaged residual pressures based on different Raman peaks for zircon and quartz inclusions are obtained to be ca. 0.6 GPa and ca. 0.65 GPa respectively. Using the equation of state for zircon and quartz, the entrapment P–T conditions are constrained to be 1.7–1.9 GPa, 680–760 °C, consistent with previous estimates based on phase equilibria. Heating/cooling experiments are performed on an entrapped zircon inclusion. A clear trend is found between the residual zircon inclusion pressure and the externally controlled temperature. We show that the residual zircon inclusion pressure sealed in garnet host is very sensitive to the entrapment temperature, and can be used as a Raman thermometer. The effects of laser heating and the thermo-elastic anisotropy of zircon inclusion are quantified and discussed.

Effect of high pressure on magnetic properties of CrMnFeCoNi high entropy alloy

Abstract The temperature and magnetic field dependences of magnetization of the Cantor CrMnFeCoNi alloy were studied at ambient and high (up to 1 GPa) external pressure. The low-temperature spin-glass-like behavior and the paramagnetic behavior at temperatures above 100 K with effective paramagnetic moment of the alloy, meff = 2.71 μB/f.u., were observed. A negative value of the paramagnetic Curie temperature, θC = −210 K, was found that confirms a presence of strong antiferromagnetic interactions of moments in the alloy. A model of the magnetic clusters with the effective moment of mcl = 0.275 μB/f.u. provides a possibility to describe qualitatively the pronounced difference between the pressure-induced decrease of magnetization of the ZFC and the FC alloys, dlnM/dP(5 K) = −15.5*10−3 GPa−1 and −39.2*10−3 GPa−1, respectively. The former value is in an excellent agreement with the value of −16*10−3 GPa−1 obtained from our quantum–mechanical calculations. The negative pressure shift of the ordering temperature of the alloy, dTord/dP = −9 K/GPa, and the pressure-induced decrease of magnetization of the alloy in paramagnetic state, dlnM/dP(90 K) = −70*10−3 GPa−1, indicate a possible itinerant character of magnetism of the alloy. The results of theoretical calculations of the total moment of the CrMnFeCoNi fcc-alloy are discussed.

CFD simulation of methane flow through an orifice plate

In this paper the effects of errors in the flow rate and discharge coefficient when Methane (CH 4 ) flow through orifice plates with flange tapping has been investigated by the Computational Fluid Dynamics (CFD). The location of vena- contracta was also estimated from CFD simulations. Outcomes of the CFD simulations are in terms of profiles of pressure and velocity. In this study a numerical investigation of the flow characteristics behavior caused by a Methane (CH 4 ) flow through an orifice configuration is reported. This study investigates the following parameters; orifice pressure drop, flow rate and shift in discharge coefficient. In the present work, k-e turbulence model with density based solver has been used to predict velocity fields, pressure loss, shift in flow rate and discharge coefficient around this device. It is observed that the flow is jet-like flow in the core field and the presence of recirculation zone.

Modulation of anticipatory postural adjustments using a powered ankle orthosis in people with Parkinson’s disease and freezing of gait

BackgroundFreezing of gait (FOG) during gait initiation in people with Parkinson’s disease (PD) may be related to a diminished ability to generate anticipatory postural adjustments (APAs). Externally applied perturbations that mimic the desired motion of the body during an APA have been demonstrated to shorten and amplify APAs; however, no portable device has been tested. In this study, a portable powered ankle-foot orthosis (PPAFO) testbed was utilized to investigate the effect of mechanical assistance, provided at the ankle joint, on the APAs during gait initiation. Research questionDoes mechanical assistance provided at the ankle joint improve APAs during gait initiation in people with PD and FOG? MethodsThirteen participants with PD and FOG initiated gait across five test conditions: two self-initiated (uncued) conditions in walking shoes [Baseline-Shoes], and the PPAFO in unpowered passive mode [Baseline-PPAFOPassive]; three “go” cued conditions that included an acoustic tone with the PPAFO in unpowered passive mode [Acoustic + PPAFOPassive], the mechanical assistance from the PPAFO [PPAFOActive], and the acoustic tone paired with mechanical assistance [Acoustic + PPAFOActive]. A warning-cue preceded the imperative “go” cue for all the cued trials. Peak amplitudes and timings of the vertical ground reaction forces (GRFs) and center of pressure (COP) shifts from onset to toe-off were compared across conditions. ResultsMechanical assistance significantly increased the peak amplitudes of the GRFs and COP shifts, reduced APA variability, and decreased the time to toe-off relative to the passive conditions. SignificanceThese findings demonstrate the potential utility of mechanical assistance at the ankle joint (with or without an acoustic cue) as a method to generate more consistent, shortened, and amplified APAs in people with PD and FOG.

Pressure shifts in high-precision hydrogen spectroscopy. I. long-range atom–atom and atom–molecule interactions

We study the theoretical foundations for the pressure shifts in high-precision atomic beam spectrosopy of hydrogen, with a particular emphasis on transitions involving higher excited P states. In particular, the long-range interaction of an excited hydrogen atom in a 4P state with a ground-state and metastable hydrogen atom is studied, with a full resolution of the hyperfine structure. It is found that the full inclusion of the 4P_1/2 and 4P_3/2 manifolds becomes necessary in order to obtain reliable theoretical predictions, because the 1S ground state hyperfine frequency is commensurate with the 4P fine-structure splitting. An even more complex problem is encountered in the case of the 4P-2S interaction, where the inclusion of quasi-degenerate 4S-2P_1/2 state becomes necessary in view of the dipole couplings induced by the van der Waals Hamiltonian. Matrices of dimension up to 40 have to be treated despite all efforts to reduce the problem to irreducible submanifolds within the quasi-degenerate basis. We focus on the phenomenologically important second-order van der Waals shifts, proportional to 1/R^6 where R is the interatomic distance, and obtain results with full resolution of the hyperfine structure. The magnitude of van der Waals coefficients for hydrogen atom-atom collisions involving excited P states is drastically enhanced due to energetic quasi-degeneracy; we find no such enhancement for atom-molecule collisions involving atomic nP states, even if the complex molecular spectrum involving ro-vibrational levels requires a deeper analysis.

Pressure shifts in high-precision hydrogen spectroscopy: II. Impact approximation and Monte-Carlo simulations

We investigate collisional shifts of spectral lines involving excited hydrogenic states, where van der Waals coefficients have recently been shown to have large numerical values when expressed in atomic units. Particular emphasis is laid on the recent hydrogen 2S-4P experiment (and an ongoing 2S-6P experiment) in Garching, but numerical input data are provided for other transitions (e.g., involving S states), as well. We show that the frequency shifts can be described, to sufficient accuracy, in the impact approximation. The pressure related effects were separated into two parts, (i) related to collisions of atoms inside of the beam, and (ii) related to collisions of the atoms in the atomic beam with the residual background gas. The latter contains both atomic as well as molecular hydrogen. The dominant effect of intra-beam collisions is evaluated by a Monte-Carlo simulation, taking the geometry of the experimental apparatus into account. While, in the Garching experiment, the collisional shift is on the order of 10 Hz, and thus negligible, it can decisively depend on the experimental conditions. We present input data which can be used in order to describe the effect for other transitions of current and planned experimental interest.

Pressure shifts in high-precision hydrogen spectroscopy. I. Long-range atom–atom and atom–molecule interactions

We study the theoretical foundations for the pressure shifts in high-precision atomic beam spectrosopy of hydrogen, with a particular emphasis on transitions involving higher excited P states. In particular, the long-range interaction of an excited hydrogen atom in a 4P state with a ground-state and metastable hydrogen atom is studied, with a full resolution of the hyperfine structure. It is found that the full inclusion of the 4P1/2 and 4P3/2 manifolds becomes necessary in order to obtain reliable theoretical predictions, because the 1S ground state hyperfine frequency is commensurate with the 4P fine-structure splitting. An even more complex problem is encountered in the case of the 4P–2S interaction, where the inclusion of quasi-degenerate 4S–2P1/2 state becomes necessary in view of the dipole couplings induced by the van der Waals Hamiltonian. Matrices of dimension up to 40 have to be treated despite all efforts to reduce the problem to irreducible submanifolds within the quasi-degenerate basis. We focus on the phenomenologically important second-order van der Waals shifts, proportional to 1/R6 where R is the interatomic distance, and obtain results with full resolution of the hyperfine structure. The magnitude of van der Waals coefficients for hydrogen atom–atom collisions involving excited P states is drastically enhanced due to energetic quasi-degeneracy; we find no such enhancement for atom–molecule collisions involving atomic nP states, even if the complex molecular spectrum involving ro-vibrational levels requires a deeper analysis.